Atmospheric Pressure Laser Ionization. An Analytical Technique for

Barnes, P. J.; Belvisi, M. G. Thorax 1993, 48, 1034−1043. There is no ... Loveless, M. O.; Phillips, C. R.; Giraud, G. D.; Holden, W. E. Thorax 1997...
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Anal. Chem. 1999, 71, 3721-3729

Atmospheric Pressure Laser Ionization. An Analytical Technique for Highly Selective Detection of Ultralow Concentrations in the Gas Phase S. Schmidt, M. F. Appel, R. M. Garnica, R. N. Schindler,† and Th. Benter*

Department of Chemistry, University of California, Irvine, Irvine, California 92697-2025

In this contribution a new analytical technique is presented for the direct mass spectrometric (MS) detection of gas-phase trace species at atmospheric pressure. Employing resonance-enhanced multiphoton ionization (REMPI) close to the inlet nozzle orifice, i.e., at high molecule densities, the sensitivity of the instrument has been increased by up to 3 orders of magnitude as compared to conventional REMPI-MS with ionization in a differentially coupled ion source. Furthermore, adiabatic cooling, resonant ionization, and mass-selective detection establish a highly selective analytical technique. Several atmospherically relevant compounds were investigated. The current detection limit for NO is 0.9 pptv, for acetaldehyde 1.7 pptv, for CO 15 pptv, and for 2,5dichlorotoluene 12 pptv. We discuss APLI with regard to applications in medical and environmental research. Gaseous trace components play key roles in a variety of chemical and physical processes. In terms of environmental concerns, it is important to describe and understand the production, transport, transformation, and removal of substances from the atmosphere. Process control, e.g., for operation of municipal waste incinerators, can prevent or minimize production of toxic substances such as dioxins.1-4 In medical research it was recently shown that time-resolved analysis of disease-related trace components in the exhaled air of humans (e.g., NO) may substantially improve noninvasive medical diagnostic techniques.5-8 * To whom correspondence should be addressed: (e-mail) [email protected]; (fax) (949) 824 3168. † Institut fu ¨ r Physikalische Chemie, Universita¨t Kiel, Ludewig-Meyn Strasse 8, D-24098 Kiel, Germany. (1) Tanada, T. N.; Velazquez, J.; Hemmi, N.; Cool, T. A. Combust. Sci. Technol. 1994, 101, 333-348. (2) Taylor, P. H.; Dellinger, B.; Lee, C. C. Environ. Sci. Technol. 1990, 24, 316-328. (3) Tanada, T. N.; Velazquez, J.; Hemmi, N.; Cool, T. A. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 1516-1527. (4) Oser, H.; Thanner, R.; Grotheer, H.-H. Chemosphere 1998, 37, 2361-2374. (5) Kharitonov, S.; Alving, K.; Barnes, P. J. Eur. Respir. J. 1997, 10, 16831693. (6) Barnes, P. J.; Belvisi, M. G. Thorax 1993, 48, 1034-1043. (7) Loveless, M. O.; Phillips, C. R.; Giraud, G. D.; Holden, W. E. Thorax 1997, 52, 185-186. (8) Baraldi E.; Azzolin, N. M.; Carra, S.; Dario, C.; Marchesini, L.; Zacchello, F. Respir. Med. 1998, 92, 558-561. 10.1021/ac9901900 CCC: $18.00 Published on Web 07/24/1999

