mined much more accurately than with the MS 12. Although hydrogen does not affect the C180/C1e0 ratio, it is generally present to at least the same concentration as the carbon monoxide and must be considered along with the other major impurity methane in evaluating the yield of carbon monoxide. Methyl chloride was not detected in any of the samples which is probably an indication that it is not formed in pyrolyses with stannic chloride. The yield results of the pyrolysis products are shown in Table 111. It is clear that the per cent of oxygen recovered as carbon dioxide is always higher than in the analogous pyrolyses with mercuric chloride. Some samples showed a slightly lower total oxygen recovery as the oxides of carbon, but in all cases these were samples which showed good oxygen recovery by either method. Table IV shows the mole fraction of the impurities and the peak on which the relative amounts were calculated. Our results indicate that the pyrolyses are not very sensitive to the amount of tin tetrachloride added, and there is no large variation in impurities with the amount of stannic chloride added. Since the change in the amount of tin tetrachloride from 20 to 100 mg did not appreciably affect the carbon dioxide yield, higher concentrations were not attempted. In fact, the lower oxygen recovery for some compounds when the larger amount of tin tetrachloride was used would indicate some caution in using indiscriminately large amounts. On the basis of our data, a ratio of 20 to 50 mg of tin tetrachloride to 150 mg of sample would seem to be optimum. Tin tetrachloride possesses the additional advantage in being a volatile liquid when anhydrous. The hydrate is a solid. This property is particularly valuable in ensuring the anhydrous condition of the additive. Tin tetrachloride can
also be sealed in break off ampoules in known amounts and transferred from the ampoule to the pyrolysis tube using standard vacuum line techniques. The ampoules can be filled using 6 to 10 mm of the tin tetrachloride vapor pressure in an appropriate volume to give the approximate weight desired. Where duplicate runs have been made, the data are reproducible to the extent that the differences in the yields observed for different compounds at different temperatures with different additives are certainly real. The overall results of our experiments would suggest the following conditions for converting organic compounds to carbon oxides for the isotopic analysis of oxygen. The pyrolysis temperature of 500 “C and pyrolysis time of one hour seem to be a generally useful condition. For compounds which might give low yields of carbon dioxide, the carbon monoxide fraction should always be collected and analyzed. In analyzing for carbon dioxide and carbon monoxide, the presence of certain hydrocarbon impurities must be checked and, if present, corrections made in the appropriate peak heights. The carbon oxide fractions should be distilled through a trap cooled to -126 “C in order to trap out all but the most volatile hydrocarbons. If mercuric chloride is used, an amount by weight equivalent to the sample weight is more desirable than a smaller quantity. Stannic chloride seems to be the additive of choice. Stannic chloride samples of 20 to 50 mg can be prepared in break ampoules and added to the pyrolysis ampoule just prior to sealing.
RECEIVED for review August 21, 1971. Accepted November 19, 1971.
Mass Spectrometri,c Isotope Ratio Measurements and Peak Area Integration Using the Peak-Switching Feature of the AEI MS-902 N. M. Frew’ and T. L. Isenhour Department of Chemistry, University of North Carolina, Chapel Hill, N.C. 27514 Precise measurements of isotope ratios using the standard voltage peak-switching circuitry of the AEI MS-902 double focusing mass spectrometer is described. Minor modification of the instrument and use of a small digital computer and data acquisition interface allow accurate peak area comparisons with precisions in the range of O.l-l%, depending on sample amount. Limitations on precision and accuracy due to instability of the electrostatic sector are discussed. Discrimination effects due to the voltage switching are observed, but can be corrected for by reference to an appropriate isotope standard. In addition to isotope ratio determinations, the system can be used for accurate trace metal determinations by integration of the ion current produced by evaporation of the volatile metal chelate from the direct insertion probe.
