Anal. Chem. 1986, 58, 2919-2923
chloro compounds were usually intense and halide (C1-) anion formation less important, this behavior is reversed with the bromo compounds. HRGC/NCI-MS SIM analyses should be especially valuable for detecting trace levels of these compounds in environmental and biological samples a t concentraions in the parts-per-trillion (pg/g) range or below. The results from the analysis of automotive emissions demonstrate high selectivity of detecting these compounds in the presence of large quantities of aromatic hydrocarbons and oxygenated compounds; the results from the PCB analysis demonstrate high selectivity also in the presence of chlorinated analogues. T h e method was also successful in detecting PBDFs and PBDDs in the presence of larger quantities of PCDFs and PCDDs in a sample from an accidental fire. Brominated compounds are important chemicals used in a variety of applications. Most of the materials containing these compounds are eventually discarded. A part of the brominated compounds is then released into the environment. In addition, significant quantities of such materials are incinerated with the additional risk of transformation into even more hazardous compounds. The usual occurrence of chloro compounds and the potential formation of mixed halogenated congeners with vast numbers of isomers further complicate the matter. Because many of these halogenated aromatic compounds have high chemical and biological stability and a potential for bioaccumulation, they are a threat to the environment. Sensitive methods are indispensable for early detection, for identifying the origin and sources of these compounds, and for taking suitable measures to prevent contaminations. Although structural information by HRGC/ NCI-MS analysis of brominated compounds is partly lost, information on the elemental composition is still provided and may be used for the trace-level detection of these compounds.
ACKNOWLEDGMENT I thank M. D. Muller and B. Scholz for making available samples and 0. Hutzinger, A. S. Kende, and C. Rappe for reference compounds. Registry No. 2,4,5,2’,4’,5’-hexa-BB, 59080-40-9; 2,4,5,2’,4’,5’-hexa-CB,35065-27-1; 1,2,3,4-tetra-BDD,104549-41-9;
2919
1,2,3,4-tetra-CDD, 30746-58-8; 2,3,7,8-tetra-BDF, 67733-57-7; l-bromo-2,3,7,8-tetra-CDD, 104549-42-0;bromo-2,3,4,5-tetra-CB, 104549-46-4;2,3,7,8-tetra-CDF,51207-31-9; 1,2,3,7,8-penta-CDF, 57117-41-6;l-brom0-2,3,7,8-tetra-CDF, 104549-43-1;2,3,7,8-tetra-CDD, 1746-01-6;1,2,3,7,8-penta-CDD,40321-76-4;deca-CB, 2051-24-3; 2,3,4,5-tetra-CB, 33284-53-6; bromopenta-CBs, 104549-47-5; bromohexa-CBs, 104549-48-6; bromohepta-CBs, 104549-49-7;hexachlorobenzene, 118-74-1;hexabromobenzene, 87-82-1;2-bromo-4,6-dichlorophenol, 4524-77-0;2,4,6-tribromo4619-74-3; Arphenol, 118-79-6;2,4,6-tribromo-3-methylphenol, odor 1254, 11097-69-1.
