phuong phap phan tich pho nguyen tu

PHM LUN

PHNG PHP PHN TCH PH NGUYN T(In Ln th 2 c sa cha v b sung)

NH XUT BN I HC QUC GIA H NI

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LI M U Php o ph pht x v hp th nguyn t l nhng k thut phn tch ha l, v ang c pht trin v ng dng rng ri trong nhiu ngnh khoa hc k thut, trong sn xut cng nghip, nng nghip, y dc, a cht, ha hc. c bit cc nc pht trin, phng php phn tch ph pht x nguyn t tr thnh mt trong cc phng php dng phn tch lng vt cc kim loi trong nhiu i tng khc nhau nh t, nc, khng kh, thc phm, v.v... Hin nay trong cng tc nghin cu bo v mi trng, phng php phn tch ny l mt cng c c lc xc nh cc kim loi nng c hi. nc ta, k thut phn tch theo ph pht x v hp th nguyn t cng c pht trin v ng dng trong khong hn hai chc nm nay. Mt s phng th nghim c trang b my o ph pht x v hp th nguyn t, hoc do Nh nc ta u t, hoc do s vin tr ca cc t chc nc ngoi theo cc chng trnh khc nhau. Mt s cn b khoa hc ca ta c c ra nc ngoi hc tp, nghin cu v o to. Song i a s khng c iu kin , nhng li cn s dng k thut phn tch ny cho cng vic phn tch ca h. Hu ht cc ti liu hay sch khoa hc v k thut ny li bng ting Anh. Mt khc, chng ta li cha c mt cun sch hoc cc ti liu c s l thuyt bng ting Vit v k thut phn tch ny. Do thc th , ng gp cho cng tc o to sinh vin trn lnh vc k thut phn tch ny ca Khoa Ha hc, Trng i hc Khoa hc T nhin - i hc Quc gia H Ni, cng nh to iu kin cho mt s cn b khoa hc, cng nhn vin phn tch ca cc phng th nghim khc c ti liu tham kho, chng ti mnh dn bin son cun sch ny. N c coi nh l gio trnh c s l thuyt v Phng php phn tch ph nguyn t. y cng l ni dung nhm phc v o to cao hc v k thut phn tch ny. y l gio trnh ting Vit u tin v k thut phn tch hin i ny nn khng th trnh khi nhng hn ch v thiu st nht nh. V th rt mong s ng gp thm kin ca cc bn b ng nghip v cc c gi quan tnm, tc gi c iu kin b sung cho hon chnh hn. Nhn dp ny chng ti xin chn thnh cm n GS. TS. J.F.M. Maessen, GS. TS. J. Kragton, GS. TS. Ban, TS. J.C. Kraak, cc k s H. Balker v J.W. Elgersma (khoa Ha, trng i hc tng hp Amsterdam, H Lan), GS. TSKH. Trnh Xun Gin (Vin Ha), TS. Nguyn Hong (HQGHN), PGS. TS. Phm Gia Hu (H Dc) v cc ng nghip B mn Ha Phn tch khoa Ha hc c nhiu kin ng gp cho ni dung ca gio trnh ny. Phm Lun

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PHN IChng 1 I CNG V PHNG PHP AES

1.1 S phn loi ph Phn tch quang ph l tn gi chung cho mt h cc phng php phn tch quang hc da trn c s ng dng nhng tnh cht quang hc ca nguyn t, Ion, phn t v nhm phn t. V d, tnh cht pht x hay hp th quang ca nguyn t, tinh cht hp th quang ca phn t, v.v... V vy ty theo quan nim, da theo nhng iu kin kch thch ph, phng tin thu ghi v quan st ph, cng nh bn cht ca qu trnh sinh ra ph m ngi ta c mt s cch phn chia thnh nhng php o khc nhau, nh php o ph pht x nguyn t, hp th nguyn t, php o ph hng ngoi,... Tuy th, nhng c hai cch phn chia sau y l ph hp hn: 1.1.1 S phn chia theo c trng ca ph Theo cch ny ngi ta c nhng phng php phn tch quang hc sau: Phng php phn tch ph nguyn t, gm c: a) Ph php x nguyn t b) Ph hp th nguyn t c) Ph hunh quang nguyn t y l ph do s chuyn mc nng lng ca cc in t ha tr ca nguyn t trng thi kh (hi) t do, khi b kch thch m sinh ra. Phng php phn tch ph phn t, gm c: a) Ph hp th phn t trong vng UV-VIS b) Ph hng ngoi (IR v NIR) c) Ph tn x Raman Ph ny c quyt nh bi cc in t ha tr ca nguyn t trong phn t, l nhng in t ha tr nm trong lin kt hay mt cp cn t do, chuyn mc nng lng khi b kch thch. Ph Rn-ghen (tia X), l ph ca in t ni ca nguyn t, gm c: + Ph pht x tia X + Ph hunh quang tia X + Ph nhiu x tia X 3Su t m b i: www.daihoc.com.vn

Ph cng hng t, gm: a) Cng hng t in t (ERMS). b) Cng hng t proton (ht nhn: NRMS)) Phng php phn tch khi ph: Ph ny c quyt nh bi khi lng ca cc Ion phn t hay cc mnh Ion ca cht phn tch b ct ra (t s m/z). y l cch phn chia c s dng rng ri v c coi nh l hp l nht v tng ng vi ngay tng php o c th.. 1.1.2 Phn chia theo di sng Nh chng ta bit, bc x in t c mi bc sng, t sng di hng ngn mt n sng ngn vi micromet hay nanomet. Do ph ca bc x in t y phi cha ht tt c cc vng sng . Nhng trong thc t khng c mt dng c quang hc no c th c kh nng thu nhn, phn li hay pht hin c ton b vng ph nh th. V th ngi ta chia ph in t thnh nhiu min (vng ph) khc nhau. l nguyn tc ca cch chia th hai ny (bng 1.1). Bng 1.1 S phn chia ph theo di sng STT 1 2 3 4 5 6 7 8 9 10 Tn vng ph Tia gama () Tia X T ngoi Kh kin Hng ngoi Sng ngn Sng Raa Sng cc ngn Tivi - FM Sng raio di sng < 0,1 nm 0,1+ 5 nm 80 + 400 nm 400 + 800 nm 1+ 400 m 400 + 1000 m 0,1+ 1 cm 0,1 - 50 cm 1 +10 m 10 + 1500 m

Trong bng 1.1, vng ph t 3 n 5 l vng ph quang hc. Cc ph ny xut hin do s chuyn mc nng lng ca cc in t ha tr ca nguyn t v phn t, khi b kch thch. Vic phn chia theo cch ny cng c nhng im cha r rng, nh trong vng t ngoi v kh kin th c c ph ca nguyn t v phn t, m bn cht ca hai loi ph v s kch thch ca hai loi li rt khc nhau. 4Su t m b i: www.daihoc.com.vn

Trn y l hai cch phn chia chnh cn hay c dng. Tt nhin cn c mt s cch phn chia khc. Nhng c nhiu nhc im v t c s dng, nn khng cp n y. S phn chia ny c tnh cht gii thiu chung ton b vng ph. Song, trong gio trnh ny chng ti ch cp n ph pht x nguyn t, v phng php phn tch ng dng tnh cht pht x ca nguyn t trng thi hi c gi l K thut phn tch ph pht x nguyn t. 1.2 S xut hin ph pht x nguyn t 1.2.1 Tm tt v cu to nguyn t Hng ngy chng ta thng quen vi nhng tn gi, st (Fe), ng (Cu) ch (Pb), bc (Ao), vng (Au), nhm (Aj), km (Zn),... l nhng nguyn t ha hc. n nay ngi ta bit trn 110 nguyn t ha hc khc nhau. Nhng v mt ha hc v theo thuyt ca alton, nguyn t ha hc bao gm nhng nguyn t ca cng mt loi v nguyn t l phn t nh nht cn gi c tnh cht ha hc ca nguyn t. Nguyn t ca mi nguyn t ha hc c cu to khc nhau nn chng c tnh cht khc nhau. Quyt nh tnh cht vt l v ha hc ca chng l cu to ca lp v electron trong nguyn t, c bit l cc in t ha tr. Nguyn t ca mi nguyn t ha hc u c xy dng t mt ht nhn nguyn t v cc electron (in t). Trong nguyn t, ht nhn gia, cc in t chuyn ng xung quanh ht nhn theo nhng qu o (orbital) tng i. Ht nhn chim th tch rt nh trong khng gian ca nguyn t (khong 1/10.000 th tch nguyn t), nhng li chim hu nh ton b khi lng ca nguyn t. Nu coi ng knh nguyn t l 10-8cm, th ng knh ht nhn ch chim khong 10-12cm. Nh vy, lp v ca nguyn t ngoi ht nhn l rt rng, n chnh l khng gian chuyn ng ca in t. S chuyn ng ca in t trong khng gian ny rt phc tp. N va tun theo quy lut ca chuyn ng sng, li va tun theo quy lut chuyn ng ca cc ht vi m. Song trong mt iu kin nht nh v mt cch tng i, ngi ta vn tha nhn cc in t chuyn ng trong khng gian ca nguyn t theo cc qu o. Nhng theo quan im hin i ca c lng t th l cc m my electron. Trong lp v nguyn t, in t phn b thnh tng lp ng vi s lng t chnh ca nguyn t (n). Trong tng lp li c nhiu qu o ng vi s lng t ph 1 ca nguyn t. l cc phn lp. Nhng theo nguyn l vng bn th in t bao gi cng chim v lm y nhng qu o c mc nng lng thp trc. Sau mi n nhng qu o c mc nng lng cao hn. Th t sp xp l: ls, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, v.v... (hnh 1.1a).


Hnh 1.1a S phn b nng lng trong nguyn t (1)-cu to nguyn t: (2)-phn b nng lng trong lp v nguyn t.


Qu trnh pht x v hp th ca mt nguyn t. E0: Nng lng trng thi c bn; cm: Nng lng trng thi kch thch; + hv v + E: Nng lng kch thch; - hv: Tia pht x.

Hnh 1.1 b S phn b nng lng trong nguyn t v sinh ph 1.2.2 S xut hin ph pht x Trong iu kin bnh thng, cc in t chuyn ng trn cc qu o ng vi mc nng lng thp nht. Khi nguyn t trng thi bn vng, trng thi c bn. trng thi ny nguyn t khng thu v cng khng pht nng lng. Nhng nu cungcp nng lng cho nguyn t th trng thi khng tn ti na. Theo quan im ca thuyt lng t, khi trng thi kh, in t chuyn ng trong khng gian ca nguyn t, c bit l cc in t ha tr, nu chng nhn c nng lng bn 7Su t m b i: www.daihoc.com.vn

ngoi (in nng, nhit nng, ha nng,...) th in t s chuyn ln mc nng lng cao hn. Khi nguyn t b kch thch. N tn ti trng thi kch thch. Nhng trng thi ny khng bn vng. Nguyn t ch lu li trng thi ny nhiu nht l 10 8 giy. Sau n lun lun c xu hng tr v trng thi c bn ban u bn vng. Ngha l gii phng nng lng m chng hp th c trong qu trnh trn di dng ca cc bc x quang hc. Bc x ny chnh l ph pht x ca nguyn t, n c tn s c tnh theo cng thc: E = (En E0) = hv hay: E = trong : En v E0 l nng lng ca nguyn t trng thi c bn v trng thi kch thch n; h l hng s Plank (6,626.10-7 erk.s) hay h = 4,1.10-15 eV.s; c l tc nh sng (3.108 m/gy) - 2,99793.108 m/gy; v l tn s ca bc x ; l bc sng ca bc x . Trong biu thc trn, nu gi tr E l nm ta c qu trnh hp th v khi gi tr E dng ta c qu trnh pht x ca nguyn t. Nhng trong nguyn t, s chuyn mc ca in t t mc nng lng En khng phi ch v mc E0, m c rt nhiu s chuyn mc t En v cc mc khc E01, E02, E03 cng vi mc E0. Ngha l c rt nhiu s chuyn mc ca in t c lng t ha, v ng vi mi bc chuyn mc ta c 1 tia bc x, tc l mt vch ph. Chnh v th m mt nguyn t khi b kch thch thng c th pht ra rt nhiu vch ph pht x. Nguyn t no c nhiu in t v c cu to phc tp ca cc lp in t ha tr th cng c nhiu vch ph pht x. Mt cch tng trng ta c th m t qu trnh kch thch v pht x ca nguyn t theo m hnh trong hnh 1.2. Nu dng my quang ph thu chm ra pht x , phn li v ghi li cc chm tia pht x do nguyn t pht ra ta s c mt di ph t sng ngn n sng di. l ph pht x ca nguyn t ca cc nguyn t v n l ph vch. Nh vy, ph pht x nguyn t l sn phm sinh ra do s tng tc vt cht, m y l cc nguyn t t do trng thi kh vi mt ngun nng lng nhit, in... nht nh ph hp. Nhng trong ngun sng, khng phi ch c nguyn t t do b kch thch, m c c Ion, phn t, nhm phn t. Cc phn t ny cng b kch thch v pht ra ph pht x ca n. Tt nhin l trong mc khc nhau ty thuc vo kh nng kch thch ca ngun nng lng. V vy, ph pht x ca vt mu lun bao gm ba thnh phn: 1. Nhm ph vch. l ph ca nguyn t v con. Nhm ph vch ny ca cc nguyn t ha hc hu nh thng nm trong vng ph t 190-1000nm (vng UVVIS). Ch c mt vi nguyn t kim hay kim loi kim mi c mt s vch ph nm ngoi vng ny. 2. Nhm ph m. l ph pht x ca cc phn t v nhm phn t. V d 8Su t m b i: www.daihoc.com.vn

(1.1)

hc

ph ca phn t MeO, CO v nhm phn t CN. Cc m ph ny xut hin thng c mt u m v mt u nht. u m pha sng di v nht pha sng ngn. Trong vng t ngoi th ph ny xut hin rt yu v nhiu khi khng thy. Nhng trong vng kh kin th xut hin rt m, v lm kh khn cho php phn tch quang ph v nhiu vch phn tch ca cc nguyn t khc b cc m ph ny che lp. 3. Ph nn lin tc. y l ph ca vt rn b t nng pht ra, ph ca nh sng trng v ph do s bc x ring ca in t. Ph ny to thnh mt nn m lin tc trn ton di ph ca mu. Nhng nht sng ngn v m dn v pha sng di. Ph ny nu qu m th cng s cn tr php phn tch. Ba loi ph trn xut hin ng thi khi kch thch mu phn tch v trong phn tch quang ph pht x nguyn t ngi ta phi tm cch loi bt ph m v ph nn. l hai yu t nhiu. 1.3 Nguyn tc ca php o ph pht x (AES) T vic nghin cu nguyn nhn xut hin ph pht x, chng ta c th khi qut phng php phn tch da trn c s o ph pht x ca nguyn t phi bao gm cc bc nh sau: 1. Trc ht mu phn tch cn c chuyn thnh hi (kh) ca nguyn t hay Ion t do trong mi trng kch thch. l qu trnh ha hi v nguyn t ha mu. Sau dng ngun nng lng ph hp kch thch m hi chng pht x. y l qu trnh kch thch ph ca mu. 2. Thu, phn 1i v ghi ton b ph pht x ca vt mu nh my quang ph. Trc y, ph c ghi ln knh nh hay phim nh. Chnh my quang ph s lm nhim v ny. Nhng nhng trang b hin i ngy nay c th thu v ghi trc tip cc tn hiu cng pht x ca mt vch ph di dng cc lc trn bng giy hay ch ra cc sng cng vch ph trn my in (printer), ghi li vo a t ca my tnh. 3. nh gi ph ghi v mt nh tnh v nh lng theo nhng yu cu t ra. y l cng vic cui cng ca php o. Chnh v vy, ng vi cc nhim v v nguyn tc ny, thc hin mt php phn tch da theo ph pht x ca nguyn t ngi ta phi cn mt h thng trang b cng gm ba phn tng ng nh th. A. Trang b c bn (ti thiu phi c): Phn 1: Ngun nng lng ha hi, nguyn t ha mu v kch thch ph ca mu phn tch, c ph ca nguyn t phn tch. Phn 2: My quang ph thu, phn li v ghi li ph pht x ca mu phn tch theo vng ph ta mong mun. Phn 3: H thng trang b nh gi nh tnh, nh lng v ch th hay biu th cc kt qu. Mt cch tng qut, ta c th m t nguyn tc ny theo s hnh 1.3.