THISPAPER INVESTIGATES the use of an AEI-902 double focusing high resolution mass spectrometer in conjunction with a small digital computer and analog-to-digital converter to accurately determine integrated peak areas and isotope ratios utilizing the standard peak-switching facilities of the instru1 Present address, Chemistry Department, Woods Hole Oceanographic Institution, Woods Hole, Mass. 02543
ment. Isotope ratio measurements of the high accuracy and precision frequently required in geochemical applications, such as age determinations, and in studies of kinetic isotope effects are normally performed on instruments specifically designed for accurate, reproducible signal intensity comparisons (1-7). The features typically incorporated into their design include either duplicate ion collection systems for simultaneous measurements ( l ) ,or provisions for programmed magnetic-field switching using magnet current pre-selectors or field sensitive devices (2, 3). A system for voltage peak-switching with a single focusing instrument has produced isotope ratio measurements comparable to the best magnetically switched data (1) J. R. Roboz, “Introduction to Mass Spectrometry,” John Wiley and Sons, New York, N.Y., 1968, p 453. (2) W. Compston, J. F. Lovering, and M. J. Vernon, Geochim. Cosmochim. Acta, 29, 1085 (1965). (3) V. R. Murthy, R. A. Schmitt, and P. Rey, Science, 167, 47C (1970). (4) P. A. Arriens and W. Compston, Itzt. J. Mass Spectrom. Ion Phys., 1, 471 (1968). (5) J. W. Taylor and E. P. Grimsrud, ANAL.CHEM., 41,805 (1969). (6) G. R. Alexandru, Reg. Sci. Imtr~rm.,39, 1571 (1968). (7) J. L. Powell and S . E. DuLong, Science, 153, 1239 (1966). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
659
ULB1
ION ACCELERATING VOLTAGE SUPPLY
I
-
540 V
'.....I
MRGNETIC ANALYZER
(A)LOW MRSS-AUTO-HIGH MASS [ B )
1 SEC
1 SEC
--I
FLYBACK
VOLTAGE TO AUXIL. SWEEP COIL AND TO X-PLATES OF OSCILLOSCOPE
E.S.A. PLATE A N D ION ACCELERATING VOLTAGES (P.LB1)
m
\ ') ) '7'
OSCILLOSCOPE AMPLIFIER CAIN CONTROLS A 6 B (RLC1 A N D RLCZ)
OSCILLOSCOPE BRIGHTNESS AMPLIFIERS
-+ +
SWEEP
T! BATTERY
*\
L SEC
TIME SCALE
Va
Va
-
AVa
B
A
ON
+
OFF
ON
OFF
Va
Va t A V a
A
B
ON
OFF
(-2)
AANDB
OSCILLOSCOPE DISPLAY
RLCl
h
RLC2 OSCILLOSCOPE DISPLAY
Figure 1. MS-902 mass measurement system (8)
(4). The type of source varies according to the nature of the sample under study, but is commonly of the triple or V filament thermal ionization type for inorganic solids or the electron impact type for substances with an appreciable vapor pressure. Particular attention is given the configuration of the source with respect to optimizing stability characteristics and to minimizing mass discrimination effects. Many instruments also incorporate provisions for convenient and rapid alternate introduction of sample and reference compounds for purposes of monitoring instrumental drift (5, 6). Isotope ratio instruments typically show high abundance sensitivities and routinely provide measurements with a reproducibility of 0.1 % o r less. The MS-902 was designed with a different primary use in mind-namely, qualitative studies requiring high resolution while retaining moderate sensitivity. As a double focusing instrument, because of the inherent instability of the electrostatic sector, the MS-902 does not possess the extreme stability necessary for quantitative work of the highest precision. In addition, this particular instrument does not allow for strictly simultaneous detection of ion currents arising from different mass fragments. Despite these obvious limitations, however, a number of arguments can be advanced for investigating the possible utilization of the MS-902 as an isotope ratio instrument of moderate accuracy and precision: first, it is extremely versatile in terms of sample introduction systems and, when operated under conditions of low to moderate resolution, it possesses the high sensitivity necessary for small samples ; second, it incorporates a convenient peak-switching feature which allows for rapid alternate readout of ion currents from widely separated mass fragments; last, it is becoming increasingly common as an all-purpose instrument (usually interfaced to a small computer) at installations doing general service work, but with limited budget outlays. It would be highly desirable to provide not only for routine qualitative and high resolution information, but also for quantitative isotopic assays, especially of biological materials, with a single instrument. The work described here demonstrates that by a minor modification of the peak-switching circuitry and with computerized data acquisition and signal averaging, the MS-902 provides isotope ratio determinations of useful precision ; peak area comparisons can be made with precisions ranging from O.l-1X depending on sample size, with a typical reproducibility of 0.5 % for samples in the microgram range. 660