LITERATURE CITED (1) Sanders, H. J. Chem. Eng. News 1978, April 24, 22-36. (2) Blum, A.; Ames, B. N. Science (Washington, D.C.)1977, 195, 17-23. (3) Buser, H. R. Ph.D. Thesis, University of UmeA, UmeA, Sweden, 1978. (4) O’Keefe, P. W. EHP, Environ. Health Perspect. 1978, 23,347-350. (5) Buser, H. R. Environ. Sci. Technol. 1986, 20, 404-408. (6) Choudhry, G. G.; Hutzinger, 0. I n Current Topics in Environmentaland Toxicological Chemistry;Gordoo and Breach Science Publishers: New York, 1983; Vol. 4. (7) Miiller, M. D.; Buser, H. R. Environ. Sci. Technol., in press. ( 8 ) Moore, J. A.; McConnell, E. E.; Dalgard, D. W.; Harris, M. W. Ann. N . Y . Acad. Sci. 1979, 320, 151-163. (9) Poland, A.; Greenlee, W. F.; Kende, A. S. Ann. N . Y . Acad. Sci. 1979, 320,214-230. (10) Poland, A.; Glover, E.; Kende, A. S. J. Bioi. Chem. 1976, 251, 4936-4946. (11) Stalling, D. L.; Smith, L. M.; Petty, J. D.; Hogan, J. W.; Johnson, J. L.; Rappe, C.; Buser, H. R. I n Human and Environmental Risks of Chlorinated Dioxins and Related Compounds; Tucker, R. E., Young, A. L.. Gray, A. P., Eds.; Plenum: New York, 1983; pp 221-240. (12) Buser, H. R.; Rappe, C.; Bergqvist, P. A. EHP, Environ. Health Perspect. 1985, 60,293-302. (13) Czuczwa, J. M.; Hites, R . A. fnviron. Sci. Technol. 1986, 20, 195-200. (14) Carter, L. J. Science (Washington, D . C . ) 1976, 192, 240-243. (15) Crow, F. W.; Bjorseth, A.; Knapp, K. T.; Bennett, R. Anal. Chem. 1981, 53,619-625. (16) Dougherty, R. C. Anal. Chem. 1981, 53, 625A-636A. (17) Hass. J. R.; Friesen, M. D.; Harvan, D. J.; Parker, C. E. Anal. Chem. 1978, 50, 1474-1479. (18) Dannan, G. A.; Moore, R. W.; Aust, S. D. EHP, Environ. Health Perspect. 1978, 23, 51-61. (19) Buser, H. R.. Wadenswil, Switzerland, 1986, unpublished results. (20) Beynon, J. H. Mass Spectrometry and Its Application to Organic Chemistry; Elsevier: Amsterdam, 1960: p 298. (21) Scholz, B., personal communication, 1986.
RECEIVED for review April 28, 1986. Accepted July 18, 1986.
Selective Analysis of Nitro- and Nitroso-Containing Compounds by Laser Ionization Gas Chromatography/Mass Spectrometry Richard B. Opsal’ and James P. Reilly* Department of Chemistry, Indiana University, Bloomington, Indiana 47405 A series of NO-containing compounds are Introduced into a Capillary column gas chromatograph. The GC effluent Is Irradiated by 193-nm pulses from an ArF exclmer laser, and the ions generated are mass analyzed. I n contrast wlth conventional electron Impact mass spectrometry, substantial quantities of NO’ are observed, and the analytical signlflcance of this is considered.
Laser ionization mass spectrometry is a powerful and well-established technique for studying molecular spectroscopy (1-7). When used in conjunction with a capillary column gas Present address: Max-Planck-Institut fur Quantenoptik, Garching bei Munchen, West Germany.
chromatograph (GC), it is also an attractive analytical technique (8-10). Although its sensitivity and selectivity have been established (8, 9),laser ionization GC/MS has not gained widespread popularity. The complexity of the apparatus and the large number of experimental parameters that must be controlled obviate its application to straightforward problems. The inherent promise of the method lies in its potential application to problems that cannot be dealt with using conventional analytical techniques. One example of such a problem might be the analysis of samples containing several molecular isomers. A second example would be the development of a detector that selectively responds to compounds containing nitro or nitroso substituents. Most work to date has focused on polyaromatic hydrocarbons and various alkylbenzenes that laser ionize very efficiently, due to their strong W absorptions and low ionization
0003-2700/86/0358-2919$01.50/0 0 1986 American Chemical Society
2920
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
A
Table I. Composition of Test Mixtures in Elution Order" Nitroaromatics and Isophorone (Chem Service No. PPI-SM, in Methanol) (1) (2) (3) (4)
nitrobenzene isophorone 2,6-dinitrotoluene 2,4-dinitrotoluene
Nitrosamines (Chem. Service No. PPN-7M, in Methanol) (1) N-nitrosodimethylamine (2) N-nitrosodi-n-propylamine (3) N-nitrosodiphenylamineb
Phenols (Chem Service No. PP-2M, in Methylene Chloride)
Ii I
L
Figure 1. Diagram of the gas chromatograph/reflectrontimesf-flight mass spectrometer. Views A and B are orthogonal: GV, gate valve, CTI-8,
vacuum cryopump.
