Download Mass Identified Mobility Spectra of p

Mass Identified Mobility Spectra of p-Nitrophenol and Reactant
Ions in Plasma Chromatography
F. W. Karasek,* S. H. Kim, and H. H. Hill, Jr.
Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada. N2L 3G 1
An interfaced plasma chromatograph/mass spectrometer
(ALPHA-II) permits Identification of the positlve and negative
ionic specles associated with ionlc peaks In the plasma
chromatographic mobility spectra of pnitrophenol. The posltive ions observed and their reduced mobility values ( K O )are
MNO+ (1.67), MH+ (1.80) and the negative ion is [M HI- at
(1.87). These are essentially the ions that are predicted from
chemical ionization mass spectrometry princlples and mobility
data alone. An ion observed at mass 108 (KO1.91) results from
a thermal decomposition product of pnitrophenol. The major
reactant ions were found at m/e 18, 30, 37, and 65, with
probable structures of NH4+, NO', (H20)*H+and ( H20)2N2H+.
The 37 and 65 ions occur at the same mobility. Comparison
of reduced mobllity values, KO, between two independent
laboratories gives an agreement within experimental reproducibility of f0.02 for these spectra.
-
The technique of plasma chromatography (PC) allows observation of the characteristic positive and negative ionic
mobility spectra of nanogram and picogram quantities of organic compounds a t atmospheric pressure. The mobility
spectra give qualitative and quantitative information for organic compounds much like that from infrared spectra. The
method involves reaction of organic molecules with ions and
electrons generated by a 63Nisource in a nitrogen carrier gas,
followed by mobility separation of the products in an ion-drift
spectrometer. A recent review and its references describe the
technique and summarize its capabilities ( I ) .
Plasma chromatography is particularly well-suited for selective detection of gas chromatographic effluents a t subnanogram levels (2, 3 ) . To use P C as a qualitative detector,
it is very desirable to have available a large number of reference mobility spectra as well as a knowledge of the general
type of spectra produced for different classes of compounds.
A number of mobility spectra have been reported previously
as part of a general study of the applicability of the plasma
chromatographic technique to the detection of GC effluents
(4-15). While the reference mobility spectra can be used alone
as fingerprint patterns for identification, their value is enhanced by assigning identification to the ionic peaks involved.
In previous spectra, identity of the ionic peaks observed has
been postulated based on an apparent relationship of positive
mobility spectra to chemical ionization (CI) mass spectra (I6),
combined with information obtained from an approximate
mobility-mass relationship. The procedure we have followed
is to postulate ionic assignments in the mobility spectra by
comparing these spectra to spectral patterns in their counterpart CI mass spectra. The CH4 CI mass spectra are generally the most available; but when CI spectra are not available,
then the type of CI mass spectra produced by compound
classes is used for guidance. The reasonableness of the ionic
assignments made is then checked using an approximate
mobility-mass curve ( I , 17). By application of this procedure,
the positive mobility spectra of most compounds exhibit
prominent ions of the MH+, M+, (M - H)+, (MNO)+ type,
along with those fragment ions most abundant in the CI mass
spectra.
Only those compounds undergoing electron attachment
reactions producing stable ions exhibit negative mobility
spectra. The ionic assignments made for these spectra are
accomplished using data from studies of associative and dissociative electron capture for the gas chromatographic electron capture detector and mobility considerations of the observed ions. The negative mobility spectra appear to be primarily the ions of M-, (M - H)-, along with simple dissociated fragment ions (1, 18).
Although ion identities postulated by these indirect
methods are very useful in advancing our understanding and
further development of the PC method and instrumentation,
the procedure does not give a positive identification to each
ion. For identity of t h e exact mass associated with an ionic
peak in the mobility spectra, an interfaced plasma chromatograph/mass spectrometer (PC/MS) system is needed. The
instrumentation and experimental procedure for the PC/MS
system have been reported previously (19). Using such
PC/MS instrumentation (Figure l),the authors have determined the mass associated with each ionic peak in the mobility
spectrum of both the reactant ions and product ions for heroin
and cocaine (15).These identified ions agree closely'with those
predicted in the ion mobility spectra using the approximate
method as indicated in Figure 2. The PC/MS data agree most
closely with the CI mass spectra obtained in charge exchange
reactions where N2/NO reactant gas is used (20).