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
LOW
HIGH
MASS
MASS
LOW MASS
Figure 2. Schematic representation of peak-switching operation
The system developed has allowed measurements of chromium isotope distributions in lunar materials and, by isotope dilution methods, quantitative determination of the chromium content of geological materials, including Apollo 12 samples. INSTRUMENTAL CONFIGURATION Peak-Matching Circuitry (8). The peak-switching facility of the MS-902 is designed to allow extremely accurate comparison between the mje value of a known fragment with that of an unidentified fragment; its operation is based on the principle that, at constant magnet current, the relative mass difference, AM]M, between the reference and unknown ions is equal to the relative change in the E.S.A. plate volatge, AVjV,, and in the accelerating potential, AVjV,, necessary to cause the two ions to traverse the same path in the mass spectrometer (Figure 1). Instrumentally, this is accomplished using a design patterned after the arrangement of Nier (9). In this arrangement, the peak-switching unit produces a sawtooth waveform which is applied to the coil of a small auxiliary magnet adjacent to the flight tube, and to the X-sweep of the cathode ray tube display; the output of the electron multiplier is fed to the Y-plates of the oscilloscope; the ion beam is thus swept repetitively across the collector slit, and the amplified ion current displayed on the oscilloscope. The sawtooth timebase of the peak-switching circuit also drives a series of contact relays, which are described in detail in the MS-902 operating manual. (It is these relays which provide for coordination of the data collection process with the sweep operations of the mass spectrometer.) The pertinent relay contacts and their functions are as follows: RLA2. Blanks out the oscilloscope display on the flyback sweep. Operates at 1 Hertz. RLB1. Switches the E.S.A. plate voltage and the ion accelerating voltage from full value to a lesser value as determined by the setting of a bank of precision resistors (decade box), thereby causing the reference (low mass) peak to be displayed alternately with the unknown (high mass) peak. Operates at 0.5 Hertz with the LOW (8) MS-902 Mass Spectrometer Instruction Manual, A.E.I. Scientific Apparatus, Ltd., Manchester, England. (9) K. S. Quisenberry, T. T. Scholman, and A. 0. Nier, Phys. Rev., 102, 1071 (1956).
INITIALIZATION
Table I. Integration of Interface Power Supply Integral of Integral of Standard high mass low mass deviation, signal signal Ratio Mean
L
z
SET HIGH/LOW MASS S M R A G E POINTERS
t
ACCUMULATE CONVERTED SIGNAL
2927978 2928050 2928204
0.99998 1.00000 0.99999
2928192 2927876 2927853 2927479 2927685
2928249 29 2785 5 2927848 2927421 2927709
0.99998 1.00001 1.00000 1 .oooo2 0.99999
0.99999
INITIATE CONVERSION
DJUSTABLE DELAY
2927931 2928044 2928172
ERROR MESSAGE
t
OPERATOR ADJUSTMENT
I
ONVERSIONS
1 YES
SWEEP COMPLETE STORE SWEEP INTEGRAL
NTERRUPT
PAPER TAPE OUTPUT OF INDIVIDUAL
OPERATOR ADJUSTMENTS
Figure 3. Flow diagram of data collection software MASS-AUTO-HIGH MASS switch in the AUTO position. RLCl and RLC2. Switches signal input alternately to one of two oscilloscope amplitude gain control circuits (sets A and B) in synchronism with RLB1. (Normally used to adjust peak intensities for accurate mass comparisons) A schematic representation of the peak-switching operation and the resultant oscilloscope display is shown in Figure 2. The peak-matching mode thus provides a convenient method not only for precise mass comparisons, but for accurate peak area comparisons by using the output of the oscilloscope amplifier as the input to the data acquisition interface, as in this application. Conversion Interface. A Digital Equipment Corporation PDP-8 model computer was connected to the MS-902 through a data acquisition interface designed and built by Applied Data Research, Inc., Princeton, N. J. This interface is marketed along with a software package for high speed acquisition and reduction of high resolution mass spectral information (IO). No portion of the ADR software package was utilized in the work described here. Features incorporated into the interface are a twelve-bit analog-to-digital converter (DEC ADC-l), a variable oscillator for control of conversion rate, and adjustable threshold and bias controls for signal conditioning. A 12-bit flag register is used to sense external scan control signals from the mass spectrometer; these are normally provided for by hardware supplied by ADR. For measurement of peak areas (low us. high mass), however, the relays of the peak-switching circuitry are used to (10) Mass Spectrometer Data System Model MSDS-I1 Operating Manual, ADR Ref. No. 68035, Applied Data Research, Inc., Princeton, N.J.
