In-Line Catalytic Derivatization Method for Selective Detection of

These aromatic skeletons are detected selectively by REMPI-TOFMS. ... In real-world samples, numerous isomers are present, often in extremely complex ...
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Anal. Chem. 1999, 71, 4148-4153

In-Line Catalytic Derivatization Method for Selective Detection of Chlorinated Aromatics with a Hyphenated Gas Chromatography/Laser Mass Spectrometry Technique: A Concept for Comprehensive Detection of Isomeric Ensembles Ralf Zimmermann,*,†,‡ Egmont R. Rohwer,§ and Hans Jo 1 rg Heger†

Institut fu¨r O ¨ kologische Chemie, GSF-Forschungszentrum fu¨r Umwelt und Gesundheit, D-85764 Oberschleissheim, (Neuherberg), Germany, Lehrstuhl fu¨r O ¨ kologische Chemie und Umweltanalytik, Technische Universita¨t Mu¨nchen, D-85748 Freising, Germany, and Department of Chemistry, University of Pretoria, ZA-0002 Pretoria, Republic of South Africa

The combination of gas chromatography (GC) and laserbased resonance-enhanced multiphoton ionization-timeof-flight mass spectrometry (REMPI-TOFMS) represents a three-dimensional analytical method, using the gas chromatographic retention time, the wavelength of the ionization laser for REMPI, and the molecular mass as analytical parameters. In this work, a novel analytical scheme for detection of chlorinated aromatic compounds, including isomeric ensembles, by GC/REMPI-TOFMS is presented. The concept uses an in-line hydrodechlorination catalyst for post- or precolumn derivatization of chlorinated aromatic compounds. The chlorinated aromatics are quantitatively reduced, forming their respective aromatic skeletons. These aromatic skeletons are detected selectively by REMPI-TOFMS. The first results for substance class selective detection of chlorinated benzene isomers are given, and potential applications in the field of the analysis of compounds such as polychlorinated dibenzo-p-dioxins and -furans are discussed. Laser-induced resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS) represents a highly selective and sensitive two-dimensional analytical technique. The REMPI-TOFMS technique readily combines UV spectroscopy and mass spectrometry.1-4 Intermediate states of target molecules can be selectively excited by laser photon absorption when the wavelength of the laser is in resonance with a UV transition. Excited molecules subsequently are ionized by absorption of an * Corresponding author: At the Institut fu ¨r O ¨ kologische Chemie, GSFForschungszentrum fu ¨ r Umwelt und Gesundheit, D-85764 Oberschleissheim, (Neuherberg), Germany; (e-mail) [email protected]; (tel) ++49 (0)89 (3187-4454; (fax) ++49 (0) 89 317. † GSF-Forschungszentrum fu ¨ r Umwelt und Gesundheit. ‡ Technische Universita ¨t Mu ¨ nchen. § University of Pretoria. (1) Lubmann, M. Lasers in Mass Spectrometry; Oxford University Press: New York, 1990. (2) Boesl, U. J. Phys. Chem. 1991, 95, 2949. (3) Cool, T. A.; Williams, B. A. Combust. Sci. Technol. 1992, 82, 67. (4) Zimmermann, R.; Boesl, U.; Weickhardt, C.; Lenoir, D.; Schramm, K.-W.; Kettrup, A.; Schlag, E. W. Chemosphere 1994, 29, 1877.