This same type of study with similar results is reported here
for p-nitrophenol. The mobility spectra of both reactant ions
and product ions for p-nitrophenol were also obtained on a
simple plasma chromatograph (Beta VI) providing an independent check on the accuracy of previously reported reduced
data.
mobility (KO)
EXPERIMENTAL
The p-nitrophenol was obtained from Aldrich Chemicals, reagent
grade. One mg was dissolved in 10 ml of methanol and 1 ~of1this solution was allowed to evaporate onto the tip of a platinum wire. This
wire was then inserted into the injection port of a Beta VI plasma
chromatograph and the ion mobility spectra were recorded. The experimental conditions for mobility spectra obtained with the Beta
VI were as follows: 350 ml/min of nitrogen drift gas; 40 ml/min of nitrogen carrier gas; gate widths of 0.2 ms; an electric field gradient of
214 V/cm; a drift tube and injection port temperature of 204 "C;
pressure, 727 Torr; scan time, 2 min; electrometer sensitivity,
A. The drift length of the Beta VI instrument is 6 cm.
Mass spectral, total ion mobility spectral, and mass-identified ion
mobility spectral data were obtained using the Alpha I1 plasma
chromatograph-mass spectrometer (Franklin GNO Corporation,
West Palm Beach, Fla.). The Alpha I1 instrument (Figure 1) consists
of a Beta VI1 plasma chromatograph coupled d,irectly to a specially
modified Extranuclear Laboratories quadrupole mass spectrometer
( 2 1 ) .The Beta VI1 plasma chromatograph employs a dual grid tube,
similar to that of the Beta VI model used by the authors for all earlier
work. The mass spectral data were obtained by holding both the drift
tube gates open, allowing all the ions formed in the plasma chromatograph to continuously drift down the tube and into the quadrupole
mass spectrometer. The mass spectrometer was then scanned to
produce a mass spectrum of the ions present. Total ion mobility
spectra were obtained by operating the first grid of the plasma chromatograph in the normal gating fashion with the second grid continuously open, and monitoring the drift times of the ions with the
total ion monitor of the mass spectrometer. Finally, mass-identified
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8 , JULY 1976
1133
I
PC
I CARRIIR
CAI
M I
PC/MS
oAWii
n
h
PC DillClOR
MULllPLllR CATNODi-
I
R l A C l A N l IONS
Figure 1. Schematic d[agram of Alpha I1 PClMS combined system
The Alpha II consists of a Beta VI1 plasma chromatograph coupled directly to
a specially modified Extranuclear Laboratories quadrupole mass spectrometer
Figure 3. Mass spectrum of reactant ions observed by PClMS system
with both PC gates open and signal averaging 2048 scans of 0.1 s each
Heroin
C I (CHI)
Table I. CH, CI Mass Spectra of p-Nitrophenol
I 370
268
I
-
Cocaine
.*
I -
368
C I (CH4)
MH
I304
+
I
182
I
I
l!b
400
2W
250
I
I1
Heroin
... L
PC/ms
Abundance,
%
180
168
141
140
2.4
2.6
7.0
100.0
134
124
123
94
1.6
1.3
2.7
1.2
1 303
I
1
I
RESULTS AND DISCUSSION
I
350
4M)
3101
M+
I
*b h I& do i i o cbo
REDUCED MOBILITY (cm2/V-m)
Figure 2. CI mass spectral patterns using both CH4 and Np/NO (20)
reactant gases are compared to PC/MS mobility spectral patterns for
heroin and cocaine. The M+ ions appear in both the Ng/NO CI and
PC/MS spectra
ion mobility spectra were obtained by operating the plasma chromatograph with the above grid gating procedure, but tuning the mass
spectrometer to respond only to those ions having a specific m/e.
The sampling technique used with the Alpha I1 instrument was the
same wire insertion method described above. Because of the low signal
levels being observed in the mass spectrometer, repeated sequential
samples were injected to maintain sample concentration sufficiently
high while multiple 20 ms scans (as many as 16 384) were signal averaged. The operating parameters were as follows: nitrogen drift gas
flow, 500 ml/min; nitrogen carrier gas flow, 100 ml/min; gate widths,
0.2 ms; electric field gradient, 214 V/cm; drift tube and injection port
temperature, 204 OC; and pressure, 763 Torr. The drift length of the
Beta VI1 plasma chromatograph is 8 cm from the injection grid to
detector, but the total drift length of the Alpha I1 system from the
injection grid t o the mass spectrometer detector is somewhat longer
since the orifice interfacing and ion lens of the mass spectrometer add
extra length to the drift space. The drift length of this combination
was calculated from its observed drift times and those obtained with
the 8-cm drift length of the Beta VI1 plasma chromatograph for the
same ion. These calculations give a drift distance of 9.64 cm for the
PC/MS instrument.
