Comprehensive Gas ChromatographyâTime-of-Flight Mass

Anal. Chem. 2006, 78, 6364-6375

Comprehensive Gas Chromatography-Time-of-Flight Mass Spectrometry Using Soft and Selective Photoionization Techniques Stefan Mitschke,†,‡ Werner Welthagen,†,‡ and Ralf Zimmermann*,†,‡,§

Analytical Chemistry, Institute of Physics, University of Augsburg, D-86159 Augsburg, Germany, Institute of Ecological Chemistry, GSF-National Research Centre for Environment and Health, D-85764 Neuherberg, Germany, Environmental Chemistry, BIfA-Bavarian Institute of Applied Environmental Research and Technology GmbH, D-86167 Augsburg, Germany

The hyphenation of gas chromatography and mass spectrometry (GC/MS) revolutionized organic analysis. In GC/ MS coupling, usually electron impact ionization is applied, and molecules are identified by their fragment pattern. Although mass spectrometry in principle is a separation method, it is used predominantly as a spectrometric technique. However, if soft (i.e., fragmentation-free) ionization techniques are applied, the inherent separation character of MS is emphasized, which has similarities to a GC boiling point separation. By combining polar column GC separation and fast soft ionization time-of-flight mass spectrometry technology, a comprehensive separation of complex petrochemical samples can be obtained (GC × MS approach). Compounds of comparable physicalchemical properties are characteristically grouped together in a two-dimensional retention time-m/z representation. This resembles the separation characteristics of comprehensive two-dimensional gas chromatography (GC × GC) and, thus, represents a novel multidimensional separation approach. In this work, a gas chromatograph equipped with a polar separation column was coupled to a home-built laser ionization time-of-flight mass spectrometer. Laser-based, single-photon ionization was used for universal soft ionization and resonance-enhanced multiphoton ionization for selective ionization of aromatic compounds. A novel capillary-jet inlet system was used for the coupling. Multidimensional comprehensive analysis of complex petrochemical hydrocarbon samples using gas chromatography coupled to mass spectrometry with soft and selective photo ionization sources is first demonstrated. In the analysis of real world samples, analytical chemists often are confronted with an enormous chemical complexity. A thorough analysis of ultracomplex samples, which readily may contain * Corresponding author. E-mail: [email protected]. † University of Augsburg. ‡ National Research Centre for Environment and Health. § Bavarian Institute of Applied Environmental Research and Technology GmbH.

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several thousands of different chemical species, is required for current research activities in the environmental, biological (i.e., metabolomics), and health sciences as well as for industrial applications in, for example, the petrochemical, pharmaceutical, or food fields. Identification and quantification of individual compounds or compound classes is even more difficult if numerous closely related chemical species are present in a wide range of concentrations. Particularly with petrochemical, environmental, or biological samples, a sufficient separation of trace analytes is often impossible using individual basic analytical procedures. Therefore, it is necessary to develop new analytical methods or find alternative ways of using existing techniques. Hyphenated analytical techniques, formed by direct combination of different separation or spectrometric methods, are used to increase the analytical power in comparison to the isolated techniques1 and represent the current state of the art in chemical analysis. A common “hyphenation concept” is the coupling of a (chromatographic) separation technique to a spectrometric detection and identification technology. By coupling spectrometry to chromatography, two-dimensional analytical techniques are generated in which retention time is used as the first-dimension variable with, for example, wavenumbers (cm-1) in Fourier transform infrared spectroscopy, light wavelengths (nm) in ultraviolet-/visible light absorption (UV/vis) spectroscopy and mass-to-charge ratios (m/ z) in mass spectrometry as second-dimension variables. The most common example of this is gas chromatography or liquid chromatography coupled to mass spectrometry (GC/MS, LC/MS). Hyphenated techniques, however, also include the coupling of two separation techniques, such as in “heart cut” twodimensional gas chromatography (GC/GC)2 and liquid chromatography coupled to gas chromatography (LC/GC)3. These techniques separate compounds in a first chromatographic step (i.e., the first dimension), with a subsequent transfer of an eluent fraction for further analysis in an additional chromatographic separation step (i.e., second separation dimension). “Heart cut” methods increase separation efficiency in an additive manner (m (1) Giddings, J. C. Anal. Chem. 1984, 56, 1258-1270. (2) Marriott, P. J.; Shellie, R. Trends Anal. Chem. 2002, 21, 573-581. (3) Grob, K. On-Line Coupled LC-GC; Hu ¨ thig Buch Verlag: Heidelberg, Germany, 1991. 10.1021/ac060531r CCC: $33.50

