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ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
voltammetry a t a dropping mercury electrode would seem to be one of the most attractive electroanalytical methods available for routine analysis, particularly when coupled with computerized instrumentation t o correct for background current.
- 200nA
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LITERATITRE CITED
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Figure 4. Influence of a m a l g a m formation. Conditions a s for Figure 3 except that the initial potential is -0.85 V vs. Ag/CI and scan direction
is positive
NaC1. Using a 10-s pre-sweep delay and the anodic stripping approach decreased the limit of detection to 2 X 1 0 M. In view of t h e high sensitivity, low noise level, simplicity of operation, and rapid data acquisition available with this approach, the technique of fast sweep differential pulse
(1) E. Barendrecht in "Electroanalytical Chemistry", A J. Bard, Ed., Marcel Dekker, New York. 1967. Vol. 2, pp 53-109. (2) T. R. Copeland and R. K . Skogerboe. Anal Chem , 46, 1257A (1974). (3) A . M. Bond. in 'Modern Polarographic Methods in Analytical Chemistry", Marcel Dekker, New York, In press. (4) J. B. Flato, Anal. Chem.. 44(11), 75A (1972). ( 5 ) J G. Osteryoung. J. H. Christie, and R A. Osteryoung. SOC.Chim. Belge, 8 4 , 647 (1975). (6) J H. Christie and R A Osteryoung. J . Electroanal. Chem., 49, 301 (1974). (7) A M Bond and B S . Grabaric. Anal Chim. Acta. 8 8 , 227 (1977). (8) H. Schmidt and M. von Stackelberg, Modern Polarographic Methods", Academic Press, New York/London, 1963. (9) H. Blutstein and A. M. Bond, Anal. Chem.. 48. 248 (1976). (10) K C. Burrows, M. P. Brindle. and M. C. Hughes, Anal. Chem., 49, 1459 (1977) (11) S. C Rifkin and D. H. Evans, Anal. Chem.. 48, 248 (1976). (12) S. C. Rifkin and D. H. Evans, Anal. Chem., 48. 2174 (1976). (13) K . F. Drake, R. P. Van Duyne, and A. M. Bond, J . Electroanal Chem.. 89. 231 (1978). (14) H E. Kelier and R . A. Osteryoung, Anal. Chem., 43. 342 (1971). (15) N . G. Velghe and A. Ciaeys. J . Electroanal. Chem.. 35, 229 (1972) (16) A. M. Bond and R. J. O'Halloran. J . Nectroana/. Chem., 68, 257 (1976). (17) A. M. Bond and B. S. Grabaric. Anal. Chem., submitted for publication.
RECEIL-ED for review .July 12, 1978. Accepted October 10, 1978.
Fingerprinting and Partial Quantification of Complex Hydrocarbon Mixtures by Chemical Ionization Mass Spectrometry L. Wayne Sieck National Bureau of Standards, Washington.
D.C. 20234
A modification of chemical ionization mass spectrometry, which involves photoionization and cyclohexane as the source of the reagent ion, has been used to develop a technique for discriminatory "fingerprinting" of neat liquid fossil fuels. The method provides a 2-min turn-around time between samples and batch introduction, with no requirements for prior separation or fractionation. Depending upon the conditions chosen, the technique may also be extended to the partial quantification of aromatic and olefinic sample components.
other complex hydrocarbon mixtures. Realization of a tractable procedure would provide the unique benefits associated with MS instrumentation, including sub-minute analysis times, high sensitivity, and very small sample size requirements. This article describes a novel discriminatory technique for screening of batch hydrocarbon mixtures using a modification of CI mass spectrometry, and gives some preliminary supportive data for extension of this technique to quantification of aromatic and olefinic components.