1134
mie
M+
304
360
.
1
Abundance,
%
The mass spectrometer was calibrated using FC-43 and the electron
impact ionization source, which is not actuated when ions from the
plasma chromatograph pass through in transit to the mass spectrometer detector. The calibration was checked prior to obtaining
these mass spectra; calibration stability is accuiate to within f O . l amu
for any 8-h period (22).
All data reported for the Alpha I1 instrument were taken by signal
averaging a given number of 20-ms scans in a Nicolet signal averaging
computer (FT-1072; Nicolet Instruments Inc., Madison, Wis. 53711).
Where indicated in the figures, the data were subjected to multiple
3-point smoothing operations.
Both the carrier and drift gases used were Linde High Purity Grade
cleaned prior to use by passage through separate filter units packed
with 60/80 mesh Linde Molecular Sieve 13X.
The CH4 CI mass spectrum in Table I was obtained using a Hewlett-Packard 5982A GC/MS under control of a 5933A computer system with the source temperature at 190 "C and pressure a t 1Torr.
m/e
E
ie
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
Reactant Ions. Usually three reactant ion peaks occur in
the mobility spectrum when a relatively pure nitrogen gas
containing 10 to 50 ppm water vapor is used as a carrier gas.
The relative abundance of these ion peaks depends primarily
upon the water concentration and the temperature, and also
somewhat on the presence of reactive trace impurities such
as NH3 and NO. The ions represented by these three peaks
were clearly defined by Carroll et al. as ( H z O ) ~ N H ~ + ,
(H20),NOf, and (H20),H+ (23).At 200 "C, each of the three
peaks represent rapidly reacting equilibrium mixtures in
which hydrated ions of like structure, i.e. (H20),H+, exhibit
a drift time dependent upon the average value of n.The most
abundant reactant ion is (H20),H+, where n = 2 and 3.
The PCIMS data for reactant ions obtained in this study
are in accord with the conclusions of Carroll. The mass spectrum of the reactant ion mixture shown in Figure 3 reveals the
presence of ions at m / e 18, 30, 37, 46, 65, and 93. Figure 4,
showing the drift times of individual ions, indicates the ion
peak in the mobility spectrum associated with each. The
presence of the mle 37 and 65 ions in the same mobility peak
could be explained by the equilibrium: (H20)2N2H+ +
(HzO)zH+ Nz. Although its mobility was not determined,
the ion present at mle 93, which corresponds in mass to
(H20)2(N&H+, could also be involved in this equilibrium.
Although CO and C2H4 could be present and have the proper
+
n
PC/M5
ION5
RlAClANl
Figure 5. Mass spectrum of pnitrophenol observed by PC/MS system
with both PC gates open and signal averaging 1024 0.2 mass scans
5PCCITIC IONS
m/..ao
o ' i ' i ' iD R I'F T 8T I M E -10 m a e c 12
I
I
I
I
I
I
14
I
"
16
'
18
Figure 4. Total (upper) and individual single ionic species of reactant
ions monitored by PC/MS combined system at 204 OC
The total ion spectrum is the signal averaged trace of 4096 20-ms scans: each
specific ion spectra is the signal averaged trace of 16 384, individual 20-ms
scans
mass to form the m/e 65 and 93 ions, Nz appears to be the most
reasonable choice because i t is the major component in the
drift space by a factor of a t least lo6. While these studies on
the exact nature of the reactant ions increase understanding
of the basic phenomena occurring in the plasma chromatograph, all these positive reactant ions are effective in forming
product ions with the sample compound injected, and analytically a consistent reproducibility is their essential characteristic.
p-Nitrophenol. By continuously admitting all the ions
formed in the PC tube into the mass spectrometer through the
orifice, a mass scan reveals the product ions formed by the
p-nitrophenol sample to be m / e 169,140 and 108 (Figure 5).
These correspond to MNO+, MH+, and (M - 31)+ ions. A
mobility spectrum can be obtained by gating the first grid of
the PC tube in the normal manner leaving the second grid
continuously open, and tuning the mass spectrometer to admit
all ions to its detector. The mobility spectrum so produced is
seen in the upper portion of Figure 6. Obtaining a mobility
spectrum in the same manner but tuning the mass spectrometer to the specific ions of mle 169,140, and 108 in turn,
produces a specific ion mobility spectrum of each ion and reveals the ion associated with each peak in the total ion mobility spectrum. These specific ions are associated with ion
peaks of the expected mobility.
