Anal. Chem. 1983, 55,955-958
phosphorus contents which cannot be detected by ordinary colorimetry without preconcentration, such as solvent extraction. Although the use off the present system is limited to the visible region (due to poor internal reflection of LCC at shorter wavelengths), this LCC is extensively applicable in colorimetry. Registry No, P, 7723-14-0; HzO, 7732-18-5.
LITERATURE CITED (1) White, J. U. #J. Opt. SOC.Am. 1942, 32,285. (2) White, J. U.; Alpert, N. L.; DeBele, A. G. J . Cpt. SOC.Am. 1955, 45, 154. (3) Chrk, G. C., Ed. “The Encyclopedb of Spectroscopy”; Reinhold: New York, 1960; p 471. (4) Herrmann, R.; Dean, J. A. “Flame Emission and Atomic Absorption Spectrometry”; Marcel Dekker: New York, 1971; pp 88, 128.
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(5) Ramlrez-Munoz, J. “Atomic Absorption Spectroscopy”; Elsevler: Amsterdam, 1968; p 142. ( 6 ) Fuwa, K.; Vallee, B. L. Anal. Chem. 1983, 35, 942. (7) Ando, A.; Fuwa, K.; Vallee, B. L. Anal. Chem. 1970, 42, 818. (8) Fuwa, K. “Spectrochemical Methods of Analysis: Quantitative Analysis of Atoms and Molecules”; Wlley: New York, 1971; p 189. (9) Fujlwara, K ; Wel, L.; Uchikl, H.; Shlmokoshl, F.; Fuwa, K.; Kobayashl, T. Anal. Chem. 1982, 5 4 , 2029. (10) Zeegers, P. J.; Smith, R.; Wlnefordner, J. D. Anal. Chem. 1988, 4 0 , 26A. (11) Rubeska, I.; Svoboda, V. Anal. Chlm. Acta 1985, 32, 253. (12) Gelaer, 2. \/. Specfrochlm. Acta, Part8 1970, 258, 669. (13) Pecsok, R. L.; Shields, L. D.; McMilllam, I. G. “Modern Methods of Chemical Analysis”, 2nd ed.; Wiley: New York, 1976; p 167.
RECEIVED for review October 26,1982. Accepted January 24, 1983* This work was by Grant No*56030049 from the Ministry of Science, Culture, and Education, Japan.
CORRESPONDENCE Multiphoton Ionization in Fourier Transform Mass Spectrometry Sir: Multiphoton ionization (MPI) has proven to be an invaluable tool in molecular spectroscopy ( 1 ) . Studies in which the ions produced by MPI are mass analyzed are rapidly gaining popularity (2-13). Initial studies have shown that variable and sonietimew remarkably extensive fragmentation can occur particularly when the MPI process involves a resonance transition (2-4). A number of workers have sought mechanistic explanations for the efficiency of the MPI process and the dramatic fragmentation behavior that has been observed (5-8). From an analytical point of view, the combined MPI/mass spectrometry approach has a number of advantages. When a molecule absorbs a t the wavelength used, resonance-enhanced MPI can serve as a highly efficient and very selective ionization method. On the other hand, nonresonance MPI in the UV is a fairly general process which produces electron-impact-like mass spectra for common organic molecules (9). Multiphoton experiments may help alleviate many of the problems encountered in photoionization mass spectrometry with vacuum ultraviolet sources. The ionization efficiency during the light pulse can be very high and the softness of ionization can be controlled by varying laser power and wavelength. Several recent studies suggest the utility of MPI/mass spectrometry in chemical analysis ( 10-1 3). Ion cyclotron resonance (ICR) and its high-performance daughter, Fourier transform mass spectrometry (FTMS), have been successfully used with various light sources. Ion photodissociation (14, 24) and laser desorption (15, 18-22) experiments in ICR and in FTMS have been reported and suggested that MPI might be a viable ionization source in FTMS. This paper reports the successful observation of multiphoton ionization in FTMS and comments on the compatibility and advantages of the combined approach. EXPERIMENTAL SECTION All data were obtained on a prototype Nicolet FT/MS-1000. Details of experimental techniques associated with Fourier transform mass spectrometry (16,17)including laser desorption (20-22), collision-induced dissociation (25,26),and high resolution (27-29) have been described elsewhere. For the multiphoton ionization experiments, a modified 1 in.3 cell (23) of the design used for solid probe laser desorption experiments was employed.