0,0014
provide the control stimuli necessary for coordinating the data acquisition process. An extra set of contacts on relay RLC/3 is used to route a - 10 volt dc voltage to the interface; each transfer of RLC/3 thus sets the appropriate bits in the external flag register, to be used subsequently by the PDP-8. Software. A flow diagram of the data collection program developed for this work is given in Figure 3. After initialization of program parameters, the program loops until a stimulus from the mass spectrometer is received, signaling the beginning of a scope flyback. The flag register is tested for high or low mass bits on, and, after a timed flyback delay, conversion is initiated at an approximate rate of 1000 samples/sec. The conversion rate is determined by delay loops within the program to ensure that any instability in the oscillator or variations in the transfer period of RLC/3 do not affect the reproducibility of the integrated peak areas. Successive conversions during a given sweep are checked for an overflow condition and accumulated for storage in the appropriate low or high mass buffer as two 12-bit words. Provision is made for interrupts on the part of the operator, either to make adjustments to the mass spectrometer or to terminate data collection. The program provides for output of individual sweep integrals on paper tape for later processing, or for averaging of the stored integrals in groups of ten and output of the averaged values. All programming was done in the PDP-8 PAL-I11 assembly language; copies of the program as well as details concerning the use of the AD1 interface are available from the authors upon request. EXAMINATION OF ATTAINABLE ACCURACY AND PRECISION
In order to determine suitability for isotope ratio measurements, the major components of the modified mass spectrometer-digital interface system described in the previous section were examined with respect to attainable accuracy and precision. The most important factors which might conceivably impose limitations on the quality of the measurements include: the accuracy of the analog-to-digital conversion step and the adequacy of the data collection program; the stability of the oscilloscope amplifier and possible introduction of bias from the dual amplifier gain circuitry (low and high mass); the stability of the electron multiplier; the stability of the electrostatic sector and the ion accelerating voltage; and the constancy of the source characteristics and possible discrimination effects. As a test of the conversion interface and associated software, the power supply of the interface was used as an input in lieu of an actual signal from the mass spectrometer. This input was then treated as the output of the oscilloscope amplifier, with the peak-switching circuitry operating to control the data collection process, and several time-averaged integrals of the “high” and ‘‘low’’ mass signals were collected. The values obtained are listed in Table I. Comparison of the ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
661
Table 11. Peak Area Comparisons (Mass Ratio = 1.000000) for Various Multiplier and Bandwidth Settings PFTBA Multiplier Bandwidth Relative Standard peaks cycles/sec Mean ratio error, deviation, setting, kV Scope amplitude controls A, B 2191219 2191219 2 19/219 2 191219 4261426 4261426
x 100 x 100 x 100
1.40 1.88 1.88 1.88 2.15 2.25
x 100 x 100 x 100
x 100 x 100 x 100
x 100
x 100 x 100
10 5 10 20 10 10
0,9999 1.0056 0.9997 0.9995 0.9994 1 ,0024
-0.01 +0.56 -0.03 -0.05 -0.06 f O .24
0.30 0.10 0.10 0.15 0.15 0.17
Table 111. Discrimination Effect Due to Voltage Switching for PFTBA Fragments Where R(apparent) = D X R(true) Ratio of intensities Exact mass ratio Discrimination factor D True Apparent PFTBA peaks (P 1lP)
+
119/120 2191220 2641265 4641465 5021503 6141614
1.008432 1.004582 1.003801 1.GO2162 1.001999 1.001634
0.0216 0.0432 0.0578 0.1011 0.1011 0.1335
