Anal. Chem. 1998, 70, 3957-3963
Determination of Halogenated Hydrocarbons by Helium Microwave Plasma Torch Time-of-Flight Mass Spectrometry Coupled to Gas Chromatography Brian W. Pack, Jose´ A. C. Broekaert,† John P. Guzowski, John Poehlman, and Gary M. Hieftje*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405
A helium microwave plasma torch (MPT) was coupled to time-of-flight mass spectrometry (TOFMS) for the detection of halogenated hydrocarbons separated by capillary gas chromatography (GC). The GC-MPT-TOFMS system offered excellent stability over the course of the experiments and avoided mass spectral peak distortions caused by spectral skew. In the initial studies, empirical formulas based on the halogen-to-carbon ratio were predicted utilizing a flow cell apparatus. The MPT proved to be very robust and could handle large amounts of organic vapor. Results from this study indicate that, for both aromatic and aliphatic halogenated hydrocarbons, the ratios of carbon to chlorine signals correlate well (r ) 0.994) with the ones expected from their chemical composition. This study was later extended to include chromatographic separation. For a series of homologous aliphatic halogenated hydrocarbons, a correlation coefficient of 0.999 was obtained for both peak heights and peak areas obtained from a single chromatogram. A novel Nichrome wireheated transfer line was developed to ensure that the capillary column was heated efficiently from the GC oven to the MPT and then through the length of the MPT up to the microwave plasma itself. No appreciable peak broadening and no detectable memory effects were associated with the heated transfer line. The GC-MPT-TOFMS system offered equal sensitivity for I, Br, and Cl. Absolute detection limits for the halogenated hydrocarbons ranged from 160 to 330 fg, constituting an improvement by a factor of 5-35 over earlier results obtained with MIPs supported in a TM010 cavity and combined with quadrupole-based mass spectrometry. In addition, the effect of molecular gases on the MPT performance was investigated. Up to about 1% (v/v) of either oxygen or hydrogen in the central channel helium flow attenuated the signal levels for both carbon and chlorine, with the larger loss seen in the chlorine signal. Element-selective detection by atomic spectrometry has proven to be important in both gas and liquid chromatography. Especially useful in this application has been the microwave-induced plasma † On leave from Department of Chemistry, University of Dortmund, D 44221 Dortmund, Federal Republic of Germany.
S0003-2700(97)00534-9 CCC: $15.00 Published on Web 08/05/1998
© 1998 American Chemical Society
(MIP), which was first coupled to gas chromatography (GC) in 1965.1,2 Unlike most other plasma sources, the MIP can be operated with several different support gases, commonly Ar or He. Further, the MIP requires very low gas flow rates and instrumentation that is simpler and less expensive than those required by other sources.3 All of these attributes make the MIP a desirable choice as an ionization or excitation source when coupled to GC for element-specific detection. For element-specific detection in chromatography, a helium MIP (He-MIP) is commonly utilized since it can employ the GC carrier gas directly. Also, the He-MIP is more efficient than an Ar-MIP in exciting halogen atomic emission lines in the visible region of the spectrum.4-6 Unfortunately, the first He-MIPs could be stably operated only at reduced pressure. In 1976, however, the introduction of the TM010 microwave cavity provided a means of maintaining a stable He-MIP at either atmospheric or reduced pressure.7,8 Since that time, the TM010 cavity has been used extensively in gas chromatography and is now incorporated in a commercial atomic emission detector for GC. This instrument has become a routine tool for pesticide residue analysis and for other GC work;9 recently, it has found favor also in applications involving metal speciation.10,11 Recently, there has been considerable interest in using the