From the CH4 CI mass spectral data in Table I, and mobility considerations, the MH+ and MNO+ ions are easily
predictable. The MH+ ion is the same in both CI and PC/MS
data. The characteristic MNO+ ion is the equivalent of the (M
29)+ and (M 41)+ ions in the CH4 CI spectrum. However
the ion at mass 108 in the mobility spectrum does not appear
in the CI spectrum. The experimental evidence points strongly
+
+
'
'
,
6
1
1
8
1
1
10
1
1
12
I
2
y
I
I
14
o
I
16
'
I
..
-1.0
,
-169
I
18
I
I
20
DRIFT TIME-msec
Figure 6. Total and specific ion mobility spectra observed for p n i t r o phenol with the PC/MS system
The (M - 31)+ ion originates from a thermal decomposition product of pnitrophenol due to the sampling technique employed. The spectra are the signal
averaged trace of 16 384 individual 20-ms mobility scans
to this ion originating from a thermal decomposition product
of the p-nitrophenol. Figure 7 shows the mobility spectra of
p-nitrophenol obtained with the simple Beta VI plasma
chromatograph. The presence of the (M - 31)+ a t K Oof 1.91
is barely discernible in these data. This positive mobility
spectrum was obtained immediately after a single sample
injection. While the temperature for the Alpha I1 PC/MS data
was the same (204 "C), the inlet system differed from that of
the Beta VI instrument, and the longer time required to obtain
the more extensive data taken with the lower signal levels
involved in the mass spectrometer necessitated repeated,
sequential injections of sample to maintain the necessary
signal levels. These factors appear to have introduced sizeable
amounts of a thermal decomposition product leading to the
mle 108 ion in the Alpha I1 data.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
1135
~~
Table 11. Mobility Values (KO)
of p-Nitrophenol and
Reactant Ions
Ion
Mass
Alpha
p-Nitrophenol
r
PO lTlV
4
1
+
k
B
Ib
i
11
'
1'1
Figure 7. The mobility spectrum and KOvalues obtained with the Beta
plasma chromatograph agree closely with those obtained with the
Alpha II PC/MS system. The (M - 31)+ decomposition product ion,
which would appear at KO 1.91, is almost completely absent
VI
1
r:
',
I
l i
r -
;
'
i
'
;
'
8
'
;
0
12
D R I F T T I M E - msec
14
16
18
20
Figure 8. The specific negative ion mobility spectrum of pnitrophenol
observed with t h e PC/MS system for m/e 138 is coincident with t h e
total ion mobility spectrum; 16 384 individual 20-ms mobility spectral
scans were signal averaged and subjected to 3 three points smoothing
operations
Only one ion appears in either the mass spectrum, total ion
mobility spectrum, or the mle 138 specific ion mobility
spectrum in the negative ion PCIMS spectrum (Figure 8).This
quite clearly is the N02C6H40- phenoxide ion reported by
Dzidic (24). The comparative reduced mobility ( K O data
)
in
Table I; gives a value of 1.87 for this ion, which compares favorably to that of 1.86 given for the phenoxide ion by Dzidic
from our previous data. The mobility data in Table I1 compare
values obtained for the same sample under completely different conditions with two different instruments in separate
laboratories. The agreement of these values within the reproducibility of the method confirms the reliability of previously reported K Ovalues obtained with the simple Beta VI
plasma chromatograph in our laboratories. The discrepancy
in K Ovalues for supposedly the same ions of p-nitrophenol
1136
I
IV
DRIFT TIME ( m r u l
i
Ion
Peak
I1
111
IMNO)+
E
IIGNO
MNO'
MH'
(M-31)'
(M-H).
Reactant
ions
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
KO-reducedmobility
(cm2 /V-sec)
Beta
VIWater-
169
140
108
138
PC/MS
1.67
1.80
1.92
1.87
1.67
1.79
1.91
1.86
18
30
37
65
3.04
2.69
2.39
2.39
3.03
2.67
2.37
2.37
loo
presented by Dzidic in Table I1 of his recent paper was attributed to errors in temperature measurements between our
and the GNO laboratories ( 2 4 ) .In view of these more definitive data, the differences most probably arise from a confusion
between the (M - H)- and MH+ ions. The GNO mobility of
1.77 reported in the Dzidic paper for the N02C2H40- corresponds quite well with that of 1.80 observed here for the
(NO&&OH)H+ ion.
