Anal. Chem. 1989, 61, 2669-2673
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Determination of Polycyclic Aromatic Hydrocarbons Using Gas Chromatography/Laser Ionization Mass Spectrometry with Picosecond and Nanosecond Light Pulses Charles W. Wilkerson, Jr., Steven M. Colby, a n d J a m e s P. Reilly*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405
The effect of laser pulse duration on the ionization efficiency of a variety of polycyclic aromatic hydrocarbons is studied by use of laser ionization gas chromatography/mass spectrometry. For compounds that undergo rapid excited-state reiaxation, the use of ultrashort (picosecond) light pulses increases the probability of ionizatlon relative to relaxation, resulting in improved ionization efficiency for these species.
INTRODUCTION Laser multiphoton ionization has been employed as an effective ion source for the mass spectrometric determination of many compounds (1-10). In this technique, which offers some unique advantages compared to other ionization methods, analyte molecules are first excited by a UV laser to a low-lying electronic state, and then a second UV photon is absorbed to generate the ion. Under appropriate experimental conditions, laser ionization can be a very efficient process, and when combined with the high ion transmission of a. time-offlight mass spectrometer, excellent sensitivity can be achieved ( 2 , 4 ) . The time-of-flight instrument records an entire mass spectrum with each laser pulse. This eliminates the need to scan a mass analyzer and provides a multiplex advantage that also enhances sensitivity. High sensitivity is particularly useful when the analyte is present in small concentration or is in the ion source for only a short period of time, such as in gas chromatography/mass spectrometry (GC/MS). A gas chromatograph is used for sample introduction in our experiments to ensure that trace impurities, which can drastically alter observed ionization yields (11, 12), are separated from the compounds of interest. The GC also provides a quantitative and reproducible method for introducing multiple analytes into the ion source, thereby enabling a direct measurement of their relative ionization efficiencies under nearly identical conditions. In addition to the mass selectivity obtained in GC/MS, laser ionization can provide an additional degree of molecular selectivity, since the laser wavelength can be tuned so that certain analytes produce very strong ion signals, while other species may show little or no ionization (2, 3 ) . A number of previous investigations have explored the influence of laser conditions on the ionization yields of various species (1-3,13-15).In the course of these studies it has been observed that ionization efficiencies can vary dramatically from compound to compound. Some of the reasons for this behavior are obvious. Molecules have different absorption characteristics and different ionization potentials. I t is therefore not surprising that changing the laser wavelength would result in significant changes in relative ionization yields. Other reasons are more subtle. For example, the neutral precursor molecules can undergo dissociation after the absorption of the first photon. The resulting fragments may or may not be laser ionizable. Benzaldehyde is an example of a molecule that shows this kind of behavior, and its multiphoton ionization characteristics have been studied in detail ( 1 1 , 14, 15). Excited neutral molecules can also undergo nonradiative relaxation. If an electronically excited molecule
Table I. Selected Properties of the PAH Analytes Studied
MW, amu
IP," eV
t
molecule
nm)
nm)
nm)
naphthalene biphenyl acenaphthene fluorene phenanthrene anthracene fluoranthene pyrene
128 154 154 166 178 178 202 202
8.12 8.27 7.66 7.78 8.03 7.43 7.72 7.53
2600 15400 1000 11500 87000 40000 11000 8500
7000 3000 5500 12000 16000 1500 10000 20000
200 =O 1500 4000 150 200 3600 7000
(240
aValues from Murov, S. L. Handbook cel Dekker: New York, 1973.