© 1999 American Chemical Society

The determination of gas-phase trace components in complex mixtures is an ongoing challenge for the skills of analytical chemists. In present-day practice, a variety of techniques are used, ranging from sampling on liquid absorbers to in situ detection of unstable isotopes (for recent compilations see, e.g., refs 9-11). Analytical instrumentation includes but is not limited to absorption spectrometers, luminescence detectors, chromatographic devices, charged particle detectors, etc. The quality of a quantitative measurement depends on the following: (i) the temporal resolution, i.e., the overall instrumental response time with respect to concentration changes in the analyte, (ii) the selectivity, i.e., detection of individually selected species or detection of “component families”, (iii) the instrumental sensitivity toward concentration changes, and (iv) the detection limit. Modern in situ applications require instrumental response times ranging from several minutes, e.g., for the detection of atmospheric trace constituents in remote areas, to the millisecond regime for motorcycle resolved analysis of engine exhaust. Relevant “trace” gas concentrations range from parts per million (volume fraction, ppmv) to or below the part per trillion (pptv) level. With respect to sensitivity and versatility, mass spectrometry has proven to be a powerful analytical technique. The most popular method of ionization is electron impact (EI). The advantages of this multipurpose technique stand opposite to the lack of selectivity during ionization. Furthermore, fragmentation processes render the interpretation of EI mass spectra more difficult. The latter may be overcome by reducing the electron energy close to respective ionization potentials, but this generally results in substantial loss of sensitivity. Furthermore, operation of hot filament driven EI sources at elevated pressures is restricted. Alternative ionization methods, such as resonance-enhanced multiphoton ionization (REMPI) or the various types of chemical ionization schemes, e.g., atmospheric pressure ionization (API)12,13 and the ion flow tube technique,14,15 may well elude the limitations (9) Fox, D. L. Anal. Chem. 1997, 69, 1R-13R. (10) Clement, R. E.; Yang, P. W.; Koester, C. J. Anal. Chem. 1997, 69, 251R 287R. (11) Hill, H. H.; Lopez-Avila, V. Anal. Chem. 1997, 69, 289R-305R. (12) Siefering, K.; Berger, H.; Whitlock, W. J. Vac. Sci. Technol. A 1993, 11, 1593-1597. (13) Spicer, C. W.; Chapman, E. G.; Finlayson-Pitts, B. J.; Plastridge, R. A., Hubbe, J. M.; Fast, J. D.; Berkowitz, C. M. Nature 1998, 394, 353-365. (14) Spanel, P.; Smith, D. Int. J. Mass Spectrom. Ion Processes 1997, 167/168, 375-388.

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of EI. With respect to laser techniques, resonant ionization has several advantages, such as (i) high selectivity of the ionization process, particularly in combination with supersonic gas jet expansions, and (ii) for a number of species, efficient REMPI pathways are known leading to high instrumental sensitivities and low detection limits (for recent compilations, see e.g., refs 1618). Conventional pulsed inlet systems for mass spectrometric investigations generally consist of a gated valve incorporated in a pumping stage, which is differentially coupled to the analyzer. In numerous studies, these systems have been used for spectroscopic investigations. Most commonly, a supersonic gas expansion is skimmed and ionization occurs further downstream in the ion source. Depending on the actual arrangement, the nozzle orificelaser beam distance is in the order of several centimeters. Recently, Oser et al.4,19 used a different approach to reduce the nozzle-ionization region distance. They used a cross-beam type setup. Here, the jet is directly expanding into the ionization chamber without use of any skimming devices. Nozzle-laser beam distances were decreased to 20 mm; i.e., ionization occurred just behind the continuous portion of the expansion (“jet”) and consequently their technique was called “jet-REMPI”. Another approach employing resonant ionization was reported by Lee et al.20 These authors developed a NO analyzer based on selective (1 + 1) REMPI. The generated NO+ photoions were detected without further mass selection directly by means of a channel electron multiplier. For NO, a detection limit as low as 16 pptv has been determined. However, the authors also noted background signal fluctuations, which interfered with the NO measurement, particularly at low concentration levels. Hippler and Pfab21 reported on the two-color REMPI of jet-cooled NO for ultratrace detection using an ionization cell equipped with biased Pt electrodes. In ref 17, Pfab gives a limit of detection of 50 pptv for this technique. In this paper, we report the development of an atmospheric pressure laser ionization (APLI) source which is based on REMPI in pulsed gas expansions ultimately close to the expansion origin. The incorporation of the gated valve as an integral part of the ion source enabled us to use nozzle-laser beam distances of g0.2 mm. Thus, ionization occurred in the high-pressure continuous region of the gas expansion where the loss in beam density is negligible compared to conventional cross-beam sources. In addition, photoions are extracted from the ionization region within the propagation direction of the expanding neutral gas jet, which restrains ion-molecule interactions. A comparable setup has been described very recently by Imasaka22 but no quantitative information on the obtained detection limits was given. (15) Lindinger, W.; Hansel, A.; Jordan, A. Int. J. Mass Spectrom. Ion Processes 1998, 173, 191-241. (16) Ashfold M. N. R.; Howe J. D. Annu. Rev. Phys. Chem. 1994, 45, 57-82. (17) Pfab, J. Laser-Induced Fluorescence and Ionization Spectroscopy of GasPhase Species. In Spectroscopy in Environmental Science; Clark, R. J. H., Hester, R. E., Eds.; John Wiley & Sons Ltd.: New York, 1995; Chapter 4. (18) Smith, S. J., Eberly, J. H., Gallagher, J. W., Eds.; Multiphoton Bibliography; NBS Publication LP-92; 1984, 1989, 1990; Suppl. 4-6. (19) Oser, H.; Thanner, R.; Grotheer, H.-H. Combust. Sci. and Technol. 1996, 116/117, 567-582. (20) Lee, S.-H.; Hirokawa, J.; Kajii, Y.; Akimoto, H. Rev. Sci. Instrum. 1997, 68, 2891-2897. (21) Hippler, M.; Pfab, J. Chem. Phys. Lett. 1995, 243, 500-505.