CONCLUSIONS
Data shown here for p-nitrophenol and previously for
heroin and cocaine using an interfaced PCIMS system indicate that product ions do not appear to be involved to any
great extent in rapid equilibrium exchange reactions in the
ion-drift spectrometer as reported by Carroll for the hydrated
reactant ions, and that a n indirect method can predict reasonably well the type of ions involved in ion peaks in the PC
mobility spectrum of a compound. The method functions well
even in the presence of a thermal decomposition product.
Limited data suggest that better results are obtainable when
CI mass spectra produced using an N21NO charge exchange
ionizing gas are available, possibly because the ion-molecule
reactions involved more closely approach those occurring in
the plasma chromatograph. Considering the relative simplicity
of the PC instrument, compared to the PCIMS, an identity
assignment of product ions associated with mobility peaks by
an indirect method appears to give reasonable and useful results for analytical use. An independent comparison of reobtained in two independent
duced mobility values (KO)
laboratories gives an agreement within the experimental reproducibility of f0.02.
ACKNOWLEDGMENT
The CH4 CI mass spectrum of p-nitrophenol was obtained
by J. Michnowicz of Hewlett-Packard Instrument Company
using the H P 5982Al5933A GCIMSlComputer system. The
mass-identified mobility data were obtained through the
courtesy of Franklin GNO Corp., West Palm Beach, Fla., using
the Alpha I1 plasma chromatograph/mass spectrometer instrument. We wish to acknowledge M. J. Cohen, C. Wernlund,
and R. F. Wernlund for direct assistance in obtaining the data
and J. H. Wolfe, R. C. Kindel, D. Taylor, and R. F. Wernlund
for design and construction of the Alpha I1 instrument.
LITERATURE CITED
F. W. Karasek, Anal. Chem., 48, 710A (1974).
M. J. Cohen and F. W. Karasek, J. Chromatogr. Sci., 8, 330 (1974).
F. W. Karasek and S. H. Kim, J. Chromatogr., 99, 257 (1974).
F. W. Karasek and R. A. Keller, J. Chromatogr. Sci.. 10, 626 (1972).
(5) F. W. Karasek and D. M. Kane, J. Chromafogr. Sci.. 10, 6 7 3 (1972).
(1)
(2)
(3)
(4)
(6) F. W. Karasek, D. M. Kane, and 0. S.Tatone, J. Chromatogr., 87, 137
(1973).
(7) F. W. Karasek, D. W. Denney, and E. H. DeDecker, Anal. Chem., 46,970
(1974).
(8) F. W. Karasek and D. M. Kane, Anal. Chem., 46, 780 (1974).
(9) F. W. Karasek and D. M. Kane, J. Chromatogr., 93, 129 (1974).
(10) F. W. Karasek and D. W. Denney, J. Chromatogr., 93, 141 (1974).
(1 1) F. W. Karasek and D. W. Denney, Anal. Chem., 46, 1312 (1974).
(12) F. W. Karasek and S.H. Kim, Anal. Chem., 47, 1166(1975).
(13) F. W. Karasek, D. E. Karasek, and S. H. Kim, J. Chromatogr., 101, 345
(1975).
(14) F. W. Karasek, A. Maican, and 0. S. Tatone, J. Chromatogr., 110, 295
(1975).
(15) F. W. Karasek, H. H. Hill, and S. H. Kim, J. Chromatogr., 117, 327
(1976).
(16) F. W. Karasek, D. W. Denney, and E. H. DeDecker, Anal. Chem., 46,970
(1974).
(17) G. W. Griffin, I. Dzidic, D. I. Carroll, R. N. Stillwell, and E. C. Horning, Anal.
Chem., 45, 1208 (1973).
(18) S.N. Lin, G. W. Griffin, E. C. Horning, and W. E. Wentworth, J. Chem. Phys.,
60. 4994 11974).
(19) F. W.Karasek, M. J. Cohen, and D. I. Carroll, J. Chromatogr. Sci., 9,390
(197 1).
(20) I. Jardine and C. Fenselaw, Anal. Chem., 47, 730 (1975).
(21) T. Giehler, Extranuclear Laboratories, Inc. P.O. Box 11512, Pittsburgh, Pa.
15238, personal communication, September, 1975.
(22) R. F. Wernlund, Franklin GNO Corp., West Palm Beach, Fla., 33402, personal communication, September, 1975.
(23) D. 1. Carroll, I. Dzidic, R. N. Stillwell, and E. C. Horning, Anal. Chem., 47,
1956 119751.
