Anal. Chem. 1999, 71, 5199-5205
Column Performance and Stability for High-Speed Vacuum-Outlet GC of Volatile Organic Compounds Using Atmospheric Pressure Air as Carrier Gas Andrew J. Grall and Richard D. Sacks*
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109
The development of lightweight, portable GC instrumentation is handicapped by the need for compressed carrier gas to drive the separation. The use of air as carrier gas eliminates the need for compressed gas tanks. If a vacuum pump is used to pull carrier gas and injected samples through the column, atmospheric pressure air can be used as carrier gas. Vacuum outlet operation also improves performance for high-speed separations by reducing detector dead time and by shifting optimal carrier gas velocity to higher values. Under vacuum outlet conditions using atmospheric pressure air as carrier gas, a 6-m-long, 0.25-mm-i.d. capillary column can generate ∼12 500 theoretical plates, and a 12-m-long column can generate ∼44 000 plates but with a 3-4-fold increase in separation time. The principal issues in column selection for highspeed GC with air as a carrier gas are efficiency and stability. Several bonded and nonbonded stationary phases were evaluated for use with air as carrier gas in the analysis of volatile organic compounds of interest in airmonitoring applications. These include dimethylpolysiloxane, 50% phenyl-50% methyl polysiloxane, 50% cycanopropylphenyl-50% methyl polysiloxane, trifluropropyl polysiloxane, poly(ethylene glycol), and dicyanoallyl polysiloxane (nonbonded). The dimethyl polysiloxane and the trifluropropyl polysiloxane columns showed good efficiency and no significant deterioration after 5 days of continuous operation with air as carrier gas. The 50% phenyl-50% methyl polysiloxane and the 50% cycanopropylphenyl-50% methyl polysiloxane columns showed poorer efficiency, and the poly(ethylene glycol) and dicyanoallyl polysiloxane columns showed excessive deterioration in air. Gas chromatography (GC) is often the method of choice for monitoring volatile organic compounds (VOCs) in the environment. The popularity of GC is based on a favorable combination of very high selectivity, good accuracy and precision, wide dynamic concentration range, and high powers of detection. Airquality monitoring is an important environmental application of GC. In general, measurements are made in nonlaboratory settings, and portable and transportable instrumentation is required. Current trends in the development of GC instrumentation for air monitoring include increasing analysis speed (sample throughput) and reducing instrument size and weight. 10.1021/ac990573y CCC: $18.00 Published on Web 10/14/1999
© 1999 American Chemical Society
Analysis speed is increased by the use of relatively short,1-3 narrow-bore4-6 capillary separation columns, special inlets, which produce very narrow injection plugs,7-9 and high-speed detection and data acquisition. Instrument size and weight have been reduced by the use of microfabricated components10-13 and through extensive engineering.14,15 The size and weight of portable GC instrumentation often is limited by the need for compressed gas containers to provide carrier and detector gases. Typically, refillable containers are used, which can hold about a one-workday supply of gases. The elimination of detector gases requires the development of detectors that use no gases other than the carrier gas to transport eluted sample components through the detector. Microfabricated thermal conductivity detectors are available for portable instrumentation.10,11 However, both VOC detectability and selectivity are poorer than with other common GC detectors. Recent developments in microfabricated sensor technology12,13,16,17 may provide practical devices with enhanced selectivity that require no additional operating gas supplies. The elimination of carrier gas containers can be accomplished by the use of air as the carrier gas.18 Air has not been extensively explored as a GC carrier gas because of poor column efficiency, particularly for fast separations and because of the deleterious (1) Sacks, R.; Smith H.; Nowak, M. Anal. Chem. 1998, 70, 29A. (2) Gaspar, G.; Annino, R.; Vidal-Madjar, C.; Guichon, G. Anal. Chem. 1978, 50, 1512. (3) Cramers, C. A. J. Chromatogr. 1981, 203, 207. (4) Van Es, A. High-Speed Narrow Bore Capillary Gas Chromatography; Huthig Buch Verlag: Heidelberg, 1992. (5) LeClercq, P. A.; Scherpenzeel, G. J.; Vermeer, E. A. A.; Cramers, C. A. J. Chromatogr. 1982, 241, 61. (6) LeClercq, P. A.; Cramers, C. A. J. High Resolution Chromatog. 