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additional laser photon. Figure 1 depicts the principle of the twophoton REMPI process with the energy level diagram of the benzene molecule. Benzene, as well as some other aromatic compounds such as phenol, dibenzo-p-dioxin, dibenzofuran, naphthalene, biphenyl, fluorene, or toluene, can be efficiently ionized by a one-photon-resonant-two-photon-ionization REMPI scheme. The first photon is resonant with the lowest excited molecular singlet state (S1 state) and the ionization potential (IP) is lower than twice the energy of the S1 state. Therefore it is possible to use a single laser wavelength for both excitation and ionization. The optical resonance condition is responsible for the high selectivity of the REMPI process. Therefore REMPI-TOFMS instruments even allow the direct, on-line monitoring of trace chemicals from real-world samples (e.g., on-line monitoring of traces of organic compounds in combustion off-gases.5,6 The optical selectivity of the REMPI process depends on the sample inlet technique. Effusive inlet of the gas chromatography (GC) eluent into the ionization chamber results in a medium, somewhat substance class selective laser ionization process.7,8 For example, many polycyclic aromatic hydrocarbons (PAH) can be ionized with the laser wavelength 248 nm, while the majority of other compounds remains invisible.6 If the inlet is performed via a supersonic jet expansion, the analyte molecules are rotationally and vibrationally cooled and their UV bands are narrowed considerably. Due to the sharpened UV spectroscopic features in the supersonic jet expansion, even isomer selective ionization can often be performed.3,4 Furthermore, REMPI-TOFMS can readily be used as a highly selective detector for gas chromatography. Some work on detec(5) Zimmermann, R.; Heger, H. J.; Boesl, U.; Kettrup, A. Rapid. Communic. Mass Spectrom. 1997, 11, 1095. (6) Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H.; Kettrup, A.; Boesl, U. Anal. Chem. 1999, 71, 46. (7) Rhodes, G.; Opsal, R. B.; Meek, J. T.; Reilly, J. P. Anal. Chem. 1983, 55, 280. (8) Dobson, R. L. M.; D′Silva, A. P.; Weeks, S. J.; Fassel, V. A. Anal. Chem. 1986, 58, 2129. (9) Imasaka, T.; Okamura, T.; Ishibashi, N. Anal. Chem. 1986, 58, 2152. (10) Zimmermann, R.; Heger, H. J.; Kettrup, A.; Schramm, K.-W.; Rohwer, E. R.; Ortner, E. K.; Boesl, U.; Schlag, E. W. J. High-Resolut. Chromatogr. 1997, 20, 461. 10.1021/ac990177f CCC: $18.00

© 1999 American Chemical Society Published on Web 08/27/1999

Figure 1. REMPI scheme of benzene, using supersonic jet cooling and a laser wavelength of 259.12 nm. The rotational Q-branch of the ν610 vibrational state of the S1 (1B2u) r S0 (1Ag) electronic transition is used as intermediate state for highly selective and sensitive detection of benzene.

Figure 2. GC/supersonic jet-REMPI-TOFMS chromatograms of a mixture of 2,3- and 2,4-dichlorotoluene. At a laser wavelength of 279.35 nm, the 2,4-dichlorotoluene is selectively ionized (top), while with 273.82 nm, the 2,3-dichlorotoluene isomer can be detected (nearly) selectively.

tion of PAH with GC/effusive beam-REMPI-TOFMS has been reported.7,8 Different supersonic jet inlet valves for coupling highresolution laser spectrometry and GC have been reported in the literature.9-11 Recently we presented a GC/jet valve-REMPITOFMS coupling, performing the high selectivity of supersonic jet spectrometry together with a good sensitivity without disturbing GC and REMPI performance. The achieved detection limit for toluene is 200 fg at a S/N ) 2 level.10 The high selectivity of this GC/supersonic jet-REMPI-TOFMS system allows discrimination of isomers (see Figure 2). However, if isomeric ensembles should be analyzed in a comprehensive manner with a GC/ REMPI-TOFMS method, the high selectivity of REMPI may be disadvantageous, as isomeric compounds often exhibit great differences in the structure of the UV transitions. Thus, each single isomer has an individual response factor at a given laser wavelength, making a straightforward quantitative GC/REMPI-TOFMS analysis of isomeric ensembles very difficult. On the other hand, real-world samples are very complex, often including isobaric (11) Koester, C.; Grotemeyer, J.; Schlag, E. W. Z. Naturforsch. 1990, 46a, 1285.