B. Trang b hon chnh: C thm cc b phn sau: 1. B t ng bm hay a mu vo o. 11Su t m b i: www.daihoc.com.vn

2. H my tnh v phn mm ca n.

Hnh 1.3 S nguyn l ca qu trnh phn tch AES



1.4 i tng ca phng php phn tch ph pht x Bn cnh mc ch nghin cu vt l quang ph nguyn t, php o ph pht x nguyn t l mt phng php phn tch vt l da trn tnh cht pht x ca nguyn t trng thi hi xc nh thnh phn ha hc ca cc nguyn t, cc cht trong mu phn tch. V vy n c tn phn tch quang ph ha hc. Phng php ny c s dng phn tch nh tnh v nh lng cc nguyn t ha hc, ch yu l cc kim loi trong mi i tng mu khc nhau, nh a cht, ha hc, luyn kim, ha du, nng nghip, thc phm, y dc, mi trng... thuc cc loi mu rn, mu dung dch, mu bt, mu qung, mu kh. Tuy phn tch nhiu i tng, nhng thc cht l xc nh cc kim loi l chnh, ngha l cc nguyn t c ph pht x nhy, khi c kch thch bng mt ngun nng lng thch hp; sau l mt vi kim nh Si, P, C. V vy, i tng chnh ca phng php phn tch da theo php o ph pht x ca nguyn t l cc kim loi nng nh trong cc loi mu khc nhau. Vi i tng kim th phng php ny c nhiu nhc im v hn ch v nhy, cng nh nhng trang b thu, ghi ph ca chng, v ph ca hu ht cc kim li nm ngoi vng t ngoi v kh kin, ngha l phi c thm nhng trang b phc tp mi c th phn tch c cc kim. 1.5 Cc u im v nhc im Phng php phn tch quang ph pht x nguyn t s d c pht trin rt nhanh v c s dng trong nhiu lnh vc ca khoa hc, k thut cng nghip, nng nghip v i sng v n c nhng u im rt c bn: a) Phng php ny c nhy rt cao. Bng phng php ny nhiu nguyn t c th c xc nh t n nhy t n.103 n n.104 %. Nhng vi nhng trang b hin i v vi nhng ngun kch thch ph mi (ICP) ngi ta c th t n nhy t n.10-5 n n.10-6 % i. Vi nhiu nguyn t m khng cn phi lm giu mu phn tch (bng 1). V th n l phng php kim tra, nh gi tinh khit ca nhiu ha cht v nguyn liu tinh khit cao, phn tch lng vt cc kim loi nng c hi trong i tng thc phm, nc gii kht, mi trng. Trong khi vi nhng i tng ny th phng php ha hc khng th no t c. b) Phng php ny gip chng ta c th tin hnh phn tch ng thi nhiu nguyn t trong mt mu, m khng cn tch ring chng ra khi nhau. Mt khc, li khng tn nhiu thi gian, c bit l phn tch nh tnh v bn nh lng. c) Vi nhng tin b ca k thut hin nay v vi nhng trang b hin nay t c, th phng php phn tch theo ph pht x nguyn t l mt php o chnh xc tng i cao. Trong nhiu trng hp, vi nng nh (c ppm) m sai s ca php o l di 10%. Tt nhin vi nhng trang b v my mc c in th sai s c th ln hn. Song thc t n b nhng trang b hin i y li. 14Su t m b i: www.daihoc.com.vn

d) Phng php phn tch theo ph pht x l mt phng php phn tch tiu tn t mu, ch cn t 1 n vi chc miligam mu l . c bit l k thut ph pht x. e) Phng php phn tch ny c th kim tra c ng nht v thnh phn ca vt mu nhng v tr (ch) khc nhau. V th cng c ng dng kim tra ng nht ca b mt vt liu. f) Trong nhiu trng hp, ph ca mu nghin cu thng c ghi li trn phim nh, knh nh hay trn bng giy. N l nhng ti liu lu tr v khi cn thit c th nh gi hay xem xt li m khng cn phi c mu phn tch. Bn cnh nhng u im nu, phng php ny cng c mt s nhc im v hn ch nht nh. Trc ht phi ni n l: Phng php ny ch cho chng ta bit c thnh phn nguyn t ca mu phn tch, m khng ch ra c trng thi lin kt ca n trong mu. chnh xc ca php phn tch ph thuc vo nng chnh xc ca thnh phn ca dy mu u v cc kt qu nh lng u phi da theo cc ng chun ca cc dy mu u c ch to sn trc. Mc du c mt s nhc im v hn ch, nhng phng php phn tch quang ph pht x nguyn t ngy cng c pht trin v ng dng rng ri trong nhiu lnh vc khc nhau xc nh lng vt cc nguyn t trong cc i tng mu khc nhau. l mt phng php phn tch nhanh, c chnh xc bo m v nhy kh cao, c bit thch hp cho phn tch t him, cc loi nc, khng kh, v.v. 1.6 Kh nng v phm vi ng dng Phng php phn tch quang ph pht x nguyn t ngy nay gi vai tr quan trng trong ha hc phn tch. Cng vi s pht trin mnh m ca cc ngnh khoa hc k thut, c bit l vt l v ha hc, s pht trin ca k thut o v ghi tn hiu, lm tng kh nng ng dng to ln ca n. Bng phng php ny ngi ta c th xc nh nh tnh, bn nh lng v nh lng c hn nm chc kim loi v gn mt chc nguyn t kim trong cc i tng mu khc nhau (v c v hu c). Phng php phn tch ny tr thnh cng c phn tch nguyn t c lc ch nhiu lnh vc, nht l sau khi c ngun kch thch ICP. a) Phn tch quang ph pht x trong ngnh ha v cng nghip ha hc. N l cng c cc nh ha hc xc nh thnh phn nh tnh v nh lng ca nhiu cht, kim tra tinh khit ca cc ha phm, nguyn liu v nh gi cht lng ca chng. N cng l mt phng php xc nh cc ng v phng x v nghin cu cu trc nguyn t. b) Phn tch quang ph pht x nguyn t trong a cht. Ngay t khi mi ra i, phng php ny c cc nh a cht s dng phn tch cc mu qung phc v cho cng vic thm d a cht v tm ti nguyn khong sn. V th ngnh a cht ca tt c cc nc u c phng phn tch quang ph pht x rt hin i v hon chnh. 15Su t m b i: www.daihoc.com.vn

c) Phn tch quang ph pht x trong luyn kim. Luyn kim cng l mt ngnh s dng phng php phn tch quang ph pht x u tin vo mc ch ca mnh trc c ngnh ha. Chnh tnh cht nhanh chng v nhy ca phng php ny l mt iu rt cn thit i vi ngnh luyn kim. N c th l cng c gip cc nh luyn kim xc nh ngay c thnh phn ca cc cht ang nng chy trong l luyn kim; qua m h c th iu chnh nguyn liu a vo ch to c nhng hp kim c thnh phn mong mun, kim tra thnh phn, kim tra nguyn liu. d) Phn tch quang ph pht x trong tiu ch un hc. Trc y con ngi tng rng kh c th hiu bit c thnh phn ha hc ca cc hnh tinh xung quanh tri t chng ta. Nhng phng php phn tch quang ph pht x ra i m rng tm vi cho con ngi. Bng phng php ph pht x v kt hp vi mt s knh thin vn, cc nh thin vn c th quan st c thnh phn ca cc nguyn t ha hc ca cc hnh tinh khc nh mt trng, cc v sao. Chnh nhng kt qu phn tch thnh phn ca cc mu t do v tinh ly t mt trng v ni ln ngha ca php o ph pht x trong lnh vc nghin cu thin vn. V nhng kt qu phn tch thc t cc mu l rt ph hp vi nhng s liu thu c trc y qua phn tch tia sng t mt trng bng h thng my quang ph v knh thin vn. e) Phn tch quang ph pht x trong nng nghip, y v sinh hc. y l nhng ngnh khoa hc s dng phng php ny cm li nhiu kt qu rc r, c bit l trong vic nghin cu th nhng, nghin cu cc nguyn t vi lng trong t trng, trong cy trng, trong phn bn ca nng nghip, hay nghin cu thnh phn thc n phc v chn nui, phn tch nguyn t vi lng trong mu, serum, nc tiu, phc v cha bnh. f) Phn tch quang ph trong cc lnh vc khc. Ngoi nhng lnh vc nu, phng php phn tch quang ph pht x nguyn t cn c mt s ngnh khc s dng nh l mt cng c phn tch. V d nh trong ngnh dc kim tra mt s kim loi c hi trong cc dc phm (Pb, Cu, Zn, Hg,...). Ngnh nghin cu c s, cng an, to n, ngoi thng, cng l nhng ni s dng phng php phn tch quang ph trong cc lnh vc nghin cu v nng php o ph nguyn t ln ngang tm ca thi i.

Hnh 1.5 S phn chia cc loi ph theo di sng 16Su t m b i: www.daihoc.com.vn

Chng 2 S KCH THCH PH PHT X NGUYN T

2.1 Yu cu v nhim v ca ngun kch thch Trong phn tch quang ph pht x nguyn t, ngun sng c gi l ngun kch thch ph v c mt vai tr ht sc quan trng. V nh ngun nng lng kch thch ngi ta c th chuyn vt liu mu phn tch thnh trng thi hi ca cc nguyn t v kch thch m hi pht sng (pht x), ngha l ngun nng lng nh hng trc tip n kt qu ca php phn tch. V vy ngun nng lng mun dng c vo mc ch phn tch quang ph pht x nguyn t cn phi thc hin c hai nhim v sau y: Trc ht ngun nng lng phi ha hi, nguyn t ha v chuyn c hon ton cc nguyn liu mu phn tch vo vng phng in (plasma). C nh th thnh phn ca m hi trong plasma mi ng nht vi thnh phn ca vt mu. ng thi ngun nng lng phi c nng lng ln (nhit ) c th kch thch c tt nht cc nguyn t ca nguyn t cn phn tch i n pht x ra ph ca n. Song nh th vn cha hon ton . V mt phng php phn tch ch c gi tr khi n c nhy v lp li cao. Chnh v th, ngoi hai nhim v trn mt ngun sng mun dng c lm ngun kch thch ph pht x n cn cn phi tha mn mt s yu cu nht nh sau y: 1) Trc ht ngun sng phi m bo cho php phn tch c nhy cao v cng ca vch ph phi nhy vi s bin thin nng ca nguyn t phn tch; nhng li khng nhy vi s dao ng ca iu kin lm vic. 2) Ngun nng lng phi n nh v bn vng theo thi gian, m bo cho phng php phn tch c lp li v n nh cao. Ngha l cc thng s ca ngun sng chn nht thit phi duy tr v lp li c. 3) Ngun nng lng phi khng a thm ph ph vo lm ln vi ph ca mu nghin cu. Nu khng s lm kh khn thm cng vic nh gi nh tnh v nh lng v c khi lm sai lc c kt qu phn tch. 4) Ngun kch thch phi c s cu to khng qu phc tp; nhng li c kh nng thay i c nhiu thng s, c th chn c nhng iu kin ph hp theo tng i tng phn tch hay tng nguyn t. 5) Yu cu cui cng l ngun nng lng kch thch phi lm tiu hao t mu phn tch v trong mt s trng hp phi khng lm h hi mu phn tch, nh trong 17Su t m b i: www.daihoc.com.vn

kim tra thnh phm. Trn y l nhng yu cu chung, nu ngun sng tha mn c y cc yu cu l mt iu rt l tng. Song trong thc t khng c c iu kin l tng nh th. V vy, mi ngi lm phn tch quang ph pht x cn phi hiu bit tng tn v ty theo tng trng hp c th m xem yu cu no cn c ch nghim ngt trc ht v yu cu no c th chnm chc c. 2.2 Cc loi ngun kch thch ph pht x Do nhng yu cu nu trn, nn ch c mt vi loi ngun nng lng c dng lm ngun kch thch cho phng php phn tch quang ph pht x. l: Ngn la n kh; H quang in dng xoay chiu v mt chiu; Tia la in; Tia lc; Plasma cao tn cm ng (ICP); tia X. Trong cc loi ngun nng lng ny, ngn la n kh, h quang v tia la in c dng t lu, nhng c nhy khng cao (ngun sng c in). Tia lc v ICP l nhng ngun nng lng cho nhy cao. Nhng tia lc t c dng. V c c tia lc ph hp cho mc ch phn tch phi c nhng trang b phc tp, tn km. Ring ICP l ngun nng lng hin ang c ng dng ph bin v c nhiu u vit. N l ngun sng mi c pht trin trong vng hn chc nm li y. Li khng t nh ngun lc. V nhy ca cc ngun nng lng ny chng ta c th xem trong hnh 1.4 v bng ph lc A. 2.2.1 Ngn la n kh 2.2.1.1 Cu to v c im ngn la n kh Ngn la n kh l ngun nng lng u tin c dng trong phn tch quang ph pht x nguyn t, ngay t khi phng php ny ra i. Bunsen v Kirschoff l nhng ngi u tin dng ngun sng ny phn tch cc kim loi kim v kim th. Nhng do s n gin, n nh, nhy tng i v r tin, nn ngy nay n vn c s dng ph bin. 1. c im. Ngn la n kh c nhit khng cao (1700 32000C), c cu to n gin, nhng n nh v d lp li c cc iu kin lm vic. Do c nhit thp, nn ngn la n kh ch kch thch c cc kim loi kim v kim th. V ng vi loi ngun sng ny ngi ta c mt phng php phn tch ring. l phng php phn tch quang ph ngn la (Flame Spectrophotometry). Song v bn cht n vn l ph pht x ca nguyn t trong ngn la. Cc cht kh t to ra ngn la ca n kh thng l mt hn hp ca hai kh (1 kh oxy ha v 1 kh nhin liu) c trn vi nhau theo mt t l nht nh. Bn cht v thnh phn ca hn hp kh quyt nh nhit ca ngn la v hnh dng cu to ca ngn la. Di y l mt vi hn hp kh c s dng to ra ngn la trong php o ph pht x: 18Su t m b i: www.daihoc.com.vn

Bng 2.1 Nhit ca mt s loi n kh STT 1 2 3 4 5 Loi hn hp kh t Axetylen v oxy Butan v oxy Kh t v oxy Hydro v oxy Axetylen v khng kh Nhit (oC) 2400-3100 2000-2550 2200-2500 2100-2300 2000-2450

2. Cu to. V hnh dng cu to v s phn b nhit ca ngn la, n kh gm ba phn nh sau (hnh 2.3 v 2.4): Phn mt (a). Phn trong cng st ming n l phn ti (a). Trong phn ny cht t c trn v nung nng chun b t chy phn hai (b). Nhit trong phn ny thp (di 700-12000C). Phn hai (b) l li ca ngn la. Trong phn ny xy ra cc phn ng t chy cht kh. N khng mu hay c mu xanh nht v nhit l cao nht. Chnh nh b l ch c nhit cao nht ca ngn la. Nu t bng hn hp kh axtylen v oxy hay khng kh nn th nhit ca ngn la c cung cp nh phn ng: 2C2H2 + 2O2 = 4CO + 2H2 + Q Phn th ba (c) l v ca ngn la. Trong phn ny thng xy ra cc phn ng th cp, c mu vng v nhit thp. N to thnh ui v v ca ngn la.

