Anal. Chem.
Table 11. Reported Copper Isotopic Abundance Ratios Measured by TIMS sample form electroplated C u electroplated C u C~(N03)2 Cu(NO3J2 Si02
+ + HW4 CU(NO~+ ) ~Si02 + H3PO4
electroplated C u Cu(N03J2
filament detector
obsd value
ref
triple triple double single
SEM
2.3039" Faraday 2.2752O Faraday 2.256 f 0.010 SEM 2.28 f 0.01
4
single
SEM
5
double double
Faraday 2.2739 f 0.0019" tWe Faraday 2.2448 f 0.0007b t w
2.278 f 0.005
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1988,6 0 , 1815-1818
2 2
3
a T h e absolute value o f the isotopic abundance ratio is 2.2440 f 0.0021. bAverage value o f eight measurements (la). Ctw, t h i s work.
work are listed in Table I. The obtained isotopic ratios are within the error range of the certified value. This indicates that the low-temperature TIMS may give directly the real value of the isotopic ratio; the correction factor is not necessary, unlike the electroplating method. The average value for eight measurements of t h e NBS sample by the low-temperature TIMS is obtained in this work as 2.2448 f 0.0007. It is listed in Table I1 together with the result for the electroplated sample obtained in this work and the historically reported values for copper isotopic ratios. The reason for the ionization of copper at such a low temperature is still unknown. It is estimated that some selfcatalytic reactions take place on the surface of an ionizing filament a t approximately 900 "C, when the evaporated copper(I1) nitrate molecules reach the surface.
NBS standard sample. The result of the isotopic analysis of the electroplated sample agreed with the reported data within the experimental error. The copper(I1) nitrate sample was found to produce Cu ion beams at two different conditions of an ionizing filament: the low-temperature region (approximately 900 "C) and the high-temperature region (approximately 1600 "C). Using the lower temperature condition with the copper(I1) nitrate sample is considered preferable for the measurement of the copper isotopic ratios.
ACKNOWLEDGMENT We wish to express our gratitude to T. Nagumo, H. Kanno, T. Katoh (The National Defense Academy), M. Nomura (Tokyo Institute of Technology), and T. Umezawa (Finnigan MAT Instruments, Japan) for their kind cooperation and valuable discussion given to this work. Registry No. Cu(NOJZ, 3251-23-8; 63Cu,14191-84-5; 65Cu, 14119-06-3.
LITERATURE CITED Walker, E. C.; Cuttitta, F.; Senftle, F. E. Geochlm . Cosmochlm . Acta 1958, 15, 183.
Shields, W. R.; Murphy, T. J.; Garner, E. L. J . Res. Nafl. Bur. Stand., Sect. A 1964. 68A. 589.
Kanzaki, T.; Yokozuka, S.; Kakihana, H. Bunseki Kagaku 1967, 76, 7. Murozumi, M.; Abe, Y. BunseklKagaku 1975, 24, 337. Murozumi, M.; Nakamura, S.; Ito, K. Bunseki Kagaku 1976, 25, 706. Broekman, A.; van Raaphorst, J. G. Fresenius' 2.Anal. Chem. 1984, 3 7 8 , 398. Beary, E. S.;Brletic, K. A.; Paulsen, P. J.; Moody, J. R. Analysf (London) 1987, 712, 441. Fujii. Y.; Hosoe, M.; Okamoto, M. 2.Nafurforsch., A : Phys., Phys. Chem., Kosmophys. 1986, 41A, 769. Mori, Y.; Nomura, T.; Kakihana, H. Shifsuryo Bunseki 1962, No. 27,
129. Kanno, H. Bull. Chem. SOC.Jpn. 1971, 4 4 , 1808.
CONCLUSION The heating conditions and sample chemical forms for copper isotope analysis by TIMS were examined by using a
RECEIVED for review January 11, 1988. Accepted April 20, 1988.