T h e recent literature of analytical chemistrv has been characterized by t h e exploitation and refinement of mass spectrometry (MS) as a versatile tool for identification of organic compounds. T h e major emphasis has been in the application areas of chemical ionization (CI), electron impact (EI), and field ionization (FI), particularly involving the interfacing of gas chromatographs for pre-separation (CI and E1 only). However, with t h e exception of an extensive feasibility study involving FI investigation of oil sampleq carried out a t the Stanford Research Institute (I),it appears that little. if any, effort has been directed toward the possible utilization of mass spectrometers, operating without a n ) auxiliary equipment (GCs, LCs, etc.), for forensic purposes such as the "fingerprinting" of liquid fossil fuels, industrial solvents, and
EXPERIMENTAI, Mass Spectrometric Instrumentation. The NBS high pressure photoionization mass spectrometer, which is described in detail elsewhere (2), was utilized as a test facility for development and refinement of this technique. Figure 1 shows a schematic of the heart of this unit, which consists basically of a sample introduction system, a photoionization and reaction chamber. an intense vacuum ultraviolet light, source to simulate low energy electron impact, and a quadrupole mass filter and associated E M detection system. The reaction chamber has a volume of approximately 3 cm' and LiF optical material, and may he heated to 200 "C. Since ionization is induced by photoabsorption there is no need for electron entrance and exit apertures in the chamber itself. Therefore, the loss of neutral flow components occurs only through the circular ion exit,pinhole (0.2 mm diameter in the present measurements). The chamber is also operated under field-free conditions; i.e., no repellers or imposed magnetic fields. The light sources are of the microwave-powered
This article not subject to U.S. Copyright. Published 1978 by the American Chemical Society
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C+ + B, and so forth, providing the IP of €3 < IP of A, the TP of C < IP of R. etc, I t is this "cascade" mechanism which
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Schematic of t h e NBS high pressure ohotoionization mass spectrometer Figure 1.
rare gas resonance type developed at NBS (.3, and provide intense outputs a t 11.7 eV (argon), 10.0 e\' (krypton). or 8.4 e\' (xenon). depending upon the particular rare gas chosen. The quadrupoltr unit has an operational range extending to approximately 600 ami,. and the detected ion currents may be handled in either digital or analog fashion using peripheral data handling systems which are computer-interfaced. Two 800 1 -s- diffusion primps equipped with cryogenic baffles are used to provide a pressure differential of lo5 between the reaction chamber and the bulk of the mass spectrometer, and chamber pressure regulation (variahle over the range 0.001 to 2 Torr) is achieved either by a remotely controlled automatic pressure controller or via manipulation of manually operated micrometering valves. The sample handling system used in the present measurements, which has remotely operated valves and both liquid and gaseous injection ports, will he discussed separately.
RESULTS AND DISCUSSION General Concept. Consider a complex mixture containing a variety of aliphatic, olefinic, and aromatic hydrocarbons. which may or may not, contain heteroatom substituents. Each of these compounds will be characterized by a unique ionization potential (IP) and a unique proton affinity (PA). As a consequence of these thermochemical properties, some of t h e mixture components will be reactive t,owards selected CI reagent ions, designated RH+, and some will not. When R H has a lower TP t h a n component A , charge exchange.
RH+ + A
-
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-
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will be endothermic. However, other components will participate in either or both of these initial reactions (involving t h e reagent ion) depending upon the chemical identity of RH (RH') and favorable energetics. At relatively high sample concentrations, secondary reactions will also occur among the various components prior t o ion diffusion from t h e CI R + + A, A+ + C C+ + A, R + + C chamber; Le., A+ + B
-
-
provides the discriminatory basis for the screening technique since t h e fractional yields of A', B+, C+, D+ ... found in t h e composite CT spectrum will be extremely sensitive t o the relative concentrations of t h e corresponding neutral components which actually participate in the consecutive reaction scheme irrespective of whether charge exchange, proton transfer, or other channels dominate the consecutive ion kinetics. Furthermore, there is no reason to assume that the rate coefficients for all of t h e possihle exothermic ion--molecules reactions are t h e same. which tends t o amplify small differences in relative concentrations in multicomponent samples. T h e CI reagent gas chosen was cyclohexane. This choice was based on several factors, including its volatility, high optical extinction coefficient and photoionization quantum yield at 10.0 eV (123.6 nm, K r resonance lamp) ( 4 ) ,and t h e fact t h a t t h e molecn!ar ion, C6HI2+.which is t h e only ion prodiiced a t this wavelength, does not react chemically with