Do s phn b nhit ca ngn la nh vy, nn kh phn tch phi a mu vo vng tnm c nhit cao nht. l vng b ca ngn la v trong vng ny cc qu trnh s n nh, v hiu sut kch thch ph s cao nht.


2.2.1.2 n nguyn t ho mu (burner head) L b phn t chy hn hp kh to ra ngn la. Tuy c nhiu kiu n khc nhau, nhng u c nguyn tc cu to ging nhau (hnh 2.4 v 2.5). l h thng nguyn t ha mu. Song bt k mt loi n no mun dng c cho php phn tch quang ph, ngoi nhng yu cu nu (mc 1) n cn phi m bo trn u hn hp kh trc khi t v ngn la phi chy ngoi ming n. Hnh 2.4. n v h thng NTH mu ca hng Philips (1) n nguyn t ha mu; (2) Mng bo him; (3) ng thi phn mu tha; (4) ng dn cht oxy ha; (S) ng dn mu vo bung aerosol ha; (6) ng dn cht chy C2H2; (7) Vin bi to bi aersol. H nguyn t ha mu bao gm: + Bung ht mu v to th si kh (1) + n nguyn t ha mu (2)

Hnh 2.5 H thng nbiulai theo k thut pneumatic K: Kh mang (oxy ha); S: ng dn mu; F: Kh chy; Q: Cnh qut quay u; G: Mng bo him; A: ng dn th aetosol ln n nguyn t ha. 2.2.1.3 Qu trnh kch thch ph trong ngn la Trong phn tch quang ph pht x, nu dng ngn la lm ngun kch thch th mu phn tch phi chun b dng dung dch. Sau nh mt h thng phun 20Su t m b i: www.daihoc.com.vn

(Nebulizer system) a dung dch ny vo ngn la di dng th sng m (th si kh - aersol) cng vi hn hp kh t. Khi vo ngn la, di tc dng ca nhit , trc ht dung mi bay hi, li cc ht bt mu mn ca cc cht phn tch, ri n c t nng v chuyn thnh hi ca nguyn t v phn t. Trong ngn la cc phn t hi ny chuyn ng, va chm vo nhau, trao i nng lng cho nhau,... Kt qu ca cc qu trnh lm cc phn t b phn li thnh nguyn t, b Ion ha v b kch thch. Nh vy trong ngn la tn ti c nguyn t t do, phn t, Ion, nhm phn t v cc in t. Trong tp hp ch yu ch cc nguyn t t do b kch thch v pht x. V th ph pht x ngn la l ph ca nguyn t trung ha. Nguyn nhn gy ra s kch thch ph y l s va chm ca cc nguyn t vi cc in t c ng nng ln trong ngn la. V th nhit ngn la cng cao th cc in t c ng nng cng ln. V th n l s kch thch nhit. Do c ch ca qu trnh kch thch trong ngn la l: 1. Trc ht l dung mi bay hi, li cc ht mu dng bt. Tip theo l bt mu b nung nng, nng chy. Sau l cc qu trnh nhit ha ca cht mu xy ra theo tnh cht ca n, v gm c. 2. Hai qu trnh chnh: Nu Eh < Ent th mu s ha hi, nguyn t ha to ra cc nguyn t t do, chng b kch thch v pht x c ph pht x (AES). Men Xm(r).Th. Men Xm(k) mMe(k) + nX(k) Me(k) + E Me(k)* Me O + n(Hv) Nng lng K Chm tia pht x C ch ny (c ch 1) cho nhy v n nh cao ca s pht x. Cc hp cht mui halogen (tr F), axetat, mt s nitrat, sunphat... ca kim loi thng theo c ch ny. Nu Eh > Ent th mu s b nguyn t ha trc, sau ha hi thnh nguyn t, ri b kch thch ph sinh ra ph pht x (AES). MenXm(r) Mn(r) + Xm(r) mMe(k) + nX(k) Me(k) + E Me(k)* Me O + n(Hv) Nng lng K.Th. Chm tia pht x C ch ny (c ch 2) cho nhy v n nh km so vi c ch 1 ca s pht x. Cc hp cht mui SIO2-3, PO3-4, F-, mt s nitrat, sunphat,.. ca kim loi thng theo c ch ny. Do c im ca 2 c ch chnh , nn trong phn tch ngi ta thng to iu 21Su t m b i: www.daihoc.com.vn

kin s kch thch ph xy ra theo c ch 1 c li. iu gii thch ti sao ngi ta hay dng cc mui halogen, v axetat ca kim loi kim lm nn, hay cht ph gia cho mu phn tch. 3. Qu trnh ph Trong ngn la thng c cc qu trnh ph km theo hai qu trnh chnh, cc qu trnh ny ty iu kin ca ngn la m xut hin trong mc khc nhau, v n u khng c li, nn cn c loi tr trong phn tch. Cc qu trnh l: - To ra cc hp cht bn nhit, ch yu l cc monoxit (dng MeO). - S Ion ha to ra cc Ion, ch yu l s Ion ha bc 1. - S hp th bc x ca nguyn t (qu trnh t o). - S pht x ph nn lin tc ca ht rn v electron b nung nng. Ni chung, cc qu trnh ny u khng c li. Ngi ta phi tm bin php loi tr. V d: + hn ch s hnh thnh hp cht bn nhit kiu MeO, ngi ta thm vo mu mui Clorua ca kim loi kim (nh KCI, CsCL) lm nn, kch thch ph theo c ch 1, hoc kch thch ph trong mi trng kh tr argon. + Hay loi tr s Ion ha ca nguyn t phn tch, ngi ta thm vo mu cht ph gia l mui halogen kim loi kim c th Ion ha thp hn th Ion ha ca nguyn t phn tch. Nh th nguyn t phn tch s khng b Ion ha na. So vi cc loi ngun kch thch khc, ngn la l ngun kch thch n gin, nhy trung bnh (0,1 - 10g) v tng i n nh. Nhng cng ca vch ph cng b nh hng bi mt s yu t nht nh, nh: + Trc ht l dng lin kt ca cc nguyn t trong cht mu. + Th hai l nh hng ca cc gc axit. Ni chung, cc axit d bay hi th nh hng ny l nh. + Th ba l cht nn hay thnh phn ca dung dch mu, v vy dung dch phn tch v dung dch mu chun u phi c cng thnh phn vt l v ha hc, nht l cht nn phi cng loi. + nh hng th t l cc phn ng ha hc xy ra trong ngn la. c bit l nh hng ca cc phn ng to ra cc hp cht bn nhit. Cc hp cht ny thng lm gim cng ca vch ph, ngha l lm gim nhy ca phng php phn tch. l khi qut chung, tt nhin cc yu t trn khng phi bao gi cng xut hin ng thi v tc dng nh nhau trong mi trng hp, m ty tng trng hp yu t no l ni bt v cn c ch . Song mt iu cn nh l nhit ngn la l yu t quan trng nht, n quyt nh cc qu trnh xy ra trong khi kch thch ph. 22Su t m b i: www.daihoc.com.vn

2.2.2 H quang in 2.2.2.1 c im v tnh cht 1. H quang l ngun kch thch c nng lng trung bnh v cng l ngun kch thch vn nng. N c kh nng kch thch c c mu dn in v khng dn in. Ty thuc vo cc thng s (A, C, R) ca my pht h quang v loi in cc ta chn m h quang c nhit t 3500 60000C. Vi nhit ny nhiu nguyn t t cc nguyn liu mu khc nhau c th c ha hi, nguyn t ha v kch thch ph pht x. Nhit ca h quang ph thuc rt nhiu vo bn cht ca nguyn liu lm in cc. V th trong mt iu kin nh nhau, h quang in cc than (graphite) c nhit cao nht (bng 2.2). Cng dng in trong mch h quang l yu t quyt nh nhit ca h quang. 2. H quang l ngun kch thch cho nhy tng i cao, v trong h quang mu phn tch c ha hi tng i d dng hn trong cc loi ngun nng lng khc, c bit l h quang dng mt chiu. Nhng cng do tnh cht ny m php phn tch dng h quang c n nh v lp li km ngn la v tia in. y h quang dng xoay chiu cho kt qu n nh hn h quang dng mt chiu, v th h quang dng xoay chiu c s dng nhiu hn. Bng 2.2 Th Ion ha v nhit h quang ca mt s kim loi Nguyn t lm in cc C Zn Fe Cr Al Th Ion ha ca nguyn t (eV) 11,30 9,40 7,80 6,90 5,67 Dng in h quang (A) 10 10 10 10 10 Nhit ca h quang (oC) 6000 5200 4800 4400 4000

3. H quang l ngun kch thch c s mch tng i n gin, nhng li c nhiu thng s c th iu chnh c chn c mt nhit tng i ph hp cho php phn tch mi cht. 4. H quang l s phng in gia hai in cc c th thp (di 260V) v dng cao (t 8 - 20A). Th v dng ca mch h quang c lin h vi nhau theo cng thc:U = A+ U B v I m = R1 + R2 Im

Trong A v B l cc hng s, R1 l in tr plasma h quang. Cng thc ny gii thch ti sao h quang km bn vng. V th phi a vo mch h quang mt 23Su t m b i: www.daihoc.com.vn

bin tr ph R2 tht ln (R2 >> R1) hn ch s thay i cng dng in trong mch v gi cho h quang n nh. 2.2.2.2 Cu to v hot ng ca h quang Tuy c nhiu loi my pht h quang khc nhau, nhng v nguyn tc cu to, tt c cc my pht h quang u gm hai phn chnh. - Phn 1 l mch pht h quang (mch chnh), n c nhim v cung cp nng lng ha hi, nguyn t ha mu v kch thch m hi i n pht x. Nu h quang mt chiu th l mch ca dng in mt chiu (hnh 2.6). Nu l h quang xoay chiu, th l dng in xoay chiu. - Phn 2 l mch ch huy (mch iu khin), c nhim v ch huy phn mch chnh hot ng theo nhng thng s c chn cho mt mc ch phn tch nht nh. Theo s hnh 2.6, nu cng tc K c ng v tr AA, ta c h quang dng xoay chiu. Cn khi v tr BB' ta c h quang dng mt chiu. S hot ng ca h quang din ra nh s (s hnh 2.6): Khi ng cng tc K', ngha l a in vo my, qua h thng bin tr Ri ta chn c th lm vic ph hp a vo bin th Tr1 tng th. Khi th hai u cun th cp ca bin th ny t n hng ngn vn (2000-7000V). Hai u cun ny c ni vi cun cm Ll, t in C1 v khong n ph E. Lc t C1 c tch in v khi t n th Vf chn th c mt tia in c nh qua khong E, nh th lp tc trn mch dao ng L1c1 xut hin mt dao ng cao tn. Do mch dao ng L2C2 cng ghp vi mch dao ng LlC1, nn trn mch L2C2 cng xut hin mt dao ng cao tn cm ng nh th. Nhng s vng ca L2 l ln hn nhiu L1, nn th trn hai ct ca t C2 cng s ln hn C1. Th ny li xut hin tc khc khi c tia in nh E v iu khin s phng in qua khong F ca hai in cc h quang. Nh th mt chu k phng in th nht c thc hin v kt thc, ri li tip din n chu k phng in th hai, th ba,... v c th din ra cho n khi no ta ngt cng tc K', tc l ngng s phng in ca h quang.


Khi h quang lm vic, in cc F s c t nng v di tc dng ca in trng cc in t bt ra khi b mt in cc nm, chng c gia tc v chuyn ng v cc dng. Nhng in t ny c ng nng rt ln, v khi chuyn ng trong plasma gia hai in cc, cc in t va chm vo cc phn t khc (nguyn t, Ion, phn t...) trong plasma F, n truyn nng lng cho nhng phn t . Khi trong plasma c phn t b Ion ha, b phn li v b kch thch. Kt qu li c thm in t t do na cng vi mt s nguyn t, Ion, phn t b kch thch. Nh vy chnh cc phn t mang in tch dng v nm duy tr dng in qua ct kh ca plasma. Gia hai in cc F cc phn t b kch thch s sinh ra ph pht x ca n. Trong h quang mt chiu in t lun lun p vo in cc dng, nn in cc dng nng hn in cc nm. Cn ngc li, trong h quang dng xoay chiu, hai in cc li c t nng nh nhau. H quang dng mt chiu thng xut hin s st th hai u ca in cc. Hiu ng ny xut hin rt r rng trong thi gian u ca s phng in v h quang c dng thp (di 5A). Nhng sau c nng lng ca ngun kch thch b vo th s st th s gim i. Nguyn nhn ca s st th ny l do s xut hin lp in kp hai u ca in cc v lp in kp ny c t trng ngc vi t trng chnh ca h quang. Vi h quang xoay chiu, do dng in lun lun i chiu, nn s st th xut hin khng r rng v rt t c tc dng. chnh l l do gii thch ti sao s kch thch ph pht x bng h quang dng in xoay chiu lun lun n nh hn s kch ph trong h quang dng in mt chiu. 2.2.2.3 Nhit ca plasma h quang 1. Yu t th nht, yu t chnh quyt nh nhit ca h quang l cng dng in trong mch h quang chnh. Trong mt khong nht nh ca nhit , khi tng cng ca dng in th nhit ca h quang cng tng theo. Ngha l ta c: T = k. I (2.2) Trong k l h s t l, T l nhit ca h quang (OC). Nhng n mt gi tr nht nh ca cng dng in I0, th nhit h quang tng rt chm theo s tng nhit (hnh 2.7). Gi tr I0 c gi l dng in gii hn ca h quang. Nhng cng dng in trong mch h quang thay i tng i mnh. V ct kh ca plasma lun thay i c v di v b rng, do im phng in trn b mt in cc lun lun di ng. l nhc im ca h quang. Do ngi ta phi dng in cc i c hnh nn ct. V vy, mun cho cng dng in I trong mch h quang n nh (t thay i) ngi ta phi mc trong mch h quang h in tr R2 tht ln, s thay i ca R1 do ct kh gy ra t b nh hng n I. V th trong cc my pht h quang ngi ta thng lm mt h in tr ni R2 rt ln v cng dng in c tnh theo cng thc:


I=

UR1

+ R2

(2.3)

trong : R1: in tr ct kh ca plasma gia hai in cc. R2: in tr ni ca my pht h quang. U: Th ca ngun in ca my pht t ln hai u in cc F. 2. Yu t th hai quyt nh nhit ca h quang l bn cht ca nguyn liu lm in cc. Ni chung, nguyn liu no kh bay hi v cu to t nhng nguyn t ha hc c th Ion ha cng cao, th h quang dng in cc nguyn t s cho nhit cng cao. V th h quang in cc than (graphite) s c nhit cao nht so vi cc h quang in nguyn liu khc trong cng mt iu kin dng in th nghim (bng 2.2). 3. Yu t th ba l th Ion ha ca cc cht c trong mu phn tch, ch yu l cht nn v nhng cht c nng ln. Ni chung, nguyn t no (nht l cht nn ca mu) c th Ion ha cng nh th cng lm gim nhiu nhit ca h quang, ngha l cc kim loi kim c tc dng mnh nht, sau n cc kim loi kim th. Chnh v th th ngi ta thng thm cc mui halogen ca kim loi kim vo mu gp phn iu chnh nhit ca plasma ca h quang cho s kch thch ph ca cc nguyn t phn tch. Bn cnh th Ion ha, th nng ca cc nguyn t cng c nh hng n nhit ca h quang, c bit l cc nguyn t c th Ion ha thp s lm gim nhit ca h quang cng nhiu, khi nng ca n trong plasma cng ln. 4. Trong plasma ca h quang thng lun lun xy ra cc phn ng ha hc v vt l. l qu trnh phn li, Ion ha, t hp phn t v s kch thch ph. Nhng qu trnh ny thng l cc qu trnh nhit ng hc v m hi trong plasma cng c mt nhit nht nh ng vi nhng thng s xc nh ca my pht h quang c chn. Nhit m bo cho plasma c mt p lc in t xc nh duy tr s kch thch ph.