1985, 8, 764. (7) Klemp, M.; Peters, A.; Sacks, R. J. Environ. Sci. Technol. 1994, 28, 369A. (8) van Es, A.; Janssen, J.; Bally, R.; Cramers, C.; Rijks, J. HRC & CC, J. High Resolut. Chromatogr. Chromatogr. Commun. 1987, 10, 273. (9) Liu, Z.; Phillips, J. J. Microcolumn Sep. 1989, 1, 249. (10) Angell, J.; Terry, S.; Barth, P. Sci. Am. 1983 248 (4), 44. (11) Lee, G.; Ray, C.; Siemers, R.; Moore, R. Am. Lab. 1989, 21 (2), 110. (12) Microsensor Systems, Inc. Bowling Green, KY 42103, MSI 301 Product Specifications, 1996. (13) Environmental Sensor Technology, Newbury Park, CA, 91320, Model 4100 Product Specifications, 1997. (14) Ehrmann, E. U.; Dharmasena, H. P.; Carney, K.; Overton, E. B. J. Chromatogr. Sci. 1996, 34, 533. (15) Overton, E.; Dharmasena, H.; Carney, K. Field Anal. Chem. Technol. 1997, 1 (2), 87. (16) Groves, W. A.; Zellers, E. T.; Frye, G. C. Anal. Chim. Acta 1998, 371, 131143. (17) Park, J., Zhang, G.-Z., and Zellers, E. T. Am. Ind. Hyg. Assoc. J. in press. (18) Smith, H.; Zellers, T.; Sacks, R. Anal. Chem. 1999, 71, 1610.
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effects of oxygen on the stationary phases commonly used for GC. The relatively high viscosity of air requires the use of higher inlet pressures, and this degrades column performance. The low values of binary diffusion coefficients of VOCs in air relative to values in hydrogen or helium result in low optimal carrier gas velocities and more rapid losses in efficiency with increasing carrier gas velocity. These conditions are not favorable for fast separations. The objections to air as carrier gas regarding stationary-phase durability are mediated by improved column manufacturing. The availability of bonded stationary phases may offer enhanced durability regarding air oxidation. It is often assumed that air oxidation will be a more significant problem for more polar stationary phases and at higher column temperatures. However, few data are available that compare different phases. The fact that the GC analysis of air-borne VOCs does not require very high column temperatures is also a potential advantage regarding column durability. Improved column performance for high-speed GC (HSGC) is achieved by the use of vacuum outlet GC techniques.18,19-21 The use of a vacuum pump to pull carrier gas and injected samples through the column also obviates the need for compressed gas containers. When a column is operated at subambient outlet pressure, gas-phase diffusion coefficients increase with the result that optimal carrier gas velocity is shifted to higher values. This results in faster separations. In addition, subambient column outlet pressure results in higher carrier gas velocity in the detector, which reduces dead time.18 This is important for the relatively narrow peaks attainably with the emerging technologies for HSGC. This report considers the use of atmospheric pressure air as carrier gas for the high-speed separation of VOCs of interest in air-monitoring applications.16,17 The advantages and limitations of vacuum outlet operation are described with respect to air as carrier gas. Column selection is based on studies of efficiency, durability, and selectivity. Several polar and nonpolar stationary phases are considered. For the studies reported here, a laboratory-bound instrument was used to generate conditions favorable for the study of column performance without major concern of instrumental limitations and artifacts. To this end, high-speed inlet and detection systems were employed, which contribute no significant extracolumn band broadening. EXPERIMENTAL SECTION Apparatus. The HSGC instrument used for column performance and durability studies has been described.18 Figure 1 shows a simplified illustration of the instrument. All components are mounted on a Varian 3700 GC (Varian Instruments, Walnut Creek, CA). A cryofocusing inlet system CI (Cryointegrator model L, Chromatofast Inc., Ann Arbor, MI)7,22 is used to collect VOCs from gasbag samples and inject samples as vapor plugs with widths of 5-10 ms. While this inlet system is not realistic for field-portable instrumentation, it was used in order to evaluate column performance without significant extracolumn band broadening. This laboratory inlet system is based on a capillary metal trap tube, which is cooled for sample collection and focusing and rapidly (19) Leclercq, P.; Cramers, C. HRC & CC, J. High Resolut. Chromatogr. Chromatogr. Commun. 1987, 10, 269. (20) Hail, M.; Yost, R. Anal. Chem. 1989, 61, 2402. (21) Puig, L.; Sacks, R. J. Chromatogr. Sci. 1991, 29, 158. (22) Klemp, M.: Akard, M.; Sacks, R. Anal. Chem. 1993, 65, 2516.