high and low mass integrals leads to an average ratio of 0.99999 with a standard deviation of 0.00001 ; the deviations observed are well below the ripple of the power supply itself and illustrate the effectiveness of the signal averaging process. The relative accuracy obtained (-0.001 %) was taken as an indication that the timing of the data collection process was indeed independent of the actual sweep times of the peakswitching relay, and that no bias is introduced from this source. The combined stabilities of the electron multiplier and the oscilloscope amplifier, as well as optimum bandwidth and amplitude settings were assessed by means of measurements on fragments of the reference compound perfluoro-tri-nbutylamine (PFTBA). With the electron beam and accelerating voltage on, the instrument was allowed to stabilize for 20 minutes, and a small amount of PFTBA was admitted to the source via the standard inlet system. The decade resistance was set for a mass ratio of 1.000000 and, with the display mode on low mass, the peak of interest was brought into focus on the oscilloscope; switching to the auto mode then allowed comparisons of the area of a single peak on both the high and low mass modes. The results for various electron multiplier and bandwidth settings are shown in Table 11. The optimum bandwidth setting was determined to be 10 cycles/sec; at settings less than 10 cycles/sec, some peak distortion occurs, while wider bandwidth settings unnecessarily decrease the precision of the measurements. Little bias due to differences in the scope amplitude controls is evident-Le., the two circuits appear to be well matched on the XlOO setting. The relative error compared to the ideal ratio of 1.000 is acceptably small, ranging from -0.01 to -0.06 for multiplier settings up to 350 (2.15 kV). Above settings of 400 (2.25 kV), the relative error increases con662
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
0.0221 0.0221 0.0222 0.0446 0.0450 0.0448 0.0595 0.0599 0.0594 0.1046 0.1047 0.1042 0.1049 0.1049 0.1048 0.1381 0.1386 0.1384
1.023 1.023 1.028 1.032 1.042 1.037 1.029 1.036 1.028 1.035 1.036 1.031 1.038 1.038 1.037 1.034 1.038 1.037
1.025 1.037 1.031 1.034 1.038 1.036
siderably; a slow decrease in precision is also observed with an increase in multiplier gain, Although not shown here, other experiments indicated that deviations increased fairly rapidly with the use of more sensitive scope amplitude settings than X100; this implies that an increase in multiplier gain is more effective in dealing with low signal intensities. The most serious limitations on the accuracy and precision are the relative instabilities of the electrostatic sector and the ion accelerating potential (normally 8 kV) and the effect of changes in the accelerating potential on the stability of the source. Problems arise for two reasons: first, rapid voltage switching precludes perfect stabilization; while small fluctuations in the magnet current are not expected to contribute significantly to measurement error due to the large inertia involved, random fluctuations in the accelerating voltage, or the E.S.A. plate potential tend effectively to expand or compress the peak envelope along the time axis, resulting in a relatively large standard deviation in the measured ratios ; second, the large changes in the accelerating voltage which occur during the peak-switching operation affect the characteristics of the source itself-k., its efficiency as a producer of ions is altered by some undetermined factor; this leads, of course, to a bias. A dependence of ion beam intensity on accelerating voltage has been demonstrated for thermal ionization sources (4), and a qualitatively similar behavior is expected with the electron impact source used here. A discrimination effect is evident from a series of measurements on the major fragments in the spectrum of PFTBA and their corresponding peaks as listed in Table 111. The intensity ratios range in magnitude from approximately 0.0200 to 0.1300, while mass ratios vary over a small range from 1.008000 to 1.002000. Each ratio was measured indepen-
Table IV. Analysis of Variance-Daily 979.