compounds. Therefore the high selectivity of REMPI (even of supersonic jet-REMPI) is desirable in terms of minimization of potential chemical interferences. In summary, for a comprehensive analysis of an isomeric ensemble, the individual congeners should be optically indistinguishable for REMPI, while efficient discrimination is maintained against all other potential chemical interferences. In this contribution, we suggest the use of specific derivatization gas chromatographic approaches for labeling isomer or compound classes for comprehensive REMPI-TOFMS detection.12 A large number of reaction or derivatization gas chromatographic techniques for chemical analysis are described in the literature.13,14 Among other reasons, the derivatization reactions are applied to enhance volatility, to stabilize thermally labile compounds, to improve detection,15 or to reduce the complexity of a sample.12,19 In principle, the labeling of specific compound classes can be performed by a reaction that couples a REMPIionizable chromophore with the analytes of interest. For example, aldehydes or ketones may be transferred into corresponding, REMPI-detectable, aromatic hydrazone derivatives,12 similar to the 2,4-dinitrophenylhydrazine (DNPH) derivatization15,16 for detection of aldehydes and ketones with GC-ECD, GC-FID, or GC/MS. The analysis of chlorinated aromatics, which are present in many environmental or emission samples in trace quantities, is particularly challenging. In real-world samples, numerous isomers are present, often in extremely complex matrixes. In the literature, a so-called “carbon skeleton” approach for survey analysis of chlorinated aromatics was reported.17-19 This technique uses a hydrodechlorination reactor, located in the GC injector port, for quantitative reduction of all chloroaromatic species. If palladium at a temperature of 300 °C is used as catalyst with hydrogen as reagent and carrier gas, the aromatic carbon skeletons are formed quantitatively (see Figure 3A). Thus, for example, all isomeric polychlorinated dibenzo-p-dioxins are reduced to nonchlorinated dibenzo-p-dioxin and all chlorinated benzenes to benzene. In general, FID detectors are used to detect the dechlorinated carbon skeletons together with the other sample constituents. The carbon skeleton technique applied in a precolumn position does not give any information about the isomeric composition. However, the catalytic dehalogenation concept is ideally suited for the desired “unification” of chloroaromatic isomers for REMPI-TOFMS detection, as all isomers transform to the same carbon skeleton. By coupling a gas chromatograph, equipped with a “dehalogenation” device, to a REMPI-TOFMS detector, a fast survey analysis of chloroaromatic isomeric ensembles can be performed. The analytical information is similar to that gained from the carbon skeleton approach, but the benefits of using REMPI-TOFMS as detector are the high sensitivity and the selectivity against possible (12) Zimmermann, R.; Rohwer, E. R.; Heger, H. J.; Boesl, U.; Kettrup, A. Verfahren zum Nachweis von Substanzen und Substanzklassen. German Patent DE 19754161, European Patent pending. (13) Blau, K.; Halket, J. Handbook of Derivatives for Chromatography, 2nd ed.; John Wiley & Sons: New York, 1993. (14) Knapp, D. R. Handbook of Analytical Derivatization Reactions; John Wiley & Sons: New York, 1979. (15) Lehmpuhl, D. W.; Birks, J. W. J. Chromatogr., A 1996, 740, 71. (16) DeGraff, I.; Nolan, L.; Fiorante, A. The Reporter (Supelco Inc.); 1996, Vol. 15 (5), p 3. (17) Beroza, M.; Sarmiento, R. Anal. Chem. 1964, 36, 1744. (18) Asai, R. I.; Gunther, F. A.; Westlake, W. A.; Iwata, Y. J. Agric. Food Chem. 1971, 19, 396M. (19) Cooke, Nickless, G.; Prescott, A. M.; Roberts, D. J. J. Chromat. 1978, 156, 293.