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With APLI, we obtain detection limits in the low-pptv region for nitric oxide, carbon monoxide, acetaldehyde, and several aromatic hydrocarbons sampled directly from ambient air. Only very minor interferences from other atmospheric constituents being present in large excess were noted. EXPERIMENTAL SECTION The major goal of our research was to significantly enhance the REMPI detection limit in mass spectrometric applications. The general approach was a shift of the ionization volume to the highpressure continuous-flow regime of the expansion where high particle densities predominate. Second, the sampling stage should allow for measurements of reactive or thermally unstable species. Thus, commercially available “dead-end” gated valves as well as the use of any metal surfaces in the inlet system were not applicable. All experiments described were conducted using time-of-flight (TOF) mass spectrometers. This type of mass-selective device is preferentially used in combination with laser ionization techniques, since complete mass spectra are acquired for each ionization event. In addition, it should be pointed out that the ion source described here can be easily adapted to both scanning and pulsed mass spectrometers for a broad analytical use. The ion source consists of a home-built gated valve used as combined gas inlet-ion repeller and a conical extraction electrode. The extraction electrode additionally serves as neutral gas skimmer, which minimizes ion-molecule interactions at the entrance port to the mass analyzer. Figure 1 shows a schematic diagram of the experimental setup used for all quantitative results reported in this paper. Gated Valve. Gated valves frequently consist of a stainless steel body, which incorporates a solenoid-driven plunger. Commercial availability and numerous valve designs allow for a variety of applications and also enable experiments at elevated temperatures. However, for in situ measurements with a temporal resolution in the subsecond regime, these devices are not suitable since fill-pump-refill cycles are time-consuming steps. Furthermore, the sample resides inside the “dead volume” of the valve for at least several seconds. Within this time scale, reactive species such as radicals will largely react away at the surfaces. To overcome these limitations we have developed a new gated valve. It was incorporated into a stainless steel vacuum chamber (ion source) which was differentially coupled to the analyzer section (cf. Figure 1). The valve consists mainly of a dual-ported ceramic Marcor body, a Teflon membrane sealed solenoid, a movable stainless steel plunger, and a stainless steel face plate. This plate was coated toward the high-pressure sampling chamber with a Teflon film (Weicon Teflon Spray, Weidling & Sohn), which was sprayed onto the surface and annealed at 80 °C for 2 h. In separate experiments it was shown that stainless steel surfaces treated in this manner essentially showed no reactivity toward reactive species such as molecular chlorine or fluorine, gaseous nitric acid, or hydrochloric acid. The body of the valve contained two bores with an inner diameter of 6 mm that served as inlet and outlet ports. Thus it was possible to flow a carrier gas through the sampling chamber containing the sample at flow rates as high as 100 L bar h-1. The solenoid was sealed off from the sampling section using a Teflon membrane of 0.5 mm thickness. A Viton (22) Imasaka, T. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3534, 573-581.

Figure 1. Experimental setup used for the majority of the experiments in this study. All results summarized in Table 1 were obtained using this setup. Key: SHG, second harmonic generation stage; L, lens; PV, pulsed valve, arrows indicate gas flow; IS, ion source; IO, ion optics; ref, reflectron; MCP, multichannel plate detector; trr, transient recorder; TP, turbo molecular pump; PC, personal computer used as terminal and display for the acquisition computer.

