-
8/3/2019 NMR transiente
1/3
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
-
8/3/2019 NMR transiente
2/3
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 )
2 .0
- - ! - _ 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
.
10
T Z l = 0 . 6 mi ( m o i s t u r e )
1 0 2
Ii( a
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 . . . . . . . . ~ . . . . . . . . . . . . . . . . .
.
4
C
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 .
A
, 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
B
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 .
-
8/3/2019 NMR transiente
3/3
FEBRUARY, 1971 PERSYN AND ROLLWlTZ: TRANSIENT NMR 69
l n i ~ 1 I i * l [ i l l
. |' i
A
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
12o~s0B
|
~ 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]