The focused or unfocused laser beam enters and exits the analyzer cell through 1/4 in. holes centered in the front and back receiver plates. The presence of these holes degrades the overall performance noticably but not significantly. An increase in trapping voltage of 0.25-0.5 V was required to maintain signal intensity comparable to EL one-hole cell. Ions were generated in MPI experiments by using the quadrupled beam (266 nm, 4.7 eV) from a Quanta Ray NdYAG laser. Laser power was estimated with a Coherent Model 210 power meter. Pulse energies on the order of 1-4 mJ were commonly used. The pulse width was approximately 4-7 ns. Laser power and focusing were adjusted qualitatively to optimize signals for individual samples and to control the extent of fragmentation.
RESULTS AND DISCUSSION In accordance with earlier reported studies, a variety of monosubstituted benzenes, polynuclear aromatics, and inorganic carbonyls which all absorb at 266 nm were found to undergo MPI in the FTMS. Because it is possible that ionization can occur in the plasma produced when the laser beam strikes a surface and electron-impact-like ions have been observed in our laser desorbed metal chemistry studies, it was important to ensure that ionization was truly resulting from optical absorption by the sample. First of all, it was a simple matter to prevent the focused beam from hitting the cell by steering it until a clear boundary where desorption of metal ions occurred could be observed and then avoided. Secondly, the efficiency for molecules that are strong solution absorbers a t 266 nm was enhanced in the unfocused beam where no metal ion desorption was possible and nonabsorbing background gases were never observed. Also, fragmentation characteristic of the MPI process, such as the observation of Cr’ as the sole product in the laser produced mass spectrum of Cr(CO)6,is difficult to explain by any spurious ionization phenomena. Figure 1 shows the most unequivocal evidence for MPI. A mixture of 1 X lo4 torr isopropyl sulfide and 2 X torr perdeuteriodiphenyl was ionized by E1 and MPI. The E1 spectrum reflects the excess of isopropyl sulfide, while the MPI spectrum reflects the relative molar absorptivity of the two compounds a t 266 nm, which approaches 0 for isopropyl sulfide (30) and ca. 8000 for diphenyl (31). Since the ionization potentials of the compounds are nearly identical (321, it must be the differences in absorption that result in
0003-2700/83/0355-0955$01.50/00 1983 American Chernical Societv
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
956
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Flgure 1. Mass spectra of a mlxture of 2 X loF7torr perdeuterlodiphenyl and 1 X lo-' torr isopropyl sulfide. Ionlzatlon was accompllshed by (a) a 70-eV 20-171s electron beam and (b) a 266-nm laser pulse. Both spectra are averages of 50 scans.
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Figure 3. Mass spectra of 2 X IO-' torr perdeuteriodiphenyl in 2 X torr Ar ionized by (a) a 10-ms 70-eV electron beam and (b) a 266-nm laser pulse.
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Flgure 2. One shot mass spectra of azulene comparing the ionization efficiency of (a) one 10-ms 70-eV electron beam pulse with (b) one 266-nm unfocused laser pulse.