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Figure 3. (A) Schematic representation of the hydrodechlorination reaction. Aromatic compounds can be dehydrochlorinated quantitatively with hydrogen over a palladium catalyst surface at 300 °C. (B) Schematic representation of the experimental setup of the carbon skeleton-GCFID setup with a capillary-based catalyst. (D) Schematic representation of the experimental setup of the gas chromatography/REMPI-TOFMS method with postcolumn derivatization reactor for analysis of chlorinated compounds.

interferences. An analytically more powerful variant can, however, also be realized by placing the catalytic converter between the column and the REMPI detector. In this case, it is possible to preserve the information of isomer composition by GC retention times as well as the chemical class information by selective REMPI-TOFMS detection. The development of a capillary-based, in-line carbon skeleton hydrodechlorination catalyst and its first application as “selectivity” modifier for REMPI-TOFMS detection is described in the following. EXPERIMENTAL SECTION The conventional carbon skeleton technique utilizes a packedbed catalyst in the injector port of the GC (precolumn derivatization). To achieve good capillary gas chromatographic results, an alternative design of the carbon skeleton catalytic converter was realized, using a palladium-coated capillary as reactor. The catalytic converter was evaluated in a Carlo Erba HRGC 5300 gas chromatograph equipped with two injector ports, two detector ports (one occupied by a FID detector), and a port for the transfer line to the time-of-flight mass spectrometer. A 30-m DB-5 analytical column of 0.32-mm i.d. was used, with hydrogen as carrier and reagent gas for the hydrodechlorination process. The capillary reactor is made by a static coating technique: 1.5 m of a deactivated fused-silica capillary (0.32 mm id.) is filled with a 0.5% palladium acetonylacetonate solution (solvent, CH2Cl2). The capillary is sealed at the one end and evacuated from the other by means of a membrane vacuum pump. The solvent evaporates, leaving a coating of the organopalladium compound at the inner wall of the capillary. In the next step, the organopalladium coating has to be reduced to elementary palladium, which is the active catalyst. This is performed by placing the catalyst capillary into a GC oven. The GC oven is heated to temperatures of ∼200 °C, while the capillary is flushed with hydrogen. Under 4150 Analytical Chemistry, Vol. 71, No. 19, October 1, 1999

these temperatures, the hydrogen reduces the yellowish organopalladium compound and a film of catalytically active metallic palladium is formed. The end of the reaction can be recognized by a color change of the coating from yellow to black. The setup of the precolumn derivatization approach (here with FID detection) is shown in Figure 3B. For the precolumn derivatization setup, the catalyst capillary (1.5 m) is placed between injector 1 and the analytical column. A flexible heating tube (Horst GmbH with Horst GmbH HT-30 controller) of 2-m length is used to house the catalyst capillary. This flexible catalyst housing is mounted between detector port 2 and injector port 3. A short piece of deactivated fused-silica capillary runs directly from injector port 1 via (empty) detector port 2 into the flexible catalyst housing. The deactivated fused silica is connected to the catalyst capillary via a Gerstel connector with graphite seals. The other end of the catalyst capillary is connected to the separation column, which runs through the modified second injector port 3 back into the GC oven. The analytical column finally is connected to the FID at detector port 4. The temperature of the external catalyst housing is set to 300 °C, while the GC oven with the analytical column is temperature programmed. Safety Warning. All connectors are carefully tested for leaks with a thermal conductivity leak tester. Hydrogen leaking into the GC oven or catalyst housing may cause an explosion. For the postcolumn derivatization setup, as shown in Figure 3C, the flexible heating tube (Horst GmbH) simultaneously acts as a transfer line to the REMPI-TOFMS as well as a temperature controller for the catalyst capillary. Injector 1 is directly connected to the analytical column. The analytical column is connected to the catalyst capillary within the flexible heating tube. A short piece of deactivated fused-silica capillary is used to transfer the chromatographic eluent from the end of the catalyst capillary to the supersonic jet valve of the REMPI-TOFMS instrument.

