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Transient NMR Quantitative MeasurementsG.A. PERSYN2 and W.L. R O L L W l T Z ,Southwest Research Institu te, San Anton io, Texas 78228
A B S T R A C TThe transient nuclear magnetic resonance tech-nique is briefly described. Application of this tech-nique allowed quantitative observation of the protonsin the oil, water and solid material in a single cornkernel and on a corn kernel baked at 107 C. Theseresults along with measurements on lyophil ized foodsare discussed.
I N T R O D U C T I O NThe transient NMR technique is a scheme for observingthe nuclear magnetic resonance phenomena and is particu-larly suited for the meas uremen t of the characteristicrelaxa tion times T 1 and T~. T he theo ry gover ning thenuclear spin dynamics under transient conditions is foundin several standard texts on magnetic resonance (1,2). Eachrelaxation time can be measured using several differentpulse schemes depending on the particular requirements ofthe experiment. Through the determinations of theserelaxation times, the interior of the bulk matter can be seenand the microscopic nuclear magnetic interactions can berelated with the macroscopic sample properties.In the present work only T2 measurements are presentedto i dentif y the moisture and oil signals in foodstuf fs. Forvery short T 2 values, the free induc tion decay after a single90 pulse is used and for the longe r values, t he 90 -7.-180
pulse seque nce is used.T R A N S I E N T N M R M E A S U R E M E N T S
The signals received after re peate d applic ation of a 90 pulse were f ollo wed by a 180 pulse (Fig . 1). If the ti mespacing bet wee n the 90 and 180 pulses is 7", afte r th e 180 pulse, a nuclear spin echo will appear. The results ofrepeat ing the pulse sequence for differe nt time s, 7. arepresented. Echo amplitude typical of gases, liquids andsome amorphous solids decreases with the characteristictime T2, where T 2 usually lies in th e range of a fewmilliseconds to seconds. For other, more tightly bondedsolids, the spin-spin intera ctions are very s trong so th at T 2
1One of 10 papers to be published from the Symposium"Wide-Line Nuclear Magnetic Resonance" presented at the AOCSMeeting, Minneapolis, October 1969.2present Address: The Praxis Corporation, San Antonio, Texas78228.
AMPLITUDE PROPORTIONAL TOCTOTAL LIQUIDS PLUS SOLIDS
~ Ak I I j-A MPL ,TU DE P,OPORT ,AL. . . . . t ..TO TOTAL L ,Q U, DS .. , . .. ..
" : . . . . . . . . . . . / I I " '1 . . . .i lIT[0 I0 20 :30 40 50 60 70 8 0 90 I00FIG. 2. Free-decay signal from one kernel of corn at 10 usec/cm.
has a value in the microsecond range (Fig. 2). Figure 2shows the free decay signal following a 90 pulse from thehydrogen, in a kernel of corn dried at ambient tempera-tures.The amplitude of the total hydrogen signal from thecorn kernel (oil plus water, starch, gluten, etc.) is propor-tional to the max imu m value of free decay signal at 10~sec, and at 60/~sec (Fig. 2) it is proportional to the totalliquid signal (oil plus water). The complete free decay signalis limited by the inhomogeneity of the magnet (Fig. 3).The free decay liquid signal at maximum magnethomogeneity is shown in Figure 4. The contribution of thewater and oil is resolvable directly; however, the oil signaldecay is limited by magnet inhomogeneity so that the90~r-180 pulse seque nce is needed to deter mine T 2 f oroil.Plots of the logarit hm of the a mplitu de of the echo as afunc tion of time show t hat the curve of Figure 5b is thesame as curve a except that the time scale is expanded to0.2 millisec per division. Since the plots are curves ratherthan straight lines, there are multip le values of T 2 . The datais obtained by the equation:
Ao = echo amplitude = A21 exp(-2 7./T21 ) + A22 exp(-27"/T22 )where A21 is the am ount with time constant T21 ; A22 ,amou nt with time constant T22 ; T21, shorter time con-stant; T22 , longer time constan t; r, time be tween the 90 and 180 pulses.From the graphs of Figure 5, the values of thenormalized amplitudes, A21/Ao, and A22/Ao, and thevalues of the corr esponding time constants are: A 21/ Ao,77%; T21 , 0.92 millisec; A22 /Ao , 23%; T22, 22 millisec,where A o = A21 +A 22 .Comparis on of these results with those o f Figure 4shows that A 21 and T 21 corr espond to th e wa ter signal and
' [ ~ ' ' . . . . . . : ! : , ~ , , , ,
FIG. 1. Spin echoes. o I O 0 2 0 0 3 0 0 4 0 0 5 0 0FIG. 3. Free-decay signal from one kernel o f corn at 50 usec/cm.67
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68 J O U R N A L O F T H E A M E R I C A N O I L C H E M I S T S ' S O C I E T Y V O L . 4 8
X 2 " : ; ' o ' c o i o " c r ~ , ~ , o o , , c , ~ c , . . , c . . , ~
0 . 2 m i l l i s e e .FI G. 4 . Free - d eca y s ig n al s f ro m o n e k ern e l o f co rn a t 2 0 0~zsec/cm. (a) Corn kernel with the germ removed; (b) corn germ.
