A Capillary-Based Supersonic Jet Inlet System for Resonance

RSA-0002 Pretoria, Republic of South Africa. A new supersonic jet inlet system for resonance-enhanced multiphoton ionization time-of-flight mass spect...
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Anal. Chem. 2001, 73, 4171-4180

A Capillary-Based Supersonic Jet Inlet System for Resonance-Enhanced Laser Ionization Mass Spectrometry: Principle and First On-line Process Analytical Applications Klaus Hafner,†,‡ Ralf Zimmermann,*,†,‡,§ Egmont R. Rohwer,⊥ Ralph Dorfner,† and Antonius Kettrup†

GSF-National Research Center for Environment and Health, Institute of Ecological Chemistry, Ingolsta¨dter Landstrasse 1, D-85764 Neuherberg, Germany, Institut fu¨r Physik, Universita¨t Augsburg, Universita¨tsstraβe 1, D-86159 Augsburg, Germany, Abteilung Umwelt und Prozeβchemie, BIfA-Bayerisches Institut fu¨r Umweltforschung und -technik, Am Mittleren Moos 46, D-86167 Augsburg, Germany, and Department of Chemistry, University of Pretoria, RSA-0002 Pretoria, Republic of South Africa.

A new supersonic jet inlet system for resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS), based on a fused-silica capillary with an integral nozzle has been developed. The new jet inlet system generates a supersonic molecular beam that originates in the center of the ion source of the time-offlight mass spectrometer. Because of the design of the inlet system, high spatial overlap of sample and laser beam (i.e., increased detection sensitivity) and excellent jet beam qualities are achieved with good adiabatic cooling properties of analyte molecules (i.e., considerably enhanced optical selectivity of the REMPI process). Furthermore, the inlet is very robust and chemically inert and contains no moving parts. As a result of these properties, the new inlet is perfectly suited for field applications of jet-REMPI. A first field application of a mobile supersonic jet-REMPI mass spectrometer equipped with the novel inlet technique is reported; namely, the concentration of monochlorobenzene, which is an indicator for the formation and emission of toxic polychlorinated dibenzo-pdioxins/furans, PCDD/F) was measured on-line in the flue gas of a waste incineration plant. Resonance-enhanced multiphoton ionization (REMPI) with pulsed UV lasers represents a highly sensitive and selective ionization method for mass spectrometry.1-7 Time-of-flight mass * Corresponding author. E-mail: [email protected]. † Institute of Ecological Chemistry. ‡ Universita ¨t Augsburg. § BIfA-Bayerisches Institut fu ¨ r Umweltforschung und -technik. ⊥ University of Pretoria. (1) Boesl, U.; Neusser, H. J.; Schlag, E. W. Z. Naturforsch. 1978, 33a, 15461548. (2) Herrmann, A.; Leutwyler, S.; Schumacher, E.; Wo ¨ste, E. Chem. Phys. Lett. 1977, 52, 418. (3) Letokhov, S. Laser Photoionization Spectroscopy; Academic Press: Orlando, 1987. (4) Lubman, D. M., Ed. Lasers and Mass Spectrometry; Oxford University Press: New York, 1990. (5) Boesl, U. J. Phys. Chem. 1991, 95, 2949-2962. (6) Cool, T. A. In Lasers and Mass Spectrometry; Lubman, D. M., Ed.: New York, 1990. 10.1021/ac010244h CCC: $20.00 Published on Web 08/04/2001

© 2001 American Chemical Society

spectrometry (TOFMS) is ideally suited as mass analyzer for REMPI. The potential of REMPI mass spectrometry for analytical applications was recognized early.8-10 Particularly interesting are applications of the REMPI-TOFMS technique for on-line monitoring of complex gas mixtures such as, for example, combustion flue gases. A first application for monitoring automotive exhausts was reported in 1993.11,12 The first application of a mobile REMPITOFMS unit for monitoring of flue gases of an industrial incinerator was performed in 1996.13,14 The principle of the REMPI-TOFMS technique has been discussed thoroughly in the literature.3-5 Briefly, UV laser pulses (ca. 10 ns pulse duration, some mJ pulse energy, power densities of 106 -107 W/cm2 in the focus) are used for ionization of molecules by simultaneous absorption of two or more photons. The REMPI process usually is applied to ionize molecules from an effusive molecular beam or a supersonic molecular beam (jet) within the ion source of the TOFMS. In the following, only the most simple REMPI scheme, the one color, two-photon ionization process, is considered, which is applicable for detection of, for example, aromatic compounds. In this case, the ionization is performed by two photons of the same wavelength. The necessary condition for the one color, two-photon ionization process is that the total energy of two photons must exceed the ionization energy of the target molecule. The two-photon ionization cross-section is enhanced by several orders of magnitude if excited molecular states are in resonance with the photon energy. Significant ionization yields under usually applied laser power densities of (7) Zimmermann, R.; Weickhardt, C.; Boesl, U.; Lenoir, D.; Schramm, K.-W.; Kettrup, A.; Schlag, E. W. Chemosphere 1994, 29, 1877. (8) Klimcak, C. M.; Wessel, J. E. Anal. Chem. 1980, 52, 1233-1239. (9) Rhodes, G.; Opsal, R. B.; Meek, J. T.; Reilly, J. P. Anal. Chem. 1983, 53, 280. (10) Rohlfing, E. A. In 22nd Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1988; pp 1843-1850. (11) Boesl, U.; Weickhardt, C.; Zimmermann, R.; Schmidt, S.; Nagel, H. SAE Technol. Pap. 1993, 930083, 61-69. (12) Weickhardt, C.; Boesl, U.; Schlag, E. W. Anal. Chem. 1994, 66, 10621069. (13) Zimmermann, R.; Heger, H. J.; Kettrup, A.; Boesl, U. Rapid Commun. Mass Spectrom. 1997, 11, 1095-1102. (14) Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H.; Kettrup, A.; Boesl, U. Anal. Chem. 1999, 71, 46-57.