2.2.2.4 S kch thch ph trong plasma h quang in

Trong plasma ca h quang, s kch thch ph pht x c th xy ra theo cc qu trnh sau: 1. Nu l mu dung dch, th trc ht l dung mi bay hi, li cc ht bt mu. Ri cc cht (c mu bt v b ca mu dung dch cn li), bt mu b nung nng, nng chy. Sau l cc qu trnh nhit ha ca cht mu xy ra theo tnh cht ca n. C th l cc ht mu s bay hi hay chuyn khi vo plasma gia 2 in cc. Ti y s xy ra cc qu trnh chnh v ph khc nhau, ty thuc vo thnh phn v bn cht ca cht mu. 2. Nu mu l dng bt c nhi vo trong l in cc mang th c cc qu trnh trong in cc mang mu l: + S nung nng v nng chy, + S chuyn ng nhit khuch tn v i lu ca cc ht trong l in cc, + Cc phn ng ha hc phn hy ca cc cht mu km bn nhit, + S bay hi ca cc phn t cht mu vo plasma, + S vn chuyn ca cc ht mu cha ha hi vo plasm. Bn cht v thnh phn ca mu trong l in cc v nhit ca in cc l yu t quyt nh s din bin ca cc qu trnh trn. Sau l cc qu trnh trong plasma. Gm c 2 qu trnh chnh sinh ra ph pht x nguyn t (AES) v mt s qu trnh ph nh sau: A- Hai qu trnh chnh: Nu Eh < Ent th mu s ha hi, nguyn t ha to ra cc nguyn t t do, chng b kch thch v pht x c ph pht x (AES) Men Xm(r) Men Xm(k) mMe(k) + nX(k) Me(k) + E Me(k)* MeO + n(hv) Nng lng K.Th. Chm tia pht x C ch 2 ny cho nhy v n nh cao ca s pht x. Cc hp cht mui halogen (tr F), axetat, mt s nitrat, sunphat,... ca kim loi thng theo c ch ny. Nu Eh > Ent th phn t ca cht mu s b nguyn t ha trc, sau ha hi thnh cc nguyn t t do, ri mi b kch thch ph sinh ra ph pht x (AES). Men Xm(r) Men (r) + Xm(r) mMe(k) + nX(k) Me(k) + E Me(k)* MeO + n(hv) Nng lng K.Th. Chm tia pht x C ch 2 ny cho nhy v n nh km so vi c ch 1 ca s pht x. Cc 27Su t m b i: www.daihoc.com.vn

hp cht mui SiO3, PO4, F, mt s nitrat, sunphat... kim loi thng theo c ch ny. Do c im ca 2 c ch chnh , nn trong phn tch ngi ta thng to diu kin s kch thch ph xy ra theo c ch 1 c li. iu gii thch ti sao ngi ta hay dng cc mui halogen, v xetat ca kim loi kim lm nn, hay cht ph gia cho mu phn tch v mu chun, bin i nn mu gc sang nn nhn to tt hn v li cn loi tr c cc yu t nh hng, ng thi c khi cn tng c c nhy ln hng chc ln. B- Cc qu trnh ph: Trong plasma ca h quang thng c: - S hnh thnh hp cht bn nhit, ch yu l cc monoxit (dng MeO). - S Ion ha to ra cc Ion, ch yu l s Ion ha bc 1. - S hp th bc x ca nguyn t (qu trnh t o). - S pht x ph nn lin tc. Bn cht ca cht mu, thnh phn ca mu, cht ph gia trong l in cc v nhit ca in cc l yu t quyt nh din bin ca cc qu trnh trn. Ni chung cc qu trnh ny u khng c li, ngi ta tm bin php loi tr. V d: + hn ch s hnh thnh hp cht bn nhit kiu MeO, ngi ta thm vo mu mui Clorua ca kim loi kim (nh KCI, CsCl,) lm nn, kch thch ph theo c ch 1, hoc kch thch ph trong mi trng kh tr argon. + Hay loi tr s Ion ha ca nguyn t phn tch, ngi ta thm vo mu cht ph gia mui halogen kim loi kim c th Ion ha thp hn th Ion ha ca nguyn t phn tch. Nh th nguyn t phn tch s khng b Ion ha na. 2.2.2.5 Cc loi my pht h quang My pht h quang c rt nhiu, c nhiu hng cung cp, song xt v c ch mch in ca mch h quang chnh, th c th chia thnh hai loi: + My pht h quang dng mt chiu. + My pht h quang dng xoay chiu. Tuy l hai loi, nhng u c nguyn tc cu to nh nhau (hnh 2.6). Trong mi loi h quang li c nhiu ch khc nhau. Thng thng c mt ch lin tc v nhiu ch gin on. Chnh v th m chng ta c th chn c nhng gi tr nng lng khc nhau cho tng mc ch phn tch c th. 2.2.3 Tia la in 2.2.3.1 c im v tnh cht 1. Tia la in l ngun kch thch ph c nng lng tng i cao. Ty theo cc thng s ca my pht tia la in (C, L, R, T) chn, ta c th t c nhit 28Su t m b i: www.daihoc.com.vn

tnm plasma tia la in t 4000 6000OC. V th tia la in c gi l ngun kch thch cng (giu nng lng). Nn ph pht x ca tia la in ch yu l ph ca cc Ion bc 1 ca cc kim loi. 2. Tia la in l ngun kch thch tng i n nh v c lp li cao. Nhng v nhy li km h quang in. Do thi gian ghi ph cn phi di hn h quang. 3. V bn cht ca s phng in, tia in l s phng in gia hai in cc c th hiu rt cao (10.000 - 20.000kV) v dng in rt thp ( Ent th phn t ca cht mu s b nguyn t ha trc, sau ha hi thnh cc nguyn t t do, ri mi b kch thch ph sinh ra ph pht x (AES). Men Xm(r) Men (r) + Xm (r) mMe(k) + nX(k) Me(k) + E Me(k)* MeO + n(hv) Nng lng K.Th. Chm tia pht x C ch 2 ny cho nhy v n nh km so vi c ch 1 ca s pht x. Cc hp cht mui SiO3, PO4, F, mt s nitrat, sunphat,... kim loi thng theo c ch ny. Do c im ca 2 c ch chnh , nn trong phn tch ngi ta thng to iu kin s kch thch ph xy ra theo c ch 1 c li. iu gii thch ti sao ngi ta hay dng cc mui halogen v axetat ca kim loi kim lm nn, hay cht ph gia cho mu phn tch v mu chun, bin i nn mu gc sang nn nhn to tt hn v li cn loi tr c cc yu t nh hng ng thi c khi cn tng c c nhy ln hng chc ln. A2- Khi mu l thanh kim loi lm in mang + Lp cc nguyn t trn b mt in cc, di tc dng ca lc in trng in mnh (v th phng in rt cao gia 2 in cc, 15- 20 kV), nn cc nguyn t b bt ra khi mt in cc v bay vo plasma, hoc l dng hi, hay l dng ht rn. Sau c cc qu trnh sau: - Ha hi ca ht rn: - Nguyn t b kch thch: - Nguyn t b Ion ha: - Ion b kch thch: Men) + E1 Me(k) Me(k) + E2 Me(k)* Me(k) + E3 Me(k)+ Me(k)+ + E4 Me(k)+* 31Su t m b i: www.daihoc.com.vn

- Pht x ca nguyn t: - Pht x ca Ion:

Me(k)* Me(k) + n(hv) Me(k)+* Me(k) + n(hv)

Trong : n(hv) l chm tia pht x ca nguyn t hay Ion b kch thch. Cn El, E2,.. l nng lng nhit do plasma tia in cung cp. B- Cc qu trnh ph Trong plasma gm c: S hnh thnh hp cht bn nhit, ch yu l cc monoixtit (dng MeO). - S Ion ha to ra cc Ion, ch yu l s Ion ha bc 1. - S hp th bc x ca nguyn t (qu trnh t o). - S pht x ph nn lin tc. Bn cht ca cht mu, thnh phn ca mu, cht ph gia trong l in cc v nhit ca in cc l yu t quyt nh s din bin ca cc qu trnh trn. Song qu trnh ph trong plasma tia in khng mnh nh trong ngn la v trong h quang, ch c s Ion ha l ng k. V th cc kim loi kim phi o theo cc vch Ion bc 1. Ni chung, cc qu trnh ny u khng c li. Ngi ta phi tm bin php loi tr. V d vi mu dung dch: + hn ch s hnh thnh hp cht bn nhit kiu MeO, ngi ta thm vo mu mui Clorua ca kim loi kim (nh KCI, CsCL,) lm nn, kch thch ph theo c ch 1, hoc kch thch ph trong mi trng kh tr argon. Vy, loi tr s Ion ha ca nguyn t phn tch, ngi ta thm vo mu cht ph gia mui halogen kim loi kim c th Ion ha thp hn th Ion ha ca nguyn t phn tch. Nh th, nguyn t phn tch s khng b Ion ha na. 2.2.3.4 Cc loi my pht tia in V nguyn tc cu to, my pht tia la in cng c hai loi: my pht tia la in c ch huy v my pht tia la in khng ch huy. Song trong mc ch phn tch quang ph pht x, ngi ta ch dng my pht tia in c ch huy. Trong loi my pht ny, vic ch huy (iu khin) cng theo hai nguyn tc v ng vi hai cch iu khin ny ngi ta ch to hai loi my pht tia in khc nhau: + My pht tia in dng hai tia in, tia in chnh, tia in ph (hnh 2.9), tng t nh my h quang. + My pht tia in dng ngt in ng b (hnh 2.10), ngha l iu khin bng m t ng b. My pht tia in dng hai tia din (hnh 2.9) 32Su t m b i: www.daihoc.com.vn

Trong my ny c mt tia in chnh v mt tia in ch huy. Theo s hnh 2.9, khi ng mch in K, bin th Tr c np in v tng th ln hng ngn vn (V). Lc h t in C c tch in v th hai ct ca t C l th ca khong n gia hai qu cu E. Lc ny trn mch in LC khng c dng in, t C c tch in cho n khi t c th Vf (th phng in gia hai qu cu E) th tia in s nh qua hai qu cu E, t C phng in qua E lm cho trn mch dao ng LC xut hin tc khc mt dao ng cao tn, dao ng ny khng qua c L2 (hay R1); do bt buc phi nh qua khong n F (khong cch gia hai in cc chnh). Nh vy, h c mt dao ng v mt tia in nh E, th cng c mt tia in phng qua F, ngha l tia in E iu khin tia in F. Sau qu trnh c th tip din cho n khi ta ngt cng tc K. Mun cho qu trnh phng in c n nh th hai qu cu E phi c kch thc xc nh, khong cch khng i v b mt phi nhn. C nh th in th Vf mi c nhng gi tr xc nh ng vi tng cp thng s ca my pht tia in c chn, m bo cho qu trnh phng in F n nh V mt nng lng, v c hai tia in xy ra ng thi, nn nng Hnh 2.9 lng phi chia i hai khong n E My pht tia in dng hia tia in v F. V th mun c nng lng ln (nhit cao) th th Vf phi rt ln. My pht dng ngt in ng b (hnh 2.10). My pht loi ny ch to theo s ca Fressner. y s phng in c ch huy nh mt m t ng b c gn b ngt in mc ngay trn mch ca my pht tia in. My pht gm bin th Tr tng th, mch dao ng LC, khong phng in chnh F, hai cun cm L1, L2 ngn khng cho dao ng cao tn tr v bin th Tr v m t ng b M ng v m mch in ca qu trnh phng Hnh 2.10 in gia 2 in cc F. My pht tia in dng ngt in ng b 33Su t m b i: www.daihoc.com.vn

Khi cho in vo my, bin th Tr tng in, t C c tch in dn, m t M quay u c mang theo hai tip im A v A, cng quay, khi n v tr B v B1 mch in trong my c ng, lc th t C cng t cc i, khin cho tia in phi phng qua F, sau tia in li tt. Nhng v m t M quay u lin tip, nn tia in cng nh u n lin tip qua F theo s quay ca mt M tun hon n v tr B v B1 ln lt theo thi gian chu k c chn, cho n khi no ta ngt cng tc K. Nh vy, chnh m t ng b M iu khin s phng in theo cc thng s c chn. 2.2.4 Plasma cao tn cm ng ICP 2.2.4.1 c im v tnh cht y l ngun nng lng mi c s dng trong php phn tch quang ph pht x nguyn t khong 10 nm li y; song li c s dng rng ri v c hiu qu cao v n c nhng c im sau y: 1. Ngun nng lng ny c nhit cao. Thng thng tnm ca plasma c th t c nhit t 5000 10000OC nn ha hi v nguyn t ha c ht mi trng thi ca vt liu mu vi hiu sut cao. Vi plasma ny, mi nguyn t kim loi u b kch thch to ra ph pht x ca n. Cc hp cht bn nhit cng b ha hi v phn li thnh nguyn t t do, nhng trong ngun nng lng ny ph pht x ca Ion l ch yu. 2. ICP (Inductivity Coupled Plasma) l ngun nng lng kch thch ph pht x m bo cho php phn tch c nhy rt cao. Ni chung c th t t n.10-4: n.106% (0,l - 5) ngng i vi hu ht cc nguyn t. Tt nhin nhy ca php phn tch y ph thuc nhiu vo cng sut ca my pht cao tn HF to ra plasma ICP (hnh 2.11). 3. ICP l ngun kch thch ph khng nhng c nhy cao, m cn c n nh cao. N hn hn h quang v tia in. V th php phn tch dng ngun nng lng ny cho sai s rt nh; ni chung l nh hn 10% trong vng nng t n.10-3- n.10-5%.


4. Tuy c nhit cao nhng s kch thch ph pht x trong ngun nng lng ny li rt m du. N tng t nh s kch thch ph trong ngn la n kh. Chnh v th m phng php phn tch t n nh cao v sai s nh. 5. Vi ngun nng lng ny, ngi ta c th nh lng c ng thi nhiu nguyn t mt lc, nn tc phn tch rt cao (t 40 - 120 mu/gi). Mt khc, khi dng ngun nng lng ICP, vng tuyn tnh ca phng php nh lng l rt rng (t 1 - 10.000 ln). 6. c im cui cng l rt t xut hin nh hng ca cht nn (matrix effect). l im hn hn cc ngun nng lng c in nu.