potentials. The high stability of their molecular ions leads to little fragmentation and relatively simple mass spectra. The present study focuses on a variety of compounds that d o not ionize so efficiently and that fragment extensively. One of the principal advantages of laser mass spectrometry is that the degree of ion fragmentation can be influenced by varying the ionizing light intensity (2, 11-14). The possibility of inducing soft ionization is often noted and exploited. The present work examines some of the advantages derived from generating low-mass ions by fragmenting molecules, and it demonstrates that useful analytical information can be extracted in this way.
EXPERIMENTAL SECTION Figure 1 depicts our experimental apparatus. A Lumonics TE-861 ArF excimer laser provided the ionizing radiation. Ten-nanosecond pulses of up to 150 mJ of energy were available from this source. However, a 7.5-mm circular aperture transmitted only 6-9 mJ of light from the central portion of the laser beam. This was focused by a 50-cm focal length lens into the source region of time-of-flight mass spectrometer. In order to roughly maintain constant laser power, several optical flats were placed in the laser beam path and individually removed as power decreased during a series of experiments. The gas chromatograph utilized in this work was a Varian Model 3700 equipped with a capillary column and a Model 1095 on-column injector. On-column injections (0.5 pL of solution) were performed a t 30 "C. Subsequent to each, the injector was programmed a t 100 "C/min to 230 "C and held there until the oven program was complete. The oven temperature program is indicated in Figures 2, 5 , and 6. A commercial bonded-phase fused silica capillary column (J & U' Scientific, DB-1,0.25 pm film thickness, 15 m length, 0.25 mm id.) was employed. A flow controller provided a constant nitrogen carrier gas flow of 0.75 mL.atm/min. The chromatograph column was directly connected to the ion source of the mass spectrometer through an all-metal, resistively heated interface. The column extended approximately 2.5 cm beyond the interface. Standard sample mixtures were obtained from Chem Service, Inc. (West Chester, PA). Their compositions are listed in elution order in Table I. These mixtures were diluted by a factor of 10, to 10 ng/FL of each component, with spectra grade methylene chloride. The reflectron time-of-flight mass spectrometer shown in Figure 1was constructed in-house and is similar to previous designs (15, 16). After two-step acceleration (17), ions drift 0.85 m in a field-free region before entering the ion reflector. The latter consists of two decelerating regions bounded by 90% transmission nickel mesh. After exiting the reflector, ions return to the drift region and travel 0.79 m toward a microchannel plate detector. The electron current from this is collected by a brass anode, which, in conjunction with an outer grounded sleeve, forms a 50-R im-
(1) (2) (3) (4) (5)
(6) (7) (8)
(9)
phenol 2-chlorophenol 2-nitrophenol 2,4-dimethylphenol 2,4-dichlorophenol 4-chloro-3-methylphenol 2,4,6-trichlorophenol 2,4-dinitrophenol' 4-nitrophenol
(10) 2-methyl-4,6-dinitrophenolC (11) pentachlorophenol
Original solutions, 100 ng/@Lof each component. diphenylamine. In 10-fold excess.
Elutes as
pedance-matched transmission line (18). The signal is amplified by a LeCroy VVlOlATB fast preamplifier and then sent to a Transiac 2001 waveform recorder that is interfaced to an IBM 5150 computer. The mass spectrometer is evacuated by a Cryotorr 8 cryopump torr. With (CTI Cryogenics, Inc.). Its base pressure is 3 X GC effluent entering it, the spectrometer pressure is approximately 10-~ torr. Chromatographic runs generate both ion yield chromatograms and mass spectra of eluting compounds. Ion yields from every 20 laser shots are averaged together. A chromatogram point is formed by either summing all averaged digitizer channels together (yielding a total ion yield chromatogram) or by summing selected channels together (selected ion monitoring). Mass spectra are recorded only during component elution, as evidenced by the ion chromatogram.