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ca. 106-107 W/cm-2 are achieved only when the photon energy (i.e., the wavelength) is in resonance with an excited molecular state (i.e., an UV-spectroscopic band). This UV spectroscopic step introduces the selectivity of REMPI that, in a complex mixture of substances, efficiently ionizes only those that fulfill the resonance condition. With intense laser pulses, the REMPI process can be extremely efficient. Ionization efficiencies in the laser spot of some 10% have been reported.15 The ionization efficiency, however, for some species is quenched by intramolecular relaxation processes.16-18 In addition to sensitivity, a soft ionization (without fragmentation) can in most cases be achieved. In summary, the REMPI process provides highly selective ionization, as a result of inclusion of the UV-spectroscopic resonance step, combined with a high ionization yield and low fragmentation rates. The latter properties make REMPI an ideal ionization technique for on-line monitoring of trace target compounds or substance classes in complex mixtures by direct inlet mass spectrometry. However, for many analytes, the selectivity as well as the sensitivity of the REMPI technique depends strongly on the type of molecular beam source used for the inlet of the sample gas. This is obvious, because the temperature of the molecules in the gas phase is influenced by the inlet system, and the temperature of the molecules influences the shape of their UV spectra. If the gas is expanded through a restriction nozzle, a supersonic jet can be formed.19 Within the supersonic jet, molecules undergo adiabatic cooling mediated by collisions between the bulk gas molecules (rare gas, air) and the analyte molecule. The cooling is a nonequilibrium process leading to a molecular beam with isolated molecules, exhibiting different temperatures for the different degrees of freedom. In general, the translational temperature (Ttrans) of the molecules in a supersonic jet is lower than the rotational temperature (Trot), followed by the vibrational temperature (Tvib) (Ttrans < Trot< Tvib). Rotational temperature (Trot) values of only a few K are easily reached in supersonic expansions. As a result of this cooling, narrower UV spectral features and very high optical selectivity of the REMPI ionization process can be achieved. Even UV spectroscopic isomer discrimination is possible.7,16,20,21 The supersonic jet technique is commonly used with sophisticated pulsed-inlet valve systems.5-7,10,21-25 The gas pulses (typical time width ∼ 100 µs) are synchronized with the laser repetition (15) Boesl, U.; Neusser, H. J.; Schlag, E. W. Chem. Phys. 1981, 55, 193-204. (16) Tembreull, R.; Sin, C. H.; Li, P.; Pang, H. M.; Lubman, D. M. Anal. Chem. 1985, 57, 1186-1192. (17) Zimmermann, R.; Lenoir, D.; Kettrup, A.; Nagel, H.; Boesl, U. In 26th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1996; pp 2859-2868. (18) Zimmermann, R. 8th International Symposium on Resonance Ionization Spectroscopy; AIP Conference Series 388; American Institute of Physics, AIP Press: New York (1997) 1996; pp 399-402. (19) Hayes, J. M. Chem. Rev. 1987, 87, 745. (20) Tanada, T. N.; Velazquez, J.; Hemmi, N.; Cool, T. A. Combust. Sci. Technol. 1994, 101, 333-348. (21) Hager, J. W.; Wallace, S. C. Anal. Chem. 1988, 60, 5-10. (22) Tembreull, R.; Lubman, D. M. Anal. Chem. 1984, 56, 1962-1967. (23) Zimmermann, R.; Heger, H. J.; Rohwer, E. R.; Schlag, E. W.; Kettrup, A.; Boesl, U. 8th International Symposium on Resonance Ionization Spectroscopy; AIP Conference Series 388, American Institute of Physics; 1996; 119-122. (24) Oser, H.; Thanner, R.; Grotheer, H.-H. Combust. Sci. Technol. 1996, 116117, 567-582. (25) Heger, H. J.; Zimmermann, R.; Rohwer, E. R.; Dorfner, R.; Boesl, U.; Kettrup, A. J. High Resolut. Chromatogr. 1999, 22, 391-394.