Chnh do nhng c im v u vit ca ngun nng lng ICP nh th, nn n nhanh chng c ng dng rng ri v thay th cho ngun nng lng c in: tia in v h quang. Tt nhin, trang b to ra ngun nng lng ny t hn. Nhng trong iu kin ngy nay v do tnh cht u vit ca n nn cc phng phn tch quang ph pht x u c trang b loi ngun nng lng ny. 2.2.4.2 Cu to ca h thng 1CP V nguyn tc, h thng thit b to ra ngun nng lng ICP gm hai phn chnh: my pht cao tn HF v h thng to th si kh v n nguyn t ha kch thch ph ca mu phn tch. 1. My pht cao tn HF Cc my ny lm vic tn s rt cao nhng ph bin l hai tn s 27,12 MHZ v 450 MHZ. Trong my trung tn s 27,12 MHZ c s dng nhiu hn. l tn s sng mang rai. My pht cao tn HF c nhim v pht tn s cao cung cp nng lng cho cun cm cao tn u ming n nguyn t ha mu, to ra plasma nhit cao cho s ha hi mu, nguyn t ha v kch thch ph (hnh 12a). 35Su t m b i: www.daihoc.com.vn

2. Phn th hai ca h thng ICP l b phn nguyn t ha mu, bao gm b phn to soi kh mu v n nguyn t ha mu (nebulizer system v khch burner). B phn burner c ch to bng thch anh chu nhit. N gm ba ng lng vo nhau, gn chc vo nhau phn y, v mi ng u c ng dn kh vo (hnh 2.12a). Trong , ng trong cng l mt ng mao dn dn mu vo plasma nh kh mang; ng th hai l ng to ra kh plasma. Cn ng th ba l ng to kh lm lnh cho n nguyn t ha. Pha ngoi ng th ba v ngay u c cun cm cao tn bng ng. Cun cm ny c ni vi my pht cao tn HF v lun c lm lnh bng dng nc chy qua khi lm vic. ng thi chnh cng sut lm vic ca cun cm ny s quyt nh nhit ca plasma kch thch ph. Cn h thng nebulizer, v nguyn tc, hon ton tng t nh trong ngn la n kh nu trn. Kh dng trong plasma ca ICP l cc kh tr nh argon, hli, nit. Trong argon l u vit nht. y ngi ta c th dng kh mang v kh plasma l argon, cn dng nit lm lnh. Hoc c th dng ch mt kh Ar cng c. Trong tng lng kh cho c ba dng l t 13 - 18 lt/pht v c phn chia nh nhau: - Kh mang: 0,7 - 1,0 l/pht. - Kh plasma: 6 - 8 l/pht. - Kh lm lnh: 5 - 9 l/pht. Nhng ngy nay, ngi ta ci tin v dng loi mini - ICP nn ch cn n 1/3 lng kh so vi loi thng thng nu.

Hnh 2.12a H thng ngun ca ICP-AES 2.2.4.3 Nhit ca plasma ICP ICP l ngun nng lng kch thch ph c nhit cao, nhng li n nh hn so vi cc loi ngun kch thch khc, nh h quang, tia la in. Nhit ca plasma ca ngun ny c quyt nh bi dng in cm ng trong cun cm A u ca n nguyn t ha, tc l nng lng cao tn ca my pht HF. V vy, ng vi nhng thng s nht nh ca my pht HF th plasma ny c mt nhit xc nh. Trong cc thng s ca my pht HF, cng sut v tn s cm ng l hai thng s quan trng 36Su t m b i: www.daihoc.com.vn

nht. Song mi my pht thng ch dng mt tn s cm ng nht nh 27,12 MHZ hay 450 MHZ, nn cng sut ca my pht ng vai tr chnh. V th cng ca vch ph ph thuc vo cng sut ca my pht HF (hnh 2.11). Tt nhin, bn cnh cn c nhng yu t khc nh hng n nhit ca plasma ICP. Trc ht l tnh cht ca kh plasma, kh mang v tc chy ca chng vo plasma. Sau l thnh phn ca hn hp mu c dn vo plasma kch thch ph. Ni chung dng lin kt, th Ion ha ca cc nguyn t cng c nh hng n nhit ca plasma. Nhng cc yu t ny khng mnh nh trong h quang v tia in. Yu t matrix effect cng c tc dng, nhng cng khng ln nh trong ngn la n kh v trong h quang in. Cc yu t trn nh hng n nhit ca plasma, tc l nh hng n cng ca vch ph. Nhng khi chn c cng sut ln ph hp, th nh hng ny s rt nh, hay khng cn na. 2.2.4.4 S kch thch ph pht trong plasma ICP Khi ta ng mch in ca my pht cao tn HF th cun cm A u n s xut hin tc khc dng in cao tn cm ng c nng lng ln. Nng lng ny c quyt nh bi cc thng s ca my pht HF c chn. ng thi tia la K cng c bt, hn hp kh v mu c t chy, plasma nhit cao xut hin, ngha l ngn la hnh thnh. Trong ngn la nhit cao ny, trc ht dung mi bay hi ( phn trong, phn ti ca ngn la), li cht mu dng ht bt mn. Ri cc bt mu ny b nung nng, nng chy, c ha hi, phn li, b nguyn t ha v Ion ha to ra trong plasma c phn t, nguyn t, Ion, in t trng thi kh. Trong ch yu l cc nguyn t, Ion v in t. Sau cc phn t kh ny c trong plasma s b kch thch v sinh ra ph pht x. Bn cht s kch thch ph y l s kch thch nhit, n ging nh s kch thch ph pht x trong ngn la n kh. Nhng v plasma c nhit cao (nng lng ln) nn hiu sut nguyn t ha v kch thch ph cao v ph ca ngun nng lng ny vi cc kim loi kim v kim th ch yu l ph ca cc Ion bc 1. C ch ca qu trnh trong plasma ICP c th tm tt theo s sau:


Hnh 2.12b Hai loi my o ph pht x ICP (ICP-AES) HF: My pht cao tn; Pm: H thng nguyn t ha mu; L: Thu knh; T: Khe sng; G: Cch t; PM: H nhn quang in; >>: H in t; Mr: My tnh; Sig: B ch th kt qu o; AB: Cung mt phng tiu; M1, M2: cc gng chun trc v hi t. 1. Trc ht l dung mi bay hi, li cc ht bt mu. Tip theo l bt mu b nung nng, nng chy. Sau l cc qu trnh nhit ha ca cht mu xy ra theo tnh cht ca n. 2. Ba qu trnh chnh: Cc qu trnh chnh y cng tng t nh trong ngn la n kh. Nhng c khc ngn la n kh l hiu qu ha hi, nguyn t ha v kch thch ph cao hn nhiu. - Nu Eh < Ent, th mu s ha hi, nguyn t ha to ra cc nguyn t t do, chng b kch thch v pht x c ph pht x (AES). Men Xm(r) Men Xm(k) mMe(k) + nX(k) 38Su t m b i: www.daihoc.com.vn

Me(k) + E Me(k)* MeO + n(hv) Nng lng K.Th. Chm tia pht x

C ch 1 ny cho nhy v n nh cao ca s pht x. Cc hp cht mui halogen (tr F), axetat, mt s nitrat, sunphat,.. ca kim loi thng theo c ch ny. - Nu Eh > Ent, th phn t ca cht mu s b nguyn t ha trc, sau ha hi thnh cc nguyn t t do, ri mi b kch thch ph sinh ra ph pht x (AES). Men Xm(r) Men(k) + Xm(k) mMe(k) + nX(k) Me(k) + E Me(k)* MeO + n(hv) Nng lng K.Th. Chm tia pht x

C ch 2 ny cho nhy v n nh km so vi c ch 1 ca s pht x. Cc hp cht mui SiO3-2, PO4-3, F, mt s nitrat, sunphat... ca kim loi thng theo c ch ny. - S Ion ha sinh Ion, sau cc Ion b kch thch v sinh ra ph ca n. C th l: Men Xm(r) Men Xm(k) mMe(k) + nX(k) Me(k) + Ei Me(k)+ + e Me(k) + E Me(k)+* MeO + n(hv) Nng lng K.Th. Chm tia pht x V trong ngun ICP c nhit cao, nn cc kim loi c th Ion ha thp s din ra theo c ch ny, v d cc kim loi kim, kim th. 3. Qu trnh ph gm c: + To ra cc hp cht bn nhit, ch yu l cc monoxit (dng MeO). Nhng trong plasma ca ngun ICP th cc qu trnh ny xy ra khng ng k. chnh l u vit ca ICP. + S hp th bc x ca nguyn t (qu trnh t o). + S pht x nn (nhng cng nh). + S Ion ha bc hai ca nguyn t kim loi kim v cc nguyn t c th Ion ha bc hai thp.


Hnh 2.13 S qu trnh nguyn t ha mu 2.3 Nguyn tc v cch chn ngun kch thch ph Cc iu kin kch thch ph l nhng yu t c nh hng trc tip n cng ca vch ph v nhy ca phng php phn tch. V th chn ng cc iu kin kch thch ph l mt m bo cho phng php phn tch t kt qu chnh xc, tin cy v nhy cao. Mt khc, qua vic chn c iu kin kch thch ph ph hp ta c th loi tr c mt suyn t nh hng, nh matrix effect, nng cao nhy. Do chn c nhng iu kin, ngun kch thch ph ph hp cho mt i tng phn tch no chng ta phi da theo mt s nguyn tc c bn sau y: 1. Phi xut pht t i tng phn tch thuc dng mu no (mu bt, mu rn, mu dung dch hay hp kim), cng nh nhng tnh cht c l ca tng loi mu thuc loi mu d bay hi hay kh bay hi, dn in hay khng dn in. V d khi phn tch mu bt (qung, t ,...) th h quang c dng trn 10A l thun li. Song khi phn tch mu hp kim th tia in li u vit. Mu dung dch dng ICP. 2. Phi da theo tnh cht v c trng s kch thch ph ca mi nguyn t cn xc nh m chn ngun nng lng kch thch v cc thng s ca ngun kch thch. V d phn tch kim loi kim th nn dng ngn la n kh hay h quang gin on. V th kch thch ph pht x ca cc kim loi kim l khng ln (3 - 5eV). 40Su t m b i: www.daihoc.com.vn

Ngc li, vi cc nguyn t V, Zn, W,... hp cht bn nhit, li phi chn ngun kch thch c nng lng cao, h quang in dng ln hay ngun ICP. 3. Khi chn ngun kch thch phi m bo cho php phn tch c nhy cao, n nh, c th d dng phn tch c cc nguyn t c nng nh. 4. Chn ngun kch thch phi m bo cho php phn tch tiu tn t mu v khi cn phi khng ph hy mu. Yu t ny trong mt s trng hp khng yu cu, bt buc nh ba yu t trn. 2.4 Cng vch ph pht x nguyn t 2.4.1 Cng vch ph : Trong ngun pht sng hay trong plasma, nguyn t (Ao) ca mt nguyn t c kch thch t trng thi nng lng thp Eo, ln trng thi nng lng cao. Cm c biu din theo phng trnh: Ao + XE = A* (a) Vi mu phn tch c nng xc nh ca Ao th qu trnh ny lc u tng nhanh, sau chm dn v ti mt thi im nht nh th khng tng na. Ngha l s nguyn t Ao b kch thch l khng i, ng vi mt nhit xc nh ca plasma. ng thi vi qu trnh trn l qu trnh ngc li, tc l nguyn t b kch thch A* gii phng nng lng m n nhn c di dng cc tia pht x n(hvi) tr v trng thi nng lng thp bn vng ban u. Chnh qu trnh ny l qu trnh pht x ca nguyn t v sinh ra ph pht x ca nguyn t, c th c biu din theo phng trnh: A* = hv + Ao (b) Qu trnh ny lc u chm, sau tng dn theo s nguyn t A* b kch thch v n mt thi im nht nh th cng khng tng na ng vi mt nhit nht nh ca plasma. Ngha l sau mt thi gian nht nh ca s kch thch ph, th hai qu trnh trn (a) v (b) s t n trng thi cn bng. l hai cn bng ng hc thun nghch. Cn bng ny ch ph thuc ch yu vo nhit ca plasma, ngha l s nguyn t A* b kch thch v pht x l khng i, ng vi mt nhit xc nh ca plasma. Nu gi Nm l s nguyn t ca nguyn t Ao b kch thch n trng thi nng lng cao cm, th theo quy lut Bolzmans ta c:Nm = Na gm e( E m ) / Kt g0

go v gm l trng lng thng k ca nguyn t A trng thi ban u, nng lng Eo v trng thi kch thch nng lng cm; Na l s nguyn t ca nguyn t Ao c trong plasma (trng thi hi); 41Su t m b i: www.daihoc.com.vn

Em l nng lng kch thch nguyn t o t trng thi Eo ln trng thi cm; T l nhit ca plasma (oK); k l hng s Bolzmans. Nu gi Ia l cng ca vch ph do qu trnh kch thch ph ni trn sinh ra, th trong mt gii hn nht nh ca nhit plasma v nng ca nguyn t Ao trong plasma, ngi ta thy Ia ph thuc vo: - S nguyn t Ao b kch thch ln trng thi A*, (Nm). - Thi gian tn ti ca nguyn t A* trng thi kch thch, (tm). - Nng lng kch thch nguyn t A t trng thi c bn n trng thi kch thch, (Em). - Xc sut chuyn mc ca nguyn t A t trng thi kch thch nng lng Cm v trng thi ban u nng lng Eo, (Amo). V mi quan h ny c th biu din theo cng thc sau:Ia = f . g l . Amo . Aa . m .hv.e ( E ) / kT tm go

Nhng i vi mt loi nguyn t v trong mt iu kin th nghim nht nh ca plasma (ch yu nhit plasma) th cc yu t tri Amo, go, gm, cm, hv l nhng hng s. Cho nn cng vch ph Ia ch cn ph thuc vo s nguyn t b kch thch Na m thi. Nh vy ta c:I a = K1 N a

(2.6)

Vi

ka = f .

g l . Amo . Aa . m .hv.e ( E ) / kT tm go

l cng vch ph ca nguyn t. Song cng thc (2.6) mi ch cho ta bit mi quan h gia cng vch ph Ia v nng ca nguyn t trng thi hi trong plasma, m cha ch cho ta c mi quan h gia cng vch ph v nng ca nguyn t trong mu phn tch. Mun th ta phi xt mi quan h gia nng C trong mu v s nguyn t Na trong plasma. Ni chung, mi quan h ny l phc tp, n ph thuc vo nhiu yu t, nh: - Kh nng ha hi, bn cht ca cht mu, cht nn; - Thnh phn ca mu, cht ph gia thm vo; - Trng thi lin kt, tn ti ca cht mu; - Cc iu kin ha hi, nguyn t ha mu, kch thch ph; - Mi trng kch thch ph, v.v... Tuy nhin ngi ta thy t thc nghim, trong mt phm vi nht nh ca nng 42Su t m b i: www.daihoc.com.vn