© 2006 American Chemical Society Published on Web 07/28/2006

+ n, where m and n are the respective peak capacities or separation efficiencies). These powerful techniques, however, are time-consuming when the entire chemical composition of a sample should be analyzed. This requires a multitude of runs in which each “heart cut” covers a different section of the first-dimensional run. The current two-dimensional hyphenated techniques used in the analysis of highly complex mixtures, such as gas chromatography and mass spectrometry or “heart cut”-hyphenated chromatographic techniques, are already pushed close to their principal detection and separation limits. Therefore, only gradual further improvements can be expected, and thus, new analytical concepts need to be developed to achieve a more significant improvement. The recently introduced comprehensively coupled separation techniques offer a very large increase in separation power. Comprehensive coupling between separation techniques means by definition that the full separation or resolution capability of the individual separation systems is retained in the combined system, and the whole comprehensive separation takes place within the time frame of the primary separation step. If the individual separation efficiencies (i.e., the peak capacities) of the primary and secondary separation technique are n and m, the theoretical separation efficiency of the comprehensively coupled system is given by the multiplication of the individual separation system efficiencies (n × m). In this context, comprehensive twodimensional gas chromatography (GC × GC)4 thus can be viewed as a “heart cut” GC system with continuous fractionating and second dimension analysis over the first dimension in such a way that the obtained fractions are short enough not to reduce the resolution obtained in the first separation dimension. Comprehensively coupled multidimensional techniques, however, have more advantages than just increasing separation efficiency. Namely, the possibility of ad hoc differentiation between various compound classes or compound groups with similar physical or chemical properties needs to be mentioned here. This is demonstrated by Giddings’s theory on the concept of twodimensional separation systems.1 Giddings showed theoretically that the key property of a separation method, which determines whether it can show the inherent structure of a mixture being separated, is the method’s dimensionality. The dimensionality of a mixture is, thus, the number of independent variables for every member of the mixture. When a mixture is then separated on the basis of these independent separation variables, each compound will separate to a unique location on a separation plane. However, because the compounds are composed of molecules with discrete but related structures, the compounds must distribute over the dimensional separation space to discrete locations, which are also related to each other. Comprehensive two-dimensional gas chromatography (GC × GC),4 demonstrates Giddings’s theory quite well. In GC × GC, separation is achieved using two more or less orthogonal stationary phase - analyte interactions. Nonpolar stationary phase columns are most commonly used for the separation in the first dimension, resulting in the elution order of organic compounds being dominated by a temperature program. Separation is thus predominantly driven by volatility (i.e., boiling point separation). In the second dimension, which is run at quasi-isothermal (4) Phillips, J. B.; Beens, J. J. Chromatogr., A 1999, 856, 331-347.

conditions, the separation properties are rather focused on the chemical or physical interactions of the analyte with the stationary phase. The second-dimension separation in most applications is based on so-called polar separation using medium polar or polar columns. Note that the term “polar” separation is due to various physical-chemical interactions which, depending on the molecular interaction of the stationary phase and the analyte, can include dipole-dipole interactions, hydrogen bonding interactions or polarizability effects. The analytes of complex mixtures that have similar volatilities and polar interactions would then show up at similar locations on the two-dimensional chromatographic separation plane. This results in ordered rows or groups of compound peaks with similar chemical or physical properties.5 These ordered chromatograms make compound identification more rational and allow for fast screening of samples. The concept of two-dimensional gas chromatography has been applied by several research groups around the world2,4-7 and has initiated many other similar two-dimensional separation schemes, such as comprehensive liquid chromatography gas chromatography (LC × GC)8 or comprehensive supercritical fluid chromatography gas chromatography (SFC × GC).9 COMPREHENSIVE COUPLING OF GC AND MS: THE GC × MS APPROACH In contrast to comprehensively coupled separation techniques (e.g., GC × GC), the second dimension in hyphenated twodimensional technologies (e.g., GC/MS) often represents a spectroscopic identification technology. Because separation in this dimension is possible only through deconvolution of the spectral information, two-dimensional data representation does not result in ordered groups of signals representing different compounds with similar chemical/physical properties. In this context, MS exhibits interesting properties. On one hand, MS can be applied as a spectrometric technique, whereby the fragmentation fingerprint pattern of a molecule is used to determine its identity. On the other hand, MS separates molecular ions and fragments according to their mass, thus representing, literally, a separation technique. The separation aspect of MS is, however, in the most typical case (i.e., electron impact ionization, EI with 70-eV kinetic energy) dominated by the spectrometric aspect due to the fragmentation of the molecules. Venkatramani et al.10 proposed that systems such as coupled gas chromatography and mass spectrometry can also be regarded as comprehensive twodimensional systems (GC × MS). This is particularly true, however, if soft ionization methods such as photo ionization (PI), chemical ionization or field ionization are used for the ion generation in MS. Soft ionization avoids the formation of fragments and, therefore, fully reveals the “separation character” of mass spectrometry (i.e., molecular ions are separated by their mass). Thus, in principle, the comprehensive coupling of chromato(5) Beens, J.; Tijssen, R.; Blomberg, J. J. Chromatogr., A 1998, 822, 233-251. (6) Dimandja, J. M. D.; Stanfill, S. B.; Grainger, J.; Patterson, J. D. G. J. High Resolut. Chromatogr. 1999, 23, 208-214. (7) Welthagen, W.; Schnelle-Kreis, J.; Zimmermann, R. J. Chromatogr., A 2003, 1019, 233-249. (8) Janssen, H. G.; de Koning, S.; Brinkman, U. A. Anal. Bioanal. Chem. 2004, 378, 1944-1947. (9) Venter, A. Ph.D. Thesis; University of Pretoria: Pretoria, South Africa, 2003. (10) Venkatramani, C. J.; Xu, J.; Phillips, J. B. Anal. Chem. 1996, 68, 14861492.