tip equipped stainless steel plunger sealed the nozzle orifice in the closed position. The surface was treated as described above. For all experiments, a face plate with a nozzle orifice diameter of 200 µm was used. The solenoid chamber was connected to the outlet port of the valve, ensuring a proper operation with respect to relative pulse length and opening time. As a result, additional plunger driving forces due to different pressures in the solenoid and sampling chamber were avoided. An additional O-ring sealed electrical feed-through was used to apply variable ion-repelling potentials (maximum + 5 kV) to the face plate. In most runs, the repeller voltage was set to 600 ( 150V. Typical background pressures in the ion source of 98 vol %), were diluted with argon after several pump-freeze-degassing cycles. Nitrogen monoxide was obtained from a steel cylinder (Messer Griesheim, >97 vol %) and roughly purified from nitrogen dioxide contaminations by flowing the gas through a 30-cm glass column filled with iron(II) sulfate. The resulting colorless gas was used after several pump-freeze cycles for the experiments. NO2 was obtained from Aldrich (>98 vol %). To eliminate any NO or N2O3 contamination, 200 mbar of NO2 was pressurized with 800 mbar of oxygen and the mixture was allowed to react overnight. After several pumpfreeze cycles at 200 K, a pure white crystalline powder was finally recovered from the gas phase. NO and NO2 concentrations were monitored in the carrier gas flow with a commercial NOx luminescence analyzer (Monitor Labs, model 8840) which was coupled to the downstream port of the gated valve. The analyzer had a detection limit of 1 ppbv. Synthetic air (N2/O2, 4:1) and argon carrier gases were obtained from Messer Griesheim with a stated purity of 5.0. RESULTS AND DISCUSSION For a critical assessment of the performance of the APLI source, several key parameters related to the use of REMPI as the ionization technique were addressed in separate experiments. (i) The optical resolution of the photoionization spectra was used as a measure of the cooling efficiency in the continuous region of the gas expansion and compared to results that were obtained with the conventional differentially pumped skimmed molecular beam technique. (ii) The effective ionization volume was determined in experiments in which the overall order of the REMPI transition was varied, e.g., (1 + 1) or (2 + 1) processes. The latter involves excitation of the resonant state through a virtual transition. Thus, strongly enhanced laser power densities were needed for efficient photoion production. In this case, the laser beam had to be tightly focused, which also reduced the effective ionization volume. (iii) To assess the extent of ion-molecule and ion-ion reactions in the very early stage of the gas expansion, changes of the relative peak abundances and mass resolution in the acquired spectra were closely examined as a function of laser power density. Also, the instrument response toward extensive unimolecular fragmentation processes in the high-pressure excitation zone was studied. To investigate each parameter individually, selected chemical species were employed for the different experiments. (1) Selectivity of the Ionization Process: REMPI of NO. Nitric oxide has a comparably low ionization potential of 9.26 eV.24 (24) Reiser, G.; Habenicht, W.; Mu ¨ ller-Dethlefs, K.; Schlag, E. W. Chem. Phys. Lett. 1988, 152, 119-123.

3724 Analytical Chemistry, Vol. 71, No. 17, September 1, 1999

Figure 2. Typical medium-resolution REMPI spectrum of nitric oxide obtained with the APLI source. Excitation occurred 2 mm downstream of the nozzle. Laser step width, 0.005 nm.

The first electronic A state has a lifetime of ∼220 ns,25 which leads to a high probability of absorbing further photons from this level in intense laser fields. NO does not photodecompose into neutral atoms when irradiated below 191 nm.26 Furthermore, it has a threshold fragmentation energy of 20.12 eV for the N + O+ channel.27 Unimolecular decomposition of NO+ due to absorption of further photons is unlikely even when high photon densities are used. This behavior makes NO an ideal molecule for the study of REMPI processes. This is reflected in the large number of reports available in the literature in which two-color (1 + 1′)21,28 as well as one-color (1 + 1)25,29 (2 + 1),30 (3 + 1), (2 + 2),31 and nonresonant excitation has been described. We have conducted (1 + 1) REMPI experiments to investigate the cooling properties of the expanding gas pulse close to the nozzle orifice and to study the effect of relatively large ionization volumes on the mass resolution of the analyzer. In Figure 2 is shown a (1 + 1) REMPI spectrum of NO recorded at parent mass 30 in the wavelength range 224 nm < λ < 227 nm covering the heads of the A 2Σ+ r X 2Π1/2 and A 2Σ+ r X 2Π3/2 subbands. A sample concentration of 50 ppbv was present in a flow of argon at 1 atm total pressure. All experiments were carried out at a fixed nozzle-laser beam distance of 1 mm. In comparison with the results reported by Lee et al.,20 we note significant discrepancies in the relative REMPI signal intensities in the wavelength range 225-227 nm. Figure 3 illustrates the incompatibility of our results with the latter. For their results, shown in Figure 3a, Lee and co-workers inferred a rotational temperature of Trot ∼ 30 K at a nozzle-laser beam distance of 22.5 mm. By comparison with the simulated (1 + 1) REMPI spectrum given in Figure 3b, we arrive at a value for Trot that is close to ambient temperatures. The spectrum in Figure 3b was calculated by assuming a rotational temperature of Trot ) 300 K. (25) Zacharias, H.; Halpern, J. B.; Welge, K. H. Chem. Phys. Lett. 1976, 43, 4144. (26) Calvert, G. J.; Pitts, J. N. Photochemistry; John Wiley & Sons: New York, 1966. (27) Erman, P.; Karawajczyk, A.; Rachlew-Kallne, E.; Stromholm, C. J. Chem. Phys. 1995, 102, 3064-3076. (28) Rottke, H.; Zacharias, H. J. Chem. Phys. 1985, 83, 4831-4844. (29) Zacharias, H.; Anders, A.; Halpern, J. B.; Welge, K. H. Opt. Commun. 1976, 19, 116-119. (30) Guizard, S.; Chapoulard, D.; Horani, M.; Gauyacq, D. Appl. Phys. B 1989, 48, 471-477. (31) Zakheim, D.; Johnson, P. J. Chem. Phys. 1978, 68, 3644-3653.

