Anal. Chem. 1991, 63, 299-304 Caldwell, K. D. Anal. Chem. 1988, 60. 959A. Janca, J. FkH-Fbw fractbnatlon: Anaiysls of Macromohxules and Particles; Marcel Dekker: New York, 1988. Qiddings. J. C.; Yang, F. J.; Myers, M. N. J. Vlrol. 1977, 27, 131. W i n g s , J. C.; Lin, 0. C.; Myers, M. N. J. C o M Interface Sci. 1978, 65, 67. Gddings. J. C.; Chen, X.; Wahiund, K.-G.; Myers, M. N. Anal. Chem. 1987, 59, 1957. Barman, B. N.; Myers, M. N.; Giddings, J. C. Powder Techno/. 1989, 59,53. Wahlund, K.-G.; Litzb. A. J. Chromatcgr. 1989. 467, 73. Caidweii, K. D.; Karaiskakis, G.; Myers, M. N.; Glddings, J. C. J. Pharm. Sci. 1981, 70, 1350. Yonker, C. R.; Jones, H. K.; Robertson, D. M. Anal. Chem. 1987, 59, 2573. Thompson, G. H.; Myers, M. N.; Giddings, J. C. Anal. Chem. 1969, 4 7 , 1219. Giddings, J. C. I n Size Exclusion Chromatcgraphy; Hunt, B. J., HoMing, S . . Eds.; Blackie and Son: Glasgow, 1989; Chapter 8. Kirkland, J. J.; Rementer, S. W.; Yau, W. W. Anal. Chem. 1988, 6 0 , 610. Gao, Y. S . ; Caidwell, K. D.; Myers, M. N.; Glddlngs, J. C. Macromolecules 1985, 78. 1272.
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(21) Schimpf, M. E.; Myers, M. N.; W i n g s , J. C. J . Appl. Polym. Sci. 1987, 33, 1170. (22) GLddings, J. C. Sep. Sci. Techno/. 1984, 79, 831. (23) Giddings, J. C.; Yang, F. J.; Myers, M. N. Anal. Blochem. 1977, 87, 395. Corresponding author.
Guangyue Liu J. Calvin Giddings* Field-Flow Fractionation Research Center Department of Chemistry University of Utah Salt Lake City, Utah 84112 RECEIVEDfor review July 3,1990. Accepted October 12,1990. This work was supported by Grant CHE-8800675 from the National Science Foundation.
TECHNICAL NOTES Vapor Sampling Device for Direct Short Column Gas Chromatography/Mass Spectrometry Analyses of Atmospheric Vapors Neil S. Arnold,* William H. McClennen, and Henk L. C. Meuzelaar Center for Micro Analysis and Reaction Chemistry, University of Utah, Salt Lake City, Utah 84112 INTRODUCTION A number of methods are currently used for atmospheric vapor and gas sampling with mass spectrometric detection and identification. Direct mass spectrometry (MS) sample introduction methods include fixed molecular leaks (I), atmospheric pressure ionization ( 2 , 3 ) ,trap and desorb (4),and membrane separation (5), while gas chromatography/mass spectrometry (GC/MS) methods employ trap and desorb (6), direct bubbler solvent injection (7,8), sample loops (9), and pressurized gas plug introduction (10). Approaches vary depending upon whether MS, tandem MS, or GC/MS analyses are desired. Direct MS and tandem MS analyses typically give quick response times and high repetition rates but are often sensitive to interferents, including atmospheric constituents, while GC/MS analyses offer greater specificity but with typically slower results. The value of short capillary column GC/MS has been demonstrated for rapid analyses of many compounds ( I I , 12) including thermally labile and polar compounds (13). Recent work by Hail and Yost (14) has continued this trend. Such investigations have encouraged the use of a standard GC/MS transfer line on the mass spectrometer (designed primarily as a pressure drop from atmosphere to vacuum) as a short (l-m) capillary GC column for separation of atmospheric components. Further, the enhanced sensitivity capabilities of the ion trap mass spectrometer (Finnigan-MAT) (15,16), to detect sample quantities on the order of 100 fg ( I 7)in both full-scan MS and MS/MS modes, allow for direct sample introduction without preconcentration. The presently described direct atmospheric sampling inlet and methodology attempts to bridge the gap between the slow GC/MS and interferent-sensitive direct MS approaches by utilizing "transfer line" chromatography for a separation of sample components and ion trap mass spectrometery for rapid detection and identification.