, . !O,r~0. ~
0 .5 i ~ ' t . . . . . .T 2 2 = 2 2 m s
( o i l s )
- - ! - _ x
0 ,0 I I 20t i m e i n m i l l i s e c o n d s
FI G. g . Gra p h o f t h e s p in - ech o a m p l i t u d e a s a f u n ct io n o f t im ef o r a k ern e l o f co rn . ( a ) 2 m i l l i sec /d iv i s io n ; ( b ) 0 . 2 m i l l i s ec /d iv i sio n .
T Z l = 0 . 6 mi ( m o i s t u r e )
1 0 2
x - - - - - xo
0.3 lo 20l , im e n M i l l i s e c o n ( [ 8FI G. 6 . Gra p h o f t h e s p in - ech o a m p l i t u d e a s a f u n ct io n o f t im ef o r a k ern e l o f co rn b a k ed f o r 4 . 7 5 h r a t 1 0 7 C . ( a ) 2 m i l l i s ec /d iv i -s io n . ( b ) 0 . 2 m i l l i s ec /d iv is io n .
B/ 1 0 t= S ./C M ,~ i i . . . . t . . . . . . . . . . . . .
" : " ' i . . . . . . . . ~ . . . . . . . . . . . . . . . . . .
I J B J I I i i
' " [ " V . . . . . I . . . . ~ . . . . I ~ . . . . . . . .\ / . s o ~ . s . / c . I, ~ ~ , , , =
FI G. 7 . Free - d eca y s ig na ls f ro m o n e g ro u n d co rn k ern e l . A . On eg ro u n d k ern e l co rn u n b a k ed ; B . 4 3 /4 h r a t 1 0 7 C; C . 2 0 h r at1 0 7 C .
, I I 1 1 1 1. . . . . i . . . . : F . . . I . . . . I , , , I . . . . I . . . .
l | ~ , i i , , , , i i v i , , w ! i ' " I , I i i , , ,\a e o n s t
A S T D E C A Y
| , | | = ~ , J l l l l. . . . ' ' ~ " " " . . . . I . . . . : . . . . . ' . . . ., - - . ~ , ~ L o w D E C A Y
J Z O ~ s IFI G. 8 . Free - d eca y s ig n al s f ro m ly o p h i l l i zed m a s h ed p o t a t o es . B .Ga in x 1 0 .
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FEBRUARY, 1971 PERSYN AND ROLLWlTZ: TRANSIENT NMR 69
l n i ~ 1 I i * l [ i l l
. |' i
l u l l n a a a n O I I a. 1 | 1 1 l UI , U I I! U l | ! nnv ,
_ _ m ~ n n ~ w m m nmmmn~ ~ ~ B N E
~ 1 ~ n a n a i i l ! 1 1 | n i I i i i , a
%,Ii i i i] i i i i i i 1 ! I I I I
FIG. 9. Free-decay signals from lyophiUized gravy and meat astaken from the package and equilibrated at 55% RH. (a) Uppertrace, gravy equilibrated at average relative humidi ty of 55%; (b)lower trace, gravy as taken from sealed package. (c) upper trace,meat equilibrated at average relative humidity of 55%; (d) lowertrace, meat as taken from sealed package.A22 and T22 to the oil signal.When the corn kernel was ground and baked at 107 Cfor 4.75 hr the spin-echo amplitude curve of Figure 6 wasobtained. With this treatment both the oil and the waterwere reduced in amplitude and the total liquid signalcontained 34% oil and 66% water. When the baking timewas increased to 20 hr, the water was reduced so that in thetotal liquid signal 45% of it was oil and 55% water. The freedecay signals obtained from this baking are shown in Figure
7. Figure 7a is the signal, with two horizontal sweep rates,from ground, unbaked kernel. The curves of Figure 7b wereobtained after 4.75 hr of baking at 107 C. After 20 hr, thecurves of Figure 7c were obtained.The free decay obtained from the hydrogen nuclei inlyophilized mashed potatoes is shown in the oscilloscopephotographs of Figure 8. The upper trace in Figure 8 is theoscilloscope presentation at a low gain so that all of thedecay signal can be observed. The fast-decay signal from thehydrogen in the carbohydrate and protein has a relaxationtime , T 21 = 9,5 /./sec. The slowly decaying signal is barelyobserved. The lower trace in Figure 8 is the same as theupper trace except that the gain has been increased by afactor of 10 to show the slowly decaying signal. When theslowly decayin g signal is plotted o n semilog paper the valueof T22 is found to be 48/lsec.Lyophilized gravy and Swiss steak were exposed to anatmosphere with a relative humidity of 55%. The gravynearly doubled the amplitude of its long-decay compon entwhile the short-decay amplitude remained the same asshown in Figure 9a and b. The meat absorbed the mostmoisture from the 55% relative humidity atmosphere asshown in Figure 9c and 9d. In fact, it nearly tripled itsmoisture as noted by the increase in the amplitude of thelong-decay component. This brief study indicates thedifferences in the hygroscopic properties of these samplesand shows the useful data which may be obtained from acomple te study involving measure ments be fore, during andafter the lyophilization process. The transient NMR methodshould also give information related to the long-rangestability of the lyophilized foods, the effects of storage andstorage conditions and deterioration.
REFERENCES1. Abragam, A., "Principles of Nuclear Magn etism" Oxfo rdUniversity Press, New York, 1961.2. Slichter, C.P., "Principles of Magnetic Reso nan ce," Harper andRow Publishers, New York, 1963.
[Received January 19, 1970]