RESULTS AND DISCUSSION Figure 2A is a chromatogram of the nitroaromatic and isophorone mixture using our gas chromatograph with its conventional flame ionization detector (FID). Irradiation of the nitroaromatics and isophorone with ArF laser light ionizes all components as shown in Figure 2B. Figure 3 contains mass spectra from this chromatogram. Fragmentation is apparently extensive due t o t h e high photon flux utilized in these experiments. The prominent ion for three of the compounds is m / z 30, Le., NO+. Number 2 corresponds to isophorone, which does not contain a nitro group. The base peak in this case is due t o C+. Although this extensive fragmentation renders molecular identification impossible, since no molecular or high-mass ions are present, it is quite obvious from these mass spectra whether or not a molecule contains an NO group. Monitoring only the m / z 30 peak should yield a n NO+selective detector. Figure 2C illustrates this point. Three peaks are observed: those due to nitrobenzene, 2,4-dinitrotoluene, and 2,6-dinitrotoluene. Isophorone is not detected since it does not contain an NO group. As is evident from comparing Parts B and C of Figure 2, monitoring this single mass greatly increases the signal-to-noise ratio, since very little of the background ion signal occurs in this region of the spectrum. It is instructive t o compare ArF laser-generated mass spectra with those obtained by electron impact. T h e latter are presented in Figure 4. In contrast to the extensive
58, NO. 14, DECEMBER 1986
ANALYTICAL CHEMISTRY, VOL.
2921
2 i
B
1
- 7 - 1
IO
20
40
60
EO
100
ION MRSS ( d z l
130
160
190
Figure 4. Electron impact mass spectra of nitroaromatic and isophorone mixture: (1) nitrobenzene, Atlas of Mass Spectral Data : Stenhagen, E., Abrahamsson, S.,McLafferty, F. W., Eds.; Wiley-Interscience: New York, 1969; (2) isophorone; (3) 2,6dinitrotoluene; (4) 2,4-dinitrotoluene, Eight Peak Index of Mass Spectra; The Mass Spectrometry Data Centre, Royal Society of Chemistry: Nottingham, United Kingdom, 1983.
C
0 5 10 15 20 Figure 2. Chromatograms of nitroaromatic and isophorone mixture, 5 ng of each component of each corriponent indicated in Table I: (A) flame ionization detection, (B) total ion chromatogram with ArF laser ionization (8.1-7.1 mJlpulse, m l z 10-590 summed), (C) selected ion chromatogram with ArF laser ionization (7.1-6.0 mllpulse, only m l z 30 monitored).
K7
I
4
7 3
2
2
1
10
20
30
40
50
I O N MRSS I m l r )
60
eo
3
C
I00
Figure 3. Mass spectra recorded while chromatogram peaks from Figure 28 eluted. See Table I for component identification.
fragmentation produced with high-intensity ArF radiation, for which the majority of ions are below m / z 40, most ions generated by electron impact have masses larger than 40. Furthermore, the m / z 30 ion, NO+, is not a t all prominent with electron impact ionization. Chromatographic detection by selected ion monitoring a t this mass would obviously not work well with a conventional mass spectrometer. Figure 5 is analogous to Figure 2, but contains chromatograms of the nitrosamine mixture. Figure 5A is a reference FID chromatogram of the mixture. Parts B and C of Figure 5 are ArF laser-generated chromatograms obtained by monitoring ions of all masses and only those a t m / z 30, respectively. Peaks 1 and 2 are due to N-nitrosodimethylamine and N-nitrosodi-n-propylamine. Interestingly, N-nitrosodiphenylamine does not appear in Figure 5C. The reason for this is that N-nitrosodiphenylamine decomposes to diphenylamine in the chromatographic process (19) and di-
30
30
0
5
80 10
130 15
180 2'0
TemplOCl Timelminl
Flgure 5. Chromatograms of nitrosamine mixture, 5 ng of each component indicated in Table I: (A) flame ionization detection, (B) total ion chromatogram with ArF laser ionization (8.6-6.0 mJ/pulse, m l z 10-590 summed), (C) selected ion chromatogram with ArF laser ionization (7.9-6.0 mJ/pulse, only m l z 30 monitored).