The home-built, compact REMPI-TOFMS instrument is described in detail elsewhere.10 Briefly it consists of a differentially pumped, three-chamber vacuum system. In the first chamber (jet expansion chamber, p ≈ 10-3 Torr), the home-built GCsupersonic molecular beam interface is placed. This interface is driven by a pulsed solenoid microvalve (General Valve Series 9). Memory effects in the interface are minimized by a special arrangement of the capillary, which is positioned very close to the valve piston and the expansion orifice, ensuring an extremely low dead volume and minimal contact of the analytes with valve parts. The special design of the interface and the operational conditions are described in detail in the literature.10 The interface transforms the continuous GC eluent flow into short (∼300 µs) supersonic molecular beam pulses at a repetition rate of 20 Hz. Only the carrier gas (hydrogen) is used as expansion gas. The center region of the supersonic jet expansion passes through a skimmer into the second chamber. The skimmed supersonic jet pulses intersect a Whiley McLaren type ion source (p ≈ 10-5 Torr). In the center of the ion source, the jet gas pulses are irradiated with laser pulses for REMPI ionization. The ions formed by REMPI are extracted into the reflectron time-of-flight mass analyzer (TOF chamber, p ≈ 10-7 Torr). The signal from the dualchannel plate detector is preamplified (Stanford Research Systems SR 240) and transferred via boxcar integrators (Stanford Research Systems SR 250) and a A/D converter to a PC. The data acquisition routine is written in LabView. The timing of the experiment (triggering of laser, valve and data acquisition) is performed by an electronic delay generator (Stanford Research Systems DG 350). An excimer pumped dye laser (Lambda Physik FL 2002) is used for generation of tunable UV laser pulses for selective REMPI ionization. RESULTS The high overall selectivity achievable with the GC supersonic jet-REMPI-TOFMS system is demonstrated in Figure 2. A mixture of 2,3- and 2,4-dichlorotoluene (2,3-DCT and 2,4-DCT) was analyzed by the GC supersonic jet-REMPI-TOFMS instrument using isomer-selective REMPI ionization schemes. The 2,4-DCT exhibits a vibronic resonance transition at 279.35 nm while the 2,3-DCT can be ionized selectively at its 273.82-nm transition. Using the laser wavelength of 279.35 nm (upper trace in Figure 2), 2,4-DCT can be ionized without any interference from the 2,3DCT. Using the resonant 273.82-nm transition of 2,3-DCT for REMPI, ∼5% of 2,4 DCT is co-ionized with the 2,3-DCT (lower trace in Figure 2). In this case, another transition should be used if higher ionization selectivity is required. The optical REMPI spectra of the dichlorotoluenes are given in the literature.10 In the first series of hydrodechlorination experiments, a classical precolumn arrangement was used in order to check the performance of the developed capillary reactor. The FID was used as detector and the catalyst was kept at 300 °C. Results were achieved12 that are similar to those of Cooke et al. and others.17-19 However, the application of the capillary-based catalyst has advantages in comparison with the classical, injector-based packedbed catalyst as reduced adsorptive sample losses as well as increased chromatographic performance are expected. The reduced zone broadening is of special importance in the postcolumn position, as described below, where no refocusing can occur after catalytic conversion. Figure 4 shows a typical carbon skeleton

Figure 4. Precolumn hydrodechlorination GC-FID chromatograms of a mixture of dibenzo-p-dioxin and 1-monochlorodibenzo-p-dioxin with inactive catalyst (left) and active catalyst (right).

Figure 5. GC/postcolumn hydrodechlorination-REMPI-TOFMS chromatogram of a mixture of o-, p-, and m-dichlorobenzenes (∼200 ng each) recorded with a laser wavelength of 259.12 nm and selective ion monitoring (SIM) at 78 amu. The three peaks observed correspond to the hydrodechlorinated benzene peaks from the respective three dichlorobenzenes.