I. Dzidic, D.’I , Carroll, R. N. Stillwell, and E. C. Horning, Anal. Chem., 47,
1308 (1975).
RECEIVEDfor review February 3,1976. Accepted March 24,
1976. The work of the authors is supported by National Research Council of Canada, Grant No. A5433.
Analysis of Organophosphorus Compounds at the Parts-perMillion Level by Phosphorus-31 Fourier Transform Nuclear
Magnetic Resonance Spectroscopy
Thomas W. Gurley and William M. Ritchey*
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44 106
A technique has been developed employing 31P Fourler
transform NMR to observe and quantitate seven Organophosphorus compounds (organlc phosphates, phosphonates, and
thiophosphates) at the lower ppm concentration range
( 10-3-10-4 M). Although these concentratlons have been
observed in extended time experiments, this technique was
developed to reduce the experimental time. The standard
experlmental tlme is 25 min. The phosphorus compounds were
studied to determine the nuclear Overhauser enhancement
and the spin-relaxation times. The development work Involved
the selection of a suitable relaxagent to reduce the spln-lattlce
relaxationtlme of each phosphorus nucleus. As the relaxation
times are reduced, the pulse interval for a 90’ pulse also can
be reduced which reduces the experimental time requlredto
achieve comparable signal-to-nolse ratios. The relaxagents
observed Included iron( 111) complexed wlth acetylacetone,
ethylene glycol, and some dithiocarbamates,other transition
metal ions, a free radical, and G d ( f ~ d ) ~ .
The development of more sensitive techniques for chemical
analysis has been and will continue to be the goal of many
researchers. This paper deals with a continuing effort in the
area of quantitative analysis using 31PNMR. This research
has been directed towards the detection and subsequent
characterization of organophosphorus compounds in our
aqueous environment. Initial work has been published on the
analysis of inorganic phosphates present in water and earlier
references to quantitative work are included therein ( I ) .
Sensitivity Redefined. With the advent of the pulsed
Fourier transform (FT) capability and other signal averaging
techniques, the “sensitivity” of a given technique had to be
defined in terms of experimental time. Figure 1gives a theoretical plot which depicts this relationship. The signal-to-noise
( S I N ) of an FT experiment is directly proportional to the
square root of the number of pulses (top of Figure 1). Given
a 2% solution, one finds that, after 25 pulses, a SIN of 30 is
produced. By increasing the number of pulses by a factor of
4 to 100 pulses, the SIN increases by a factor of the square root
of 4 or 2 to give a SIN of 60. If this experiment was performed
using a series of 90° pulses and TI>>Tz*,an adequate pulse
interval would have to be employed to ensure adequate restoration of M , toward its equilibrium value of M a or a steady
state condition would occur with a greatly attenuated signal.
However, if T I =Tz*,then a more rapid pulse repetition rate
would be satisfactory (2). For the best quantitative results,
the repetition rate must be slow enough to allow the nuclei to
fully relax before a subsequent pulse is applied. Therefore, the
experimental time would be a function of the spin-lattice
relaxation time, T I . If the pulse interval required was 50 s
(bottom of Figure l),the experimental time for a 25-pulse
experiment would be 20 min. However, if the pulse interval
was only 5 s, the experimental time would be 2 min and a 0.5-s
interval would allow a 12-s experiment, all with identical SIN.
It becomes obvious that the optimum condition is approached
when the spin-relaxation time is less than 0.5 s.
Methodology. The methodology which has been developed
and employed in this study involves the use of paramagnetic
ions a t very low concentrations to reduce the T1 to be nearly
equivalent to Tz*. Figure 2 is a theoretical plot of SIN vs.
concentration of paramagnetic ions assuming a relatively short
pulse interval and a given experimental time (fixed number
of pulses). In region A, a steady state is achieved where very
little of the M , is restored to M , since T I >> Tz*and therefore
an attenuated signal is obtained. As the concentration of
paramagnetic ions increases the SIN also begins to increase
since the steady state achieved by reduction of T1 involves a
greater restoration of M , and therefore an enhanced signal
(region B). The maximum of the curve is the concentration
of paramagnetic ions to obtain the optimum SIN. However,
at higher concentrations, the plot will begin to show a decrease
in SIN due to a reduction in the spin-spin relaxation time, T2.
As this occurs, the line width of the observed signal begins to
increase and SIN decreases. At very high paramagnetic ion
concentrations, the signal could be broadened severely enough
to disappear completely into the noise, SIN = 1 (region D).
This area indicated to the right of the optimum SIN should
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
1137