MIP not only as an atomic emission source but also as an ion source for mass spectrometry (MS). A MIP was first coupled to an MS system in 1981.12 However, it proved to be necessary to use aerosol desolvation; even then, the MIP was perceived to suffer from more severe matrix effects than the Ar ICP. (1) McCormack, A. J.; Tong, S. C.; Cook, W. D. Anal. Chem. 1965, 37, 14701478. (2) Bache, C. A.; Lisk, D. S. Anal. Chem. 1967, 39, 786-789. (3) Long, G. L.; Ducatte, G. R.; Lancaster, E. D. Spectrochim. Acta, Part B 1994, 49B, 75-87. (4) Carnahan, J. W.; Caruso, J. A. Anal. Chim. Acta 1982, 136, 261-267. (5) Spencer, B. M.; Raghani, A. R.; Winefordner, J. D. Appl. Spectrosc 1994, 48, 643-646. (6) Van Dalen, H. P. J.; Kwee, B. G.; De Galan, L. Anal. Chim. Acta 1982, 142, 159-171. (7) Beenakker, C. I. M. Spectrochim. Acta, Part B 1976, 31B, 483-486. (8) Beenakker, C. I. M. Spectrochim. Acta, Part B, 1977, 32B, 173-187. (9) Uden, P. C. Element-Specific Chromatographic Detection by Atomic Spectroscopy; American Chemical Society: Washington, DC, 1992. (10) Lobinski, R.; Adams, F. C. Anal. Chim. Acta 1992, 262, 285-297. (11) Lobinski, R.; Adams, F. C. Trends Anal. Chem. 1993, 12, 41-49. (12) Douglas, D. J.; French, J. B. Anal. Chem. 1981, 53, 37-41.
Analytical Chemistry, Vol. 70, No. 18, September 15, 1998 3957
These limitations now appear to be due in large part to the use of a TM010 cavity. The TM010 plasma has an extremely low tolerance to molecular species and, therefore, requires venting of the solvent peak. Further, even small amounts of carbon deposited on the quartz discharge tube devitrify it and alter the tuning characteristics of the plasma.13 Such problems do not occur with the microwave plasma torch (MPT).14,15 The MPT produces a flamelike plasma, with a central channel through which analyte material is introduced. This feature promotes efficient analyte-plasma interaction. The MPT has also proven to be extremely robust and resistant to the introduction of molecular species.16 Plus, the plasma is formed at the edge of a conductive tube in which a quartz tube with smaller diameter can be placed concentrically and used for the introduction of aerosols or vapors.17 The MPT was employed early as a source for both atomic emission spectrometry15 and atomic mass spectrometry18 and has been coupled successfully with several alternative sample introduction techniques.19-21 In the present study, the MPT was investigated as an ion source for time-of-flight MS (TOFMS). TOFMS is ideally suited for GC analysis. The transient nature of the signal in GC demands that the entire spectral range of interest be covered in a nearly simultaneous fashion; if not, the changing concentration in an eluting peak will cause a massdependent distortion in the measured signals. The TOFMS in use at this laboratory has been demonstrated to possess this ability.22,23 It offers the advantages of complete elemental and isotopic coverage and higher resolution than quadrupole instruments ordinarily used in ICPMS.24,25 With the advantages cited above for the MPT over the MIP (in a TM010 cavity) and the TOFMS over the quadrupole, the coupling of an MPT with TOFMS seems ideally suited for GC detection. The present study explores the coupling of GC with MPTTOFMS and their use for the element-selective detection of the halogens. The greatest effort was devoted to the effective coupling of the MPT to the TOF instrument; of lower priority was optimization of the GC separation. The study aims at an evaluation of analytical figures of merit, including power of detection and the precision of elemental ratio determinations. These features (13) Valente, A. L. P.; Uden, P. C. J. High Resolut. Chromatogr. 1993, 16, 275288. (14) Jin, Q.; Zhu, C.; Borer, M. W.; Hieftje, G. M. Spectrochim. Acta, Part B 1991, 46B, 417-430. (15) Jin, Q.; Wang, F.; Zhu, C.; Chambers, D. M.; Hieftje, G. M. J. Anal. At. Spectrom. 