EXPERIMENTAL SECTION A schematicdiagram of the vapor inlet system appears in Figure 1 (18). It comprises three concentric tubes whose internal flows
control the vapor sampling process. The innermost tube is the transfer line column (0.18- or 0.15-mm-i.d. fused silica capillary column) to the mass spectrometer. The intermediate tube consisted of 0.53-mm-i.d. deactivated fused silica capillary column (Supelco) cut to extend approximately 2 cm beyond the end of the inner tube. The outer tube was 3-mm-0.d. quartz or 1/8-in. glass-lined steel (GLT, SGE). All other fittings are either standard components of an ion trap detector (ITD 700, Finnigan-MAT) transfer line housing or are modified Swagelok fittings as shown in Figure 2. The width and frequency of the injection pulse onto the column are controlled by a computer, but the actual switching speed of the inlet is limited by compressible volumes and flow restrictions in the overall plumbing (including the valves). The system is t y p i d y run with 10-30 mL min-' of gas sampled through the large-diameter outermost tube. Between injections, 1-5 mL min-' of helium carrier gas is expelled from the intermediate-diameter (0.53-mm) tube into this flow in order to prevent atmospheric access to the capillary column. During the injection, the intermediate tube flow is reversed to provide direct atmospheric access to the head of the capillary GC column. Further, these flows are adjusted so that the flow into the large tube is unchanged by the sampling position., The transfer line capillary consisted of 1.1m of a 0.18-mm-i.d. fused silica capillary column coated with 0.4-pm DB-5 stationary phase (J&W Scientific)or a similar length of 0.15-mm-i.d. column coated with 1.2-pm film thickness CP SIL 5 CB (Chrompack). With a pressure drop from ambient (0.85 atm at 4400-ft elevation) to vacuum (approximately Torr), the flow rates were 4 mL min-' (270 cm s-') and 1.5 mL m i d (200 cm s-l), respectively, for the two capillary inside diameters at 25 OC. Transfer line isothermal operating temperatures ranged from 25 to 250 "C depending upon the target compound volatility. The inlet temperature was also varied for different target compounds and was typically operated more than 25 "C higher than the column temperature.
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Two different ion trap type mass spectrometers were used for the reported analyses. A Finnigan-MAT ITD 700 was modified extensively (19)to provide analysis capabilities compatible with an ion trap mass spectrometer (Finnigan-MAT) system in a more portable instrument that has been used for analyses in both laboratory and field situations. An unmodified ITD 700 system with version 3.01 software was also used. The primary difference between the two is that the use of the axial modulation option on the ion trap mass spectrometer system allows higher helium flow rates and better detection limits. (The 0.18-mm4.d. column can only be used on the modified system, and an improvement of about 1 order of magnitude is obtained.) Software has been written in FORTH and Assembler languages to control the inlet via a digital I/O board (Metrobyte DASCON 1or Keithley System 570) from the same PC-compatible compuer running the data acquisition.