of the Mach disk location or jet terminus xM, and the translational temperature Tt,i of the gas at a distance i from the nozzle. Tt,i is obtained from

Tt,i γ -1 2 ) 1+ Mi T0 2

(

Figure 3. Comparison of the experimental NO REMPI spectrum with the literature and simulated data. (a) Spectrum obtained by Lee et al. (b) Simulated spectrum, assuming a rotational temperature of Trot ) 300 K. (c) Room-temperature spectrum reported by Jacobs et al. (d) Simulated spectrum, Trot ) 40 K. (e) Experimental spectrum from this work, cf. Figure 2.

This estimate is further supported by experimental results obtained by Jacobs et al., shown in Figure 3c. Their roomtemperature (1 + 1) REMPI spectrum is nearly identical to the results reported by Lee and co-workers and our simulated spectrum (Trot ) 300 K). On the other hand, excellent agreement is found between the simulated spectrum plotted in Figure 3d and our experimental spectrum, which is shown for comparison in Figure 3e. The best agreement between the theoretical and experimental spectrum resulted when a rotational temperature of 40 K was used for the simulation. For all calculations, molecular parameters given in refs 32-34 were used. It is very likely that the REMPI signals in the spectrum given by Lee et al. originated predominantly from resonantly ionized background NO molecules, which were in thermal equilibrium with the cell walls. We conclude from this discussion that, even at the relatively small nozzle-ionization region distance of 1 mm in our setup, efficient rotational cooling has already occurred. To further elucidate this efficiency, we use the well-known thermodynamic approach to describe gas expansions. A thorough account of the theory and practical application of supersonic beam expansions is given in the excellent review edited by Scoles.35 A compilation of practical equations can be found in ref 36. Useful quantities to characterize a supersonic jet expansion are the Mach number M, the terminal position of the continuous flow region xT, the position (32) Heath, B. A.; Robin, M. B.; Kuebler, N. A.; Fisanick, G. J.; Eichelberger, T. S. J. Chem. Phys. 1980, 72, 5565-5570. (33) Erman, P.; Karawajczyk, A.; Rachlew-Kallne, E.; Stromholm, C.; Larsson, J.; Persson, A.; Zerne, R. Chem. Phys. Lett. 1993, 215, 173-178. (34) Mallard, W. G.; Miller, J. H.; Smyth, K. C. J. Chem. Phys. 1982, 76, 34833492. (35) Scoles, G., Ed. Atomic and Molecular Beam Methods; Oxford University Press: Oxford, 1988. (36) Goodman, L.; Philis, J. Multiphoton Absorption Spectroscopy. In Applied Laser Spectroscopy: Techniques, Instrumentation, and Applications; Andrews, D. L., Ed.; VCH Publishers: New York, 1992.

-1

)

(1)

