Anal. Chem. 2003, 75, 1400-1409
Portable Gas Chromatograph with Tunable Retention and Sensor Array Detection for Determination of Complex Vapor Mixtures Chia-Jung Lu,† Joshua Whiting,‡ Richard D. Sacks,‡ and Edward T. Zellers*,†,‡
Department of Environmental Health Sciences and Department of Chemistry, Center for Wireless Integrated MicroSystems, University of Michigan, Ann Arbor, Michigan 48109-2029
A prototype portable gas chromatograph that combines a multiadsorbent preconcentrator/focuser, a tandem-column separation stage with individual column temperature control and junction point pressure modulation, and a detector consisting of an integrated array of polymercoated surface acoustic wave microsensors is described. Using scheduled first-column stop-flow intervals and independent temperature programming of the two columns, it is possible to adjust the retention of eluting analyte vapors to maximize vapor recognition with the microsensor array and minimize the time of analysis. A retention window approach is combined with Monte Carlo simulations to guide retention tuning requirements and facilitate pattern recognition analyses. The determination of a 30-vapor mixture of common indoor air contaminants in 470 140 75 >450 52 >410 66 59 20 >490 15 18 21 7.8 8.4 8.8 6.5 6.6 5.4 32 4.3 2.4 3.7 2.2 4.3 1.5 2.3 2.1 >290 1.8 0.5 2.9 6.2 0.3 0.7 0.4
>470 >500 >490 >450 >790 195 >640 >440 130 >490 155 >440 130 >280 >310 >300 >250 >271 >260 >200 >260 140 >250 >242 >260 150 >210 >230 >300 78 >110 >93 52 53 >55 39
continues for 5.5 min at 0.18 L/min, which is the highest flow rate achievable. After collection of the 1-L sample volume, the PCF is purged with clean, dry air drawn through the scrubber for 50 s (0.15 L) to remove coadsorbed water vapor from the Carboxen 1000 (note: the other adsorbents do not retain water vapor to any significant extent).7 The six-port valve is switched back to the inject position, and the PCF is heated to inject the next sample. Pressure-tuning segments and column-temperature changes can be inserted at any point in the separation cycle. The maximum column temperature employed for this study was 150 °C.8 Following collection of an initial sample, the sampling and separation cycles can be synchronized to reduce the total duty cycle. Calibration Procedure. Test atmospheres were prepared by injecting liquid solvents into a Tedlar bag prefilled with a known volume of clean, dry air. All bags were seasoned by preexposure to low concentrations of the test compounds followed by repeated flushing with clean air until no detectable concentration was observed in samples collected and analyzed by EPA method TO17 (see below). Target compounds (Table 1) were obtained from Aldrich (Milwaukee, WI) at g98% purity except for 2-methylheptane (99%) and octamethylcyclotetrasiloxane, (D4, 98%) (Acros/ Fisher, Pittsburgh, PA), 2-methylfuran and 2,5-dimethylfuran (99%, Lancaster, Windham, NH), 4-phenylcyclohexene (98%, Chem1404 Analytical Chemistry, Vol. 75, No. 6, March 15, 2003
LOD (ppb)
sampco, Trenton, NJ), and decamethylcyclopentasiloxane (D5, 98%, Gelest, Tullytown, PA). Concentrations were calculated by assuming complete evaporation for volatile liquids. The concentrations ranged from 20 to 50 ppm for compounds with vapor pressures less than 4 Torr and 200-300 ppm for compounds with vapor pressures higher than 4 Torr. For solid chemicals, such as p-dichlorobenzene and naphthalene, several milligrams were placed in a 1-L Tedlar bag, flushed with clean air, filled to ∼0.7 L, warmed gently with heat gun, and left at room temperature overnight. Aliquots of the headspace air were then transferred to other Tedlar bags by gastight syringe and diluted as needed. For this testing, the instrument inlet was fitted with a union tee with one end connected to the side-mounted scrubber and the other end sealed with a septum. Calibrations were performed initially with noncoeluting subsets of 12 vapors from Table 1. Highconcentration vapor spikes of 0.2, 0.5, 1.0, and 2.0 mL were introduced into the sample stream, which, for a sample volume of 1 L, correspond to equivalent concentrations of 4-600 ppb for spikes of 20-300 ppm. For example, the mass introduced by 1 mL of vapor at 50 ppm is equivalent to 50 ppb in 1 L (i.e., 50 ppb‚L). Five replicate analyses were performed at each challenge concentration. The sample volume of 1 L was selected on the basis of a series of preliminary tests performed on two 390-MHz SAW oscillators coated with PIB and BSP3 and sealed in a cell similar
to that used for the instrument’s sensor array. Results indicated that detection limits in the low- or sub-ppb range could be obtained under similar operating conditions with the current array.25 EPA method TO-17 was used to confirm vapor concentrations during the instrument calibration.26 Stainless steel sampling tubes (Scientific Instrument Service SIS, Inc., Ringoes, NJ) packed with 80 mg of Carbotrap C, 120 mg of Carbopack B, and 60 mg of Carboxen 1000 (Supelco) were preconditioned prior to use. The method was calibrated using solvent spikes prepared in CS2 and injected into the tubes in volumes that covered an appropriate range of mass (0.8-200 ng) to match that collected from the test atmospheres. Duplicate TO-17 reference samples were collected at 0.1 L/min for 5 min in parallel with those collected by the instrument and analyzed using a short-path thermal desorption module (TD-4, Scientific Instrument Services Inc.) connected to the GC injection port (GC-FID HP6890, Agilent, Santa Clarita, CA). Peak areas from the SAW sensors were integrated using Grams 32 software (Galactic