5200 Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
Figure 1. Experimental system used for vacuum outlet GC with atmospheric pressure air as carrier gas: CI, cryofocusing inlet system; C, capillary separation column; PID, photoionization detector; VP, vacuum pump; V, toggle valve; CG, carrier gas inlet; R, capillary pneumatic restrictor; P1 and P2, digital pressure gauges.
heated for injection onto capillary column C. Because of pneumatic restrictors in the inlet system, it could not be operated with ambient pressure air as carrier gas, and compressed tank air was delivered to the inlet and the pressure adjusted to give a column head pressure of 1 atm (101 kPa). Pressure gauge P1 monitors the column head pressure, and restrictor R provides a bleed flow of air to prevent back diffusion of sample into the pressure transducer. Detection was provided by a photoionization detector PID (model PI 52-02A, HNU Systems, Newton, MA) equipped with a 10.2-eV lamp. The detector cell volume is less than 100 µL. The detector outlet is connected to a laboratory vacuum pump VP (CENCO, model HYVAC 14, Central Scientific Co., Chicago, IL). Pressure gauge P2 monitors the detector pressure. Toggle valve V is closed during sample collection in the cryofocusing inlet device and opened for sample injection and separation. Detector pressure was maintained at ∼0.3 psia (2.1 kPa). Previous studies have shown that the detector contributes no significant dead time at this pressure.18 These studies also indicated that significantly higher pressures could be used with lower-dead-volume detectors without significant loss in column performance. This should allow for the use of much smaller, lower power vacuum pumps, which will be required for field-portable instrumentation. Materials and Procedures. The stationary phases considered in this study include dimethylpolysiloxane (DMS; J&W Scientific, Folsom, CA), 50% phenyl-50% methyl polysiloxane (PMS; J&W), 50% cycanopropylphenyl-50% methyl polysiloxane (CPS; J&W), trifluropropyl polysiloxane (TFS; Restek, Bellafonte, PA), poly(ethylene glycol) (PEG; Quadrex, Woodbridge, CT), and dicyanoallyl polysiloxane (CAS; Supelco, Bellafonte, PA). The stationaryphase thickness was 0.25 µm for all columns. All phases were bonded except for the CAS. Column lengths of 6 and 12 m were used. Test mixtures were prepared from pure compounds (neat). Concentrations ranged from 20 to 40 ppm (v/v) in air. Table 1 lists the mixture components. All components were reagent grade or better. Equal volumes were injected into a 3.8-L Tedlar gas sampling bag (Chromatography Research Supplies, Inc., Addison, IL) and diluted with clean, dry air. About 0.5 mL of sample was collected in the inlet system for each injection. Tank air used as carrier gas was purified with filters for water vapor and hydrocarbons. The detector was interfaced to a 350-MHz Pentium II computer through a 16-bit A/D board (C10-DAS1602/16, Computer Boards, Inc., Middleboro, MA). A data sampling rate of 100 Hz was used
Table 1. VOCs Used in Test Mixture label
compound
bp (°C) label
1 2 3 4 5 6 7 8 9
acetone 2-butanone benzene isopropyl alcohol trichloroethylene heptane toluene tetrachloroethylene octane
56.2 79.6 80.1 82.4 86.7 98.4 110.6 121.1 126
10 11 12 13 14 15 16 17 18
compound
bp (°C)
chlorobenzene ethylbenzene p-xylene m-xylene o-xylene styrene nonane bromobenzene mesitylene
130 136.2 138.3 139.1 144 145.2 150.8 155 165