with T0 being the stagnation temperature of the gas. Using (eq 1) together with data given in refs 35 and 36, the following values are calculated for our setup, with typical conditions of P0 ) 1 atm, P ) 10-8 atm, D ) 0.02 cm: xm > 100 cm, MT ≈ 28, xT ≈ 0.5 cm and Tt ≈ 1 K (Assumptions made are monatomic ideal gas, γ ) 5/3 ) constant, and isentropic and continuum flow.) However, the ionization region is closer to the nozzle than xT. At this position, x ) 0.1 cm, M ≈ 9, and Tt ≈ 10 K. Although the rapid rotational-translational equilibrium makes relatively low rotational temperatures possible, the rotational temperature Tr actually achieved is always higher than Tt. A rough estimate of Tr ) 30 K is calculated from the data given in ref 35. This value is consistent with the relative rotational line intensities in our experimental NO REMPI spectrum and also with the calculated spectrum (cf. Figure 3). The power dependence of the NO REMPI signal at λ ) 226.25 nm [Q11 + P21(1/2) line of the A 2Σ r X2Π1/2(0,0) transition] in the range 50-500 µJ exhibited a nearly linear correlation. A slope of 1.04 ( 0.06 (2σ) was calculated from the data. This result indicates that under the present experimental conditions, i.e., moderately focused laser beam and pulse energies, the first excitation step from the ground state to the intermediate A state is almost fully saturated (see, e.g., ref 36 or 37 and references herein). In contrast to Lee et al. and Jacobs et al., who observed only “partial saturation” of the A state using unfocused laser radiation, the monitoring of the laser pulse energy in our setup is far less critical. Jacobs et al.38 and also Lee et al. have suggested that problems may arise from the application of a focused laser beam, which are caused by large gradients of laser intensity in the ionization region. We do not see any interferences related to the moderately focused laser beam. It should be noted that the effective ionization volume in our setup has an upper limit of 1 mm3 whereas Lee et al. estimated a 5000-mm3 excitation region for their instrument. The advantage of having installed a mass-selective detector enabled us to monitor NO+ ion signals at m/z ) 31 (mainly 15N16O) and m/z ) 32 (14N18O) together with m/z ) 30 (14N16O). Figure 4 shows the expanded region of the NO (1 + 1) REMPI spectrum in the region 226.10-226.40 nm. Signals of m/z 30, 31, and 32 were recorded simultaneously. A concentration of 20 ppbv NO with natural isotopic abundance was present in a flow of argon. Using an isotopic distribution of 16O:17O:18O ) 1:0.0004:0.002 and 14N:15N ) 1:0.0036 39 the abundances of the differently labeled nitric oxide molecules are calculated as 14N16O:15N16O:14N18O ) 1:0.004:0.002. Since [NO] ) 20 ppbv, the respective concentrations (37) Lin, S. H.; Fujimura, Y.; Neusser, H. J.; Schlag, E. W. Multiphoton Spectroscopy of Molecules; Academic Press Inc.: London, 1984. (38) Jacobs, D. C.; Madix, R. J.; Zare, R. N. J. Chem. Phys. 1986, 85, 54695479. (39) De Laeter, J. R.; Heumann, K. G.; Rosman, K. J. M. J. Phys. Chem. Ref. Data 1991, 20, 1327-1337.

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Figure 4. Mass-selected REMPI spectra of nitric oxide with the isotopic composition 14N16O, 15N16O, and 14N18O. The signals recorded at m/z ) 31 and m/z ) 32 are enhanced by a factor of 400 and 600, respectively. [14N16O] ) 20 ppbv, leading to a calculated [15N16O] of 80 ppt and [14N18O] of 40 ppt.

of 15N16O and 14N18O are 80 and 40 pptv. Even at this low concentration level, all features of the REMPI spectrum shown in Figure 2 can be reproduced for the isotopically labeled species. We observed a shift of the absorption lines to the red, with increasing mass. Very recently, Wang et al.40 measured the UV absorption lines of NO with the isotopic composition 14N16O, 15N16O, and 14N18O. Excellent agreement is found between the absorption and REMPI data reported in this paper. Although 14N16O was present in 500-fold excess over 14N18O, no mass peak overlap occurred. Both high spectral selectivity and mass resolution of the instrument strongly suppressed the 14N16O signals. This finding clearly shows that ionization volumes in the order of 1 mm3 do not significantly reduce the mass resolution of the analyzer (see also below). The detection limit was determined from measurements of m/z ) 32 (16N18O) as a function of the concentration of naturally labeled NO. Thus, it was not necessary to dilute the analyte concentration down to or even below the pptv range. A detection limit (S/N ) 3) of 0.9 pptv (accumulation of 200 laser shots at 10 Hz ) 20 s integration time) is obtained for 16N18O. Compared to the detection limit of 16 pptv reported by Lee et al., this value has been improved roughly by a factor of 20. Using our conventional skimmed molecular beam setup, which has a nozzle-ionization source distance of 55 mm, we were able to detect NO at a concentration limit of 1.1 × 1010 molecule cm-1 or ∼0.4 ppbv. Compared to our result obtained with APLI given above, this detection limit has been increased by a factor of roughly 400. Argon was used as carrier gas in these experiments to avoid any possible reactions of NO with molecular oxygen. It has to be pointed out that no significant changes in the REMPI spectrum, selectivity, or sensitivity were noted upon switching from argon to synthetic air as the carrier gas. Addition of up to 50-fold excess [NO2] did not interfere with the NO measurements. Only a brief explanation is given for this finding, since a detailed discussion of the analysis of NO/NO2 mixtures by APLI MS will follow in an upcoming contribution. The low impact of excess [NO2] on the NO measurements results mainly from two effects. First, the nascent NO fragment from NO2 photolysis at 226 nm is produced in a distribution of excited vibrational states, each of which is also (40) Wang, D. X.; Haridas, C.; Reddy, S. P. J. Mol. Spectrosc. 1996, 175, 73-84.