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graphic and mass spectrometric separation is possible (GC × MS). Wang et al. have recently demonstrated that with appropriate gas chromatography separation parameters, that is, a polarity separation (column), coupled to mass spectrometry with soft ionization, the hyphenated system is independent enough to allow a comprehensive GC × MS separation approach.11 However, even with the use of a column that separates compounds according to selective interaction, such as polarity or polarizability, the effect of the volatility of compounds cannot be eliminated, since gas chromatographic separations are usually also driven by a temperature program. The resulting plot of chromatographic retention time against molecular mass thus results in inefficient use of the two-dimensional separation plane. In classical GC × GC (i.e., with a polar second column), this effect does not occur to this extent, because the second column is operated at quasi-isothermal conditions, emphasizing the orthogonal analyte-column stationaryphase interaction-driven aspect of the separation. In GC × MS, however, a more efficient use of the two-dimensional plane can be obtained by adjusting the retention time axis to a relative axis at which the retention time of the two-dimensional plot is normalized to the n-alkanes.11 Using this procedure, the volatility aspect of the chromatographic separation is eliminated. The used retention time transformation is a Kovats Index-like adjustment because it refers to the n-alkanes as the reference point. Relative chromatographic retention times are now used as the first separation dimension, and the mass of the molecular ion is used as the second dimension. These two dimensions are highly orthogonal; the criteria for a comprehensive gas chromatography mass spectrometry coupling (GC × MS) are met. PHOTOIONIZATION MASS SPECTROMETRY In the work presented here, soft laser-based photo ionization techniques are applied for usage as a second dimension on GC × MS. These ionization methods with inherently high selectivities are single photon ionization (SPI)12,13 and resonance-enhanced multiphoton ionization (REMPI)14-18 methods. In photo ionization, the ionization selectivity depends on the light wavelengths used. The use of different wavelengths may be viewed in this case as a further selectivity dimension that can be added to support peak identification by selectively ionizing different species or compound classes. REMPI mass spectrometry is the combination of two powerful detection techniques: ultraviolet (UV) laser spectroscopy and mass spectroscopy. The selectivity of REMPI depends on the UV absorption bands as well as on the ionization potential threshold. In single photon ionization, the target molecules are ionized in a single absorption step using vacuum ultraviolet (VUV) photons with sufficient energy to ionize the target molecule. Some (11) Wang, F. C. Y.; Qian, K.; Green, L. A. Anal. Chem. 2005, 77, 2777-2785. (12) Butcher, D. J.; Goeringer, D. E.; Hurst, G. B. Anal. Chem. 1999, 71, 489496. (13) Mu ¨ hlberger, F.; Wieser, J.; Morozov, A.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2005, 77, 2218-2226 (14) Lubman, D. M., Ed. Lasers and Mass Spectrometry; Oxford University Press: New York, 1990. (15) Lubman, D. M. Anal. Chem. 1987, 59, 31A-40A. (16) Boesl, U.; Neusser, H. J.; Schlag, E. W. J. Chem. Phys. 1980, 72, 43274333. (17) Boesl, U.; Weinkauf, R.; Weickhardt, C.; Schlag, E. W. Int. J. Mass Spectrom. Ion Processes 1994, 131, 87-124. (18) Zimmermann, R.; Lenoir, D.; Schramm, K.-W.; Kettrup, A.; Boesl, U. Organohalogen Compd. 1994, 19, 155-160.