Variations in Observed 5aCr/52Cr for NBS Isotope Reference Standard = 10 (S1) and 144 (Sz)]
Fo,ol = 2.45 [Degrees of Freedom
Sample mean
Estimated std dev of sample
Size of sample
Sample mean
Estimated std dev of sample
8 18
0.1122 0.1131
0.0006 0.0007
15 10
0.1130 0.1138
0,0008 0.0011
18 18 18 15
0.1137 0.1129 0.1142 0.1130
O.OOO4
15 15 5
0.1130 0.1142 0.1148
0.0006 0.0008 0.0014
Size of sample
Overall mean
0.1134 0.0005 0.0007 0.0006
dently three times as a series of 50 sweeps for each mass by shutting down the electron beam and accelerating voltage between runs. The data in Table I11 indicate that even small ratios can be measured with moderately high precision, but that the observed ratios differ from those calculable from the abundance of lacand probability considerations by a discrimination factor D , such that R(apparent) = D X R(true) (1) The discriminations observed are too large to be attributed to actual mass discrimination since such effects have an approximate inverse square root functionality of the mass ratio. The data in Table 111also indicate that over the small range of mass ratios studied, the discrimination factor D is relatively constant and that corrections could be applied by reference to an isotope standard with a similar mass ratio. Depending on changes in source parameter adjustments, the value of D is expected to vary with time. Since the required frequency of calibration os. the isotope standard depends on the relative variations of D over the short and long term, a study over several months was made of a particular isotope ratio using NBS Standard Reference Material 979, a chromium isotope standard available as Cr(NO& (11). This was converted to the Cr(II1) trifluoroacetylacetonate, a volatile chelate, and the Cr(tfa)2+ fragment monitored for the 5aCr/52Cr ratio over a period of four months. [Microgram amounts of the Cr(tfa)a were introduced to the source using the direct insertion probe.] The mean of each series of measurements taken over this period along with the estimated standard deviations is reported in Table IV. The overall mean of the data has been normalized to the true value of 0.1134. A one-way analysis of variance was performed to determine whether systematic differences between the sample means were statistically significant. The estimate of the standard deviation based on variations within the samples themselves is 0.0007 (0.6%), whereas, the standard deviation as estimated from the variations of the means is considerably larger at 0.0025 (2.2%). This leads to an estimate of F of 13.93 as compared to a tabular value of 2.45 for F at the 99% confidence level. The implication is that while deviations within any given run are acceptably low for most purposes, systematic day-to-day differences occur which require daily calibration against the isotope standard. POTENTIAL APPLICATIONS
The work described is part of a larger investigation into the analytical uses of volatile metal chelates of the fluorinated ___
(11) W. R. Shields, T. J. Murphy, E. J. Catanzaro, and E. L. Garner, J . Res. Nat. Bur. Sfatid.,70A, 193 (1966).
Estimate of Estimate of std dev Sz std dev SI based on based on variation variation within among means samples F
0,0025
0.0007
=
S,z/S22
13.93
beta-diketones for the mass spectrometric determination of trace metals in a variety of materials, including metal alloys, geological samples, and biological materials (12, 13). The volatility and thermal stability characteristics of many metal beta-diketonates make them ideal for introduction into mass spectrometers with the electron impact type of source. While analytical determinations by this method allow detection of extremely small amounts of the metal, the small quantities of sample to be handled introduce difficulties not inherent in conventional quantitative mass spectrometric measurements; in the latter case, measurements are usually performed with the sample being contained in a reservoir at relatively constant pressure. Recorded peak heights are then related to the partial pressure of the sample in the reservoir by means of previously determined sensitivity coefficients. In dealing with samples in the microgram range or lower, it is infeasible to use inlet systems of large volume because of the tremendous loss in potential sensitivity. Instead, the sample is best introduced by means of a direct insertion probe which allows small amounts of the compound to be placed directly adjacent to the ionizing electron beam. With the rapid voltage-switching described here, maintenance of a constant sample pressure is not required. Use of the probe technique along with the peak-switching circuitry of the MS-9 series instrument and computerized data collection will allow sensitive quantitative determinations for a variety of metals as volatile chelates with a high degree of precision. As reported elsewhere ( I d ) , extremely small samples of materials from the Apollo 11 and 12 missions have been examined for significant variations in chromium isotope distributions with uncertainties of 1 % or less. In addition, the system described in this paper has been applied to isotope dilution methods for determining the chromium contents of several U.S. Geological Survey standards and lunar materials, with accuracies shown to be about 1 % for microgram amounts of the metal (15). It should be pointed out that this technique is not restricted to isotope ratio measurements as such; it can also be applied to the “integrated ion current” method of Majer et al., to (12) B. R. Kowalski, T. L. Isenhour, and R. E. Sievers ANAL. CHEM., 41, 998 (1969). (13) J. L. Booker, T. L. Isenhour, and R. E. Sievers ibid., p 1705. (14) R. E. Sievers, K. J. Eisentraut,D. J. Griest, M. F. Richardson, W. R. Wolf, W. D. Ross, N. M. Frew, and T. L. Isenhour, Proc. Apollo X I I Luti. Sci. Conf., in press, (1971). (15) N. M. Frew, J. J. Leary, and T. L. Isenhour, AYAL.CHEM., 44, 665 (1972). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
663
acetone, Majer has reported accuracies on the order of 10-15 % (20). As a brief test of the applicability of the present computerized system to the integrated ion current method, identical 100-nanogram samples of cobalt as Co(fod)s were determined using the peak-switching mode. Data collection was initiated just prior to lowering of the probe and terminated when the signal was estimated to be less than two or three times the noise level. The molecular ion peak of PFTBA (m/e 614) was used as a reference on the low mass sweep. The pressure of the PFTBA in the source was not held constant during the course of the determinations, and a variation of a factor of approximately six was allowed. The results of five identical determinations are shown in Table V; a large gas interference effect due to the partial pressure of the reference compound is evident, and, at first glance, the data appear unusable. If, however, the integrals obtained for each cobalt sample are plotted against their respective PFTBA integrals as in Figure 4, a linear relationship is obtained. The data points can be used as a calibration curve by calculating a linear least squares fit and using the coefficients obtained to correct subsequent unknown samples to zero pressure of PFTBA. Duplicate 100-nanogram samples treated in this way gave 98.3 and 102.3 nanograms with relative errors of - 1.7 %and +2.3 %, respectively.