Industries, Salem, NH) following smoothing using a 110-point running average. The average peak areas from the five replicates were regressed onto the average concentrations determined from the reference samples, forcing the regression line through the origin. EDPCR/Monte Carlo Model. To assist in evaluating the vapor recognition capabilities of the array, Monte Carlo simulations were performed and the data generated were analyzed with EDPCR. The Monte Carlo model employed has been described elsewhere.16,27 Five hundred simulated (i.e., error-enhanced) sensor responses were generated for each vapor and treated as unknown test samples, each of which was assigned an identity via EDPCR using the library of response patterns established from the calibration data. Average recognition rates were compiled and compared for different separation tuning conditions. The values for the various error terms in the Monte Carlo model were determined experimentally by repeated measurements under controlled conditions. EDPCR and Monte Carlo software routines were written in-house in Visual Basic and run on a personal computer. RESULTS AND DISCUSSION Initial Instrument Calibration and Limits of Detection (LODs). Figure 3 shows the three-channel chromatogram for one of the 12-component calibration mixtures, which includes compounds ranging in vapor pressures from 1.5 (D5) to 231 Torr (acetone), using the same temperature program for both columns (see figure caption). Due to the sensitivity differences among the three sensors, the y-axis scales for the ECEL and PIB sensors have been enlarged for ease of visualization. Peak shapes are generally good, though some tailing and broadening are evident for the less volatile compounds. Peak widths are somewhat (25) Lu, C.-J.; Zellers, E. T.; Grall, A.; Whiting, J.; Sacks, R. D. Design and Preliminary Characterization of a Portable Instrument with Tunable Selectivity for Analyzing Complex VOC Mixtures. 198th Meeting of the Electrochemical Society, Phoenix, AZ, October 22-27, 2000; Abstr. 1095, p 104. (26) U.S. EPA. TO-17, Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air, 2nd ed.; Report EPA/625/R-96/010b; U.S. Environmental Protection Agency, Washington, DC, 1997. (27) (a) Zellers, E. T.; Pan, T.-S.; Patrash, S. J.; Han, M.; Batterman, S. A.; Sens. Actuators, B 1993, 12, 123-133. (b) Zellers, E. T.; Batterman, S. A.; Han, M.; Patrash, S. J. Anal. Chem. 1995, 67, 1092-1106.
Figure 3. Twelve-compound chromatograms from each sensor in the array using a temperature program (same for both columns) of 35 °C (ambient temperature) for 120 s, 50 °C for 70 s, 70 °C for 120 s, and 130 °C for 140 s.
broader than observed in previous tests with this array connected to a bench-scale GC with the same PCF.7 This is apparently due to a slightly slower PCF heating rate in the instrument, a lower desorption flow rate (2.5 vs 4 mL/min), and additional inlet broadening from the dead volume associated with the PCFcolumn connection. The less volatile compounds may also suffer broadening from nonspecific adsorption on the postcolumn system surfaces. Calibrations were performed over a range of preconcentrated masses and equivalent vapor concentrations that depended on the vapor. For the more volatile compounds, the concentration range was extended beyond an initially planned maximum of 100 ppb‚L in an attempt to address the relatively low sensitivities of the PIBand ECEL-coated sensors (see below). Plots of peak area versus vapor concentration (from the TO-17 method) were generally linear over the ranges examined (regression r2 values were >0.98 in most cases and >0.96 in all cases), and the slopes from the three sensors were used to construct response patterns for vapor recognition. For 10 of the 36 compounds, the response at the highest concentration fell below the linear trend at the lower concentrations. The upper concentration range for each of these cases is given in Table 1. This behavior was not observed for the other compounds over the ranges tested. After probing possible sources of the nonlinearity, it was determined to be due to the sensors rather than other components of the system (e.g., PCF breakthrough, leaks, pump flow rate variations). The precision of responses among the five replicates varied substantially among the different compound-sensor combinations. For example, compounds such as perchloroethylene and n-nonane had stable and reproducible responses from all sensors, while compounds such as acetone and mesitylene had large variances at high concentrations. For the least volatile compounds (e.g., n-dodecane and n-tridecane) extracolumn wall adsorption may have been contributory. LODs (Table 1) were determined by multiplying the standard deviation of the smoothed baseline for each sensor by a factor of 3 and then dividing by the average slope from calibration. It is recognized that this provides an optimistic estimate of the LOD because it does not take full account of slope variations. The BSP3coated sensor consistently provides the highest sensitivity and Analytical Chemistry, Vol. 75, No. 6, March 15, 2003
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Table 2. Recognition Rates from Monte Carlo/EDPCR Analyses of Critical Coeluting or Nearest-Neighbor Compounds in the Chromatograms Shown in Figure 4 window
retention time (s)
compound(s)
A B C
98-123 123-153 178-203
n-octane/perchloroethylene 2-hexanone/butyl acetate nonane
D E F
235-270 301-324 357-430
mesitylene D-limonene D5/naphthalene/dodecane D5/naphthalene naphthalene/dodecane D5/dodecane
lowest LODs, with LOD values of e10 ppb for 26 of the 36 compounds and