1990, 5, 487-494. (16) Camuna-Aguilar, J. F.; Pereiro-Garcia, R.; Sanchez-Uria, J. E.; Sanz-Medel, A. Spectrochim. Acta, Part B 1994, 49B, 545-554. (17) Pack, B. W.; Hieftje, G. M. An Improved Microwave Plasma Torch for Atomic Emission Spectrometry, Submitted to Spectrochim. Acta, Part B. (18) Wu, M.; Duan, Y.; Hieftje, G. M. Spectrochim. Acta, Part B 1994, 49B, 137148. (19) Pereiro, R.; Wu, M.; Broekaert, J. A. C.; Hieftje, G. M. Spectrochim. Acta, Part B 1994, 49B, 59-73. (20) Duan, Y.; Wu, M.; Jin, Q.; Hieftje, G. M. Spectrochim. Acta, Part B 1995, 50B, 1095-1108. (21) Madrid, Y.; Wu, M.; Jin, Q.; Hieftje, G. M. Anal. Chim. Acta 1993, 277, 1-8. (22) Mahoney, P. P.; Ray, S. J.; Li, G.; Hieftje, G. M. Electrothermal VaporizationInductively Coupled Plasma-Time-of-Flight Mass Spectrometry. Submitted to Anal. Chem., 1996. (23) Myers, D. P.; Li, G.; Yang, P.; Hieftje, G. M. J. Am. Soc. Mass. Spectrom. 1994, 5, 1008-1016. (24) Myers, D. P.; Mahoney, P. P.; Li, G.; Hieftje, G. M. J. Am. Soc. Mass. Spectrom. 1995, 6, 920-927. (25) Furuta, N. J. Anal. At. Spectrom. 1991, 6, 199-205.
3958 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
Table 1. Instrumental Components gas chromatograph capillary column
GC Perkin-Elmer F45 headspace analyzer 30-m DB-5, 0.25-mm i.d. (J&W Scientific)
Microwave Plasma Microtron 200 Mark 3 (ElectroMedical Supplies, Ltd.) microwave plasma torch laboratory constructed19 microwave power cable LDF4-50A (Cable Wave Systems) microwave generator
500 MHz digital oscilloscope constant fraction discriminator rate meter Quadra 950 computer LabVIEW 2 gated integrator boxcar averager
Data Acquisition Tektronix model TDS520 Tennelec TC 454 Quad CFD Ortec 9349 Log/Lin rate meter Macintosh National Instruments Stanford Research Systems
are explored for halogenated aliphatic and aromatic hydrocarbons which are representatives of a relevant class of substances for environmental studies. EXPERIMENTAL SECTION Instrumentation (Table 1). The design of the MPT was altered somewhat17 to improve its performance as an ion source for mass spectrometry. First, the central tube in the MPT that was originally constructed of copper was replaced by one made of quartz. This modification enabled a helium discharge to be sustained at input microwave power levels from 100 to 200 W, whereas the earlier design produced an unstable helium discharge that spontaneously changed to a filament-type plasma at low power levels or when a GC effluent was introduced into it.15 Also, the original MPT had a large slot in its side that permitted the microwave coupling loop to be slid up and down and also promoted cooling of the torch by the atmosphere. Unfortunately, this slot also increased air entrainment into the plasma. As reported by other investigators,26 air entrainment reduces the ionization potential of the plasma to that of NO (9.25 eV), which degrades its ability to ionize the halogens efficiently. In the modified MPT design, the slot was removed, and a Swagelok fitting was added at the base of the torch. This alteration allowed a laminar sheath of nitrogen to be introduced around the plasma to reduce air entrainment into the plasma. With the sheath gas in operation, the dominant ions in the background mass spectrum become He+, He2+, N2+, and N+ (see Figure 1). The relatively uncluttered spectrum below 40 amu simplifies the determination of sulfur and phosphorus by reducing isobaric overlaps from NO and O2, respectively. The mass spectrum of carbon tetrachloride vapor (Figure 1) does not indicate accurately whether the N2+ or the N+ peak is most intense, since ions in this entire region of the spectrum are being intentionally deflected from the path to the detector.27 Ideally, ions with m/z 14 and 28 would be deflected independently, but the design of the present deflection circuit mandates that the entire span between m/z 14 and 28 be deflected. This requirement precluded the determination of fluorine in the current study. The nitrogen sheath gas was used in all of the halogen determinations. (26) Olson, L. K.; Caruso, J. A. Spectrochim. Acta, Part B 1994, 49B, 7-30. (27) Myers, D. P.; Hieftje, G. M. Microchem. J. 1993, 48, 259-277.