cyclohexane itself ( r S ) . Furthermore, t h e IP of cyclohexane, 9.83 eV ( 6 ) ,is much higher than the collective IPS of most of the olefinic and all of the aromatic compounds which are likely t o he present in typical solvents and liquid fossile fuels. This situation is shown in graphic form in Figure 2, which displays the IPSof three families of hydrocarbons, insofar as they have been established with reasonable certainty, vs. t h e carbon numbers within a given family. T h e error bars indicate the spread in IPS for structural isomerr, as well as imprecision in the literature values themselves. Rased on the relative IP of cyclohexane, it would appear, a t the very least. that CsHI2+ would be a suitahle charge exchange reagent for ionization of higher molecular weight aliphatic:;, olefins, aromatics, etc., likely t o be present in t h e samples of interest. Also note (Figure 2) t h a t there is a gradual decrease in t h e IPS of individual family members as one goes to higher carbon numbers within a homologous series. This trend suggests t h a t a n encounter between, for example, a CiH8+ ion (toluene), produced under CI conditions via the initial reaction C6H12+ + C7H8-* CiH8+ + C6HIZ,and neutral naphthalene would result in further charge exchange t o give CloHx+;C7H8++ C,,H, C,,,H8+ + C7H8. A s outlined above, these consecutive react,ions occurring a t higher sample concentrations provide t h e kinetic basis for the "cascade" fingerprinting technique. Analytical Details. T h e specific procedure chosen for fingerprinting involved generation of a n approximately 3% mixture of the unknown in cyclohexane, a CI chamber pressure of 0.2 Torr, and a 2-min turnaround time between successive samples. These conditions were achieved via t h e following sequence (refer to Figure 3). (1) Syringe injection of 5 PI, of an undiluted liquid sample into t h e 1-1, spherical volume A (heated t o 373 K) which is +
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A
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interfaced with the photoionization chamber via a manually operated micrometering valve. This valve was preadjusted to provide 0.2 Torr in the reaction chamber when 27 Torr of vapor was present in volume A. ( 2 ) Immediate admission of pure cyclohexane vapor into the 1-L sample volume from a 3-L reservoir containing cyclohexane maintained a t its equilibrium vapor pressure at 0 "C (27 Torr) by opening isolation valve C. The time required to reach a total steady-state pressure of 27 Torr (sample cyclohexane) in volume A, which gave 0.2 Torr in the CI chamber, was approximately 20 s. (3) Initiation of mass spectrometric scanning and simultaneous recording of the composite mass spectrum. T h e quadrupole unit was preset t o sweep from mle 90 to mle 240 in 40 s. (4) Removal of the sample mixture from volume A by closing isolation valve C and opening the high conductance valve D, which interconnects the sample volume and a high capacity (300 L-s-') vacuum system equipped with cryogenic baffles. T h e time required to reach a base pressure < Torr in the sample volume was approximately 40 s. ( 5 ) After closing valve D, the system was ready for another injection. I t is important to note that the analysis need not to have been terminated after a single MS scan due to the very slow rate of sample loss, which occurs only through the 0.2-mm diameter ion exit pinhole of the CI chamber. Repetitive scans of a single sample mixture over 10-15 min periods gave no perceptible change in the composite mass spectra. Fingerprinting Results. Measurements were initially carried out with gasoline samples obtained from local service stations. (Manufacturers' names are used only to define the nature of the technique. Their usage does not imply endorsement of their products by the National Bureau of Standards). Figure 4 gives a reproduction of the complete CI spectra of two urlleaded gasolines obtained by the procedure outlined above. These tracings were taken in analog mode using a multichannel UV recording oscillograph with only semipermanent copy paper. For clarity of presentation, the peak intensities shown were obtained by inking over the signal obtained on the least sensitive recording channel. The gross features of the spectra may be summarized as follows. (1)Major peaks under these conditions correspond to alkyl substituted benzenes having the empirical formulas C9Hl, ( m / e 120), CI0Hl4( m / e 1341, and CllHI6 ( m / e 1481, as well as naphthalene ( m / e 128) and methylnaphthalene (mle 142). ( 2 ) Intense peaks (on a relative basis) also appear in the C7 through C9 olefinic manifolds; i.e. heptenes, heptadienes, octenes, octadienes, and so forth. (3) T h e spectra are characterized by an almost complete absence of measurable signals from saturates, including those isomeric heptanes, octanes, and nonanes which are the major components in each sample. T h e predominance of peaks from higher molecular weight aromatics, as well as those associated with olefinic components,
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is to be expected since it has been established (7) that the efficiency (rate) of the charge exchange reaction
-