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rate (typically, 10-20 Hz). This procedure increases the duty cycle (i.e., the ratio of the amount of supplied sample gas to the amount of sample gas probed by REMPI) and minimizes the gas load of the vacuum system; however, the commonly used pulsed valve inlets have some serious drawbacks for practical analytical applications. Because of their bulky appearance, pulsed nozzles cannot be mounted between the acceleration plates of the ion source. Thus, large distances, r, between the expansion nozzle and the ionization region are typical (r g 20 mm) in conventional supersonic jet-REMPI-TOFMS setups. Because of the large distance, r, between the nozzle and the ionization region, the density of the sample gas pulses δ, which is proportional to the inverse of the square of the distance r (δ ∼ r-2), is reduced considerably. Furthermore the nozzles need to be heated for several analytical applications, such as, for example, gas chromatography couplings,26 spectroscopy of semivolatile compounds, or for direct inlet of complex, “dirty” gases, for example, for on-line analysis of coffee-roasting off-gas.27 At higher temperatures, pulsed jet nozzles tend to lose reliability and require permanent careful readjustment; thus, it is sometimes difficult even in the laboratory to perform analytical measurements with this types of pulsed inlet nozzles. For industrial process analytical applications, the situation is worse. Because of these problems with conventional supersonic jet nozzles, in our group, an alternative molecular beam inlet technique was used for industrial REMPI-TOFMS process monitoring projects. This technique is based on a capillary for generation of an effusive molecular beam. The technique has been described thoroughly in the literature.13,14 Briefly, a heated hollow needle (made of, e.g., stainless steel) reaches directly into the center of the TOFMS ion source (i.e., between the repeller and the first extraction electrode). Within the hollow needle runs a fused-silica capillary with chemically deactivated inner walls. The capillary itself acts as the restrictor between the vacuum of the ion source (10-5 Torr) and ambient pressure of the sampling system. A gas flow of ∼2-10 mL/min is drawn through the fused-silica capillary by the ion source vacuum. An effusive molecular beam is formed at the end of the capillary into the center of the ion source.28 The ionizing laser pulses cross the effusive molecular beam directly underneath the capillary tip (r ∼ 1-2 mm), and the formed ions are extracted. Because of the narrow design of the inlet needle and selection of its electric potential to match that of the parallel potential field contours in the ion source, disturbances of the electrostatic fields in the TOFMS ion source are kept low. Sensitivities in the low parts-per-trillion volume region have been reached with this type of inlet and, as a result of the simple setup and constantly running flow through the quartz capillary, memory contamination in the inlet system is minimized. In summary, the capillary-based, continuous effusive molecular beam inlet is reliable and rugged and has been applied to REMPI-TOFMS measurement campaigns at industrial incinerators and food processing units.13,14 The major drawback of the technique is the loss of optical selectivity when compared to the jet inlet technique. Because there is no expansion via a restriction nozzle, the analyte molecules in (26) Zimmermann, R.; Boesl, U.; Heger, H. J.; Rohwer, E. R.; Ortner, E. K.; Schlag, E. W.; Kettrup, A. J. High Resolut. Chromatogr. 1997, 20, 461-470. (27) Zimmermann, R.; Heger, H. J.; Yeretzian, C.; Nagel, H.; Boesl, U. Rapid Commun. Mass Spectrom. 1996, 10, 1975-1979. (28) Wegner, P. P., Ed. Molecular Beams and Low-Density Gas Dynamics; Marcel Dekker: New York, 1974.

Figure 1. Setup of the capillary-based micro nozzle in the ion source of the TOF mass spectrometer. The laser focus is ∼1 mm (∼20× orifice diameter) below the microjet orifice. At this position, the cooling properties are fully developed.

the effusive molecular beam are not cooled adiabatically. For warm molecules, only a reduced optical selectivity of the REMPI process is achievable. In this paper, a new supersonic molecular microbeam source is presented that combines the advantages of the conventional gated supersonic jet valve technique and the simple effusive, continuous capillary-based inlet. The heart of the supersonic jet inlet microsystem is a fused-silica capillary having a conically tapered restriction nozzle at its tip. The restriction nozzles exhibit a perfectly smooth quartz glass surface. With this type of nozzle, a continuous supersonic jet expansion of high quality can be generated at very low carrier gas flow rates (2-10 mL/min, comparable to the effusive molecular beam sources described above). The new supersonic jet sources are integrated into the REMPI-TOFMS spectrometer in the same manner as the effusive molecular beam sources (see Figure 1).14 In addition to laboratory results, which demonstrate the cooling properties of the new inlet, first results from an industrial process analysis field application are also presented. The new continuous sample inlet for MS applications has been patented.29 Pyrex glass supersonic jet nozzles for laser induced fluorescence (LIF) applications have been described previously. The latter devices were based on an 8-mm-o.d. Pyrex glass tubing with a 160 µm pinhole restriction nozzle30 or a 1-mm-o.d. capillary that terminated in a 150 µm pinhole31 and were applied for both LIF spectroscopic (LIF30) and analytical (LIF-gas chromatography coupling31) applications. EXPERIMENTAL SECTION Manufacture of Nozzles. Figure 2 shows microscope pictures of the nozzle manufacturing steps (see also ref 32). The jet inlet (29) Zimmermann, R.; Rohwer, E. R.; Dorfner, R.; Boesl, U.; Kettrup, A.: German patent application, 1999. (30) Warren, J. A.; Hayes, J. M.; Small, G. J. Anal. Chem. 1982, 54, 138-140. (31) Hayes, J. M.; Small, G. J. Anal. Chem. 1982, 54, 1202-1204. (32) Guthrie, E. J.; Schwarz, H. E. J. Chromatogr. Sci. 1986, 24, 236-241.