Figure 1. Mass spectrum of carbon tetrachloride obtained with the flow cell sample introduction technique and with a digital oscilloscope used for data acquisition. Peaks arising from air entrainment have been eliminated by a flowing sheath of nitrogen. Peak undershoot is caused by ringing in the high-speed amplifier used in this study. Table 2. Operating Conditions Microwave Plasma forward power reflected power central channel flow rate plasma gas flow rate sampling distance TOFMS aluminum sampling aperture diameter stainless steel skimmer aperture diameter third stage aperture diameter first stage pressure second stage pressure drift region pressure repeller frequency
200 W 250 °C) with a current-regulated voltage supply.
of lengthened analyte residence time in the plasma. This flow rate was used for both the steady-state and GC measurements.
Plasma positioning was the most important of the optimization variables. Since the analyte density is highest in the central channel of the discharge, the plasma must be centered on the sampler to yield the strongest and stablest signals. The distance between the top of the MPT and the sampling plate was varied between 5 and 10 mm. The resulting signal was greatest and most stable with a distance of approximately 7 mm between the top of the MPT and the sampler. Detection limits and empirical formula data were collected at this sampling distance. Empirical Formula Determinations. The determination of empirical formulas in GC-MIP-AES has received a considerable amount of attention, as it provides unambiguous peak identification in chromatograms.32 Because a TOFMS extracts all ions from the source (MPT) simultaneously, it simplifies the determination of empirical formulas by avoiding spectral skew.33 With quadrupole-based (or any scanning) instruments, the analyte concentration in an eluting peak changes as the spectrum is scanned, causing mass-dependent errors in signal amplitudes. This “spectral skew” is often difficult to correct and can lead to substantial errors in ion ratios, especially for sharp GC peaks. In contrast, the TOFMS system monitors the entire mass spectral range of interest in a nearly simultaneous fashion, so spectral skew can be avoided. The data in this portion of the study were acquired by means of the flow cell and the Tektronix TDS520 500-MHz digital oscilloscope. In Figure 4a is illustrated the ability of the MPTTOFMS to determine precisely Cl/C ratios in several halogenated hydrocarbons. The experimental ratios for aromatic, aliphatic, di-, tri-, and tetrasubstituted halogenated hydrocarbons all agree well with the Cl/C ratios derived from their empirical formulas; the linear regression yields a correlation coefficient of 0.994. When a series of aliphatic hydrocarbon homologues is examined alone (see Figure 4b), a correlation coefficient of 0.996 is obtained. Effect of Added Gases. Foreign gases added to a plasma can sometimes enhance its analytical performance. For example, a signal increase is typically observed when hydrogen is added to an ICP.34 The same benefit might exist for the MPT. In addition, in GC-MIP work, oxygen is commonly added to the plasma gas to serve as a scavenger for carbon. The addition of oxygen also prolongs the lifetime of the quartz discharge tube, reduces long-term memory effects, and lessens peak broadening caused by carbon deposition on the quartz. Of course, the MPT plasma is not contained in a tube as is the plasma sustained by a TM010 resonator, so the addition of oxygen is not necessary for this purpose; however, oxygen might serve to thermalize the plasma. The flow-through cell was utilized for sample introduction in this experiment. It was found that the addition of either 1% oxygen or 1% hydrogen to the central channel flow resulted in a loss of signal for both chlorine and carbon; however, the loss is greater for chlorine than for carbon. Presumably this effect occurs because both molecular gases serve to thermalize the discharge, thereby lowering its effective ionization temperature. (32) Sullivan, J. J.; Quimby, B. D. J. High Resolut. Chromatogr. 1989, 12, 282286. (33) Holland, J. F.; Enke, C. G.; Allison, J.; Stults, J. T.; Pinkston, D.; Newcome, B.; Watson, J. T. Anal. Chem. 1983, 55, 997A-1012A. (34) Sesi, N. N.; MacKenzie, A.; Shanks, K. E.; Yang, P.; Hieftje, G. M. Spectrochim. Acta, Part B 1994, 49B, 1259-1282.