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selectivity in SPI can thus be obtained by adjusting the energy of the photons to either exceed or fall short in the ionization potential (IP) of the target molecules. In this study, both ionization methods, REMPI and SPI, are used with time-of-flight mass spectrometry (TOFMS)14,19,20. Mass spectrometric techniques based on the soft photoionization methods SPI and REMPI are well-suited for online analytical applications.21-23 This is due to the high sensitivity, selectivity, and inherent softness of the laser-based ionization methods, such as SPI and REMPI. Obviously, the REMPI-MS and SPI-MS also represent interesting detectors for chromatographic methods. In particular, gas chromatography REMPI-TOFMS combinations have been often described.24-27 However, up to now, the intention of the preceding work was more focused on increasing the selectivity for specific applications. EXPERIMENTAL SECTION For the present study, an Agilent 6890N gas chromatograph (Agilent, U.S.A.) was connected via a heated transfer line (2 m, 300 °C) to the REMPI/SPI-TOFMS instrument28 which was previously used for various studies on tobacco smoke29,30 and contamination in the steel-recycling process22 (Figure 1). The setup is described in detail elsewhere;31 therefore, only a brief description is given here. Fundamental Nd:YAG laser pulses with a wavelength of 1064 nm and a repetition rate of 10 Hz are frequency tripled to a wavelength of 355 nm and used for simultaneous generation of ultraviolet and vacuum ultraviolet laser pulses for REMPI and SPI ionization, respectively. In doing so, 89% of the energy is guided into a β-BBO (β-barium borate) crystal of an optic parametric oscillator laser (GWU, Germany) to generate laser pulses in a range from 205 to 2500 nm. For the experiments in this article, three wavelengths (250, 275, and 300 nm) are used so that detection of several aromatic compounds is possible. A minor fraction (11%) of the laser beam is used for frequencytripling in a rare gas cell (xenon) to generate 118-nm laser pulses. The VUV beam with a wavelength of 118 nm is separated from the fundamental beam to avoid post-fragmentation of ions due to multiphoton absorption and directed into the ion source for the single photon ionization of the organic molecules in the gas sample. (19) Boesl, U.; Zimmermann, R.; Weickhardt, C.; Lenoir, D.; Schramm, K.-W.; Kettrup, A.; Schlag, E. W. Chemosphere 1994, 29, 1429-1440. (20) Lubman, D. M.; Kronick, M. N. Anal. Chem.,1982, 54, 660-665. (21) Mu ¨ hlberger, F. Doktorarbeit; Technische Universita¨t Mu ¨ nchen, Mu ¨ nchen, 2003. (22) Cao, L.; Mu ¨ hlberger, F.; Adam, T.; Streibel, T.; Wang, H. Z.; Kettrup, A.; Zimmermann, R. Anal. Chem. 2003, 75, 5639-5645. (23) Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H.; Kettrup, A.; Boesl, U. Anal. Chem. 1999, 71, 46-57. (24) Opsal, R. B.; Reilly, J. P. Anal. Chem. 1988, 58, 1060. (25) Dobson, R. L. M.; D’Silva, A. P.; Weeks, S. J.; Fassel, V. A. Anal. Chem. 1986, 58, 2129-2137. (26) Imasaka, T.; Okamura, T.; Ishibashi, N. Anal. Chem. 1986, 58, 2152-2155. (27) Zimmermann, R.; Lermer, C.; Schramm, K.-W.; Kettrup, A.; Boesl, U. Eur. Mass Spectrom. 1995, 1, 341-351. (28) Mu ¨ hlberger, F.; Zimmermann, R.; Kettrup, A. Anal. Chem. 2001, 73, 35903604. (29) Mitschke, S.; Adam, T.; Streibel, T.; Baker, R. R.; Zimmermann, R. Anal. Chem. 2005, 77, 2288-2296. (30) Adam, T.; Streibel, T.; Mitschke, S.; Mu ¨ hlberger, F.; Cao, L.; Baker, R. R.; Zimmermann, R. J. Anal. Appl. Pyrol. 2005, 74, 454-464. (31) Mu ¨ hlberger, F.; Hafner, K.; Kaesdorf, S.; Ferge, T.; Zimmermann, R. 2004, 76, 6753-6764.

Figure 1. (a) Experimental setup of the gas chromatography-laser mass spectrometry system. The inset shows the continuous supersonic jet nozzle that is used for interfacing the gas chromatograph with the mass spectrometer. Both photoionization methods, namely, resonanceenhanced multiphoton ionization and single photon ionization, can be applied for ionization. (b) REMPI and SPI ionization schemes: (bottom) In the SPI process, vacuum ultraviolet photons are used for ionization. Single photon absorption delivers the necessary ionization energy. This method ionizes universally all compounds with ionization energies below the respective photon energy. (top) In the REMPI process, two UV photons are absorbed simultaneously. Suited UV absorption bands in resonance with the one photon energy are prerequisite. This method is particularly well suited to ionize aromatic compounds.

Two computer-controlled beam blockers are used to select between the REMPI and SPI beams, allowing alternating application of REMPI and SPI with a corresponding frequency of 5 Hz.31 Both beams are focused underneath the inlet needle in the ion source. The generated molecular ions are extracted into the flight tube of the reflectron time-of-flight mass spectrometer (Kaesdorf Instruments, Germany). The TOF mass spectra are recorded via two transient recorder PC cards (Acqiris, Switzerland, 250 MHz, 1 GS/s, 128 k). The two cards are synchronized to achieve the necessary dynamic range within the recorded mass spectra. The recorded mass range is 27-430 m/z. Data processing is done by a LabView-based (National Instruments, U.S.A.) home-written software package. Some limits of detection of the instrument for on-line applications and a proof of linearity can be found in refs 31 and 32. A 30-m, 0.25-mm-i.d., 0.25-µm d.f, BPX50 (SGE, Australia) is used as the separation column with a temperature ramp of 5 °C/min from 40 to 360 °C and helium carrier gas at 200 kPa (constant pressure mode, 30 cm/s linear flow at 25 °C). A 0.1-µL diesel sample is injected “splitless” in a heated inlet at 325 °C. A Fischer-Tropsch diesel sample (source: SASOL Ltd., Sasolburg, RSA) with an alkane range from C4 to C28 was used as a test sample for demonstration. A diesel sample is the matrix of choice, since it is a well-studied mixture containing a variety of compounds of different polarity (mono-, di-, and triaromatics) and a boiling point range ideal for most GC separations. Furthermore an n-alkane standard containing C6-C22 was used for the (32) Hafner, K. Ph.D. Thesis; Technische Universita¨t Mu ¨ nchen, Mu ¨ nchen, 2004.