of
t
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c (299
Photochemistry; Mar-
relaxes to a lower-lying state before it absorbs a second, ionizing photon, a loss of ionization efficiency may be observed. Some relaxation phenomena, such as internal conversion and intersystem crossing, can occur on very rapid, subnanosecond, time scales (I, 16, 17). In theory, one could overcome this problem by increasing the intensity of the laser pulse, thereby increasing the rate of photoionization relative to the rate of spontaneous relaxation. However, in practice this has not been of great utility. Previous experiments have shown that as the pulse intensity is increased, dissociation of both neutrals and ions becomes a dominant process (13). On the other hand, if the laser pulse is short compared to the time scale of these relaxation events, the use of high intensities is not necessary. In this case, photoionization should be favored compared to excited-state relaxation, and the ionization efficiency of a molecule with a very short excited-state lifetime should improve. The goal of the current study is to use both nanosecond and picosecond laser pulses to investigate the effect of laser pulse duration on the ionization efficiency of several polycyclic aromatic hydrocarbons. The measurements undertaken here employ species that have been studied in the past ( 2 , 4 ,7) but involve the use of different light wavelengths and pulse durations. EXPERIMENTAL SECTION Naphthalene (Eastman), biphenyl (MCB), acenaphthene (MCB),fluorene (MCB),phenanthrene (New England Nuclear), anthracene (MCB),fluoranthene (City Chemical Co.), and pyrene (Aldrich) are all used as received. For some experiments, stock solutions of the various analytes are prepared in methylene chloride (Fischer, Spectranalyzed), and these solutions are combined and diluted to give a final sample containing =I ng/gL of each component. In other experiments, a standard polycyclic aromatic hydrocarbon sample (Accu-Standard M-610), containing all of the desired analytes except biphenyl, is used, and biphenyl is spiked into this sample to give a final concentration of 10 ng/wL of each component. A summary of selected properties of the molecules studied is given in Table I. The analytes are separated, using a Varian 3700 GC with on-column injection, on a 30 m/250 gm i.d. fused silica capillary column (Supelco SPB-1). The nitrogen carrier gas flows at a rate of e0.6 mL/min. A heated interface, which is held at a constant temperature of 250 or 320 "C (depending on the sample used), connects the GC to the mass spectrometer. The end of the column is positioned between the
0003-2700/89/0361-2669$01.50/0 0 1989 American Chemical Society
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Figure 1. Schematic layout of experimental apparatus. The dotted line shows the path of the picosecond light pulse.
first two accelerating grids of a linear time-of-flight mass spectrometer that has been described earlier (2). A schematic drawing of the experimental apparatus is shown in Figure 1. The mass spectrometer is evacuated by a liquid-nitrogen-trapped Varian VHS-6 diffusion pump. Spectrometer pressure during the exTorr. Dual microchannel plates perimental runs is 4.6 X (Varian) configured in tandem are used for ion detection. The detector signal is amplified (LeCroy VVlOlATB) and then digitized by using a high-speed waveform recorder (Biomation6500). The digital signal is stored in a microcomputer for data display and analysis. A pico/nano pulsed Nd:YAG laser (Quantel YG571-c) is used as the light source in all experiments. Conversion of the 1.06-pm NdYAG fundamental to the UV fourth harmonic at 266 nm is accomplished by two steps of frequency doubling (in KD*P and KDP). In nanosecond mode, the laser produces =IO ns (fwhm) Q-switched pulses. When operated in picosecond mode, the active/ passive mode-locked system produces pulses of 18 picosecond (fwhm) duration at 1.06 pm. We estimate that the pulse duration at 266 nm is e l 0 ps, although this has not been directly measured. Stimulated Raman shifting in hydrogen gas (18)is used to obtain additional UV wavelengths, specifically, the first anti-Stokes (240 nm) and first Stokes (299 nm) lines of the fourth harmonic of the Nd:YAG. This technique for generating multiple wavelengths of coherent W light from a fixed frequency pump laser offers the advantages of simplicity, ease of operation, and ruggedness compared to tunable dye lasers, and may be more suitable for routine analysis than a dye laser. The details of our Raman shifting technique will be presented in a separate publication (19). A 250 mm focal length lens is used to focus the beam into the ionization region of the mass spectrometer, and the focal point is positioned to intersect the analytes as they elute from the GC. The diameter of the beam at the point of ionization is 4300 pm. The laser pulse energy is measured with a Laser Precision RjP-735 joulemeter mounted on the instrument so as to intercept the beam as it exits the spectrometer.