result obtained with the catalyst capillary in precolumn position. Two chromatograms of a mixture of 100 ng of dibenzodioxin (DD) and 100 ng of 2-monochlorodibenzo-p-dioxin (2-MCDD) are shown. The chromatogram on the left was recorded with a hydrodechlorination catalyst temperature of 100 °C. At this temperature, the catalyst is inactive but still allows transfer of DD and 2-MCDD through the capillary. Both compounds, DD and 2-MCDD, are visible in the chromatogram. The chromatogram on the right was recorded with an active catalyst at working temperature (300 °C). Due to the quantitative reduction of 2-MCDD to DD, only the enlarged DD peak is visible in the chromatogram. Neither 2-MCDD nor hydrogenated DD products are detectable. Similar experiments have been performed with different chlorinated aromatics, such as chlorinated biphenyls, benzenes, phenols, and others, showing that the performance of the capillary-based catalyst is comparable to the more common packed-bed catalysts.17-19 In a second series of experiments, the novel combination of the hydrodechlorination catalyst with the GC/REMPI-TOFMS was evaluated. The catalyst was mounted in the postcolumn configuration of Figure 3C; i.e., the catalytic converter was located between the analytical column and the jet inlet system of the Analytical Chemistry, Vol. 71, No. 19, October 1, 1999

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Table 1. Relevant Chlorinated Substance Classes with Respective Hydrodechlorination Products (Carbon Skeletons) and Parameters for REMPI-TOFMS Detection chlorinated congener/ isomer ensemble

hydrodechlorination product (carbon skeleton) and molecular mass (amu)

feasible one-color REMPI, wavelengtha

REMPI-TOFMS detection limit for carbon skeletonb

polychlorinated benzenes, PCBz polychlorinated phenols, PCPh polychlorinated toluenes, PCT polychlorinated biphenyls, PCB polychlorinated naphthalenes, PCN polychlorinated dibenzofurans, PCDF polychlorinated dibenzo-p-dioxins, PCDD

benzene, 78 phenol, 94 toluene, 92 biphenyl, 154 naphthalene, 128 dibenzofuran, 168 dibenzo-p-dioxin, 182

259.12 3 275.0 3 260.21 10 274.57 21 301.6 3 297.29 22 296.09 22

direct inlet, 90 pptv3 direct inlet, 60 pptv3 e GC/REMPI-TOFMS, 200 fgc 10 GC/REMPI-TOFMS, 5 pge 7 GC/REMPI-TOFMS, 200 fgd 7

a Intermediate state in the S system (for optimal jet selectivity). b REMPI-TOFMS detection limits are strongly influenced by laser wavelength 1 and power as well as from the respective experimental setup. c With jet inlet and 260.21 nm for REMPI. d With effusive inlet and 248 nm for REMPI. e Off-line derivatization step (e.g., silylation) necessary for good chromatographic properties.

REMPI-TOFMS spectrometer. Here the chlorinated compounds are dechlorinated successively upon their arrival at the catalyst and the REMPI-TOFMS selectively detects a given aromatic skeleton. The information about chemical constitution (i.e., degree and position of chlorination) is still available from the retention times of the gas chromatographic run. For example, all chlorinated benzenes will be detectable with a laser wavelength of 259.12 nm (REMPI resonance of benzene; see Figure 1) at the mass of 78 amu (molecular mass of benzene) but will be distinguished by their retention times. Figure 5 shows the GC/postcolumn hydrodechlorination-REMPI-TOFMS chromatogram of a mixture of ∼200 ng of o-, p-, and m-dichlorobenzene. After injection, the dichlorobenzenes are separated according to their GC properties. Subsequently, the three chlorinated benzene isomer peaks are transformed into benzene within the hydrodechlorination catalyst. The eluted benzene peaks are detected selectively and, importantly, with the identical ionization efficiency by REMPI-TOFMS. For this purpose, the laser wavelength was tuned onto the S1 ν610 benzene resonance at 259.12 nm.1 The 78 amu mass trace was recorded. (Note that, under the applied REMPI ionization conditions (∼106 W/cm2), benzene is not fragmented upon ionization.) DISCUSSION The presented hyphenation of the GC carbon skeleton method with resonance-enhanced laser mass spectrometry allows highly selective detection of chlorinated aromatics. One very important advantage of the outlined approach for isomer ensemble detection is the preservation of the high REMPI selectivity to discriminate against potential interferences (see Figure 2). The expected limits of detection of a fully developed system will be at least in the same range as achieved for polycyclic hydrocarbons with GC REMPI (100-fg region). The intrinsic physical problems,20 arising when efficient REMPI ionization of highly chlorinated compounds is attempted (e.g., quenching due to intersystem crossing), are avoided in an elegant manner. It has to be stressed that with the combination of chemical (carbon skeleton), optical, and mass selectivity a tunable substance classselective detector has been realized here for the first time. It should be emphasized that, unlike most chlorinated congeners,20 (20) Zimmermann, R.; Lenoir, D.; Kettrup, A.; Nagel, H.; Boesl, U. 26th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1996; p 2859.