evaluation of fragmentation in the ion source and as a reference for shifting the retention times, as described later. The diesel sample has also been analyzed using GC × GC-FID for comparison with the GC × MS results. For the GC × GC measurements, a nonpolar column (30-m, 0.25-mm i.d., 0.25-µm d.f, RTX1 (Restek, Germany)) was used for the first dimension, and a polar column (1-m, 0.1-mm i.d., 0.1-µm d.f, RTX-wax (Restek, Germany)) was used for the second dimension. RESULTS AND DISCUSSION Three modes of GC-photoionization MS interfaces are described in the literature:27 effusive gas inlets, pulsed supersonic beams, and continuous supersonic beams. In an effusive inlet, the end of the heated capillary column is typically introduced directly into the ion source.17,25 With this technique, some of the selectivity potential of the REMPI photoionization method is reduced. This is due to the thermal excitation of vibrational and rotational intramolecular motions of the molecules in the hot eluent. This results in strong overlapping of the UV absorption bands. Effusive inlets are, however, simplistic and have small gas flows into the vacuum as well as a high sensitivity. In supersonic beam couplings, the eluent is mixed with an expansion gas (e.g., He or Ar) and expanded through a restriction nozzle into the vacuum of the ion source. The molecules are cooled to very low temperatures in this process, and thermal excitation of the vibrational and rotational intramolecular motions is reduced.27,33-35 The UV spectra (33) Stiller, S. W.; Johnston, M. V. Anal. Chem. 1987, 59, 567-572. (34) Hayes, J. M. Chem. Rev. 1987, 87, 745.


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of these cooled molecules often feature very sharp absorption bands that could be compared to those obtained in IR spectroscopy fingerprinting and allow selective ionization of target compounds.27,35 Usually, supersonic jets are pulsed to reduce the gas load of the vacuum. For continuous jet expansions, differential pumping steps need to be implemented to handle the high gas flow into the vacuum system. Supersonic jets are also used to reduce fragmentation in electron impact ionization.36 In this work, in addition to a conventional effusive inlet, a novel continuous micro capillary jet system 37 was used for the first time as a GC × MS interface. This device combines the cooling power of conventional jet systems with the low gas flow of the effusive inlet system. A detailed description of micro capillary jet inlet systems can be found in Hafner et al.37 The jet inlet nozzle was made from a commercial deactivated fused-silica capillary (deactivated gas chromatography capillary without stationary phase) with an i.d. of 0.53 mm. The tip of the capillary column was carefully melted until closure while rotating it to achieve rotational symmetry. Subsequently, the closed capillary tip was abraded with fine sandpaper to produce a well-defined nozzle opening. The nozzle was abraded to a desired gas flow rate of ∼10 mL/ min. The cooling efficiency of the used nozzle was measured by scanning the A-X absorption band of nitric oxide, at 226.1 nm with a backpressure of 2 bar, by UV-wavelength scans measured with the previously described REMPI-SPI-TOFMS instrument. The resulting spectra are compared with simulated spectra (LIFBase software38) to evaluate the cooling efficiency (rotational temperature) of the supersonic jet. The respective REMPI spectra of NO obtained by the installed jet with selected buffer gases are shown in Figure 2. For the GC carrier gas, helium, the resulting rotational temperature was determined to be 200 K. For further investigation of the jet performance, similar measurements were carried out using the improved cooling efficiency of argon and nitrogen. The data and related simulation shown in Figure 2 indicate a rotational temperature of 60 and 110 K for argon and nitrogen, respectively. The jet capillary inlet into the ion source was heated throughout to 523 K. First studies with an effusive system exhibited that some thermally induced fragmentation of fragile compounds, such as alkanes, occurs (Figure 3b). When the capillary jet was installed, practically no residual fragmentation was detectable. The resulting fragmentation-reduced spectrum of the alkane nonane is shown in Figure 3c. Note that the gas flow into the instrument with the supersonic micronozzle is not reduced with respect to the effusive inlet. (i.e., no decrease in sensitivity). In Figure 3a, the 70-eV EI spectrum of nonane is shown for comparison (data source: NIST39). The striking advantage of photoionization methods is their capability to efficiently ionize organic molecules without fragmentation (soft ionization). Because the ionization efficiency depends on the photon flux and advanced high-flux light sources, such as lasers or novel lamps,13 are readily available, high sensitivities can (35) 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. (36) Amirav, A.; Danon, A. Int. J. Mass Spectrom. Ion Processes 1990, 97, 107113. (37) Hafner, K.; Zimmermann, R.; Rohwer, E. R.; Dorfner, R.; Kettrup, A. Anal. Chem. 2001, 73, 4171-4180. (38) Luque, J. LIFBASE Software 2.0; http://www.sri.com/psd/lifbase/, 2005. (39) http://webbook.nist.gov/chemistry/; June 2005 release; NIST, 2005.