RESULTS It is important to bear in mind that the purpose of the current study is to observe the effect of laser pulse duration on the relative ionization efficiencies of different molecules. With our current laser system, the amount of light produced in nanosecond mode is considerably greater than in picosecond mode. As a result, the absolute ion signal is invariably larger, and the signal to noise ratio tends to be better, in the longer pulse measurements. Nevertheless, the purpose of the short-pulse experiment is to improve our understanding of why ionization efficiencies vary as they do from one molecule to the next. 240-nm Wavelength. Figure 2 shows laser ionization chromatograms of the polycyclic aromatic hydrocarbon sample recorded by using the 240-nm first anti-Stokes line of the Nd:YAG fourth harmonic. Figure 2A displays the chromatogram obtained with nanosecond light having an average
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RETENTION TIME Figure 2. Chromatograms of polycyclic aromatic hydrocarbon sample at 240-nm wavelength: (A) nanosecond laser ionization, 100 @/pulse, 0.6-pL injection volume (=7 nglcomponent); (B) picosecond laser
ionization, 2 pJlpulse, 2-pL injection volume. Elution order is naphthalene, biphenyl, acenaphthene,fluorene, phenanthrene, anthracene, fluoranthene, pyrene. pulse energy of 100 pJ, Figure 2B shows the chromatogram recorded by using picosecond light a t the same wavelength but a t a significantly lower pulse energy of 2 pJ. Some loss of chromatographic resolution is observed in the picosecond experiment, this being due to a larger injection volume (2.0 pL compared to 0.6 pL in the nano experiment). Note that in the nanosecond chromatogram the area of the biphenyl peak is ==4times smaller than the area of the naphthalene peak. The picosecond result is dramatically different, the biphenyl signal being =13 times larger than the naphthalene response. Fluorene and anthracene also exhibit enhanced ionization efficiency with the picosecond pulses in comparison with naphthalene. 299-nm Wavelength. The laser ionization chromatograms of the polycyclic aromatic hydrocarbon sample obtained by using the Stokes Raman shifted light a t 299 nm are shown in Figure 3. Again, Figure 3A gives the nanosecond data acquired by using a laser pulse energy of 616 pJ, while Figure 3B shows the chromatogram obtained in the picosecond experiment at an average pulse energy of 52 pJ. Note that the chromatograms at this wavelength appear completely different from those recorded with the 240-nm light, demonstrating that molecular selectivity can be obtained by prudent selection of laser wavelength, even without using supersonic beam sample introduction to reduce spectral congestion. Phenanthrene and anthracene, both of which ionize well a t 240 nm, yield very small ion signals for both picosecond and nanosecond pulse durations at this wavelength, as do both naphthalene and biphenyl. By comparison with naphthalene, fluorene and fluoranthene appear to ionize much more efficiently with picosecond light. The relative ionization efficiencies of acenaphthene and fluorene seem to reverse on going from nanosecond to picosecond conditions, and this observation will be discussed in the following section. 266-nm Wavelength. Chromatograms obtained by using the fourth harmonic of the Nd:YAG laser a t 266 nm are presented in Figure 4. The nanosecond result, with an average pulse energy of 710 pJ, is shown in Figure 4A, while the pi-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 23, DECEMBER 1, 1989
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RETENTION TIME Figure 3. Chromatograms of polycyclic aromatic hydrocarbon sample at 299-nm wavelength: (A) nanosecond laser ionization, 616 dlpulse, 0.6-pL injection volume: (B) picosecond laser ionization, 52 pJ/pulse, 0.6-pL injection volume. The elution order is the same as in Figure 2.
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RETENTION TIME Figure 4. Chromatograms of polycyclic aromatic hydrocarbon sample at 266-nm wavelength: (A) nanosecond laser ionization, 71 d/pulse, 0.6-pL injection volume; (B) picosecond laser ionization, 41 pJ/pulse, 0.6-pL injection volume. Elution order is the same as in Figure 2, although retention times are not identical due to use of a dlfferentGC
temperature program.
cosecond chromatogram, recorded by using 410 cJ/pulse laser energy, is shown in Figure 4B. The nanosecond results presented here are somewhat different than those reported by Gross for a few of these analytes (7). It is not clear why this is the case. While the nanosecond and picosecond results are quite similar at this wavelength, one noteworthy exception is the improvement in fluoranthene ionization yield with shorter light pulses. Since the primary goal of this work was to determine the ionization yield for these molecules as a function of laser pulse width, the full capability of the mass spectrometric detection system was not exploited. Single ion monitoring to improve both sensitivity and selectivity has been demonstrated using laser ionization MS (5,6),but was not employed in this study. Ion signals with flight times corresponding to a mass range of 1-210 amu were integrated to obtain the data presented above. This time window discriminated against scattered laser light and high mass background, although very little of the latter was observed and it was not necessary to perform background subtraction. The amount of ion fragmentation was found to be strongly dependent on the laser intensity. In these experiments, the lowest light intensities were at 240 nm, and in this case fragment ions were responsible for less than 10% of the total ion signal. However, at the 266- and 299-nm wavelengths, more energetic pulses of light were available, particularly under nanosecond conditions, and fragment ions made up 30-9070 of the ion yield. Since the fragments are believed to result from photofragmentation of parent ions, it is appropriate to include them in the total ion sum.