the total absence of isopropyl sulfide ions in the MPI spectrum. The analytical characteristics of the MPI-FTMS combination were qualitatively investigated. Even at the low powers employed, ionization efficiency was very high. Signal levels comparable to typical E1 experiments could be generated on more strongly absorbing samples. Figure 2 shows a direct comparison of mass spectra obtained from a single laser pulse and a 10-ms electron beam pulse. Usable signals were obtained for compounds whose molar absorptivity is around 10000 at pressures lower than 2 X lo4 torr. Signal intensity was found to increase with pressure up to a point where space charge
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effects began to degrade linearity and mass resolution. The working ranges of pressure and laser power were highly sample dependent. The potential for high selectivity by MPI is demonstrated not only in Figure 1 but also in Figure 3 where perdeuteriodiphenyl is detected in a 100-fold excess of argon. Also, enhancement in ionization efficiency due to differences in molar absorptivity can be large as shown in Figure 4. Here, perdeuteriodiphenyl is ionized in the presence of a 5-fold excess of toluene. The solution molar absorptivities of the compounds are 8000 and 130, respectively (31). The relative increase of fragmentation with laser power is exemplified in Figure 5 for diethylaniline. One obvious advantage of MPI is the ability to control the softness of ionization by varying laser power or wavelength without a loss in ionization efficiency. Most previous MPI/MS studies have involved the use of time-of-flight (TOF) mass spectrometers. Sector instruments and quadrupoles can be used but involve a poor duty cycle
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983 * 957
a)
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Figure 5. MPI spectra of diethylaniline: (a) unfocused beam, ca. lmJ/pulse, (b) urifocused beam, ca. 2.5 mJ/pulse, and (c) focused beam ca. 2.5 mJ/pulse.
since several laser shots are required per atomic mass unit. As another instrument which can acquire a complete mass spectrum from a single ionizing pulse, the FTMS may have a number of advantages over the TOF mass spectrometer. Resolution of 1000 fwhm at m / z 164 has been observed in a wide band MPI spectrum from 15 to 1601) amu using 32k data points. High Iesolution of greater than 21000 fwhm was readily achieved in the heterodyne mode at m / z 128 for the torr (Figure 6). Exact molecular ion of azulene at 2 x mass was measurable to within 20 ppm when calibrated under the same conditions. These results by no means represent the inherent lirnits in high resolution and exact mass measurement (27, 28, 33, ,34). In addition to high resolution and exact mass capabilities, ions formed in the FTMS by MPI can be subjected to a number of experiments not possible in TOF experiments. A good example isi collision-induced dissociation for structure determination and mixture analysis. The molecular ion from n-butylbenzene formed by MPI was readily isolated and collisionally activated in the FTMS. A typical low-energy CID fragmentation pattern was obtained (35). Ion chemistry, as studied by FTMS, can provide information on ion structure and it may also be possible to study the chemistry of state selected ions formed by MPI. Also, since ions can be stored under collision-free conditions for times approaching seconds, multiple light source experiments with fairly long time scales could easily be carried out. Two color ionization experiments could be performled with the laser beams coincident or delayed in time up to several seconds. We have recently demonstrated the feasibility of transient addition of high gas pressure in FTMS using a pidsed solenoid valve (33). This approach could be used to enhance the sensitivity of MPI without incurring the deleterious effects on resolution of higher pressure during detection. A conceivable experiment involves pulsed addition of a sample for ionization by MPI. After the gas pulse is pumped away, the molecular ion could be mass selected and
photodissociated at the same or different wavelength. This approach would completely eliminate any complications caused by further ionization of the neutral gas and permit high-resolution detection under optimum conditions. Due to our interest in gas-phase reactions of atomic metal ions (15,18-21), MPI experiments were performed on several inorganic complexes. In agreement with earlier work (36-38), formation of atomic metal ions was the dominant process for the complexes studied. Multiphoton ionization of Cr(CO)6, MO(CO)~, and (C5M5)Mn(CO),yielded the respective atomic metal ion as the only major fragment in each case. In general Cr+ and Mn+ in their ground states are unreactive with hydrocarbons, but excited state Cr+, formed by electron impact, has