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Figure 2. Measurement of the cooling efficiency of the continuous micronozzle supersonic jet interface. The measured REMPI spectrum (ion yield versus photon energy) of the rotational contour of the AX band of NO is shown with the respective overlaid best fitting simulated contour. With argon, a rotational temperature of 60 K is reached, whereas with nitrogen and helium, 110 and 200 K, respectively, are achieved.

be achieved in photoionization mass spectrometry. Furthermore, PI methods, such as REMPI, can be highly selective, that is, can be used to selectively address specific compounds or compound classes. This results in an additional optical dimension for the GC/ MS, making it possible to further simplify the mass spectra. To demonstrate the properties of the different ionization schemes, the summed mass spectral patterns from the whole gas chromatographic run of the diesel samples were recorded. The summed mass spectra in principle shows the mass spectrum that one would obtain if the sample is directly inserted and fully vaporized in the ionization region (e.g., as in an on-line measurement application21-23). The summed mass spectra using EI (70 eV), SPI (118 nm), and REMPI (at different wavelengths: 250, 275, and 300 nm), are shown in Figure 4 a-e. The EI mass spectrum (Figure 4a) shows solely hydrocarbon fragmentation peaks; subsequently, no definite molecular information can be extracted. Thus, only the information that the samples predominately consist of aliphatic hydrocarbons can be extracted from the EI mass spectrum. In contrast, the mass spectra obtained by soft photo ionization methods show clear molecular information. The summed SPI (118 nm) mass spectrum clearly depicts the homologue row of the alkanes (marked by an asterisk (*) in Figure 4b). Other homologue rows can also be assigned. Note that each peak may consist of several isomeric compounds. Because aliphatic compounds are in higher concentration in the

Figure 3. Effect of the supersonic jet cooling on the fragmentation of alkanes in the SPI process. Two SPI mass spectra of nonane (b, c) are shown and compared to the 70-eV EI spectrum obtained from NIST (a). The upper SPI spectrum (b) is recorded with a conventional effusive inlet. The temperature of the gas molecules is ∼525 K. Some thermally induced fragmentation is observed. The lower mass spectrum (c) is recorded with the micro supersonic jet nozzle. The molecules are cooled to ∼200 K (rotational temperature), and the thermal fragmentation has vanished.

diesel mixture, the respective aliphatic homologue rows dominate the visible peaks on the spectrum. Although the REMPI method does not address aliphatic compounds because they do not have suited absorption bands in the applied UV range. Moreover, aromatic compounds can be ionized in the used UV wavelength range. In Figure 4c, d, and e, the summed REMPI mass spectra of the Fischer-Tropsch diesel sample, recorded with different laser wavelengths (250, 275, and 300 nm) are presented. A large number of homologue rows of alkylated aromatic species are observable. The differences in the three summed REMPI mass spectra visualize the wavelength-dependent differences in the REMPI efficiency for different aromatic compound classes. Whereas with a laser wavelength of 250 nm, the vast majority of the aromatic compounds are ionized (a more or less “universal aromatic ionization”), the number of accessible aromatic homologue rows decreases toward longer wavelengths. This is mainly due to the decrease of the two-photon energy with increasing wavelengths. Compounds with ionization potentials higher than the two-photon energy are excluded from ionization. Furthermore, there is an influence of the UV spectroscopy because some compound classes do not have suited UV absorption bands at all used wavelengths. The homologue rows of, for example, the isobaric compounds anthracene and phenanthrene (starting at m/z ) 178), as well as the homologue rows of fluorenes (starting at m/z ) 166) and the

isobaric compounds biphenyl and acenaphthene (starting at m/z ) 154), are detectable with all applied REMPI laser wavelengths. With a laser wavelength of 250 nm (two-photon energy: 9.91 eV) for REMPI, most polyaromatic compounds, including the naphthalenes, are ionized efficiently. Furthermore, homologue rows of compounds with a single aromatic ring, such as benzenes, naphthenes, styrenes, and indanes, are ionized. With 275 nm (twophoton energy: 9.01 eV), some of the polycyclic compounds are less prominent. Benzene (IP: 9.2 eV) and toluene (IP: 8.8 eV) are not detectable anymore, but phenol is now accessible. With a REMPI laser wavelength of 300 nm (two-photon energy: 8.26 eV), the picture has changed more drastically. All compounds with a single aromatic ring (naphthenes, benzenes, etc.) are now excluded from ionization. The two-ring aromatic naphthalenes still are ionized, but they are comparably weak due to very weak UV transition at this wavelength. Therefore, the spectrum is largely simplified, showing predominately three-ring aromatics. If the whole complexity of a sample should be revealed, however, the mass spectral information is not sufficient. The gas chromatographic separation is required to distinguish isobaric and isomeric compounds, such as, for example, the different branched and linear alkanes, the alkenes, and cycloalkanes, or the homologue row of isobaric carbon skeletons, such as phenanthrene and anthracene (starting at m/z ) 178), biphenyl, and acenaphthene Analytical Chemistry, Vol. 78, No. 18, September 15, 2006

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Figure 4. Mass spectra summed over the total gas chromatographic run of the diesel sample recorded with different ionization techniques: (a) Conventional 70 eV electron impact ionization (EI). The mass spectrum is dominated by fragments of aliphatic compounds. (b) Soft SPI ionization (118 nm/10.5 eV). The mass spectrum is dominated by the homologue aliphatic rows, most prominent the alkanes. (c) Soft REMPI ionization (250 nm/4.95 eV). The mass spectrum shows nearly all aromatic compounds, including benzene and toluene. Aliphatics are suppressed. (d) Soft REMPI ionization (275 nm/4.5 eV). The mass spectrum shows nearly all aromatic compounds with slight changes in the intensity profiles. Toluene and benzene are not visible at this wavelength, and phenol is enhanced. Aliphatics are suppressed. (e) Soft REMPI ionization (300 nm/4.13 eV). The mass spectrum shows only aromatic compounds with three or more rings. Naphthalenes and benzene derivates, as well as the aliphatics, are excluded from ionization.