nozzle was made from a commercial deactivated fused-silica capillary (deactivated gas chromatography capillary without stationary phase, Resteck GmbH, Germany) with an i.d. of 0.53 mm. The tip of the capillary was carefully melted, until closure, using a butane/O2 microtorch. During the melting procedure, the capillary was rotated to achieve rotational symmetry. The result is shown as the microscope photograph in Figure 2b. The inner diameter is conically tapered until closure. Subsequently, the closed capillary tip was abraded with fine sandpaper to produce a well-defined nozzle opening (Figure 2c). The nozzle was eroded until the desired gas flow rate upon a pressure difference of 1 bar was ∼10 mL/min (nozzle L ∼ 50 µm). Afterward, the surface around the nozzle opening was polished. It should be noted that different nozzles with other shapes have been manufactured (e.g., laval-type nozzles with a tulip-like exit part after the restriction).29 With the above-described type (see Figure 2c), however, we achieved the best results regarding the cooling efficiency. Laboratory Measurements. The laboratory measurements for characterization of the inlet system performance have been performed with a compact, home-built linear time-of-flight mass spectrometer. The system has previously been described in detail.13,14 The flange with the effusive inlet capillary has been replaced by a flange with the new capillary supersonic jet inlet. In Figure 1, a schematic sketch of the ionization region of the TOFMS system is given. The tip of the capillary reaches into the center of the TOFMS ion source. The REMPI ionization occurs directly underneath the nozzle opening; the distance between the nozzle orifice and the laser focus is ∼1 mm. The orifice-laser focus distance of 1 mm corresponds to the 20-fold orifice diameter (20 × 50 µm). At this position, the jet cooling conditions are fully developed.23 An excimer laser (LPX 100, Lambda Physik, Germany) pumped-dye laser (FL 2002, Lambda Physik, Germany) was used for generation of tunable laser radiation. The shown spectra have been recorded using a capillary supersonic inlet with a ∼ 50-µm nozzle diameter of ∼50 µm (∼ 10 mL/min inlet gas Analytical Chemistry, Vol. 73, No. 17, September 1, 2001

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Figure 2. Photographs of the capillary tips at different stages of manufacturing: a, untreated capillary as used for effusive inlet; b, capillary closed because of the melting procedure; and c, ready, capillary-based supersonic jet nozzle. The nozzle has been opened (L ∼ 50 µm) by abrasion with fine sandpaper.

flow). The analytes were seeded in dry air. The mass-selected REMPI spectra of benzene (78 m/z, Figure 3) and monochlorobenzene (112 m/z, Figure 6) were acquired by registering the area of the respective molecular ion peak in the TOF mass spectrum as a function of the dye laser wavelength. A boxcar 4174

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integration system (SR 250, Stanford Research System) was used for this purpose. The data are digitized by a triggered A/D computer interface (SR245, Stanford Research System) and transferred via a GPIB interface to a personal computer system. Home-written software (based on LabView, National Instruments) was used for data recording, laser control, and data analysis. REMPI mass spectra were recorded using a digital storage oscilloscope (LeCroy, Type 9361, Switzerland). Field Measurements. The field measurements have been performed using a home-built mobile REMPI-TOFMS system that is equipped with a reflectron time-of-flight mass spectrometer and a compact Nd:YAG dye laser (NarrowScan, Radiant Dyes, Germany). Applications of this mobile REMPI-TOFMS system for monitoring of traces of combustion byproducts by using REMPITOFMS have been reported previously.33 In the work presented here, the first REMPI-TOFMS field application utilizing the new capillary-based continuous supersonic jet inlet concept is presented. The setup of the nozzle and the ionization condition (1 mm below orifice) are in principle identical, as described above for the laboratory measurements. The only difference is that the inlet needle and the transfer line are heatable (typically 200-300 °C). Full REMPI mass spectra are recorded via a 250 MHz/500Gs/ 8bit transient recorder PC card (Aquiris, Switzerland) at a repetition rate of 10 Hz. Data acquisition and analysis is performed by a home-written program package (LabView, National Instruments). A standard gas mixture is used for quantification/calibration of the REMPI-TOFMS measurements as well as for the alignment of the instrumentation. The standard gas mixture contains known concentrations (parts-per-billion volume region) of standard substances (e.g., benzene, toluene, naphthalene, and monochlorobenzene) in air and is supplied by a home-built unit using the diffusion and permeation cell technique.34,35 The calibration gas is measured for some 10 s prior to and after each (flue gas) online measurement sequence. The calibration gas measurements are used for an external calibration procedure. Details on the quantification procedure and general setup of the on-line sampling train are given in the literature 14,33 (see also below, Figure 8). The flue gas was sampled via a quartz tube that is mechanically supported by a stainless steel tube. A sample flue gas stream of ∼1-5 L/min was drawn by a sampling pump. All outer parts of the sampling train were heated to a temperature of ∼500 K to avoid condensation of high-molecular-weight compounds (tar). All of the inner parts of the sampling train that are in contact with the flue gas were manufactured of either quartz or Pyrex glassware for the same reason. Particles are precipitated from the sampling gas stream by a surface filter made of pressed glass fiber material and a prefilter of packed glass wool. From the dedusted sampling gas stream, a small fraction (∼ 10 mL/min.) goes through a deactivated fused-silica transfer capillary to the sample inlet system. The transfer capillary runs in a heated (550 K), flexible transfer line. In the inlet system, the short piece of capillary with integral nozzle at its tip (see Figure 2c) is connected (33) Zimmermann, R.; Heger, H. J.; Blumenstock, M.; Dorfner, R.; Schramm, K.-W.; Boesl, U.; Kettrup, A. Rapid Commun. Mass Spectrom. 1999, 13, 307-314. (34) Namiesnik, J. J. Chromatogr. 1984, 300, 79-108. (35) Namiesnik, J. Chromatographia 1983, 17, 47-48.