DISCUSSION In order to understand the significance of the data presented in the previous section, it is first necessary to discuss those molecular characteristics that lead to efficient laser ionization. Clearly, in order for a resonant ionization process to occur, it is necessary for the molecules of interest to absorb the incident light. Therefore, investigating the W absorption properties of the various analytes should give an initial indication as to their expected laser ionization propensities. Unfortunately, while solution-phase UV absorption spectra
of the analytes involved in this study are well-known (17), gas-phase spectra for most have not been published. T o measure accurate molar extinction coefficients of large gasphase molecules generally requires a heated gas absorption cell. The higher temperatures needed to attain sufficient vapor pressure for such determinations typically lead to a significant amount of thermal congestion. This results in spectra that are not much better resolved than they are in solution. Furthermore, without accurate knowledge of the vapor pressure curve for a compound, information about the temperature of a heated cell is not sufficient to determine the sample concentration. In order to determine gas-phase extinction coefficients for the species in our experiment, we employ a hybrid approach. We have recorded low-resolution (=l A) gas-phase absorption spectra for each of the eight polycyclic aromatic hydrocarbons. The general appearance of these spectra is very similar to the published solution, spectra; that is, the bands are of approximately the Same width and shape. However, all are shifted by 6 to 18 nm from their solution counterparts. For this reason, we use the measured liquid phase extinction coefficients in conjunction with the shifted gas phase wavelength measurements. From the 240-nm data, it is clear that the picosecond result is dramatically different than the nanosecond result. In particular, we focus on biphenyl. In the nanosecond experiment, biphenyl ionizes poorly compared to naphthalene, while with the shorter light pulse biphenyl ionizes very well compared to naphthalene. The current nanosecond result is similar to earlier data obtained by using a 10-ns pulse KrF excimer laser ( 2 , 4 ) whose wavelength i s 248 nm. Using the method outlined above for determining gas-phase extinction coefficients, we find that biphenyl absorbs about 10 times more strongly than naphthalene a t both the KrF wavelength and 240 nm. This number correlates well with the observed chromatographic peak ratios in Figure 2B. We therefore believe that biphenyl's poor ionization efficiency in the nanosecond experiment is the result of rapid excited-state relaxation. There are several additional pieces of data to support
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this interpretation. It is well-known to physical organic chemists that molecules with “floppy” substituent groups, that is, groups that are free to rotate and vibrate with respect to the rest of the molecule, often display fast relaxation characteristics (20). The quantum yield of fluorescence for biphenyl is 0.13, which also suggests rapid nonradiative decay (21). In additon, although the radiative lifetime of biphenyl in solution is on the order of several nanoseconds ( l a ,lifetime measurements in the gas phase using a 2-ns dye laser pulse suggest that the lifetime is much shorter, perhaps on the order of hundreds of picoseconds (22). The fact that biphenyl ionizes well with picosecond light pulses establishes a lower bound on the lifetime of 10 ps. Other molecules, including fluorene and anthracene, also show improved ionization efficiency relative to naphthalene using the shorter light pulses. These species also have short radiative lifetimes in solution and are therefore also likely to be experiencing rapid excited-state relaxation. The 299-nm data also exhibit some interesting effects. Phenanthrene and anthracene, both of which ionize very well at 240 nm, ionize only sparingly at 299 nm due to their small extinction coefficients in this region of the spectrum. This observation is true for both nanosecond and picosecond experiments. The exceptional case, fluoranthene, has a short fluorescence lifetime in solution and has a nonradiative relaxation rate that is 4 times greater than its radiative rate (16). Its ionization characteristics should be quite analogous to those of biphenyl as described above. The dramatic change in relative ion yields for acenaphthene and fluorene on going from nanosecond to picosecond conditions merits further comment. In the picosecond experiment, the ratio of the chromatographic peak areas is the same as the ratio of the extinction coefficients, as one might most simply predict. It is noteworthy that in this case the laser pulse energy is rather small (52 KJ). In the nanosecond experiment, the pulse energy is substanand the reversal of the peak heights may tially higher (616 jd), be indicative of transition saturation effects. When saturation occurs, all of the strongly absorbing molecules irradiated by the laser beam are converted to ions, and the relative ionization efficiencies for all of the other species change. complicating the interpretation of the data. The analytical utility of this technique would be improved if a quantitative model could predict laser ionization efficiencies of various compounds. An initial formulation will now be presented, in which we assume that the ionization efficiency of a molecule depends primarily on four factors: [ 11 ol,the cross section for absorption of the first photon which excites the molecule from the ground electronic state into an electronically excited intermediate state; [2] u2, the cross section for absorption of the second photon which pumps the excited molecule into the ionization continuum; [3] T, the lifetime of the electronically excited intermediate state; and [4] the laser fluence, which affects the rate of stimulated absorption. We assume that the time for the absorption of both excitation and ionization photons, which is less than or equal to the temporal profile of the laser pulse ( T ~ )is, short compared to the lifetime of the resonant excited state ( s