3726 Analytical Chemistry, Vol. 71, No. 17, September 1, 1999

Figure 5. (2 + 1) REMPI mass spectrum of acetaldehyde recorded at λ ) 363.35 nm. [CH3CHO] ) 20 ppbv in air.

rotationally highly excited. Grant and co-workers41 demonstrated that the final vibrational state distribution for the one-photon photodissociation is strongly inverted in ν′′ ) 2. The A r X (2,2) transition peaks around 221.5 nm. In a recent paper, Singhal and co-workers42 gave a thorough account for the detection of NO/ NO2 mixtures using an ionization cell and one-color REMPI. Second, the rovibrationally excited NO fragments carry significant amounts of kinetic energy due to the recoil upon photolysis. The ion trajectory calculations performed for the design of the present APLI source revealed that the acceptance angle for ions carrying more than 0.2 eV kinetic energy becomes very small. Thus, the instrument efficiently discriminates between translationally hot ions from thermal ions. (2) Instrumental Response to Fragmentation Processes: (2+1) REMPI of Acetaldehyde. Acetaldehyde was employed for the study of the instrumental response to ionic fragmentation processes in the high-pressure regime. The (2+1) REMPI spectrum of acetaldehyde in the wavelength range 358 nm < λ < 368 nm was first reported by Heath et al..32 In this study a photoionization cell was employed. From the work of Fisanick et al.43 information about the photoionization mass spectrum of CH3CHO is available. In both studies simultaneous absorption of two photons resonantly excited vibrational levels in the 3s Rydberg manifold. From this level, absorption of a third photon led to ionization. Nagel et al.44 reported the (2+1) REMPI spectrum of acetaldehyde at elevated temperatures, Shand et al.45 recorded the (2 + 1) spectrum in the same wavelength range under jetcooling conditions. In contrast to NO, parent acetaldehyde ions absorb further photons and undergo ionic fragmentation processes. “Ladder climbing” and “switching”46 finally lead to the major fragment ion signals at m/z ) 15 (CH3+), 29 (CHO+), and 43 (CH2CHO+), respectively. Figure 5 shows a photoionization mass spectrum that was observed when 5 ppbv acetaldehyde was present in a flow of synthetic air. The laser wavelength was tuned to the strongest (41) Bigio, L.; Tapper, R. S.; Grant, E. R. J. Chem. Phys. 1984, 88, 1271-1273. (42) Peng, W. X.; Ledingham, K. W. D.; Marshall, A.; Singhal, R. P. Analyst 1995, 120, 2537-2542. (43) Fisanick, G. J.; Eichelberger, T. S., IV; Heath, B. A.; Robin, M. B. J. Chem. Phys. 1980, 72, 5571-5580. (44) Nagel, H.; Frey, R.; Boesl, U. SAE Tech. Pap. Ser. 1996, No. 961084. (45) Shand, N. C.; Ning, S.-L.; Pfab, J. Chem. Phys. Lett. 1995, 247, 32-37. (46) Robin, M. B. Appl. Opt. 1980, 19, 3941-3947.

Figure 6. (2 + 1) REMPI spectrum of acetaldehyde, recorded at parent mass m/z ) 44 (CH3CHO+). All other major mass peaks exhibit an identical wavelength dependence in the range 360 < λ < 364 nm (cf. Figure 6).

resonance in this spectral range at λ ) 363.35 nm (cf. Figure 6). Besides fragment ion signals, the mass spectrum also shows relatively strong parent ion signals at m/z ) 44. To further elucidate the fragmentation mechanism (ionic versus neutral fragmentation with subsequent ionization of the photoproducts; see below), REMPI spectra of all major ion signals were recorded. The parent ion signal intensity recorded at m/z ) 44 as function of wavelength is plotted in Figure 6. REMPI spectra for the major fragment ion signals at m/z ) 15, 29, and 43 correspond within experimental uncertainty to the wavelength dependence of the parent ion spectrum. This clearly demonstrates that predominantly parent ion fragmentation leads to the corresponding ion signals.43 The APLI mass spectrum of acetaldehyde is nearly identical to the spectrum reported by Fisanick et al.,43 who used an effusive inlet system in their experiments. We conclude from this result that ion-molecule or ion-ion interactions are only of negligible importance. The APLI source produces a fragment ion distribution that is similar to conventional laser ionization carried out in a classical high-vacuum environment. The detection limit measurements were conducted at 363.35 nm, monitoring the HCO+ ion fragment signal at m/z ) 29 as function of CH3CHO concentration. For a given integration time, the detection limit calculated from