been shown to react with methane (39). Both Cr+ and Mn+ formed by MPI were found to be unreactive with isobutane; and Cr+, so formed, did not react with methane, indicating the absence of any excited state population. Multiphoton ionization of MnzCOloyielded only Mn+ with no metal dimer ions being observed. It seems likely, however, that MPI of complexes with multiple metal-metal bonds may be a possible source of metal dimer ions for gas-phase studies. Studies of gas-phase metal ions in ICR and FTMS have relied on electron impact ionization of volatile organometallics or direct laser desorption from pure metal samples as ionization sources. The latter approach has the advantage of forming exclusively metal ions and avoiding complications due to the neutral background. Formation of atomic metal ions by MPI appears to be a fairly general process and, although neutral organometallic background must be present, the metal ion is likely to be the only major primary ion. Thus, MPI may be an alternative metal ion source for special cases where laser desorption is inconvenient. Moreover, as an elementally selective ionization method, MPI has implications as an approach to trace elemental analysis in mass spectrometry (12). Finally, the possibility of forming excited state metal ions optically warrants further consideration. One of the most important advantages of FTMS as an analytical technique is high versatility in experiments that can be performed with little or no modification of the basic instrument. The ease with which MPI can be performed on our instrument is a further example. Although the scope of our results is somewhat limited by the laser powers and wavelengths available, the potential of the combined approach is clear. The use of MPI as a selective ionization method is a viable alternative in reducing chemical noise in the FTMS. In conjunction with GC/FTMS (40,4 1 ) , multiphoton ioni-
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zation at 266 nm, for example, may be an effective approach to selective detection of aromatic compounds. Allowing experiments analogous to those performed by photoionization mass spectrometry to be carried out in a simple way, MPI may increase the versatility of FTMS as an all-purpose mass spectrometer. Certainly FTMS has some attractive features when compared to TOF in an analytical MPI experiment. As a high-performance, pulsed mass spectrometer which is especially compatible with various light sources, the FTMS will enhance the marriage of optical and mass spectroscopy. Subsequent to the submission of this manuscript a report by McIver et al. has appeared also describing the detection of ions produced by MPI using FTMS (42).
LITERATURE CITED (1) Johnson, P. M. Acc. Chem. Res. 1980, 73,20-26. (2) Zandee, L.; Bernstein, R. B. J. Chem. Phys. 1979, 70,2574-2575. (3) Zandee, L.; Bernstein, R. B. J. Chem. Phys. 1979, 77,1359.
(4) Cooper, C. D.; Williamson, A. D.; Mliler, J. C.; Compton, R. N. J. Chem. Phys. 1980, 73,1527-1537. (5) Llchtin. D. A.; Datta-Ghosh, S.; Newton, K. R.; Bernstein, R. 8. Chem. Phys. Left. 1980, 75,214-219. (6) Reiiiv, J. P.: KomDa. K. L. J. Chem. Phvs. 1980. 73. 5468-5476. Carney, T.; Baer, T. J. Chem. Phys. 1961, 75,477-478. Lubman, D. M.; Naaman, R.; Zare, R. N. J. Chem. Phys., 1980, 72,
3034-3040. Seaver, M.; Hudgens, J. W.; Decorpo, J. J. Int. J . Mass Spectrom. Ion. Phys. 1980, 3 4 , 159-173. Lubman, D. M.; Kronick, M. N. Anal. Chem. 1982, 54,660-665. Rider, D. M.; Durant, J.; Anderson, S.; Zare, R. N. Presented at the 30th Annual Conference on Mass Spectrometry and Allied Toplcs, Honolulu, HI, June 6-11, 1982. Harrlson, W. W.; Rider, D. M.: Zare, R. N. Presented at the 30th Annual Conference on Mass Spectrometry and Allled Topics, Honolulu, HI, June 6-11, 1982. Hudgens, J. W.; Dulgan, M. T.; DiGiuseppe, T. G.; Wyatt, J. R. Presented at the 30th Annual Conference on Mass Spectrometry and Allied Topics, Honolulu, HI, June 6-11, 1982. Dunbar, R. C. "Gas Phase Ion Chemistry"; Bowers, M. T., Ed.; Academic Press: New York, 1979;Vol. 2. Cody, R. B.; Burnier, R. C.; Reents, W. D., Jr.; Carlin, T. J.; McCrery, D. A.; Lengel, R. K.; Freiser, B. S. Int. J. Mass. Spectrom. Ion Phys. 1980, 33,37-43.
Comlsarow, M. B. Adv. Mass Spectrom. 1980, 8 , 1698-1706. Wilkins, C. L.; Gross, M. L. Anal. Chem. 1981, 53, 166lA-1676A. Burnier, R. C.; Byrd, G. D.; Freiser, B. S. Anal. Chem. 1980, 52,
1641-1650. Carlin, T. J.; Wise, M. B.; Freiser, B. S. Inorg. Chem. 1981, 20,
2743-2745. Byrd, G. D.; Freiser, B. S. J. Am. Chem. SOC. 1982, 704,
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(21) Jacobson, D. 8.; Byrd, G. D.; Freiser, B. S. J. Am. Chem. SOC.1982, 704, 2320-2321. (22) McCrew. D. A.; Ledford, E. B., Jr.: Gross, M. L. Anal. Chem. 1982. 54, 1435-1437. (23) Comisarow, M. B. Inf. J. Mass Spectrom. Ion Phys. 1981, 37, 251-257. - . - ..