C th ta lun lun c: Na = k2.Cb (2.7) Nh vy, cng vch ph s l: Ia = a.Cb (2.8) Trong : a: k1.k2 v a c gi l hng s thc nghim. N ph thuc vo tt c cc iu kin thc nghim ha hi, nguyn t ha v kch thch ph mt nguyn t. V th trong mt php phn tch phi gi cho cc iu kin thc nghim n nh, m bo a thc s l hng s ca php o. Cn b l hng s bn cht, n ph thuc vo bn cht ca tng loi nguyn t, b ch nhn nhng gi tr bng 1 v nh hn 1, nhng ln hn 0. Gi tr ca b nh hn 1 khi nng C ln, cn khi C nh th b lun lun bng 1. Khi b nh hn 1 l lc qu trnh t hp th xy ra. Qu trnh t hp th xy ra cng nhiu (mnh) th b cng nh xa 1, tc l chnh hng s b l c trng cho qu trnh t o ca mi loi nguyn t. Do , ng vi mi nguyn t v vi mi mt vch ph pht x v trong nhng iu kin nht nh, ta lun c mt gi tr nng Co, m: + Khi vi mi Cx < Co th b lun lun bng 1. + Khi vi mi Cx > Co th b nh dn xa 1 (hnh 2.14). Trn y cp n cng ca vch ph nguyn t trung ha nhng trong plasma cn c c cc Ion. Chng cng b kch thch v sinh ra ph pht x. Qu trnh kch thch v pht x cng ging nh cc nguyn t v cng tnh ton hon ton tng t nn chng ta c cng vch ph Ion l: li = Al. Cb (2.9)

Do , mt cch tng qut, ta c th vit cng vch ph pht x: I = a.Cb (2.10) 43Su t m b i: www.daihoc.com.vn

Cng thc (2.10) l phng trnh c bn ca phng php phn tch nh lng theo ph pht x nguyn t. Phng trnh ny do Lmakins (Nga) v Saiber (c) tm ra, nn gi l phng trnh Lmakins-saiber. 2.4.2 Cng vch ph v nhit plasmaNh trn chng ta bit, cng vch ph pht x ca nguyn t c tnh theo biu thc sau: Ia = Pa.N(l-x)e-E(m)/kT

V vch Ion: Trong , vi:

Ii = Pi.N.x.e-Em/kTPa = l t am

(2.12)

. Aam .

gm hv go

iu ny cho thy cng ca vch ph ph thuc vo nhit ca plasma. Nhng, tt nhin, s ph thuc ny i vi vch nguyn t v vch Ion l khc nhau v khng n iu, m theo hm s m (e-Em/kT). Chnh hai cng thc trn d dng cho ta thy rng, nhit no ca plasma to ra c s nguyn t N(1 x) trong cng thc tnh cng vch nguyn t v s phn t (N.x) trong cng thc vch ph ln l ln nht, th lc s c c cng ca vch ph ln nht. l nhit ti u ca s kch thch ph.


Vi mt vch ph ca mt nguyn t, thc t ch c mt nhit to ra c cng I cc i. Nhit ny c gi l nhit ti hn ca vch ph v c k hiu l To. Tt nhin i vi mi vch th nhit To ny khc nhau (hnh 2.15a,b,c). Ngay i vi vch ph ca mt nguyn t, nu n tn ti trong cc nn mu khc nhau (matric khc nhau) th nhit To ny cng khc nhau (hnh 2.15c). iu c ngha l, ngoi bn cht ca cht phn tch, nhit To cn ph thuc vo c thnh phn v nn ca mu phn tch. Tnh cht ny c ngha thc t rt ln khi xc nh lng nh cc nguyn t trong mu phn tch. V trong nhng trng hp ny ta cn phi chn nhng iu kin lm vic v iu kin kch thch ph sao cho thu c vch ph c cng ln nht, tc l chn cc iu kin kch thch ph c c nhit ca plasma gn vi nhit To ca vch ph cn o, hoc bng ng c To l l tng. Tt nhin bng c l trng hp rt him, ta ch c th lm cho nhit ca plasma cng gn vi nhit To th cng tt. 2.5 Hin tng t hp th (t o) trng thi hi trong plasma, hin tng mt nguyn t c kh nng hp th tia bc x do chnh mt nguyn t khc cng loi ca chng pht ra c gi l hin tng t hp th (hay t o). Hin tng ny xy ra theo nh lut Kirschhoff. Ngha l mt nguyn t ch hp th nhng tia bc x m chnh n c th pht ra c trong qu trnh pht x. Hin tng ny thng xy ra mnh v ch yu i vi cc vch cng hng, vch cui cng ca ph pht x ca mt nguyn t, v nng cng ln, th hin tng ny xy ra cng mnh. Chnh do hin tng ny, m a n kt qu lm cho cng vch ph pht x I khng ph thuc tuyn tnh vo C, khi nng C tng gi tr Cx > Co, Gi tr Co c gi l nng gii hn, cn c quan h gia I v C l tuyn tnh. Vi mi vch ph, Co l khc nhau. Vch cng nhy th Co cng nh. ng thi, trong qu trnh pht x, cng sut pht x ca mt vch ph c tnh theo cng thc: W = f.Wo (2.13) vi f l h s hot ng pht x ca vch ph v n nhn gi tr: 0 < f < 1. Khi khng c hin tng t o, ngha l nng Cx nh hn Co th f = 1, tc l W = Wo. Cn mi trng hp khc, khi c hin tng t o th lun lun c W < Wo. Trong :Vo = Am .N a .hv.e Em kT )dv0 V

(2.14)

y: Na l s nguyn t trong n v th tch V ca plasma. Amo l xc sut chuyn mc; Cm l nng lng kch thch trng thi mi T l nhit ca plasma (oK); 45Su t m b i: www.daihoc.com.vn

hv l lng t sng c bc x ra; k l hng s Bolzmans. V ngha thc t, khi hin tng t hp th xut hin, ni chung u khng c li cho php phn tch ph pht x nguyn t. iu c ngha l trong phn tch khng nn xc nh mt nguyn t vng nng bt u xut hin hin tng t o, tc l nm ngoi vng tuyn tnh, phi pha long mu kt qu s chnh xc hn. 2.6 Bc x nn trong ph pht x Khi kch thch mu phn tch ta thu c ph pht x ca n. Ph ny gm c ph vch ca nguyn t v Ion, ph m ca phn t v nhm phn t. Sau n ph nn hay gi l bc x nn. Bc x nn thng l ph lin tc. N to thnh mt di en m trn ton b vng ph ca mu phn tch. en ca di ph nn ny mi vng ph l khc nhau. Thng thng vng sng di m hn vng sng ngn. Nhn chung, n tng dn t vng sng ngn sang vng sng di, n th hin r rt vng kh kin (360 - 500 mm). Cn vng t ngoi (UY) th ph nn thng rt nh. Trong mt mc va th bc x nn l c li v n lm nn cho vch ph xut hin ln trn m hn, ngha l lm tng cng ca vch ph. Do , trong mt s trng hp nn qu yu ngi ta thng phi chiu sng ph knh nh bng nh sng trng vi mt thi gian ph hp. Song trong nhiu trng hp ph nn thng qu mnh, c bit l trong vng kh kin v khi dng h quang. Lc ny n che lp mt vch ph ca nguyn t phn tch, nht l nng nh. Trong trng hp ny ngi ta phi tm cch kh bt bc x nn. Vy nguyn nhn no gy ra bc x nn ? T nhng kt qu nghin cu ngi ta thy c cc yu t sau y to ra bc x nn (ph nn): - S pht sng ca ht vt rn c t nng trong plasma. - Tc dng ca nh sng trng. - S bc x ca in t trong plasma nhit cao. Do in cc b t nng sng v pht x (vt rn pht sng nhit cao). Trong ba yu t ny, yu t th ba th hin khng r nt bng hai yu t u. loi tr bc x nn ngi ta c th p dng nhiu bin php khc nhau. V d vi nguyn nhn th nht, ngi ta dng mn chn trc khe sng khng cho nh sng do hai u in cc c t nng chiu vo khe my quang ph. Hoc cng c th thm cht ph gia vo mu phn tch kh bt mt phn cng nn, lm cho nn ca ph khng qu en. Nhng tt nht l kch thch ph trong mi trng kh tr Ar (argon) hay hn hp ca kh tr Ar v oxy theo nhng t l ph hp. Bin php ny loi tr c c ph nn v ph xm ca nhm CN v ph phn t H2O, mt khc li cn tng c cng vch ph, ngha l tng c nhy ca phng php phn tch. l u vit ca mi trng kh tr trong s kch thch ph pht x nguyn t. 46Su t m b i: www.daihoc.com.vn

Chng 3 MY QUANG PH V S PHN LI CHM SNG

3.1 Nguyn tc cu to ca my quang ph pht x My quang ph l mt dng c dng thu, phn li v ghi li ph ca mt vng ph quang hc nht nh. Vng ph ny l mt i ph ca vt mu nghin cu t sng ngn n sng di. Ty theo b phn dng phn li nh sng trong my da theo hin tng vt l no (khc x hay nhiu x) m ngi ta chia cc my quang ph thnh hai loi. 1. My quang ph lng knh. l nhng my quang ph m h tn sc ca chng c ch to t 1 hay 2 hoc 3 lng knh. S phn li nh sng y da theo hin tng khc x ca nh sng qua hai mi trng c chit sut khc nhau (khng kh v thy tinh hay khng kh v thch anh). 2. My quang ph cch t. L nhng my quang ph m h tn sc l mt cch t phng hay lm phn x. Bn cht ca s tn sc nh sng y l s nhiu x ca tia sng qua cc khe hp. Tuy c 2 loi my quang ph khc nhau, nhng v nguyn tc cu to th u nh nhau, u gm 3 phn chnh. l h chun trc, h phn li v h hi t chm sng (hnh 3.1). 1. H chun trc v phn u ca my quang ph. N gm mt hay mt h thng thu knh ghp vi nhau hay h gng hi t v mt khe hp (khe vo ca chm sng v c th iu chnh c) t tiu c ca h thu knh ny. H chun trc c nhim v nhn v to ra chm sng song song hng vo h tn sc phn li thnh ph. 2. H tn sc. l mt h thng lng knh hay mt tm cch t. H ny c nhim v phn li (tn sc) chm sng a sc thnh cc tia n sc, tc l phn li mt ngun sng phc tp nhiu bc sng khc nhau thnh mt di ph ca chng theo tng sng ring bit lch i nhng gc khc nhau. Nu h tn sc c ch to bng lng knh th chng ta c my quang ph lng knh. V ngc li, nu h tn sc l cch t ta c my quang ph cch t. Trong my lng knh, tia sng ngn s b lch nhiu, sng di lch t, cn trong my cch t th ngc li.




3. H bung nh. l mt h thng thu knh hay mt h gng hi t v mt mt phng tiu ca cc chm sng. H ny c nhim v hi t cc tia sng c cng bc sng sau khi i qua h phn li li vi nhau to ra nh ca khe my trn mt phng tiu. chnh l cc vch ph. Thm vo l mt s b phn ph khc my quang ph c th hot ng c chnh xc, d dng v c hiu qu cao hn. V d nh: - H thng gng hay thu knh chiu sng khe my. - H thng gi in cc. - Cc loi chn sng v lc sng trc khe my. - B phn t ng a mu. - My tnh v phn mm iu khin v x l s liu. 3.2 Cc loi my quang ph pht x 3.2.1 Lng knh v my quang ph lng knh 3.2.1.1 Lng knh v my quang ph Lng knh l mt b phn quan trng ca my quang ph lng knh. N quyt nh kh nng, tnh cht phn li nh sng ca my quang ph. Lng knh l mt mi trng trong sut, ng nht v ng hng trong mt vng ph nht nh. N c gii hn bi 5 mt phng, nh trong hnh 3.2 ta c: ABC v A,B'C' l hai mt tit din ca lng knh, n song song vi nhau. BB'CC' l mt y ca lng knh. Cc mt ABB'A, v AAICC' l hai mt bn ca lng knh. Cnh AA, gi l cnh nh ca lng knh. Gc i din vi mt y gi l gc nh ca lng knh. Mt lng knh thng c c trng bi hai i lng chnh. l gc nh A (hay cnh y) v chit sut n ca vt liu lm lng knh. Nu ta chiu mt chm sng Si vo mt mt bn ca lng knh th hin tng khc x s xy ra nh trong hnh 3.3. Trong : Si l tia ti; KH l tia l. i l gc ti v i' l gc l ca tia sng; 50Su t m b i: www.daihoc.com.vn

D l gc lch ca chm sng sau khi qua lng knh; ABC l tit din chnh ca lng knh; Di l gc lch ti thiu; y ta c cng thc c trng cho 1 lng knh l:Sin

( A + D) = n. sin( A / 2) 2

(3.1) (3.2)

dD dn 2 sin( A / 2) = . sin d d 1 n 2 sin 2 ( A / 2)

Chnh hai i lng ni trn c nh hng trc tip n s phn li chm sng, ngha l quyt nh gc lch D ca chm sng. Chnh v th khi ch to lng knh ngi ta phi chn nhng vt liu c chit sut ln v ch to gc nh ca lng knh c ln cho ph hp. Vt liu lm lng knh cng phi trong sut, ng nht v ng hng trong mt vng ph nht nh. N phi bn vi nhit v nh sng. Chit sut phi hu nh khng ph thuc vo nhit v m. Trong vng kh kin, ch to lng knh ngi ta thng dng mt vi loi thy tinh, nh thy tinh flin nng, flin nh, thy tinh krau. Trong vng t ngoi ngi ta thng dng thch anh. Nu c vng ph (UY + VIS) phi dng thy tinh c bit. 3.2.1.2 Cc c trung ca my quang ph lng knh nh gi cht lng, hiu qu v kh nng s dng ca mt my quang ph, ngi ta thng dng ba thng s c trng c bn l tn sc gc, tn sc di v phn gii. 1. tn sc gc. Trong mi my quang ph, s bin thin ca gc lch D theo di sng ca tia sng l mt c trng quan trng. N ni ln kh nng tn sc gc ca mt my quang ph v c gi l tn sc gc ca my quang ph . Trong cc my quang ph lng knh, lng knh thng c t v tr cc tiu i vi tia sng trung tnm ca mt vng ph ca my. V th tn sc gc s c 51Su t m b i: www.daihoc.com.vn

tnh theo cng thc:Dg = Dd dn 2 sin( A / 2) x .sin 2 2 d 1 n sin ( A / 2) d

(3.3)

Nu gc nh A ca lng knh bng 60 th cng thc trn s l:Dg = 2 1 n2

x

dn d

(3.4)

Nu my quang ph c m lng knh nh nhau trong h tn sc, th tn sc ca my s bng m ln ca my 1 lng knh. Cn nu cc lng knh c gc nh A khc nhau, th tn sc chung s l tng ca tn sc tng lng knh theo cch b tr. Ngha l tn sc gc chung bng tng tn sc gc ca tng lng knh c trong h tn sc ca my quang ph. Nh vy, theo cc biu thc trn, chng ta thy tn sc gc ca mt my quang ph lng knh ph thuc vo cc yu t sau: 1. S lng knh c trong h tn sc. 2. Gc nh A ca lng knh. 3. Chit sut ca vt liu lm lng knh. 4. Bin thin theo bc sng (sng ngn lch nhiu, sng di lch t). V th, mun tng tn sc gc ca mt my quang ph lng knh ngi ta phi ch to h tn sc c nhiu lng knh ghp li vi nhau. Trong thc t ngi ta thng ghp hai hoc ba lng knh. Bin php th hai l chn nhng vt liu c chit sut ln ch to lng knh v ch to cc lng knh c gc nh A ln. Nhng bin php ch to gc A ln cng ch thc hin c trong mt mc nht nh, thng thng n 90 l ln nht. V khi tng gc nh A ca lng knh th hin tng phn x ton phn mt bn ca lng knh s nhanh chng xut hin, lm mt phn chm sng s khng ra khi lng knh, v ta b mt chm tia . 2. tn sc di. tn sc gc ch cho bit s khc nhau v gc lch ca 2 tia sng, ngha l tia sng b lch i ni gc ln bao nhiu sau khi phn li, m cha ch ra c hai vch ph gn nhau s cch nhau l bao nhiu trn mt phng tiu (hay knh nh). Do , trong thc t, nh gi kh nng tn sc ca mt my quang ph ngi ta cn phi s dng thm c tn sc di. Nhng tn sc di ph thuc vo tn sc gc v tiu c ca thu knh bung nh f2, v n c tnh theo cng thc:Dl = f dl dD = 2 x d sin E d