phenylamine does not contain an NO group. The very first peak in Figure 5A-C corresponds to the methanol solvent. Its molecular ion is a t mlz 32, but loss of two hydrogens yields a fragment of mass 30 that is detectable. ArF laser ionization with selected ion monitoring thus dramatically reduces, but does not totally eliminate, the solvent peak from our chromatogram. As a final example, we investigated the phenol mixture listed in Table I. A reference FID chromatogram of the mixture is displayed in Figure 6A. An ArF laser ionization, total ion yield chromatogram appears in Figure 6B. The relative heights of various peaks in these two chromatograms
2922
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
Table 11. Mechanistic Routes Leading to ArF Laser-Induced NO+ Ion Production"
---- -- -- -.
possible photochemistry for nitroaromatic compounds RN02 RN=O + 0 RN=O R + NO RN=O RNO+ R + NO+ RNOZ RNOz+ RO + NO+ RNO, R + NO2 known photochemical processes in nitrobenzene* CeHbNOz C+jHbN=O + O C6HbNOz C6H5 + NO, possible photochemistry for nitrosamines R-N=O R + NO R-N=O RNO+ NO+ + R ionization schemes for NO and NO, NO NO+ NO, NO2+ NO+ + 0 NOz NO + 0 NO+ + 0 4
I/
B
+
I
+
C
"The arrow in each equation represents absorption by one or more ArF laser photons. *Reference21.
30
30
80
0
5
to
t
130 15
_i_.__
180
TempPCl
io
Tirnelrnin)
Figwe 6. Chromatogramsof phenol mixture ionized with the ArF laser, 5 ng of each component listed in Table I except 50 ng of 2,4dinitrophenol and 2-methyl-4,6dinitrophenol: (A) flame ionization detection, (B) total ion chromatogram with ArF laser ionization (7.6-6.0 nrl/pulse, m / z 10-590 summed), (C) selected ion chromatogram with ArF laser ionization (7.1-6.2 mJ/pulse, only rn / z 30 monitored).
are similar. Notice, however, that two impurities also laser ionize (peaks A and B). A chromatogram obtained by selected ion monitoring a t m / z 30 is displayed in Figure 6C. Although identification of the unknown compounds by examination of their mass spectra is not possible due to the extensive fragmentation, we can conclude that unknown B almost certainly contains an NO group but unknown A does not. The unique aspect of this study is the significant NO+ yield obtained by ArF laser-induced ionization of a variety of nitroor nitroso-containing compounds. Although the NO+ ion is observed in electron impact mass spectra of most of these compounds, it is not a prominent peak. Several possible mechanisms that can explain the copious production of NO+ by ArF laser ionization are listed in Table 11. These equations indicate the multiplicity and diversity of the paths that lead to NO+ ions. They fit into two categories: (1)photofragmentation of neutral molecules, followed by ionization of the neutral nitric oxide fragment, and (2) multiphoton ionization of parent molecules or large fragments, followed by photofragmentation of the ions. It has recently been demonstrated by photoelectron kinetic energy measurements that the production of C+ in the ArF laser ionization of benzene results from both of these types of processes (20), but the specific dissociation channels involved could not be identified. Production of NO from NO2 is virtually complete a t wavelengths shorter than 313 nm (21). NO absorption begins a t 226 nm with the A2Z+ X2n transition and is particularly strong between 190 and 200 nm due to the