Figure 4. Comparison of Cl/C ratios obtained from flow cell MPTTOFMS measurements to values derived from empirical formulas. (a) Ratios obtained from both aromatic and aliphatic species. Compounds used and corresponding Cl/C ratios: chlorotoluene (Cl:C 0.143), chlorobenzene (Cl:C 0.167), dichlorobenzene (Cl:C 0.333), chloroheptane (Cl:C 0.143), chlorohexane (Cl:C 0.167), chloropentane (Cl:C 0.200), chlorobutane (Cl:C 0.250), chloropropane (Cl:C 0.333), methylene chloride (Cl:C 2.0), chloroform (Cl:C 3.0), and carbon tetrachloride (Cl:C 4.0). Correlation coefficient, r ) 0.994. (b) An expanded view of the low Cl:C ratio region in (a) which consists of a homologous series of aliphatic halogenated hydrocarbons (chloropropane to chloroheptane).
Figure 5. Width (fwhm) of chlorotoluene peaks obtained at different GC oven temperatures, which reduces to the same value as for the solvent (methanol), indicating that the Nichrome wire-heated transfer line does not add appreciably to band broadening.
GC-MPT-TOFMS. To determine whether the Nichrome wireheated transfer line was performing adequately, the GC oven temperature was varied between 100 and 200 °C, and the resulting peak width (full width at half-maximum, fwhm) for the relatively high-boiling chlorotoluene was measured (see Figure 5). At the highest furnace temperature, the width of the chlorotoluene peak narrowed to the same value as that found for volatile compounds. From this finding, it can be concluded that the transfer line is not the limiting factor in governing peak width. Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
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Table 3. Detection Limits for Halogenated Compounds detection limita compound
amount injected (pg)
S/N
as halogen (fg)
as total compound (fm)
as total compound (fg)
as total compound (Mohamad et al.35) (fg)
chlorobenzene 1-chloropentane p-chlorotoluene
10.1 8.8 12.7
118.6 91.6 114.9
75.4 95.2 91.2
2.21 2.72 2.61
260 290 330
9200 b 11 000
bromobenzene bromoform
14.9 28.0
287.0 320.7
74.0 254
0.93 1.01
150 270
b b
iodobenzene 1-iodobutane
18.2 15.0
199.7 272.0
168 110
1.32 0.87
270 160
1500 b
a
Detection limits were calculated at 3σ and were based on peak heights. b Detection limits for these compounds are not reported.
Figure 7. Chromatogram obtained from 12.7 pg of chlorotoluene. Signal measured at 35 amu, corresponding to 35Cl.
Figure 6. Isotope-specific chromatograms of halogenated hydrocarbons (chlorobutane to chlorohexane) in methanol. Twin boxcar averagers used for data collection. (a) Signal from 12C. (b) Signal from 35Cl.