(3.5)

Trong di l khong cch gia hi vch ph trn mn nh (mt phng tiu); E l gc nghing ca hp nh vi quang trc bung nh ca my v do hiu s di sng ca 2 vch ph, tc l : d = (1 - 2) v = (2 + 1)/2. 52Su t m b i: www.daihoc.com.vn

Theo cng thc trn, mun tng tn sc di ca mt my quang ph lng knh ngi ta phi: - Tng tn sc gc ca n. - Tng tiu c ca thu knh bung nh f2. Nhng bin php ny ch t c trong mt gii hn nht nh. V khi f2 cng ln th cng sng ca vch ph cng gim, ngha l c v tn sc th mt v cng (hai yu t ny t l nghch vi nhau). Do , trong thc t, ngi ta phi tnh ton v chn gi tr f2 cho ph hp sao cho c tn sc ln, m cng vch ph vn m bo cn trn 80%. Tuy nhin, d nh gi v so snh tn sc di ca cc my quang ph vi nhau, trong thc t ngi ta li thng dng gi tr nghch o ca tn sc di, ngha l thay biu thc dl/d bng biu thc dl/d. Nh vy, i lng mi quy c ny cho ta bit c trn n v di ca knh nh (mm) hay mt phng tiu c cha bao nhiu n v di sng (amstrong hay nm), ngha l my quang ph no c gi tr d/dl cng ln th c tn sc di cng nh. V d hai my quang ph Ql v Q2, m Q1 c dl bng 4 A/mm v Q2 c d/dl bng 2 A/mm, th my Q2 c tn sc di gp i tn sc di ca my Q1. tn sc di ca mt my quang ph c lin quan trc tip n di ca vng ph m my thu nhn c. Vi mt vng ph nht nh tn sc di cng ln th vng ph ca my cng ri ra trn mt khong rng, ngha l mt phng tiu ln, nh th trn mt knh nh c chiu di nht nh th ch thu c mt phn ca ton vng ph ca my, nu my c tn sc ln. Nh trong v d trn ca hai my quang ph Ql v Q2, i vi vng ph t 2000 - 4000 A th ton b vng ph thu c ca my Q2 chim di gp i ca my Ql. Do vi nhng my c tn sc ln, ngi ta khng th thu c ton b vng ph ca chng ln mt knh nh, v phi cn mt knh nh qu di. Cho nn ngi ta phi ghi tng phn ca ton b vng ph ca my ln mt knh nh c di nht nh (24 hay 30 cm), ngha l i vi nhng my quang ph c tn sc ln ngi ta phi c b phn quay v hng vng ph cn nghin cu vo mn nh thu ghi. Trong my quang ph lng knh, tn sc ca chng gim theo s tng ca di sng, ngha l trong mt my, th vng sng di c tn sc nh hn vng sng ngn (hnh 3.4). 53Su t m b i: www.daihoc.com.vn

3. Nng sut phn li. Nng sut phn li (kh nng phn gii) ca mt my quang ph trong mt vng no c biu th bng t s:R=

hay

R=

d

(3.6)

Trong 1 v 2 l di sng ca hai vch ph gn nhau m cn c th tch ra thnh hai vch r rt trn hnh nh (mt phng tiu); hay d l hiu s ca 1 v 2. Nhng my quang ph c nng sut phn gii R cng ln th hai vch ph c di sng 1 v 2 ny cng nm xa nhau trn knh nh (hnh 3.5). Nhng gi tr d phi bng bao nhiu hai vch ph 1 v 2 cn c th tch ra r rt thnh hai vch trn knh nh? Nghin cu vn ny v t nhng kt qu thc nghim Rayley nu ra quy tc: Hai tia sng c bc sng gn nhau 1 v 2 ch tch ra c thnh hai vch ph r rng, nu cc tiu nhiu x th nht ca vch ny trng vi cc i nhiu x th nht ca vch kia. l khong cch ti thiu ca 2 vch ph (hnh 3.6).Trn c s , nng sut phn li ca mt my quang ph lng knh c tnh theo cng thc: Nu my quang ph c h tn sc gm m lng knh nh nhau th Rt= mRi (3.7).

Cng thc ny cho ta thy nng sut phn li ca mt my quang ph lng knh ph thuc vo nhiu yu t, nh: + S lng knh trong h tn sc (m); + Chit sut ca vt liu lm lng knh (n); + Sin ca 1/2 gc nh ca lng knh (A/2) + ng knh ca thu knh bung nh (d2); + S bin thin ca chit sut theo bc sng (dn/d) 54Su t m b i: www.daihoc.com.vn

Hnh 3.6a Tch thnh hai vch

Hnh 3.6b Khng tch thnh hai vch

V vy, nu chn cc thng s trn cho ph hp khi ch to lng knh v ch to my quang ph ta s thu c kh nng phn gii tt nht ca mt my quang ph. Nhng ng knh ca thu knh bung nh li c tnh theo cng thc:d2 = d x 2 sin( A / 2)

[1 n

2

sin 2 ( A / 2)

]

(3.8)

trong b l chiu di cnh y ca lng knh. Nh vy, nng sut phn gii ca my quang ph lng knh s l: R = m.b.(dn/d) (3.9)

Ngoi nhng yu t ni, nng sut phn li ca mt my quang ph cn b nh hng bi mt s yu t khc nh: - Tnh cht v mn (c ht) ca lp nh tng trn knh nh, nu ph c ghi ln knh nh. Vi yu t ny, nhng kt qu thc nghim cho thy rng, lp nh tng trn knh nh cng nhy th nh hng cng nhiu n kh nng phn li ca vch ph. V nhng knh nh c nhy cao thng c lp nh tng cha cc ht AgBr bt nh sng c kch thc ln. Nhng trong nhng h thng my mi ca th h th ba, yu t ny hon ton c loi tr, v ph c ghi trc tip ln bng giy hoc o trc tip m khng phi ghi ln knh nh na. - Mt yu t na nh hng n nng sut phn li ca my quang ph l rng ca khe my (khe vo ca chm sng). Ni chung, vic tng rng ca khe my s a n kt qu lm gim nng sut phn li ca my v rng ca khe my cng ln th rng ca vch ph cng cng ln. 3.2.2.3 Nng sut phn li v rng khe my Nng sut phn li ca mt my quang ph nhn chung ph thuc vo rng ca khe my. Khe my cng rng th nng sut phn li ca my cng gim. Nhng mi quan h ny l khng n gin. V th chng ta c th xt trng hp c th sau y: Nu gi a l rng ca khe my quang ph; f1 v f2 l tiu c ca thu knh chun 55Su t m b i: www.daihoc.com.vn

trc v thu knh bung nh; d l ng knh ca cc thu knh th rng hnh hc ca vch ph s c tnh theo cng thc sau: a, = a.(f2/f1) (3.10)

Trong t s f2/fl c gi l phng i ca my quang ph. Ni chung trong cc my quang ph ngi ta c gng ch to t s ny bng 1. Nh vy, vch ph l nh khng phng i ca khe my, ngha l a, = a. V cho vch ph r nt th s phn b cng phi tha mn iu kin: a, < 2s trong s = .(f2/d) (3.11b) (3.11a)

s l na rng hnh hc ca cc i th nht ca vn nhiu x. Do trong iu kin ti thiu ta phi c: a, = 2(f2/d) Hay t cng thc (3.10) v (3.12) chng ta c: a = 2(f1/d) (3.13) (3.12)

Gi tr rng ca a theo cng thc (3.13) ny c gi l rng tiu chun ca khe my quang ph v k hiu l ao, ngha l lc ny ta c: ao = 2 (fl/d) Nh vy rng tiu chun ao ph thuc vo: - di sng ca tia sng. - Tiu c f1 ca thu knh chun trc. - T l nghch vi ng knh d ca thu knh. L thuyt v thc nghim chng minh rng rng tiu chun ao ca khe my, nng sut phn gii ca mt my quang ph thc s ch bng 90 phn trm theo l thuyt. V th, mun c nng sut phn gii cao ta phi m khe my nh hn ao mt cht (10%). Nhng khi m khe my nh hn gi tr ao ta mt v cng vch ph, v trong mt gii hn nht nh, th rng khe my quang ph v cng vch ph l t l nghch vi nhau. Bng 3.1 l vi v d v rng ca khe my quang ph pht x. Cho nn tt nht vn l chn rng khe my ng bng ao hay ln hn mt t c v cng trong phn tch nh lng.


Bng 3.1 Quan h rng khe my v nng sut phn gii - cng vch ph Khe my 0,50 ao 0,90 ao 0,95 ao 1,00 ao 1,20 ao 1,50 ao 2,00 ao 2,20 ao Cng vch ph (%) 50 75 82 90 95 98 99 100 Nng sut phn gii (%) 98 96 94 90 80 75 60 50

V khi khe my rng tiu chun th ch c cc i th nht ca vn nhiu x lt vo c h thng chun trc cho nn trong cc php phn tch khng c chn khe my nh hn rng tiu chun ao. Thng thng, theo kinh nghim thc t ngi ta hay chn khe my c rng t 1ao n 1,5ao l ph hp (phn tch T chn: 0,9 - 1ao; Phn tch L chn t: 1,0 - 1,5ao). 3.2.2.4 rng ca vch ph Nu khe my quang ph c rng l th rng ca vch ph bng rng hnh hc ca khe my cng vi rng ca vn nhiu x chnh gia, ngha l ta c:s = a'+2 f2 d . sin E

(3.14)

Nu khe my rt hp, th rng ca vch ph ch l rng ca vn nhiu x chnh gia, ngha l ta c:s= f2 d . sin E

(3.15)

Hai biu thc trn cho thy, rng ca mt vch ph c lin quan trc tip rng ca khe my v nng sut phn gii ca my quang ph. Khi vch ph cng rng th nng sut phn gii cng gim. ng thi rng vch ph cn ph thuc vo: + Tiu c ca thu knh bung nh. + T l nghch vi ng knh ca thu knh v sin gc nghing ca hp nh. + T l thun vi di sng ca vch ph, ngha l cc bc sng di th vch ph cng rng hn, mc du c cng rng khe sng bng a. 57Su t m b i: www.daihoc.com.vn

Nu knh nh c t vung gc vi quang trc ca h thng thu knh bung nh th rng vch ph s l: s = a, + 2(f2/d) v s = 2v.(f2/) (3.16b) (3.16a)

cho c hai trng hp nu trn, v trong trng hp ny sin E = 1. 3.2.2.5 Cng vch ph v rng khe my Trong phn tch quang ph nh tnh v nh lng, c bit l phn tch hm lng vt ca cc nguyn t, mt yu cu quan trng l phi thu c vch ph c cng ln nht, ngha l phi bit c s truyn nh sng t ngun pht x qua khe my quang ph n vt thu cng vch ph (knh nh, etector) l nh th no. Nu gi Io l cng ca chm sng pht x ca tia sng th quang thng tng cng i vo h chun trc s l: = I o =

4

.(d / f1 ) 2 B.a.l

(3.17)

Hay l.I o = = kB.a.l I o = kB.a.l

trong : a v l l rng v chiu di ca khe my quang ph; B l huy ca chm sng c gc y l . Khi chm sng Io i qua h quang hc ca my quang ph th quang thng thc t tc dng ln knh nh (hay vt thu) s l: = a. = a

4

(d / f1 ) 2 B.a.l

(3.18a) (3.18b)

' = .kB.a.l I o =

kB.a.l

y l h s mt mt nng lng ca chm sng qua h quang hc ca my quang ph, nh thu knh, lng knh... Trong trng hp l tng th bng 1, cn mi trng hp khc u ln hn 1. Quang thng ' ny s tc dng ln knh nh vi ri l:E= ' ' = S a'.l '

(3.19a)

trong S l din tch ca vch ph v ta c: 58Su t m b i: www.daihoc.com.vn

S = a,.l' vi a,= a.f2/(f1.sin) v l' = l.(f2/fl) S = a.l.(f2/fl)2.(l/sin )

Nn din tch ca vch ph s l: (3.19b)

Do , nu nh sng l n sc v b qua hin tng nhiu x, khc x, tn x, khi chm sng Io qua h quang hc ca my quang ph th ri ca chm sng s l:E= ' d 2 = . sin S 4 ( f2 )2

(3.20)

y l trng hp l tng, n gin v gi tr ri E trong trng hp ny c k hiu l Eo. Nh vy, chm sng Io tc dng ln knh nh to ra vch ph c cng l:I = Eo .t

B4

(d / f 2 ) 2 .t. sin

(3.21)

Ngha l nu vch ph hon ton n sc th cng IM ca vch ph t l vi bnh phng khu t i ca thu knh bung nh v sin ca gc nghing ca hp nh , cng nh thi gian t m ri Eo ca chm sng tc dng ln knh nh hay vo khe o ca nhn quang in. Trn y l trng hp n gin, nhng thc t th phc tp hn, v nh sng vo my quang ph khng phi khng c cc hin tng nhiu x, tn x, cc sai st ca h thng quang hc, l nhng yu t thc t khng th loi tr c hon ton. Chnh v th m vch ph trn knh nh thng rng hn mt cht (10%) so vi trng hp tnh ton trn y. Nu gi mt cch tng qut, a1, a2, a3, l rng do cc hin tng nhiu x, tn x, tnh khng tuyt i n sc ca vch ph gy ra, th rng thc t ca vch ph phi l:

a' =

f 1 .(a. 1 + a1 , a2 , a3 ,...) sin f2

(3.22)

v ri thc t lc ny s l:E= ' B a (d / f 2 ) 2 sin = 2 4 [a + (a1 , a2 , a3 ..., )( f1 / f 2 )]

(3.23)

hay l:E = E0 . a [a + (a1 + a2 + a3 ).( f1 / f 2 )]

(3.24)


Nh vy nu a >> (a1 + a2 + a3).(f1/f2) th chng ta li c E - Eo, ngha l tr li trng hp c bit trn. Tc l cng vch ph khng ph thuc vo rng ca khe sng. Cn tt c cc trng hp khc th rng ca vch ph t hoc nhiu u c b nh hng bi rng ca khe sng. V cng vch ph trong cc trng hp lun lun phi tnh theo cng thc:I = E.t = Eo t a [a + (a1 + a2 + a3 ).( f1 / f 2 )]

(3.25)