C2n X2n and D22 X2n bands in this region. This explains why any neutral NO photofragments that are produced can be very efficiently ionized. I t is intriguing to ask whether a laser ionization chromatograph detector can be made more selective to NO-containing molecules and less susceptible to interferences, such as the methanol solvent discussed above. Use of higher laser intensities may accomplish this, since potential interferences at mass 30 should be further fragmented. Whether or not NO' will stay intact in higher fields is open to question; approx-
-
-
+ -
imately 10 eV of energy must be deposited into cold NO+ to dissociate it (see Gilmore's potential energy curves of NO (22)). Additional selectivity and sensitivity may result from choosing an optimal laser wavelength (see Bethke (23) for an absorption spectrum of NO). A 248-nm KrF laser would be a poor choice for the ionizing light source. In experiments with substantially more energetic KrF pulses, we observed no m / t 30 ions for the nitrosamine mixture and only a modest yield was obtained for some of the nitro-containing compounds. As stated above, room-temperature NO does not absorb a t wavelengths longer than 226 nm. Ascertaining whether laser ionization can be selective toward specific groups containing NO is another area of future endeavor. For example, it should be possible to distinguish nitro- from nitroso-containing compounds by exploiting the unique ultraviolet absorption characteristics of these different types of molecules. This can be accomplished with the greatest facility on cold molecules that have been expanded from supersonic nozzles (24,25). Laser photoelectron measurements (26-28) will also be attempted in order to probe whether NO+ is produced from neutral photofragment NO or from photofragmentation of larger ions. Such mechanistic information can be utilized to optimize our ionizing laser wavelength. Although the cost and complexity of the present apparatus is relatively high and its application to routine chemical analysis is not advisable or realistic, it is worth pointing out that this instrumentation is actually less expensive than most commercial hybrid mass spectrometers or F T / M S instruments. It may also be economically feasible for the following reasons: (1)A time-of-flight mass spectrometer is well-suited to this type of instrument, and this is one of the simplest and least expensive of mass spectrometers. (2) A conventional GC/MS system could be modified for laser GC/MS experiments for a cost not significantly greater than that of the laser itself. (3) Only a small excimer laser light source is really needed for these experiments, and these can be manufactured a t a modest cost. Registry No. Nitrobenzene, 98-95-3; isophorone, 78-59-1; 2,6-dinitrotoluene, 606-20-2; 2,4-dinitrotoluene, 121-14-2; Nnitrosodimethylamine, 62-75-9; N-nitrosodi-n-propylamine, 621-64-7;N-nitrosodiphenylamine, 686-30-6;phenol, 108-95-2; 2-chlorophenol, 95-57-8; 2-nitrophenol, 88-75-5; 2,4-dimethylphenol, 105-67-9; 2,4-dichlorophenol, 120-83-2; 4-chloro-3methylphenol, 59-50-7; 2,4,6-trichlorophenol, 88-06-2; 2,4-dinitrophenol, 51-28-5; 4-nitrophenol, 100-02-7;2-methyl-4,6-dinitrophenol, 534-52-1; pentachlorophenol, 87-86-5.
LITERATURE CITED (1) Johnson, P. M. ACC. Chem. Res. 1980, 13, 20. (2) Zandee, L.; Bernstein, R . 6. J . Chern. Phys. 1979, 71,1359.