Cl ) kσbl/S

(2)

C is the detection limit, σbl the standard deviation of the blank signal, and S the calibration sensitivity. As argued by Kaiser,47 we used k ) 3. The detection limit for acetaldehyde was obtained from parent ion signal summation of 200 laser shots at a repetition frequency of 10 Hz (t ) 20 s). Therefore, for acetaldehyde we obtained Cl ) 1.7 pptv. (3) ALPI with High Laser Power Densities: (2+1) REMPI of CO. Compared to NO, carbon monoxide has a considerably higher ionization potential of 14.01 eV.33 Since the origin of the first electronic excited A state appears at λ ) 154.5 nm,26 only REMPI processes including virtual states are feasible for effective ionization of CO with conventional tunable dye laser radiation, e.g., (2 + 1) ionization around 230 nm. With respect to fragmenta(47) Kaiser, H. Anal. Chem. 1987, 42, 53A.

Figure 7. (2 + 1) REMPI spectra of carbon monoxide with natural isotopic labeling. Upper trace: wavelength dependence of the 12C16O peak recorded at m/z ) 28. The lower trace was recorded at m/z ) 29 (13C-labeled carbon monoxide) and is enhanced by a factor of 50.

tion processes, the same arguments as for NO apply. Carbon monoxide was used to study resonant ionization including virtual states in more detail, i.e., high laser fields (in the order of 1010 W cm-2) and reduced excitation volumes. Possible interferences with a selective detection are (i) nonresonant ionization of the major compounds nitrogen and oxygen. Since CO and N2 have identical m/z values within the mass resolution of the TOF analyzer, N2 signals would strongly interfere with an accurate detection of CO trace concentrations. (ii) The numerous 2-photon resonances of the 3d Πg rr X 3Σg- series of molecular oxygen in the wavelength range 220 nm < λ < 235 nm48 and also 1-photon resonances of NO could also cause generation of background photoions that possibly affect the CO measurement. Although the mass analyzer separates these ions, simplified experimental arrangements without such a device would not deliver reliable results. Figure 7 shows the strong 2-photon resonance of the B r X (0,0) transition of carbon monoxide of 12C16O and 13C16O isotopic composition. To reach the comparably high ionization potential of 14.01 eV,33 even in an overall 3-photon process, frequencydoubled laser light had to be used. With our laser setup, the maximum pulse energy available was 0.5 mJ in this wavelength region. The laser beam was focused with a 10-cm focal length quartz lens leading to an estimated effective ionization volume that is more than 1 order of magnitude lower compared to the experimental conditions in the NO and acetaldehyde experiments. Carbon monoxide could be easily detected in ambient air. When the laser was scanned off-resonance to 230.00 nm, all signals at m/z ) 28 vanished. Under these conditions no other near- or nonresonant contributions, e.g., from N2, were detected at m/z ) 28. However, a small signal at m/z ) 32, possibly originating from molecular oxygen, was always present. Also, signals at m/z ) 18 and in the mass range 32 < m/z < 300 were detected. This result demonstrates that high photon densities in this wavelength range lead to significant background signal contributions. It clearly turns out that mass-selective devices are mandatory in conjunction with the APLI method. (48) Yokelson, R. J.; Lipert, R. J.; Chupka, W. A. J. Chem. Phys. 1992, 97, 61446152.

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The lack of any background signals at m/z ) 28 enabled us to monitor resonantly ionized CO+ also with high sensitivity. Upon moving the laser beam focus further toward the nozzle orifice, a detection limit of 15 pptv (30-s integration time at a repetition frequency of 10 Hz) was determined 1 mm downstream of the nozzle orifice. The same procedure as described for CH3CHO was applied to calculate the CO detection limit. This result reflects the effect of the substantially smaller effective ionization volume. Looney et al.49 also employed REMPI with in situ mass-selective detection to monitor background CO concentrations in an ultrahigh vacuum environment. These authors report a detection limit of 104 molecule cm-3, which is at least 3 orders of magnitude lower than our current value. However, the experimental parameters were very different. Since background concentrations of carbon monoxide in a vacuum chamber had to be measured, Looney et al. could use the entire focus length of the laser beam to generate CO+ ions. The focus length was ∼30 mm compared to