Figure 3. Measured rotational contours of the ν6 vibronic band of the S1 r S0 transition of benzene with the respective calculated contour and determined rotational temperature Trot: a, effusive inlet with air as carrier gas (Trot ) 300 K); b, supersonic jet with air as carrier gas (Trot ) 20 K; and c, supersonic jet with argon as carrier gas (Trot ) 3 K, additional peaks and some band broadening is caused by benzene-argon clusters).

to the transfer capillary. The dye laser of the system was tuned to the S1 origin resonance of monochlorobenzene at a wavelength of 269.82 nm, and pulse energies of ∼100 µJ/pulse were available for the measurements at the waste incineration plant. RESULTS AND DISCUSSION Laboratory Results - Test of the Nozzle Performance. To characterize the quality of the formed supersonic beam, its cooling efficiency was measured. A laser spectroscopic method was used for the determination of the rotational temperature, Trot, of benzene in a supersonic beam from the homemade jet nozzles.36,37 More specifically, the REMPI spectra of the 10ν6 vibronic band of the 1B (S ) r 1A (S ) transition of benzene in the wavelength region 2 1 1 0 258.90-259.15 nm were recorded. The shape of the rotational contour of the ν6 vibronic band contains the information about the rotational temperature (Trot). The correlation between the rotational contour and the temperature is known exactly.38 It is possible to compute the shape of the rotational contour of the ν6 (36) Zimmermann, R.; Lermer, C.; Schramm, K.-W.; Kettrup, A.; Boesl, U. Eur. Mass Spectrom. 1995, 1, 341-351. (37) Dorfner, R. Diploma thesis, Technische Universita¨t Mu ¨ nchen, Mu ¨ nchen, 1998. (38) Callomon, J. H.; Dunn, T. M.; Mills, I. M. Philos. Trans. R. Soc. B 1966, A 259, 499-532.

vibronic band by computer routines.36,39 The respective rotational temperatures can be determined by comparing the computed rotational contours with those observed experimentally from the 1B (S ) r 1A (S ) ν vibronic transition in the REMPI spectra. In 2 1 1 0 6 Figure 3, measured rotational contours of the ν6 vibronic band of benzene are shown with the corresponding calculated vibronic contours. The calculated temperatures, Trot, are shown together with the measured benzene ν6 rotational contours recorded (a) with effusive inlet, using air as carrier gas, (b) with the supersonic jet technique with air, and (c) with argon. For the low-temperature region (Trot < 50 K), the distance between the R and Q branches of the rotational contour is a very convenient and sensitive temperature indicator. This parameter was, therefore, used for the determination of Trot. In Figure 4, the calibration curve of rotational temperature against the calculated distance between the center of the R and Q branches is shown. In general, computed and experimentally obtained rotational contours correspond very closely. The supersonic jet spectrum with argon as carrier gas, as shown in Figure 3c, represents an exception. With argon as the carrier gas, a very high cooling efficiency is achieved.19 Under these conditions van der Waals clusters of benzene and argon (39) Boesl, U.; Neusser, H. J.; Schlag, E. W. In Laser Induced Processes in Molecules; Springer-Verlag, 1979; Vol. 6, p 219.

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Figure 4. Calibration curve, derived from calculations, for determination of rotational temperatures from the rotational contour of benzene ν6 vibronic band via the distance of the R and Q branches.

Figure 5. Influence of the gas flow on the cooling properties for air and argon as carrier gas.

are formed.40 The cluster species have slightly shifted resonance transitions with respect to the free benzene molecule. Furthermore, sidebands occur as a result of combinational modes with excited low-frequency argon-benzene vibrations. The argonbenzene van der Waals bonds in the clusters exhibit very low bonding energies (∼100 cm-1). Upon the REMPI excitation/ ionization, the argon-benzene clusters fragment into neutral argon atoms and benzene cations; therefore, the respective optical argon-benzene cluster transitions are also observed at m/z 78 in the mass spectrum and are, thus, superimposed on the REMPI spectrum of the free benzene molecules. Because of this, the ν6 in Figure 3c appears, on one hand, to be broadened. On the other hand, additional peaks show up as a result of the resonance transitions of the benzene-argon cluster species. The temperature Trot nevertheless can still reliably be determined by the distance between the R and Q branches of the ν6 rotational contour. The 4176

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occurrence of the argon-benzene clusters itself is an independent proof of the good cooling properties of the obtained supersonic molecular beam, because these species are only formed in undisturbed, “cold” jet expansions.40 An interesting question in this context is the dependence of the cooling properties upon downscaling of the nozzle diameter. This was tested by comparing the achieved rotational temperatures of capillary-based continuous supersonic jet inlets with different nozzle openings. To avoid influences of differences in the shape of the conical tapering of individually manufactured jet inlets, one single closed capillary, as shown in Figure 2b, was successively abraded in four stages, leading to four different nozzle openings. For each manufacturing stage, the gas flow for one bar pressure difference through the nozzle as well as the achieved (40) Weber, T.; von Bargen, A.; Riedle, A.; Neusser, H. J. J. Chem. Phys. 1990, 92, 90-96.