Anal. Chem. 1986, 58, 2923-2927 (3) Feldman, D. L.; Lengel, R. K.; Zare, R. N. Chem. Phys. Lett. 1977, 5 2 , 413. (4) Antonov, V. S.;Letokhov, V. S. Appl. Phys. 1981, 2 4 , 89. (5) DiGiuseppe, T. G.;Hudgens, J. W.; Lin, M. C. J . Phys. Chem. 1982, 86,36. (6) Owens, K. G.; Reilly, J. P. J . Opt. SOC.Am. 8 : Opt. Phys. 1985, 2 , 1589. (7) Tembruell, R.; Sin, C. H.; Li, P.; Pang, H. M.; Lubman, D. M. Anal. Chem. 1985, 5 7 , 1186. (8) Rhodes, G.;Opsal, R. 8.; Meek, J. T.; Reilly, J. P. Anal. Chem. 1983, 55, 280. (9) Sack, T. M.; McCrery, D. A.; Gross, M. L. Anal. Cbem. 1985, 5 7 , 1291. (IO) Opsal, R. 8.; Reiliy, J. P. Opt. News 1986, 72, 18. (11) Rockwood, S.;Reilly, J. P.; Hohla, K.; Kompa, K. L. Opt. Commun. 1979, 28, 175. (12) Boesl, U.; Neusser, H. J.; Schlag, E. W. J . Chem. Phys. 1980, 72, 4327. (13) Reilly, J. P.; Kompa. K. L. J . Chem. Phys. 1980, 7 3 , 5468. (14) Seaver, M.; Hudgens. J. W.; DeCorpo, J. J . I n t . J . Mass Spectrom. ion Phys. 1980, 3 4 , 159. (15) Mamyrin, B. A.; Karateav, V. I.; Shmikk, D. V.; Zagulin, V. A. Sov. Phys. JETP (Engl. Trans/.)1973, 3 7 , 45. (16) Boesl, U.; Neusser, H. J.; Weinkauf, R.; Schlag, E. W. J . Phys. Chem. 1982, 86, 4857.
2923
(17) Opsal, R. 8.; Owens, K. G.; Reilly, J. P. Anal. Chem. 1985, 5 7 , 1884. (18) Wiza, J. L. Nucl. Instrum. Methods 1979, 762, 587. (19) Trussell, A. R.; Moncur, J. G.; Lieu, F. Y.; Leong, L. Y . C. HRC CC, J . High Resolut . Chromatogr . Chromatogr. Commun. 1981, 4 , 156. (20) Sekreta, E.: Owens, K.; Reilly, J. P.,submitted for publication in Chem. Phys Lett. (21) Catvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wliey: New York, 1986. (22) Gilmore, F . R . J . Quant. Spectrosc. Radiat. Trans. 1965, 5 , 369. (23) Bethke, G. W. J . Chem. Phys. 1959, 37, 662. (24) Smalley, R. E.; Wharton, L.; Levy, D. H. Acc. Chem. Res. 1977, 10,
.
139.
(25) Amirav, A.; Even, U.; Jortner, J. Chem. Phys. 1980, 57, 31. (26) Meek, J. T.; Jones, R. K.; Reilly, J. P. J . Chem. Phys. 1980, 73, 3503. (27) Anderson, S. L.; Rider, 0. M.; Zare, R. N. Chem. Phys. Lett. 1982, 93, 11.
(28) Long, S.R.; Meek, J. T.; Reiliy, J. P. J . Chem. Phys. 1983, 79, 3206.
RECEIVED for review May 27,1986. Accepted August 1,1986. This research has been supported by the National Science Foundation and the Environmental Protection Agency. J.P.R. is a Camille and Henry Dreyfus Teacher Scholar.
Isotopic Measurement of Subnanogram Quantities of Rhenium and Osmium by Resonance Ionization Mass Spectrometry Richard J. Walker* and Jack D. Fassett National Measurement Laboratory, A 21 Physics Building, National Bureau of Standards, Gaithersburg, Maryland 20899
Resonance lonlzatlon mass spectrometry has been used to measure the lsotoplc compositions of mlcrogram and plcogram quantitles of Re and Os. The high sensltlvlty requlred for these measurements was achieved through the optimlzation of sample atomization and efficient ionization from the resuttlng gas-phase reservoir. Re and Os are absorbed from chlorlde solutions onto anion exchange beads as a means of purlfylng and concentrating the sample and then loaded onto a miniaturized Ta filament. The molecular specles of Re and Os are reduced to metal In the vacuum of the mass spectrometer by gradual heating in the presence of collodion and graphite. Both Re and Os atomize efficiently at 2173-2373 K. Os atomization is optimized further by pulsing the filament temperature to coinclde wlth the pulse rate of the laser system. Pulsed ion signals of >2 ions/s are quantifled by signal averaglng uslng a transient dlgltlzer. Ion fluxes