(starting at m/z ) 154) or indane (starting at m/z ) 118) and styrene (starting at m/z ) 104). As discussed above, the data of GC/MS data can be represented as a two-dimensional contour plot when the retention time 6370


is plotted against the m/z values. This is shown in the example for the GC-SPI-TOFMS data set (Figure 5a and b). Due to the soft ionization capability of SPI, only molecular ion signals are present, thus resembling a boiling point-type separation obtained

Figure 5. Comprehensive gas chromatography coupled to soft ionization mass spectrometry (GC × MS) representation, using SPI-mass spectrometry. (a) With the GC × MS chromatogram before transformation and (b) the chromatogram after doing the “Kovats Index”-like transformation of the retention time axes.

with a nonpolar gas chromatographic column. A retention time shift was applied to eliminate the influence of the temperature program and increase the orthogonality of the two separation axes in analogy to the work of Wang et al. 11 (Figure 5a). The retention time axis is adjusted so that the n-alkanes in the final GC × MS plot a straight line. The adjustment is achieved by doing a 10-

point polynomial fit on the retention times of the n-alkanes and then shifting the retention times so that the n-alkanes align with one another. With the retention time adjustment, the temperature program influence of the chromatographic separation is “eliminated”. Thus, the relative time scale now represents the polar separation Analytical Chemistry, Vol. 78, No. 18, September 15, 2006

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component of the gas chromatographic separation (GC × MS plot). In the GC × SPI-TOFMS plot (Figure 5b), compounds with an ionization potential below 10.5 eV (∼118 nm) are appearing. The enlarged section depicts the aliphatic compounds from C14 to C17 with the alkanes CnH2n+2; the cyclic alkanes or alkenes CnH2n; and the dienes, cyclic alkenes, or alkynes CnHn-2 and demonstrates the type of separation achieved through the comprehensive combination of GC and SPI-TOFMS. The aliphatic compounds in the sample dominate the GC × MS plot, as is expected from diesel samples. Aromatic compounds also are easily detectable, but according to their lower abundance, they appear at considerably smaller signal intensities. Note that by changing of the wavelength of the SPI process, some compound classes can be excluded from ionization. For example, SPI-TOFMS systems based on newly developed highly efficient VUV lamps are available at various wavelengths13,40 and can be used as a selective GC detector. With a wavelength of 126 nm (9.86 eV), for example, most aliphatic compounds would be excluded, and the GC × SPI-TOFMS plot would be dominated by aromatic species and conjugated unsaturated species. A similar effect, that is, the selective detection of aromatic species but at a tremendously increased sensitivity, is obtained by application of the laserbased resonance-enhanced multiphoton ionization method. With ion yields of up to the 10% range in the laser focus, REMPI is one of the most efficient ionization methods for aromatic compounds.41 In Figure 6a-c, three GC × MS plots obtained by the REMPI ionization method at wavelengths of 250, 275, and 300 nm are shown. Because REMPI addresses only aromatic compounds at the applied laser wavelength, this is an example of highly chemicalstructure-selective ionization. From the GC × REMPI-TOFMS contour plots, similar trends in compound class selectivity can be seen, as in the summed mass spectra at these wavelengths (Figure 4). With longer wavelengths, the accessible compound classes are reduced. On each of the GC × REMPI-TOFMS plots, a small section is enlarged to demonstrate the separation possibilities of the comprehensively coupled gas chromatography and soft REMPI ionization MS technique. In Figure 6a, the GC × REMPI-TOFMS plot obtained with a laser wavelength of 250 nm is depicted. Here, most aromatic compounds are addressed. The inset shows the enlarged origin area of the homologue row of the four-cyclic aromatic pyrene system (starting at 202 m/z). According to the increasing nonpolar character, the relative retention time tends to decrease with increased alkylation (i.e., increased number of aliphatic carbon atoms in the molecule), demonstrating the chemical logics behind the obtained comprehensive GC × MS separation. In the GC × REMPI-TOFMS plot obtained from the data with 275-nm laser ionization wavelength (Figure 6b), some of the two- and three-ring aromatic systems are particularly efficiently ionized. This is seen in the extracted sections showing the origin region of the three-ring aromatic homologous series of the isobaric anthracene and phenanthrene (starting at 178 m/z). The GC × REMPI-TOFMS plot obtained from the 300-nm laser wavelength (Figure 6c) shows the fewest compounds ionized, because, for example, all compounds with (40) Mu ¨ hlberger, F.; Wieser, J.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2002, 74, 3790-3801. (41) Boesl, U.; Heger, H.-J.; Zimmermann, R.; Nagel, H.; Pu ¨ ffel, P. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; John Wiley & Sons: Chichester, 2000, pp 2087-2118.