Figure 7. Photograph of the mobile REMPI-TOFMS instrument and the continuous sampling unit at the measurement site in a German waste incineration plant.

Figure 6. REMPI spectra of the origin region of the 1B2(S1) r 1A (S ) transition of monochlorobenzene, recorded with a, effusive 1 0 inlet at 500 K; and b, effusive inlet at 300K and continuous supersonic jet at a rotational temperature Trot of ∼30 K. Air was used as carrier gas. The spectra are scaled according to an equal area below the vibrationless origin band at 269.82 nm.

rotational temperature (Trot) of benzene was measured (via the REMPI spectrum of the ν6 of the S1 of benzene, see above). The measurement of Trot was, furthermore, performed for both carrier gases, argon and air. The temperature of the sample gas was 300 K. The results of these measurements are depicted in Figure 5 (line exponentially fitted). Even for the lowest flow rate of 0.25 mL/min, considerable cooling efficiencies were observed. With argon as the carrier gas, a Trot of 35 K was achieved. At a gas flow rate of 2-3 mL/min, the cooling efficiency is nearly fully developed (Trot < 10 K), and a further increase of the flow rate results only in a gradual improvement (Trot ∼ 3 K at 9.4 mL/ min). In summary, it is particularly surprising that very good supersonic beam qualities (i.e., cooling efficiencies) are achieved already at very low flow rates of 2-10 mL/.min. With conventional gated supersonic jet valves (e.g., type 9, General Valve Inc.), which

exhibit cylindrical nozzles drilled in stainless steel or ceramic material (diameter, 100-200 µm), considerably higher flow rates are required to achieve similar cooling properties. (With a pulsed inlet valve operated at 10 Hz and a typical gas pulse width of 300 µs, the total valve open time is only 180 ms/min. Assuming an inlet rate of 10 mL/min, which is surely the lower border value for pulsed and skimmed molecular beam inlet systems, the gas flux during the valve opening time is increased by a factor of 330 with respect to the continuous jet inlet) The reasons for the high cooling efficiency of the new fused-silica-based jet nozzles are the perfectly smooth inner surface of the inlet capillary and nozzle (see Figure 2c) as well as the smaller nozzle length-to-diameter ratio, which allow the development of a more ideal, undisturbed supersonic jet beam. One interesting application for the REMPI-TOFMS method is the on-line monitoring of monochlorobenzene (MCBz) from waste incineration flue gases. Monochlorobenzene is a surrogate compound for toxic polychlorinated dibenzo-p-dioxins/furans (PCDD/ F).41,42 In 1998, MCBz was first measured on-line in the flue gas of a German hazardous waste incinerator by REMPI-TOFMS using an effusive capillary inlet.33,43 In the following, REMPI spectra of MCBz recorded with a conventional effusive inlet13,14 and a novel supersonic jet capillary (41) Blumenstock, M.; Zimmermann, R.; Schramm, K.-W.; Kettrup, A. Chemosphere 2001, 42, 507-518 (42) Blumenstock, M.; Zimmermann, R.; Schramm, K.-W.; Kaune, A.; Nikolai, U.; Lenoir, D.; Kettrup, A. J. Anal. Appl. Pyrol. 1999, 49, 179-190. (43) Heger, H. J.; Boesl, U.; Zimmermann, R.; Dorfner, R.; Kettrup, A. Eur. Mass Spectrom. 1999, 5, 51-57.

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Figure 8. Setup of the sampling system for on-line measurements of flue gases from industrial thermal processes.

inlet are compared. In Figure 6, three REMPI spectra of the origin region of the 1B2(S1) r 1A1(S0) transition of monochlorobenzene, seeded in air as carrier gas, are shown. The upper trace (Figure 6a) depicts a monochlorobenzene spectrum recorded with an effusive inlet at 500 K (i.e., the gas temperature in the molecular beam, thus, is also 500 K). The middle trace (Figure 6b) is also due to an unheated effusive inlet. Here, an ambient temperature (300 K) of the effusive molecular beam is achieved. A REMPI spectrum of MCBz recorded with an effusive, unheated capillary inlet has been published previously.43 Finally, in the lower trace (Figure 6c) a spectrum of monochlorobenzene recorded with the new supersonic jet nozzle with a rotational temperature Trot of ∼30 K (determined for benzene, as mentioned above) is shown. For the laboratory measurements, the new supersonic jet nozzle was mounted on a separate flange without heating devices. Thus, the temperature of the device was 300 K; however, previous work showed that the achieved rotational temperatures in the supersonic expansion are not influenced by the temperature of the inlet nozzle (for nozzle temperature range 300-600 K).36 The spectra are scaled according to an equal area below the 1B2 (S1) origin band at 269.82 nm. Figure 6 shows that, because of the narrowing of the vibronic transitions of the cooled sample, a considerable increase in peak intensity is achieved. With conventional narrow band laser systems (∆ν ∼ 0.1 cm-1), this peak intensity increase is directly correlated to an increase of the detection sensitivity. Thus, with a capillary jet inlet, a 10-fold sensitivity increase can be achieved (500 K operation temperature) with respect to an effusive capillary inlet using the same gas volume throughput. REMPI-TOFMS detection limits below 1 ppbv have previously been achieved with the effusive molecular beam technique.14,33 Note that for effusive inlets, the gas density decreases already in the capillary, but for jet inlets the density does not start to decrease until the gas flow has passed the orifice. 4178