(24) Cassady, C. J.; Freiser, B. S. Presented at the 29th Annual Confer-
ence on Mass Spectrometry and Allled Topics, Minneapolis, MN, May
24-29, 1982. (25) Cody, R. B.; Burnler, R. C.; Freiser, B. S. Anal. Chem. 1982, 54, 96-101. (26) Cody, R. B.; Freiser, B. S. Int. J. Mass Spectrom. Ion Phys. 1982, 4 7 , 199-204. (27) Comisarow, M. B.; Marshall, A. G. J. Chem. Phys. 1975, 62,293. (28) White, R. L.; Ledford, E. B., Jr.: Ghaderi, S.; Wilkins, C. L.; Gross, M. L. Anal. Chem. 1980, 52, 1527-1529. (29) Cody, R. 8.; Frelser, B. S. Anal. Chem. 1982, 54, 1431-1433. (30) Fehnel, E. A.; Carmack. M. J. Am. Chem. SOC. 1949, 77,85-93. (31) Freidel, R. A.; Orchln, M. "UV Spectra of Aromatic Compounds": Wiley: New York, 1951. (32) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T.; J. Phys. Chem. Ref. Data, Suppl. 1977, 6 , No. I.
(33) Carlin, T. J.; Freiser, B. S. Anal. Chem. l98S, 55,571-574. (34) Whlte, R. L.; Wilkins, C. L. Anal. Chem. 1982, 54,2211-2215. (35) Mcluckey, S.A.; Sallans, L.; Cody, R. B.; Burnier, R. C.; Verma, S.;
Freiser, B. S.;Cooks, R. G. Int. J. Mass Spectrom. Ion Phys. 1982,
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Timothy J. Carlin Ben S. Freiser* Department of Chemistry Purdue University West Lafayette, Indiana 47907 RECEIVED for review November 5,1982. Accepted February 1, 1983. Support for this research was provided by the Department of Energy (DE-AC02-80ER10689)and the National Science Foundation (CHE-80026585),which provided funds to purchase the FTMS.
Identification of Geochemical Polycyclic Aromatic Hydrocarbons from Terpenes by High-Resolution Shpol'skii Effect Fluorescence Spectrometry Sir: In this paper, we demonstrate that high-resolution spectrofluorimetry (HRF) at 4.2 K in n-alkane matrices can be used to identify polycyclic aromatic hydrocarbons (PAH) derived from triterpene, which occur in the organic matter of marine and terrestrial sediments. The chrys-a (see the structure on Figure 1A) is indeed an interesting case since this molecule has bulky substituents and certainly cannot fit in the crystal lattice of n-alkane in the same way as triphenylene or coronene itself (I). We present results obtained on a synthetic sample (2)of chrys-a that show unambiguously the existence of a quasi-linear fluorescence spectrum with conventional excitation (Figure 1A). The presence of the ethylcyclopenteno part of the chrys-a molecule apparently does not hinder the inclusion at low concentration (Le., C < lo4 M) of the solute in the n-heptane crystal in sites
that are sufficiently homogeneous to give rise to high-resolution spectra. It should be mentioned that another large compound with unusual structure such as hexahydrohexahelicene, a nonplanar composite molecule, was also shown to give a Shpol'skii effect (3). In three marine sediments from the Sea of Oman (South Arabia), the series of chrysene derivatives was extracted from the total aromatic fraction by HPLC on psilica-NH2 stationary phase with n-heptane as an eluant ( 4 ) . Figure 1B shows that chrys-a can be easily identified in the sediment extract. This is, to our knowledge, the first identification by HRF of such a highly alkylated PAH. This result clearly opens a new field since it shows that such molecules, which occur widely in sediments and petroleums, can be introduced in
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