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only a single aromatic ring are now excluded from ionization. Due to weak UV absorption, the naphthalenes as well as some larger PAHs are considerably weaker at 300 nm. The inset shows the region of the homologue rows of the relatively weak naphthalene series (starting at 128 m/z) as well as the more intense fluorene series (starting at 166 m/z). A particularly interesting representation of chemical information in the GC × MS plot can be obtained if the combined SPITOFMS and the REMPI-TOFMS (250 nm) data are displayed (Figure 7a). Peaks obtained by SPI are depicted in red, and the REMPI data is displayed in blue. In this representation, the blue peaks, thus, originate from the aromatic systems and the predominant part of the red colored peaks belong to aliphatic structures. Note that the intensity of the aromatic systems is exaggerated due to the higher ionization efficiency of REMPI. However, in principle, it is possible to apply suitable scale factors to approximate the REMPI and SPI sensitivities for aromatic compounds. The additional chemical classification allows the adhoc differentiation of overlapping peaks, for example, in the naphthene region of the GC × MS chromatogram. The combination of the GC × MS separation is very comparable to the separation achieved by the conventional two-dimensional comprehensive gas chromatography approach (GC × GC). In Figure 7b, a GC × GC separation of the same diesel samples is shown. Similar groupings of different compound classes can be made in both GC × GC and GC × MS similarly to the differentiation among aliphatics, one-ring aromatics, two-ring aromatics, threering aromatics, and even higher aromatics as indicated on the GC × MS plot. Note that the GC × REMPI-TOFMS and GC × SPI-TOFMS data were obtained in separated runs in the presented work; however, the used home-built TOFMS system has the ability to record simultaneously REMPI- and SPI-TOFMS data (at 5 Hz) as well as the electron impact data (EI). Thus within a single chromatographic run, the above-mentioned aromatic/aliphaticselective GC × MS plot can be obtained as well as the conventional GC/EI-MS data. The EI-MS data allow the use of conventional fragmentation libraries for identification of sufficiently separated peaks. This, however, is not relevant for discrimination of branched hydrocarbons, since the distinction of isomers in most cases is not possible. The combination of comprehensive separation (i.e., the position of a peak in the GC × MS plane) and the chemical selectivity introduced by the application of different photoionization methods (here, REMPI and SPI for discrimination of aromatic and aliphatic structures) represents a novel separation/classification concept. The photoionization selectivity can be further changed by using different ionization wavelengths for REMPI or SPI. With, for example, different wavelengths for the REMPI ionization process, specific aromatic compound classes can be either emphasized or suppressed. Therefore, the selectivity of the comprehensive separation can be adjusted to the separation/analysis problem. An additional possibility to further increase the separation power of the analysis system is to use a high-resolution TOFMS system for determination of the elemental composition. The sensitivity of the REMPI process is extremely high for aromatic compounds41 so that despite the relatively low duty cycle MS detection, sensitivities equal to or better than with EI are obtained. The

Figure 6. Transformed GC × MS representation of the REMPI-mass spectrometry coupled to gas chromatography. The retention times being adjusted to the same polynomial function used in the GC × SPI TOFMS retention shift. Three different REMPI wavelengths were used: (a) 250, (b) 275, and (c) 300 nm.


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Figure 7. GC × SPI TOFMS and GC × REMPI TOFMS (250 nm) plotted on the same two-dimensional contour plot for an overview of the total ionized molecules. The SPI peaks are color-coded from green to blue (blue, highest intensity), and the REMPI, from green to red (red, highest intensity). The approximate location of aliphatics and the different aromatic species are encircled for comparison with GC × GC results (Figure 8).

Figure 8. Comprehensive two-dimensional gas chromatography plot of the same diesel sample, indicating approximate location of compound class groupings similar to those obtained through the GC × MS analysis (Figure 7).

sensitivity of the laser-based SPI method is lower than the REMPI process. This is due to the rather inefficient conversion of UV laser photons to VUV photons in the third harmonic generation process.21 The sensitivity of the SPI method is proportional to the applied VUV photon flux; therefore, it can be increased by using more intense VUV light sources. This is advantageous in comparison to some other soft ionization methods, such as field ionization,11 which are rather insensitive without much potential for further optimization. In context, it should be mentioned that we introduced a novel, very bright VUV lamp (electron beam pumped excimer lamp) as the light source for SPI mass spectrometry. The novel light source was already operated 6374


with time-of flight MS13,40 and quadruple MS13 for on-line applications. Very recently, an orthogonal acceleration time-of-flight mass spectrometry system was equipped with the novel photoionization technology and successfully coupled to a gas chromatograph for a GC × SPI demonstration. CONCLUSION Soft ionization mass spectrometry coupled to gas chromatography allows a comprehensive characterization of highly complex samples (GC × MS) that is similar to two-dimensional comprehensive gas chromatography (GC × GC). This investigation furthermore shows that photoionization exhibits particularly

interesting properties for comprehensive two-dimensional gas chromatography mass spectrometry separations (GC × MS). Future work will include the comprehensive coupling of GC × GC and MS (GC × GC × MS), resulting in a three-dimensional separation system. ACKNOWLEDGMENT Werner Welthagen thanks the Bavarian Research Foundation (BFS) and Stefan Mitschke thanks British American Tobacco

(BAT) for their financial support. Fabian Mu¨lberger, Thomas Adam, and Thorsten Streibel provided help with the instrumentation setup.

Received for review March 23, 2006. Accepted June 19, 2006. AC060531R


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