The mass spectrometer scan rate and mass range varied with individual experiments from 4 to 8 scan s-'. The ion trap manifold temperature was maintained at 100-200 "C for all analyses. Specific transfer line, vapor sampling, and mass spectrometer operating conditions were varied and are described with the individual sampling experiments. Vapor standards for instrument calibration were prepared by dilution into air streams of certified compressed gas standards including a mixture of 1parts per million (ppm) p-xylene in N2 (Matheson) and a mixture of 10 ppm each benzene, toluene, 0-, m-,and p-xylene, and ethylbenzene in N2 (Scott Specialty Gases). Typical dilution factors up to 2500 were obtained by mixing 0.25-0.5 mL min-' of standard mixing into compressed air flows up to 1 L min-'. Mixing the two flows was performed in 75 cm of '/,-in.-i.d. Teflon tubing followed by an additional 1 m of '/,-in.-i.d. Teflon tubing, yielding a total mixer volume of 30 mL and a minimum mixing time of 2 s prior to sampling. In field tests sponsored by the EPA, vapor standards were diluted into a 5-m-long X 2.5-cm-i.d. glass and Teflon manifold with a 2.2 L s-' total flow rate. These standards included both 50 ppm compressed gas mixtures and equilibrium headspace vapors of pure compounds injected into the manifold with a motor-driven syringe pump. The syringe pump was capable of producing manifold vapor concentrations ranging from less than 20 parts per billion (ppb) to over 10 ppm in air, depending on the compound vapor pressure, syringe diameter, and motor speed. Vapor standard calibration data were obtained by scanning from m / z 45 or 50 to m / z 200 at 4 scans s-'. The inlet and transfer line were maintained at 25 "C, while the ion trap was maintained a t 85 "C. The temperature of the mixing chamber was ambient and not controlled. The vapor inlet drew approximately 5 mL min-' from the mixer flow or up to 120 mL min-' from the EPA vapor manifold. A 0.5-s vapor sample pulse was generally used, although the EPA experiments involved varying the sample time from 330 ms to 2.5 s with a routine pulse width of 715 ms. Calibration data were obtained by integration of peaks in the m/z 91 trace for the alkylbenzene calibration compounds. A thermal bed reactor for hazardous waste studies (20) has been monitored by the modified ion trap mass spectrometer during experiments involving thermal desorption of sample compounds from test soil. Test soil experiments utilized soils loaded to 0.5 wt % of toluene, ethylbenzene, cumene, or tert-butylbenzene, Loaded soils were heated to 538 "C under a radiant heater with a preheated air flow of 0.5 L m i d above the bed of soil. The exhaust gas was sampled at 60-s intervals with 2-9 vapor sampling pulses. The inlet was operated between 150 and 175 "C, and the transfer line was maintained at ambient temperature (25 "C). The separation was performed in the 0.18-mm-i.d. column specified above. The ion trap manifold was 100 " C , and the MS was scanned from m / z 35 to m / z 200 at 4 scans s-'. Additional desorption studies of polynuclear aromatics from coal tar contaminated soils have been performed and are recounted in McClennen et al. (8). A rotary kiln simulator (21)has been monitored for gas-phase hydrocarbons during the combustion of polymeric medical supplies. The 11-g samples were loaded into the kiln and incinerated a t two different temperatures, 600 and 760 "C. Rapid on-line analyses were obtained by using the unmodified ITD system. A sample flow of 25-50 mL m i d was drawn from the kiln exhaust gases in a transition area preceding the afterburner. Vapor samples were taken a t 10-s intervals to monitor concentration transients during sample combustion. The 0.15-mm4.d. column was used in this system at ambient temperature (25 "C) with the vapor inlet at 60 "C, and the mass spectrometer was scanned from m / z 35 to m / z 120 a t 4 scans s-'.