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many relevant aromatic carbon skeletons, such as benzene or dibenzo-p-dioxin, can be ionized very efficiently in a simple onecolor REMPI scheme.21,22 In Table 1, some chlorinated classes/ compounds are listed together with the respective hydrodechlorination product and parameters for one-color REMPI detection. Thus, in principle, even relative complex mixtures should be quickly analyzable for a specific isomeric ensemble. The high overall selectivity is expected to reduce cleanup and sample preparation effort. Thus, the combined GC/REMPI-TOFMS and postcolumn carbon skeleton technique may be useful for a fast survey screening of PCDD/F, PCB, or other chloroaromatic ensembles. Here, for example, it may be interesting to work with a reduced GC selectivity, performing a separation of the PCDD/F homologue groups. This would have several advantages for a rapid screening of PCDD/F. Due to the summation of several isomeric PCDD or PCDF in one “homologue peak”, the detection sensitivity can be improved, allowing less extensive preconcentration and thus shorter sampling times. The duration of the chromatographic run also is considerably reduced in comparison to an isomerresolving HRGC run. Together with new trapping materials for thermal desorption analysis of medium-volatility components, it may be possible to set up an analytical scheme that can give information about the emitted homologue profile, e.g., from an incinerator, within the time span of ∼1 h. At this point it should be mentioned that the different sample inlet modes for REMPI (i.e., effusive beam and supersonic jet) can be applied for different analytical approaches. For less complex samples (e.g., air samples), the selectivity of the effusive inlet technique will be sufficient. This work only describes the concept and the very first results of the carbon skeleton GC/REMPI-TOFMS approach. Extensive studies on more complex samples are envisaged. To develop the presented concept into a working analytical method, the properties of the catalytic converters need to be investigated carefully. For example, future work has to include studies on catalyst poisoning and degradation. The reactivity of the catalyst degrades after some days of operation when side reactions start to occur; i.e., additional peaks appear in the chromatograms (FID detection). In summary, a concept and apparatus for in-line chemical modification (i.e., a special reaction gas chromatography ap(21) Zimmermann, R.; Weickhardt, C.; Boesl, U.; Schlag, E. W. J. Mol. Struct. 1994, 327, 81. (22) Weickhardt, C.; Zimmermann, R.; Boesl, U.; Schlag, E. W. Rapid Commun. Mass Spectrom. 1993, 7, 183.

proach) for comprehensive GC/REMPI-TOFMS detection of isomer ensembles or substance classes has been developed. ACKNOWLEDGMENT The authors acknowledge the permanent support of Prof. A. Kettrup and Dr. U. Boesl. Financial support by the Deutsche Bundesstiftung Umwelt, Osnabru¨ck, Germany is gratefully acknowledged. This work has been performed in collaboration with the Technical University Munich (Germany), the GSF Research

center (Oberschleissheim, Germany), and the University of Pretoria (Rep. South Africa). We thank the Merensky Foundation, the GSF Research Center, and the South African FRD for financial support of the international collaboration.

Received for review February 11, 1999. Accepted June 29, 1999. AC990177F

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