It was shown above that the MPT performed well in the determination of empirical formulas when steady-state sample introduction was used. Practically, this feature had to be extended to a single chromatographic run where internal standards could be employed. The chromatograms obtained from monitoring the 35Cl and the 12C channels simultaneously are shown in Figure 6a and b, respectively. The boxcar averagers were utilized for data collection, and the chromatogram was obtained under isothermal conditions for 1 µL of 10% v/v solutions of halogenated hydrocarbons in methanol. As can be seen from Figure 6, memory effects appear to be absent, even for the large solvent peak; however, the chlorohexane peak appears to exhibit some surprising asymmetry in both chromatograms, which could possibly be caused by column overload. The Cl/C signal ratios were determined from both peak areas and peak heights from a series of 15 replicate injections. The agreement between experiment and theory (obtained from the corresponding empirical formula) 3962 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
is good, as was indicated by the correlation coefficient of 0.999 that was obtained for both peak heights and peak areas. Detection limits were determined for several halogenated hydrocarbons, all under the same operating conditions. A typical chromatogram from which detection limits were calculated is reproduced in Figure 7. This chromatogram was obtained from a 1-µL injection of a 0.1% v/v solution of chlorotoluene in methanol while the halogen signal (35Cl in this case) was monitored. In the present chromatograph, as in most that employ a flash vaporization injector port, a split ratio of 100:1 was utilized; this split ratio was taken into account in the computation of detection limits. Of course, with an on-column injector, this measure would not be necessary. The resulting detection limits (Table 3) show that the GC-MPT-TOFMS system offered roughly equal sensitivity for most compounds that were examined, which included both aromatic and aliphatic species. The absolute detection limit determined for bromoform was slightly worse than that for the other halogenated species under investigation by approximately a factor of 3. This lower sensitivity is presumably because bromoform is the most volatile species that was examined, which caused it to diffuse throughout the plasma more than the other compounds. This diffusion process reduced the sampling efficiency and the subsequent sensitivity that was observed for this compound. Table 3 includes a comparison with detection limits reported by Mohamad et al.,35 who used a TM010 cavity and a quadrupole-based mass analyzer. The detection limits for all (35) Mohamad, A. H.; Creed, J. T.; Davidson, T. M.; Caruso, J. A. Appl. Spectrosc. 1989, 43, 1127-1131.
compounds common to both studies were significantly better with the GC-MPT-TOFMS system. For all of the tested species, absolute detection limits were in the low femtomole or hundreds of femtograms range when presented as the total compound or as the elemental halogen. Detection limits are reported here as absolute values but are equivalent to mass flow rates, since the peak widths at fwhm were about 1 s. Because ion counting was used in this portion of the study, the dynamic range was limited to only 3 orders of magnitude (a correlation coefficient of 0.99). At ion count rates above roughly 50% of the repeller frequency, a condition referred to as pulse pile-up occurs.36 It was observed that an injection of 160 pg produced a count rate of 12000 counts/s, already approaching the repeller frequency of 14 kHz. To avoid this pulse pile-up and to extend the dynamic range, boxcar integrators could be substituted, but with a consequent loss in sensitivity. Alternatively, a detection system tailored for use with TOFMS could be employed.37 (36) Mahoney, P. P.; Li, G., Hieftje, G. M. J. Anal. At. Spectrom. 1996, 11, 401405. (37) Holland, J. F.; Newcombe, B.; Tecklenburg, R. E., Jr.; Davenport, M.; Allisom, J.; Watson, J. T.; Enke, C. G. Rev. Sci. Instrum. 1991, 62, 69-76.
CONCLUSIONS Gas chromatography can be effectively coupled to a MPT-TOF mass analyzer for the element-specific detection of halogenated hydrocarbons. The system described here offers excellent performance in the determination of empirical formulas, in large measure because of its ability to avoid the distortions of spectral skew. Moreover, detection limits are better than those previously reported by investigators who utilized atmospheric pressure HeMIPs sustained in a Beenakker cavity. Clearly, the MPT deserves further investigation as a means of element-specific detection for chromatographic methods. ACKNOWLEDGMENT The work was supported by the National Institutes of Health through Grant R01-GM53560-01. J.A.C.B. thanks the “Deutsche Forschungsgemeinschaft” (DFG), Bonn, FRG, for travel assistance.
Received for review May 23, 1997. Accepted July 7, 1998. AC970534R
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