RESULTS AND DISCUSSION In designing this vapor sampling inlet, the desire for high resolution chromatography and low ppb detection limits has dictated the following four operating criteria for the inlet (1) the inlet should minimize atmospheric background when the inlet is not sampling (ideally this would be less than the background levels of the carrier gas); (2) the inlet should preserve the ratios of atmospheric components during sampling (including minimization of carrier gas dilution); (3) the
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inlet should respond rapidly enough that a "square" peak shape sampling pulse can be obtained over the desired range of sampling times; and (4) the inlet should minimize the disturbance of the system being sampled. The inlet (Figure 1)comprises three concentric tubes whose internal flows control the vapor sampling process. It is operated by rapidly switching the flow direction in the intermediate tube to allow the atmosphere being sampled in the outer tube to have access to the tip of the transfer line column innermost tube. The subambient pressure detector draws a continuous flow of carrier gas dependent on the column length and inside diameter, while reversible flows of helium or sampled gas in the intermediate tube control the duration of a gas sample. The outermost tube is switched so that a constant sample flow is drawn through the aperture to minimize the perturbation of the sampled system. This is particularly important for small volume systems where quantitation is easily disrupted by uncontrolled dilution. Criterion 1 (above) requires a flow of carrier gas exiting from the mouth of the intermediate (0.53-mm) tube when the inlet is not sampling. Criterion 2 dictates the use of inert, nonadsorbing materials for the inlet parts and the operation of the inlet in a way that limits kinetic molecular mass discrimination. Criterion 3, which is most critical for maintaining chromatographic resolution, dictates that plumbing volumes and flow restrictions be minimized to obtain quick response times. Finally, criterion 4 requires that the volumetric flow into the inlet be small with respect to the sampled system dimensions and/or that the volumetric flow be constant to permit quantitative dilution of the sampled system. Unlike solution-based GC injection techniques in which the sample is introduced in a condensed phase, detection limits in direct atmospheric injection techniques are concentration, not total mass, limited. This limitation, coupled with the sensitivity properties of the MS detector (a concentrationsensitive device), makes it important that the sample be presented to the detector with little or no reduction in concentration to attain the best detection limits. Band broadening is chromatography makes this impossible in an absolute sense, yet a peak that initially has a finite width can elute from the column with its maximum concentation arbitrarily close to the intial maximum if the conditions are properly selected. But because this initial peak width is its own form of band
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broadening, we maintain the concentration at the expense of resolution unless some method of deconvolution can be used. Figure 3 shows an ion chromatogram obtained for six consecutive vapor samples of toluene standard at 16 ppb in air. The standard was prepared by the syringe pump method described above and utilized the 0.15-mm-i.d. column on the modified ion trap mass spectrometer system. In the total ion current trace, the sampling points are indicated where the curve goes to zero due to saturation of the analyzer from the air pulse (in effect the solvent peak) passing through the detector. These points correspond to injection points at 25-9 intervals for the monitoring of the standard mixture. In addition to these 16 ppb data, Table I presents a set of calibration points for toluene showing the degree of sample repeatability via the percent relative standard deviation. A minimum of five consecutive vapor samples were taken at each concentration for t h e statistics. These concentrations were prepared via the syringe pump method described above, and response is measured via the peak area of the m / z 91 trace from a 0.715-s vapor sample. A linear fit to the full set of data points is obtained with a correlation coefficient R = 0.9998. Similar calibration data have been obtained for various volatile chlorinated and aromatic organic compounds at concentrations as low as 3.5 ppb @-xylene) while maintaining a signal-to-noise ratio ( S I N ) of 10 to 1 and a relative standard deviation of 22%. For various chlorinated alkanes and substituted benzenes calibrated during field tests, typical correlation values of 0.99 were obtained along with relative standard
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deviations of less than 5% for concentrations above 40 or 50 ppb and less than 25% for more dilute samples. The vapor sampling data in Figure 4 were obtained during desorption of ethylbenzene from a 500-g soil bed in air at 538 "C. A number of reaction products are identified in the figure caption. Concentrations are estimated at 50 ppb and upward for these compounds. Not included in the caption is a peak
Flgure 7. Ion profiles showing the effect of sampling time on chromatographic resolution and sensitivity. (a) Obtained from a 330-ms vapor sample and shows the separation of dichloromethane (the first peak) from the complex of 1,2dichloroethene (mlz 61) and 1,l-dichloroethane ( m l z 65). Separating these latter two compounds may be possible with even shorter sample pulses. (b) Obtained at 715 ms and illustrates the relationship between sample duration and sensitlvtty. The m l z 82 (tetrachloromethane) trace has 50% increase in peak height, which correlates to the absolute sensitivity of the instrument.