Table V. Integrated Ion Current of Five Identical 100-ng Cobalt Samples as Co(fod), Monitored against Ion Current for Reference PFTBA Units of PFTBA Units of cobalt Sample as C1zFz4N+( X as Co(fod)*+( x 13.41 10.82 7.40 6.61 1.58
1 2 3 4 5
7.76 9.01 11.73 11.76 16.95
CONCLUSIONS
,
1 0
1
4
6
0
10
UNITS PFTBA AS c,,F,,N+
11
( x
14
16
10.5)
Figure 4. Calibration of integrated ion current of Co(fod),+ us. CizFzaNf (PFTBA) enhance accuracy (16-19). In this method, the sample is evaporated from the probe while the instrument is operated in either the peak-switching or repetitive slow scan modes, and the ion current at a selected mle value is recorded on the strip recorder; the manually integrated area under the recorder trace is then taken as proportional to the amount of sample on the probe. Calibration curves are constructed using known sample amounts to relate the ion current to the area of the trace. If the peak-switching mode is used, a suitable reference compound introduced via another inlet system at constant pressure can be monitored to detect changes in instrumental conditions. Little information on the accuracy and precision of the method has been reported, but in a recent application to the determination of mixtures of microgram amounts of Zr, Hf, and Ti as the chelates of benzoyl trifluoro(16) A. E. Jenkins and J. R. Majer, Talanra, 14, 777 (1967). (17) A. E. Jenkins and M. J. A. Reade, ibid., p 1213. (18) J. R. Majer, M. J. A. Reade, and W. I. Stephen, ibid., 15, 373 (1968). (19) R. Belcher, J. R. Majer, R. Perry, and W. I. Stephen, Anal. Chim. Acta, 43,451 (1968).
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
In view of the results discussed above, the MS-902 mass spectrometer (as well as the earlier MS-9 version) is seen to have sufficient stability to make possible isotope ratio measurements of moderately high precision and accuracy when combined with computerized data acquisition and signal averaging. Modifications to the instrument itself are minor. Ratios ranging from unity down to approximately 1 to 50 can be measured with reproducibilities better than 1 %. With the use of a suitable isotope standard to compensate for instrumental discrimination and for day-to-day variations in instrumental parameters, accuracies also in the range of 0.5 to 1 are easily attainable. In addition, peak area integrations of the same accuracy and reproducibility and proportional to the total ion current produced by a compound evaporated from the direct insertion probe provide a convenient method for trace quantitative analysis of metals. ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance of L. E. Wangen and J. J. Leary in the experimental work. We also thank David Rosenthal for his cooperation in our use of the mass spectrometer facilities of the Center for Mass Spectrometry, Research Triangle Institute, Durham, N. C. RECEIVED for review June 21, 1971. Accepted November 24, 1971. Work was supported by the Materials Research Center, University of North Carolina, under contract DAHC 15-67-C-0223 with the Advanced Research Projects Agency. We also acknowledge support from the Biotechnology Resources Branch of the Division of Research Resources, NIH, under Grant PR-330. (20) M. G. Allcock, R. Belcher, J. R. Majer, and R. Perry, ANAL. GEM., 42, 776 (1970).