RH+ + A A+ + RH (1) increases dramatically as the exothermicity increases; Le., the I P of cyclohexane (9.83 eV), is effectively the same as those associated with aliphatic components, while it is far above those associated with the Ci-CI2 olefins and substituted aromatics likely to be present in these samples (see Figure 2). Furthermore, because these spectra were recorded under cascade conditions, one would expect that any aliphatic molecular ions produced in the mixtures would undergo charge exchange with components of lower IP prior to exiting from the CI chamber of diffusing to the walls. Superficially, the spectra given in Figure 4 might appear to be quite similar. However, cross-comparison of the relative intensities of the 10 most intense peaks in each tracing reveals that the two samples are readily separable. The 10-peak spectra of these two gasolines, as well as five others obtained from various sources, screened under the same conditions as those outlined above, are reproduced in Figure 5. These tracings were all obtained consecutively within a 20-min period, including a repeat injection of Exxon Regular-leaded (B). It is apparent that the individual samples are readily distinguishable by simple visual inspection, and that re-ir,-
ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
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jection of the same gasoline gives a reproducible pattern. Note also that two samples of Exxon Regular-leaded (A and B) obtained from different service stations give entirely different CI spectra. Figure 6 gives the 10-peak spectra of no. 4 fuel oils measured under the same conditions as the gasoline fingerprints. These samples were provided to NBS by the U.S. Coast Guard Research and Development Center, Groton, Conn., as part of an interlaboratory round-robin study of weathered oils carried out in late 1976. Again, the CI spectra of the six samples are easily distinguishable by visual inspection, with the exception of nos. 29 and 81, which gave identical spectra. Subsequent contact with Coast Guard personnel confirmed that nos. 29 and 81 were, in fact, the same oil, all of the others being different in this particular sample set. Also note that, as expected, the 10-peak spectra for no. 4 fuel oils include more higher molecular weight features (alkyl substituted benzenes and naphthalenes) when compared with the gasolines (Figure 5). Quantification of Aromatic Components, The kinetic basis for quantification by batch injection is essentially the same as that for fingerprinting, but requires that a set of conditions be maintained in which the CI spectrum of the sample reflects, ideally, only the product ion distribution obtained after the first ion-molecule reactions have occurred in the system. This is equivalent to terminating the cascade sequence a t step one, which corresponds to the initial reactions involving the solvent ion (in this case CsHI2+)and the various mixture components. In principle, quantification measurements could be attempted by reducing either the absolute sample concentration a t constant solvent pressure, or by keeping total pressure in the CI chamber a t a constant, with relatively high solvent/sample ratio. Either variation has the same effect since the net result is a reduction in the absolute concentration of sample components in the reaction zone which, in turn, decreases the probability for secondary reactions involving mixture components. A reduction in the total CI chamber pressure actually has a two-fold effect since ionic residence times in the reaction zone are linearly proportional to pressure above a few milliTorr. Our approach, which was chosen arbitrarily, was to vary sample concentration a t a constant solvent pressure. Figure 7 shows the major peak spectra for one of the gasoline samples vs. dilution a t a constant CI chamber of 0.2 Torr of cyclohexane (the spectrum a t 3% corresponds to no. 5 in Figure 5 ) . On the last two tracings (0.0038% and pure cyclohexane) the complete signals obtained on both the X1 and X10 in Figure 7 recording channels have been reproduced. The mass filter scan was again preset to cover the range m / p 90 to m / e 250. As expected, the major peaks are shifted t o lower molecular weight components as the sample concentration is decreased, reflecting the reduced involvement of consecutive reactions. At concentratinns S0.5%,two peaks, which increase in relative intensity a t higher dilutions. appear a t m / e 98 and 100. These
m
Figure 8. CI spectrum of a gasoline under quantification conditions (A) and spectrum of cyclohexane solvent under same conditions (B)
features are associated with methy1c;yclohexane ( m / e 98) and isomeric heptanes ( m / e 100) present as impurities in the cyclohexane, and result from the reactions
Minor peaks are also evident in the p i r e cyclohexane spectrum a t m / e 92, 106, 112, 114, 140, and so forth. Note that the distribution of these major peaks associated with this particular gasoline sample (the alkyl-suhtituted benzenes) does not change significantly when one compares the 0.03% and 0.0038% dilutions. Concentrations of this order therefore appear to satisfy the criteria for quantification a t 0.2 Torr of cyclohexane in our CI chamber, since secondary reactions involving sample components are negligible. Consequently, the charge exchange CI spectra obtained should give individual signals which are directly proportional to the relative concentrations of the various sample components which are reactive toward CeH12+. Figure 8 shows the results of a partial quantification attempt. The upper portion (A) gives the CI spectrum resulting from the injection of 1 WLof a 1 % solution of Exxon gasoline in cyclohexane into the 1-L sample volume followed by further mixing with cyclohexane at its equilibrium vapor pressure at 0 "C (27 Torr). T h e micrometering valve had again been pre-adjusted to provide a steady-state pressure of 0.2 Torr in the photoionization chamber. T h e major sample peaks are observed a t m / e 92 (CiH8+,toluene), 106 (C8Hlo+,xylenes, ethylbenzene), 120 (CgH12+.propylbenzene, methylethyl-