Cho nn khi khe my thay i th rng ca vch ph cng thay i theo. Nhng mi quan h ny khng n iu v cng khng tuyn tnh. V vy, trong mt php phn tch bt buc phi gi cho khe sng c rng khng i trong mt php o. Mt khc, cng thc (3.23) v (3.25) cn cho thy, ngoi rng ca khe sng, cng ca vch ph cn ph thuc vo c khu ca my quang ph, tc l t s d/f2. Cng vch ph s ln, nu my quang ph c khu ln. Ngha l tiu c thu knh bung nh phi nh. Nhng nu tiu c thu knh bung nh f2 nh, th li mt v tn sc di. Chnh v th trong ch to my quang ph ngi ta phi tnh tm c gi tr khu sao cho ph hp nht, nhm bo m c tn sc m cng ca vch ph cn ln c th o c v pht hin c d dng. Nu ly gi tr nghch o ca khu th ta c t s f2/d. T s ny gi l m ca thu knh bung nh. Trong thc t khi ch to cc my quang ph ngi ta thng hay chn t s ny c gi tr t 10 n 20. Tri li, khi m ln hn 20, vch ph c cng ln, nhng tn sc ca my li qu km. 3.2.2.6 sng ca vch ph pht x Nu ta pht hin v o trc tip cng ca vch ph bng mt nhn quang in, th sng ca vch ph l yu t quan trng. sng ca vch ph ph thuc vo cng ca chm sng i vo my quang ph v cng ca chm sng sau khi qua my quang ph vo nhn quang in. V th cu to, tnh cht ca h quang hc ca my u c nh hng n sng ca vch ph. Mt cch gn ng v theo thc nghim, sng ca vch ph c th c tnh theo cng thc: L = (d2/f2)2 (3.26)

Trong d2 v f2 l ng knh v tiu c ca thu knh bung nh. Nh vy, sng ca vch ph t l thun vi bnh phng ng knh ca thu knh bung nh d2 v t l nghch vi bnh phng ca tiu c bung nh f2, ngha l ph thuc vo m ca thu knh bung nh (t l f2/d2). Nu tiu c f2 cng ln th sng L ca vch ph cng gim, tc l mt v cng v ngc li. 3.2.2.7 S quang hc mt s my quang ph lng knh V d v cc s h quang hc ca mt s my quang ph pht x c ch ra trong hnh 3.7. 60Su t m b i: www.daihoc.com.vn

Hnh 3.7 S quang hc ca mt s my o ph pht x P: Lng knh; G: Cch t; M, M1, M2: Cc h gng; L: Thu knh; E: Hp nh ghi ph; T: Khe sng; S: Ngun pht x. 3.2.2 Cch t v my quang ph cch t 3.2.2.1 Gii thiu v cch t Cch t l h gm nhiu khe hp (vch) song song vi nhau v cch u nhau. Mi khe nh l mt chn sng, cn ch khng c vch nh l mt khe sng. Cch t c tc dng phn li nh sng nh lng knh. Nhng bn cht s phn li nh sng cch t khc lng knh. S phn li nh sng ca cch t l do hin tng nhiu 61Su t m b i: www.daihoc.com.vn

x ca chm sng qua khe hp. Khi s phn li nh sng ca lng knh l theo hin tng khc x ca nh sng qua hai mi trng c chit sut khc nhau.Cch t c hai loi: 1. Cch t phn x v 2. Cch t truyn x. ch to my quang ph ngi ta thng dng cch t phn x, v dng cch t truyn x s b mt nhiu nng lng khi chm sng i qua cch t. Trong mi loi trn li c cch t phng v cch t lm. T thc t chng ta s ch cp n cu to v bn cht ca cch t phn x l chnh (hnh 3.8a v 3.8b).

Cc i lng c trng cho mt cch t l hai thng s chnh sau y: - Chu k ca cch t: L khong cch gia hai vch trn cch t v c k hiu l d (hnh 3.8a). - Hng s ca cch t: L s vch trn mt n v di ca cch t v c k hiu l k, tc l s vch c khc trn 1mm. Nh vy, gia chu k v hng s ca cch t lin quan vi nhau theo biu thc: d. k = 1 (3.27)

Nu gi L l chiu di ca cch t, th tng s vch trn mt cch t s l: N = k. L (3.28)

V cu to th cch t phn x gm c hai phn. l b v mt cch t. B l tm thu tinh hay thch anh dy t 1,5 n 2 cm, c hai mt song song vi nhau v thng c din tch t 4 x 4 hay 4 x 6 n 8 x 12cm. Mt cch t l lp kim loi nhm (Al) tinh khit (99,999 %) v dy t 0,2 - 0,5 mm. Lp kim loi ny c a ln b bng con ng in phn c bit trong mi trng tr. Mt lp kim loi nhn bng v c khc ln nhiu vch song song v cch u nhau. S vch c khc ln lp kim loi ny thng l t 650 - 3600 vch/mm. S vch trn 1 mm cng nhiu th kh nng phn li nh sng ca cch t cng ln. 62Su t m b i: www.daihoc.com.vn

Nh th, nu chng ta chiu mt chm sng song song khng n sc vo mt cch t theo mt gc ti no th ch c vch trn cch t nh mt chn sng, ch khng c vch nh mt khe sng v di tc dng ca hin tng nhiu x trn b mt cch t th chm sng a sc s c phn li thnh ph. Nu gi l gc l ca tia sng, di sng l th ta lun lun c biu thc: - sin + swin = m.k. (3.29)

Biu thc ny c gi l cng thc c bn ca cch t. Cng thc ny gii thch cho ta nguyn nhn s tn sc ca cch t, v t cng thc (3.29) ta c: sin = m.k. + sin (3.30)

y l gc ti ca chm sng, m v k l khng i i vi mt cch t. Nn ng vi mi bc sng ta s c mt gi tr sin tc l c 1 gi tr . iu c ngha l cch t phn li chm sng a sc thnh tng tia n sc lch i theo tng gc 13 khc nhau. Nhng y sng di b lch nhiu, cn sng ngn b lch t hn. ng thi cch t cho ta ph c nhiu bc, ng vi cc gi tr m ca cng mt di sng S m c gi l bc ca ph cch t. Nu tia ti vung gc vi mt cch t, tc l gc ti - 0 th cng thc (3.30) s c dng l: sin = m.k. (3.31)

Trong trng hp ny gc phn x ca cc tia sng khng ph thuc vo gc ti m ch ph thuc vo hng s k ca cch t v di sng ca tia sng m thi. Trong thc t ngi ta hay t cch t v tr ny trong my quang ph. Nu gc ti = 13 th cng thc (3.30) s l: sin = (mk/2). (3.32)

y chnh l tia sng c gc ti tc dng ng vo gia knh nh ca mt vng ph ca hp nh. V th cng thc ny c s dng tnh gc ti khi chn vng ph cho mt php phn tch. cng l v tr cc tiu ca tia sng trung tnm ca vng ph cn chn trong my. Mt khc, cng chnh do hin tng nhiu x qua khe hp to ra ph ca cch t cho nn khong cch t v tr cc i cc vn sng n tnm hnh nhiu x c tnh theo cng thc: xt = m.f0.(/k) ngha l: - Cc i th 1 cch tnm mt on xt1 = (1.f2,)/k - Cc i th 2 cch tnm mt on xt2 = (2.f2. )/k 63Su t m b i: www.daihoc.com.vn

(3.33)

- Cc i th 3 cch tnm mt on xt3 = (3.f2)/k - Cc i th m cch tnm mt on xtm = (m.f2)/k. iu ny gii thch cho ta ti sao tn sc di ca cch t bc th m gp m ln bc th 1, ngha l c tn sc bc th m l:DDM = DDl m

(3.34)

Vi m l s nguyn v m > 1. 3.2.2.2 My quang ph cch t My quang ph cch t, v nguyn tc, cng c cu to nh my quang ph lng knh. Nhng y h tn sc l mt cch t, v cng dng ba i lng c trng nh trong my quang ph lng knh nh gi cht lng ca mt my quang ph cch t. C th l: 1. tn sc gc T cng thc c bn ca cch t (3.29), nu xt s bin thin ca gc lch theo di sng th ta c: cos.d = (m.k).d hay l:Dg = D mk = D cos

(3.35)

Biu thc ny chnh l cng thc tnh tn sc gc ca my quang ph cch t. Nh vy, tn sc gc ca mt my quang ph cch t s: + T l thun vi hng s k ca cch t v bc m ca ph cch t. + T l nghch vi cos ca gc phn x . Ngha l mt cch t th vi cc bc ph cng cao tn sc gc ca my cng ln. V d: - Nu m bng 1 ta c d/d = 1.k/cos - Nu m bng 2 ta c d/d = 2.k/cos - Nu m bng 3 ta c d/d = 3.k/cos v.v... Mt cch tng qut, ngha l tn sc bc m th ln gp m ln tn sc gc bc 1: 64Su t m b i: www.daihoc.com.vn

Dmg =

Dgl m

Nhng cc bc ph ca cch t li xut hin ng thi, ph ca cc bc cng cao th cng cng yu. Gn n 70% nng lng l tp trung bc 1 (vn nhiu x th nht). Chnh v th khi ghi ph phi dng knh lc ph hp loi b nhng bc ph khng cn thit. Cho nn tnh cht a bc ca ph cch t va l u im, nhng cng va l nhc im ca my quang ph cch t, v ph ca cc bc cao ta c v tn sc, nhng li mt v cng . Nn trong thc t, ngi ta ch dng ph bc 1, hay i khi n bc 2 m thi. 2. tn sc di i vi my quang ph cch t, ngi ta cng gi t s dl/d l tn sc di ca n v c tnh theo cng thc:dl 1 = m.k . f 2 d cos

(3.36)

Nh vy, tn sc di ca my quang ph cch t t l thun vi tn sc gc ca n v tiu c f2 ca h bung nh. Hay ni mt cch khc, tn sc di l: - T l thun vi hng s ca cch t, tiu c ca bung nh, v s bc ca ph cch t, v - Thay i trong quan h t l nghch vi cos ca gc phn x ca tia sng. V d t cng thc trn chng ta c: + Nu m = 1 th d/d = (1.k.f2)/cos; + Nu m = 2 th d/d = (2.k.f2)/cos; + Nu m = 3 th d/d = (3.k.f2)/cos; Ngha l di tn sc bc th m th cng gp m ln tn sc di bc 1 (cng thc 3.34). Nhng trong thc t, d so snh, ngi ta cng dng gi tr nghch o ca i lng ni trn, nh quy c trong my quang ph lng knh. Nh vy chng ta s c:1 cos = Ddm m.k . f 2

(3.37)

Cng thc (3.37) ny l tn sc di nghch o ca my quang ph cch t bc th m v ti im chnh gia ca vng ph. Cn tn sc di ti mt im X no cch tnm knh nh mt khong x s c tnh theo cng thc:Ddm = Ddm 0,00016.

10 3

x

(3.38) 65

Su t m b i: www.daihoc.com.vn

T biu thc ny ta thy tn sc di ca my quang ph cch t hu nh ph thuc rt t vo di sng ca tia sng (bng 3.2). Cng thc trn nhn du (-) khi i t tnm knh nh theo chiu di sng tng dn v nhn du (+) theo chiu ngc li. Bng 3.2 So snh tn sc ca hai my Vng ph () 2000 2500 3000 3500 4000 tn sc di nghch o trong my Cch t PGS-2, bc 1 7,390 /mm 7,380 7,370 7,356 7,340 Lng knh q 24 4,00 /mm 7,80 13,00 18,00 31,00 -

3. Nng sut phn gii Nng sut phn gii R ca mt my quang ph cch t c nh ngha nh sau:R=

d

Vi d = (2 - 1), = (2 - 1)/2 v c tnh theo cng thc sau:R=

d . d d

hay l R = m.k.L Nh vy, r rng nng sut phn gii mt my quang ph cch t t l thun vi: - Hng s k ca cch t, ngha l k ln th tn sc ln - Chiu di ca cch t L, v - S bc ph ca cch t, m. Bc ph cao th phn gii cao. Do , mun tng kh nng phn gii ca mt my quang ph cch t phi s dng nhng cch t c hng s k ln v chiu di L ln. Nhng vic tng chiu di L ch c gii hn (di nht l 15 cm). Nn ch yu l tng hng s k ca cch t, ngha l phi ch to nhng cch t c nhiu vch trn 1 mm di. ng thi phi s dng ph nhng bc cao, nh bc m=2. Song cc bc cao ca ph th cng vch ph li nh. V d: ng vi cch t c k bng 1800, th trong vng sng 3000 hai 66Su t m b i: www.daihoc.com.vn

vch ph 1 v 2 khc nhau mt gi tr sau y s c phn li thnh tng vch ring bit: Nu m = 1, ta s c = 0,02 , - Nu m = 2, ta s c = 0,01 , - Nu m = 4, ta s c = 0,005 , Ngha l bc m=1, hai vch 1 v 2 mun tch thnh hai vch r rng th phi khc nhau mt gi tr di sng ti thiu l 0,02 . Khi bc 2, hai vch ny ch cn khc nhau mt gi tr = 0,01 l tch thnh hai vch ring bit. Nhng trong thc t ngi ta t dng ph bc cao ca cch t. Ngi ta thng hay dng ph bc I v bc II, v n 70% nng lng l tp trung bc I. Cho nn trong thc t hin nay ngi ta dng nhng cch t c hng s k rt ln (k:2400-3600) ch to nhng my quang ph c kh nng phn gii cao. Hnh 3.1a, b l s quang hc ca mt s k my ph cch t. 3.3 Vng lm vic ca my quang ph y cng l mt ai lng nh gi mt my quang ph. Do tnh cht quang hc ca vt liu dng ch to h tn sc l trong sut v ng nht trong mt min nht nh, nn mi my quang ph ch c th thu, phn li v ghi nhn c mt vng sng nht nh ca ton b gii ph quang hc. Vng ph ny c gi l vng lm vic ca mt my quang ph. Vng ph lm vic ca my quang ph lng knh l do trong sut v tnh cht ca vt liu lm lng knh quyt nh v ph thuc vo cch b tr (cu to) ca my quang ph y. V d: Cc my quang ph lng knh thch anh thng c vng ph lm vic l min t ngoi gn (2000-4000 ). Cc my quang ph lng knh thu tinh c vng lm vic l min kh kin (36007800 ), v thch anh ch trong sut trong vng t ngoi; cn thy tinh ti trong sut trong vng kh kin (nhn thy). My quang ph cch t thng c vng lm vic rng hn my quang ph lng knh. Vng lm vic ca mt my quang ph cch t l cu to, tnh cht, di sng blanz b) v s bc ca cch t quyt nh. Ni chung, my quang ph cch t thng c vng lm vic rng hn my quang ph lng knh. V d: Vi my quang ph cch t PGS-2 lp cch t c k:650 v b = 2700 , c vng lm vic l: bc 1: 2000 - 7800 v bc 2: 2000 - 4000 ... Nh vy, vi my cch t th vng lm vic bc ph th m hp hn bc 1 l m ln, ngha l chng ta lun lun c: m = 1/m Vng lm vic ca mt my quang ph c lin quan cht ch n di ca c di ph m n thu c. Cc my c tn sc cng ln th di ca di ph cng ln. V th nhng my quang ph c tn sc ln, vi mt tm knh nh c 67Su t m b i: www.daihoc.com.vn

di nht nh (v d 24 hay 30 cm), chng ta ch c th ghi c mt phn ca vng ph lm vic ca my . T thc t , trong cc my quang ph c tn sc ln ngi ta phi c b phn quay cch t hng phn ph cn nghin cu vo knh nh, ngha l vng ph lm vic ca my cn phi c ghi ln nhiu tm knh. V d: Vi my Q 24 vng ph lm vic ca n ch cn ghi trn mt tm knh nh di 24 cm. Khi vng ph ny my PGS-2 ta phi cn hai tm knh 24 cm mi c th ghi ht c vi ph bc I. 3.4 Trang b ca h thng my quang ph pht x T nhng nghin cu c cp trn, chng ta c th minh ha tm tt trang b ca mt h thng my quang ph pht x gm nhng phn sau y: 1. T

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