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Because of the microscale of the nozzle and corresponding small nozzle-laser focus distance, the lateral extension of the jet molecular beam is within the laser focus (same laser focus position as in the case of an effusive capillary inlet13,14); thus, about the same number of sample molecules are captured with the laser beam, and the differences in sensitivity are due to the transition line’s narrowing, which is induced by the rotational jet cooling. The above-mentioned promising laboratory results motivated a first REMPI-TOFMS field application test with the new inlet technique. In the following, results from this first field application test with the novel Jet-REMPI concept monitoring are reported. First Field Application: On-Line, Real Time Measurement of Monochlorobenzene with Jet-REMPI-TOFMS in the Flue Gas of a Waste Incineration Plant. The measurements were performed at a 24 MW German municipal waste incineration plant, using a mobile REMPI-TOFMS spectrometer equipped with a Nd: YAG pumped-dye laser system. The dye laser was tuned to 269.82 nm, the resonance frequency for monochlorobenzene detection. In Figure 7, a photograph of the mobile REMPI-TOFMS spectrometer at the sampling point at the 24 MW waste incineration plant is given. Figure 8 shows a schematic representation of the sampling and inlet system used for the on-line measurements at the incinerator. Some details on the sampling system and the mobile REMPI-TOFMS system are given in the Experimental Section. For a thorough description, see for example, reference 33. The sampling point was located in the flue gas directly after the last section of the heat exchanger unit and prior to any flue gas purification measure. The flue gas temperature was about 500 K. In Figure 9, a REMPI mass spectrum of on-line sampled waste incineration flue gas is shown. The spectrum is due to 50 averaged, single laser shot transients (i.e., a 5-s average). The monochlorobenzene signal is clearly visible. Because no scanning experiments were possible during the field measurement campaig, the

Figure 9. Jet-REMPI-TOFMS on-line measurement result from a German waste incineration plant (24 MW thermal power) utilizing the newly developed continuous supersonic jet nozzle: on-line recorded REMPI mass spectrum of waste incineration flue gas at the resonance frequency of monochlorobenzene (λ ) 269.82 nm). Monochlorobenzene is clearly visible in the spectrum.

rotational cooling properties (Trot) have not been measured. As a result of the more complex flue gas composition (about 6% oxygen, 8% water vapor, 8% carbon dioxide in nitrogen) the cooling efficiency may be slightly lower with respect to the cooling efficiency achieved in the laboratory experiments with dry air (see Figure 3b). However, the bulk composition of flue gases from industrial incinerators is relative stable (nitrogen, oxygen, carbon dioxide), and larger fluctuations are observed only for the products of incomplete combustion, like CO, NO, or organic species (e.g., chlorobenzene and naphthalene). Because the total concentration of the products of incomplete combustion usually is far below 1%, no dynamic changes of the jet cooling properties are expectable. The monochlorobenzene signal in the mass spectrum shows isotopic peaks at 112, 113, and 114 m/z at the correct intensity ratios of about 1:0.1:0.3 (12C61H535Cl, 12C513C1H535Cl and 12C 1H 37Cl). Furthermore, some other flue gas compounds are 6 5 visible in the mass spectrum. This includes toluene (92/91 m/z) and phenol (94 m/z), as well as naphthalene (128 m/z). Toluene and phenol are, in general, far more abundant than monochlorobenzene in waste incineration flue gases.44 Because both molecules, toluene and phenol, exhibit low-frequency vibrational modes (internal rotation) and the achieved cooling is assumed to be not better than 30-50 K, some continuous absorption of the compounds cannot be avoided. A substantial ion signal is, thus, still obtained for these compounds. Naphthalene presents a different case. The REMPI-ionization of naphthalene is mediated by its intense second excited molecular singlet state at the applied wavelength. The UV-absorption spectrum of the second excited singlet state of naphthalene exhibits an intrinsically broad, continuous spectral structure. Thus, no suppression of the REMPI efficiency can be achieved by jet cooling in this wavelength range. In Figure 10, an on-line measured time sequence of the monochlorobenzene concentration and the naphthalene concen(44) Zimmermann, R.; Blumenstock, M.; Heger, H. J.; Schramm, K.-W.; Kettrup, A. Environ. Sci. Technol. 2001, 35, 1019-1030.