indicated by the question mark in Figure 4d a t 32 s relative to the sampling point, whose background subtracted spectrum is shown compared to a library spectrum in Figure 5. The unknown was identified as dichlorobenzene from this spectrum and from retention time data. The concentration is estimated near 10 ppb. The concentration estimates are based upon the previously cited data for alkylbenzenes and upon general electron ionization (EI)response characteristics of ion trap mass spectrometers. A major asset of this system is the ability to do on-line monitoring in the near real-time mode. Figure 6a shows a set of chromatograms monitoring the evolution of volatile organics from the combustion of polypropylene materials in a laboratory-scale rotary kiln. The 10-s sampling interval was sufficient to follow the transient concentrations of aromatics during the 2-min experiment. Selected ion traces a t mlz 78 and 91 mlz show the specific benzene and toluene peaks in the repetitive analyses. Figure 6b explicitly plots the quantitated evolution curves obtained from peak areas of selected ions for benzene, toluene, phenol, and styrene. Figure 7 illustrates the effect of sampling duration on resolution and sensitivity. The same mixture of dichloromethane, 1,2-dichloroethane,1,l-dichloromethane, and tetrachloromethane at 350 ppb each is used in part a with a sampling time of 330 ms and in part b with a time of 720 ms. Clearly the 330-ms time improves the resolution of the early eluting compounds, but comparison of the mlz 82 selected ion trace indicates that the tetrachloromethane, which elutes at 7 s, has only 70% of the peak height obtained a t 720 ms.
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tailing. T o date, oxidation of the inlet itself does not appear to be the main problem since it has only a thin organic phase coating for deactivation and is typically operated warmer than the transfer line to reduce adsorption. Sampling into a cool transfer line column, which is then rapidly heated (e.g., at 10-30 "C s-l) to elute less volatile components, is one possible strategy to minimize column oxidation (22, 23). A further advantage of temperature-programmed transfer line chromatography would be the much greater range of molecular sizes and types that could be separated and detected in a single run. In the end, great demands have been placed upon the detector. Very short columns and short retention times demand high scanning rates. T o date, we have utilized the ability of the transfer line to elute in a relatively short time compounds with very high capacity factors ( k values are typically from 5 to 100 in a 60-s repetitive analysis), while more typical length GC columns would examine a smaller range of k values with isothermal operation or resort to temperature programming. Thus, one way to lower the detector load has been to operate the system where the retention times are long relative to the air pulse or with long sample pulse widths so that inlet and detector system resolution and sampling rate are less critical. The ability to do direct mass spectrometric analysis of atmospheric gases and vapors at low parts per billion levels is established via a system that circumvents many of the interference problems of direct MS methods, without the time delays of traditional GC/MS methods. The inlet described provides for direct atmospheric monitoring in a way that is consistent with short column GC/MS analysis and the needs of high-resolution capillary chromatography. Further work with this inlet in our laboratory has not been restricted to ion trap mass spectrometric applications. It is being ported to a quadrupole mass spectrometer as well as to a gas-phase infrared spectrometer and an ion mobility spectrometer. (These last two applications involve either sampling from pressurized systems or the operation of the detector at reduced pressures.) The inlet itself may also not be limited to capillary gas chromatography. In fact, response times will be better in less compressible fluids, which may allow for interesting on-line monitoring applications in such areas as supercritical fluid chromatography, liquid chromatography, or capillary zone electrophoresis (CZE).