Comparison of Theoretical and Experimental Relative Sensitivities in Gas Chromatography/Chemical Ionization Mass Spectrometry Ron Orlando and Burnaby Munson* Department of Chemistry, University of Delaware, Newark, Delaware 19716
The relatlve response factors (or relatlve sensltlvltles) for a number of compounds havlng a variety of functlonal groups were obtained In gas chromatography/chemlcal ionization mass spectrometry (tetramethylsllane) and compared wlth the ratios of calculated rate constants for the reactions of the trlmethylsllyl Ion wlth these compounds. Very good agreement was observed between the experknental and theoretical values. The dlfferences between the calculated and experlmental values were generally less than the standard devlatlons of the experimental values.
One problem in the quantitation of complex mixtures by any technique is the availability of the standards that are, needed to establish either the sensitivities or the relative 0003-2700/88/0360-1815$01.50/0
sensitivities of the compounds in the mixtures. The more complex the mixture, the more difficult it becomes to obtain all of the compounds. Methods of predicting relative sensitivities of compounds or correlating sensitivities with molecular structures are of great importance in reducing the necessary number of standardization experiments. I t was shown many years ago that the total ionization for isomeric hydrocarbons in electron ionization (EI) mass spectrometry (MS) is independent of molecular structure and increases with increasing molecular weight ( I ) . I t was also shown that the total ionization (or ionization cross section) for hydrocarbons in EIMS increases linearly with increasing molecular weight (or molecular polarizability) ( 2 ) . Consequently, all of the components in a mixture are not required for calibration experiments in the analysis of hydrocarbons by gas chromatography/mass spectrometry (GC/MS) if the 0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
total ionization is used for quantitation. On the other hand, all of the compounds in the mixture are needed for calibration if the more sensitive technique of selected (or single) ion monitoring is used ( 3 ) . Low-voltage EIMS has been used for the analysis of aromatic hydrocarbons for many years, and it was shown that the sensitivities for isomeric alkylbenzenes are independent of molecular structure when the electron energy is sufficiently low that essentially only molecular ions, M+, are formed (4). In addition, correlation plots were obtained that showed the variation of sensitivities (or calibration factors) with increasing molecular weight for homologous series of different types of aromatic hydrocarbons (4,5). Consequently, one could analyze complex mixtures of aromatic and heteronuclear aromatic hydrocarbons without the necessity of obtaining calibration data for each isomer. Chemical ionization mass spectrometry (CIMS) generally produces fewer ions (frequently at higher masses) per compound than EIMS. However, calibration data at the appropriate mass for each compound in the mixture are generally obtained for quantitation (3). The total sample ionization is produced by ion/molecule reactions of the reagent ions with the sample molecules; consequently, the sensitivities (or calibration factors) are proportional to the rate constants for these ion/molecule reactions (6). The rate constants (or collision frequencies) for these reactions can be calculated either from the Langevin theory, which uses only the molecular polarizabilities (a,or from another theoretical model, which includes the molecular polarizabilities and dipole moments (8). The relative sensitivities, based on total sample ionization, can then be calculated for all compounds from the ratios of these rate constants. As part of our continuing studies on selective reagents in CIMS and the analytical applications of the reactions of trimethylsilyl ions, we wish to report some preliminary results on both the theoretical and experimental relative rate constants, or the relative sensitivities, for this reagent system with a variety of compounds. Also, we would like to propose the use of this technique as a general method for use with other chemical ionization reagent systems (reactant ions).
EXPERIMENTAL SECTION The mass spectral data were obtained on samples introduced through a Varian Moduline 2740 gas chromatograph using He as the carrier gas. The gas chromatograph is connected without a separator to a Du Pont 492B mass spectrometer, which has been modified for CI operation (9). The data were obtained by use of a Hewlett-Packard 21MX computer with a Du Pont data system. The temperature of the ion source in the mass spectrometer was varied from 100 to 260 "C. The source pressures were generally 0.40 Torr, measured with an MKS Baratron capacitance manometer connected directly to the ion source through a glass line. The repeller was kept at 0 V to maximize the ionic residence times. The tetramethylsilane, TMS (Aldrich, 99.9%), was introduced through the CI inlet system, and the helium from the gas chromatograph served as a diluent gas. The reagent gas mixtures (approximately 25% TMS) were mixed in the ion source of the mass spectrometer. The chemicals were obtained from several commercial sources and were used as obtained. The gas chromatographic and mass spectrometric experiments gave no indications of significant amounts of impurities in these compounds (