Figure 10. Jet-REMPI-TOFMS on-line measurement result from a German waste incineration plant (24 MW thermal power) utilizing the newly developed continuous supersonic jet nozzle: quantified time profiles of the monochlorobenzene and naphthalene concentrations. Monochlorobenzene is known as a reliable indicator for dioxin (PCDD/F I-TEQ) formation/emission from waste incineration plants.

tration is shown. The concentration values were determined from the integral area of the monochlorobenzene and naphthalene molecular peaks in the mass spectra using an external calibration routine, as described in references 14 and 33 and in the Experimental Section. Each point in the curve is due to the average of five single laser shot mass spectra. Because of the averaging procedure, the shot-to-shot laser intensity fluctuation is eliminated. The remaining time resolution is 0.5 s. The time profile shows a monochlorobenzene concentration that is fluctuating around an average value of ∼25 ppbv. Note that the observed fluctuations are not due to experimental reasons, such as for example, laser fluctuations, but are due to the rapid changes occurring within the combustion process. In combustion processes, rapid fluctuations of trace compound concentrations are often observable.14 For other, less dynamic processes, such as, for example, the release of flavor compounds during the coffee Analytical Chemistry, Vol. 73, No. 17, September 1, 2001

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roasting process, much smoother concentration profiles are obtained.27 Periods with significantly higher monochlorobenzene concentrations exceeding 100 ppbv were observed twice in the measurement time (at 0-30 s. and again at 360-410 s.). The naphthalene concentration curve shows a different time-to-intensity behavior with respect to the one of monochlorobenzene. This is in line with previous results, showing chlorinated and nonchlorinated aromatic species to behave differently.45,46 Thus, the simultaneous monitoring of, for example, monochlorobenzene and naphthalene gives insight into the formation behavior of two classes of toxic combustion byproducts, namely, the chlorinated aromatics and the polycyclic aromatic hydrocarbons (PAH), in the primary combustion process. In this context, the on-line analysis of monochlorobenzene in waste incineration flue gases is particularly interesting, it has been identified as a reliable surrogate for the formation and emission of dioxins (polychlorinated dibenzodioxins/-furans), PCDD/F. More specifically, monochlorobenzene is a surrogate for the international toxicity equivalent (I-TEQ), which describes the toxicity due to the 17 in 2-, 3-, 7-, and 8-position chlorinated PCDD/F congeners.33,41,42,47 Thus, REMPI-TOFMS monitoring of monochlorobenzene could help to elucidate the PCDD/F formation pathways in industrial processes and to find optimized process control measures for the primary reduction of the toxic PCDD/F. Because of the increased selectivity by the jet-cooling process, the probability of interference is reduced. Furthermore, the narrowing of the vibronic bands allows achievement of lower detection limits with regard to the effusive inlet at the same flow rate. This is clearly visible by the comparison of the jet-cooled and warm (effusive inlet) REMPI spectra of monochlorobenzene shown in Figure 6. Further advantages of both capillary type inlet techniques (i.e., effusive and the supersonic jet capillary inlet techniques) for process analytical applications are their simplicity, their chemical inertness, and of course, the possibility of realizing a very open design of the ion source. The molecular beam is formed between the repeller and first extraction electrodes and is directed into a turbo molecular pump, which is mounted beneath the ion source. Thus, minimal interaction of the molecular beam with parts of the ion source occurs. Because the fused silica capillary runs (45) Zimmermann, R.; Heger, H. J.; Kettrup, A.; Nikolai, U. Fresenius’ J. Anal. Chem. 2000, 366, 368-374. (46) Zimmermann, R.; Blumenstock, M.; Schramm, K.-W.; Kettrup, A. Organohalogen Compd. 2000, 46, 78-81. (47) Blumenstock, M.; Zimmermann, R.; Schramm, K.-W.; Kettrup, A. Chemosphere 2000, 40, 987-993. (48) Dagan, S.; Amirav, A. J. Am. Soc. Mass Spectrosc. 1996, 7, 737-752.

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within a heated hollow steel needle, even “dirty” process gases with a high content of semivolatile compounds (hydrocarbon vapors) can be expanded. The open setup of the ion source avoids the risk of contamination of the ion source region; however, it has to be mentioned that, in addition to the thorough design of the inlet technique and ionization region of the instrument, the concept and realization of the sampling system is also crucial for monitoring complex process gases (see Figure 8). Particularly important for the jet inlet is a reliable precipitation of dust particles, which otherwise may clog the nozzle. In summary, the laboratory results have shown that the capillary-based supersonic jet inlet technique allows the combination of the benefits of the conventional effusive capillary inlet,13,14 namely (i) high sensitivities due to low distances, r, between the molecular beam origin and the ionization region, (ii) very simple and rugged setup, (iii) long-term stability due to avoidance of moving parts, and (iv) low contamination risk resulting from chemical inertness (quartz) and good heatability with the benefits of supersonic jet nozzles,29 namely: (i) increased sensitivity as a result of narrowing of vibronic modes in the REMPI spectrum, (ii) increased selectivity, (iii) reduced degree of fragmentation (also important for application with electron impact ionization48). The application results from the waste incineration plant further indicate that the technique can be applied for process analytical purposes under the rough conditions present at industrial plants. ACKNOWLEDGMENT The authors thank the members of the GSF laser mass spectrometry group for contributions during the field measurement campaign and Prof. Boesl, Technical University, Munich, for continuous interest in their work. Furthermore, we thank Dr. Selzle, Technical University, Munich, for supplying the computer program for simulation of the rotational bands of benzene (symmetrical rotator). Funding from the Deutsche Bundesstiftung Umwelt, Osnabru¨ck, Germany (grant no. 12447), the German Ministry of Research, BMBF (HGF-Strategiefondsprokjekt “Stickoxidminderung”, grant no. 01SF9920/7), is gratefully acknowledged. Received for review February 28, 2001. Accepted June 19, 2001. AC010244H