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For detection of compounds at the lowest levels, the peak height becomes the limiting factor as the ion counts must exceed the noise threshold. At 8 scans s-l, the 330-ms sample time is essentially a minimum for repeatable peak areas in the early peaks; however, chromatographic plate height calculations based on later eluting peaks indicate that overlapping early peaks such as the peaks of 1,2-dichloroethane ( m / z 61) and 1,l-dichloroethane (m/z 65) could be clearly resolved with a 50-100-ms sample pulse. The linearity of inlet response with sample time has also been measured by determining the peak areas for fixed sample concentrations. A linear peak area response with sample time indicates that the volume sampled to the detector is proportional to the duration of the sample. Correlation coefficient values between 0.98 and 1 are typical for a range of sample times from 330 ms to 2.5 s. It is anticipated that sampling times as short as 10-20 ms may be obtainable for gas sampling near 1 atm. Such short sampling pulses may allow the expansion of these monitoring systems to analyses of even more volatile compounds (e.g., vinyl chloride, chloromethane, and others) without having to resort to subambient cooling or long columns (and retention times), since the principal problem with such compounds is reduced detection limits due to suppression caused by the air peaks. Of course, faster scanning detectors will be required to investigate this area. An illustration of the high end molecular weight compounds amenable to this technique appears in Figure 8. Five selected ion traces are shown for a range of polynuclear aromatics from phenanthrene to the six fused ring compounds idenopyrene and benzoperylene. These traces were obtained from a single vapor sample taken 15 s after 1 pL of 23 ppm (by weight) standard solution was deposited inside the tip of an extended (10-cm-long) outer inlet tube, which was heated to 275 "C. The transfer line temperature was 240 "C, whereas the trap manifold was kept a t 210 "C. Extended operation of the column at elevated temperatures such as these has led to accelerated column degradation and subsequent tailing (illustrated by the m/z 276 trace of Figure 7) when the inlet is used on atmospheres with high concentrations of oxygen and water. Extremely small sample sizes, on the order of 500-fg total mass in a 0.5-s vapor sample at 5 ppb, exacerabate the loss of sensitivity due to compound
ACKNOWLEDGMENT We acknowledge the invaluable advice and assistance of Michael Story and Michael Weber-Grabau (Finnigan MAT Corp.) for the modified ion trap mass spectrometer system development, of A. Peter Snyder ( U S . Army Chemical Research and Development Engineering Center) for initial system testing, of David Mickunas (U.S. Environmental Protection Agency) and Erich Ludwig for field preparations, and of Advanced Combustion Engineering Research Center participants JoAnn Lighty, David Pershing, David Wagner, Eric Eddings, Eric Lindgren, Kenneth Roberts, and Sue Anne Sheya for rotary kiln and desorption reactor studies. Registry No. Benzene, 71-43-2; toluene, 108-88-3; ethylbenzene, 100-41-4; styrene, 100-42-5;benzaldehyde, 100-52-7; benzofuran, 271-89-6; chlorostyrene, 1331-28-8; chloroethylbenzene, 1331-31-3;dichlorobenzene, 25321-22-6;xylene, 1330-20-7; phenol, 108-95-2;dichloromethane, 75-09-2; 1,2-dichloroethene, 540-59-0;1,l-dichloroethane,75-34-3;tetrachloromethane,56-23-5; phenanthrene, 85-01-8;anthracene, 120-12-7;pyrene, 129-00-0; fluoranthene, 206-44-0; benz[a]anthracene, 56-55-3; chrysene, 218-01-9benzo[d]fluoranthene,42126-84-1;benzo[k]fluoranthene, 207-08-9;benzo[blpyrene, 50-32-8; indeno[ 1,2,3-cd]pyrene,19339-5; benzo[ghi]perylene, 191-24-2.
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RECEIVED for review March 12, 1990. Revised manuscript received August 22, 1990. Accepted November 2, 1990. This work was funded by the Advanced Combustion Engineering Research Center (which is supported by the National Science Foundation, the state of Utah, 23 industrial participants, and U.S. Department of Energy), the U.S. Army Chemical Research Development Engineering Center, the US.Environmental Protection Agency, and Finnigan MAT Corporation.