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
Figure 9. GC t r a c e of t h e gasoline s a m p l e shown in Figure 8
benzenes, trimethylbenzenes), and 134, which corresponds to alkyl-substituted benzene ions having the empirical formula CloH1,. T h e composite spectrum for pure cyclohexane under these conditions is shown in Figure 8B. Gas chromatographic (GC) analyses of the 1%solution were also taken to quantify the relative concentrations of the C7 through Cg alkylated benzenes for comparison with the M S intensities a t m / e 92, 106, and 120 (see Figure 9). The particular column used (15 m x 0.508 mm i.d. SCOT column containing diatomaceous earth on fumed silica support coated with a mixture of m-bis(m-phen0xyphenoxy)benzeneand Apiezon L operated a t 80 "C, with FID) gives excellent separation of all possible isomers with the exception of 1methyl, 3-ethyl-, and 1-methyl-4-ethylbenzene. Taking the carbon number-corrected peak areas for toluene (CiHs), and all of the CsHl, and CgHl2isomers, the following fractional distribution was obtained for the particular fuel sample: CiH8 (0.257), C8Hl, (0.335), and C9H,, (0.408). Referring again to Figure 8A, and taking the MS intensities at m / e 92, 106, and 120 after correction for the impurity signals in the "pure" cyclohexane spectra a t m / e 92 and 106 (Figure 8B), the following was obtained: C7H8+(0.238), C8HloC(0.341), and CgHI2+(0.421), in essentially quantitative agreement with the GC result. Therefore, it would seem that cyclohexane might provide a suitable R H + candidate for bulk quantification of lower molecular weight substituted aromatics in complex mixtures based on carbon number (isomers are not resolved). I t is noteworthy that only 35 s were required for the entire M S measurement, and that the analogue peak at m l e 92 corresponds to -10 pg of toluene in the CI chamber at the time of the recording.
CONCLUSIONS T h e conditions chosen for fingerprinting seem to be more than adequate for rapid, discriminatory screening of all of the
neat samples which we have had on hand. An obvious extension of this method would include the intercomparison of a variety of weathered and nonweathered samples of the same fuels to determine which CI features might be common to both, and what changes in conditions, including the reagent ion source, might be required to achieve an unambiguous match of these features. Studies of this type are anticipated in this laboratory in the near future. With respect to quantification, the preliminary results are very encouraging. However, the interpretation of the CI spectrum in terms of sample composition obviously depends critically upon the identity of R H + and a knowledge of its reactivity toward the compound of interest. For example, the gasoline-aromatic content analysis just described presumes that cyclohexane ions react equally efficiently with all of the C7 through CI1 alkylated benzene isomers, which may or may not be the case. Furthermore, there is nothing intrinsically unique about cyclohexane itself as a solvent for quantification purposes, since the resultant C6H12+ion does not appear to discriminate at all with respect to aromatic structural isomers. Our future efforts in this area will therefore be directed toward the investigation of other solvents which may give conveniently-generated reagent ions which exhibit either different reactivities, or modes of reaction, with various classes of organic molecules.
ACKNOWLEDGMENT The author is indebted to Delyle Eastwood of the US.Coast Guard R. and D Center, Groton, Conn., for providing information concerning the fuel oil samples, and to Stanley Wasik of NBS for assisting in the GC analyses. LITERATURE CITED (1) M .E. Scolnick, A. C. Scott, and M. Anbar, "Methods of Identifying and Determining Source and Age of Petroleum Found in the Marine Environment", Rep. No. CG-D-67-75 prepared for Department of Transportation, United States Coast Guard, available through the National Technical Information Service, Springfield, Va. 22151. (2) L. W . Sieck, S. K . Searles, and P. Ausioos, J Am. Cbem. S O C . ,91, 7627 (1969). (3) R. Gorden, Jr., R. E. Rebbert, and P. Ausloos, Natl. Bur. Stand. ( U . S . ) Tech. Note, 496 (1969). (4) P. Ausloos and S. G. Lias, "The Chemistry of Ionization and Excitation", G. R. A. Scholes, Ed., London, 1967 p 77. (5) L. W. Sieck, S. K. Searles, and P. Ausloos. J . Pbys. Cbem., 74, 3829 11970\. (6) Ionizaiion potentials (IPS).where given, are taken from H. M. Rosenstock, K. Draxi, B.W. Steiner, and J. T. Herrong, "Energetics of Gaseous Ions", J . Phys. Cbem. Ref. Data, 6 , Suppl. 1 (1977). (7) S. G. Lias, P. Ausloos, and 2. Horvath. I n t . J . Chem. Kinetics, V I I I , 725 (1976).
RECEIVED for review June 15,1978. Accepted October 12, 1978. Research supported in part by the Office of Air and Water Measurement, National Bureau of Standards, Washington, D.C. 20234.