for all measurements. Labtech Notebook software (Laboratory Technologies Corp., Wilmington, MA) was used for data acquisition and instrument control. Grams/32 software (Galactic Industries Corp.) was used for data processing. All spreadsheet calculations were performed with Excel software. Since no useful holdup time measurements could be made with the PID, values were calculated for each column temperature using standard gasdynamic equations23 and an Excel spreadsheet. RESULTS AND DISCUSSION Vacuum Outlet Operation. With the reduction in separation time as a principal objective, HSGC usually involves the use of relatively short columns (5-15 m) and higher than usual average carrier gas velocities. For the case of atmospheric pressure at the outlet, the inlet pressure is relatively low, and the ratio of inlet to outlet pressure may be significantly lower than for conventional GC with much longer columns. For vacuum outlet GC, much larger inlet-to-outlet pressure ratios may occur. The result is much greater carrier gas acceleration in the column with vacuum outlet operation. An additional important feature of vacuum outlet operation is that gas-phase binary diffusion coefficient values scale inversely with gas density. The much greater carrier gas acceleration tends to reduce overall column efficiency; while the larger diffusion coefficients tend to improve efficiency particularly at high carrier gas velocities where resistance to mass transport in the gas phase may be the principal source of band broadening. An important advantage in the use of vacuum outlet operation for HSGC is the increased optimal linear carrier gas velocity uopt. This is the average linear velocity that produces the minimum height equivalent to a theoretical plate. This is particularly important for the case of air as carrier gas because its large viscosity and small binary diffusion coefficients relative to He and H2 results in a more rapid loss in efficiency with increasing average carrier gas linear velocity. The effects of carrier gas acceleration and changes in binary diffusion coefficients on uopt for vacuum outlet operation can be found by differentiation of the Golay equation for column efficiency.24 The resulting expression for uopt neglecting band broadening from the stationary phase and from extracolumn effects is shown in eq 1. where Dg,o is the binary
uopt ) f(k) Dg,of2/r
(1)
diffusion coefficient of a mixture component in the carrier gas at (23) Grant, D. Capillary Gas Chromatography; John Wiley & Sons: Chichester, 1996. (24) Ingraham, D.; Shoemaker, C.; Jennings, W. HRC & CC, J. High Resolut. Chromatogr. Chromatogr. Commun. 1981, 5, 227.
Figure 2. Plots of the Martin-James pressure correction parameter f2 (A), the outlet diffusion coefficient value Dg,o (B), and their product (C) vs outlet pressure for atmospheric pressure air at the column inlet. Both plots A and C are scaled along the left vertical axis.
the column outlet pressure, r is the column radius, f2 is the Martin-James gas compression parameter,25 and f(k) is a function only of retention factor k.
Dg,o ) Dg,atm (patm/po)
(2)
f2 ) 3(P2 - 1)/2(P3 - 1)
(3)
f(k) ) [48(k + 1)2/(1 + 6k + 11k2)]1/2
(4)
where P is the ratio of inlet pressure pi to outlet pressure po and the subscript atm refers to atmospheric pressure values. Since f(k) has both upper and lower bounds, the range of uopt values can be found from the limits as k goes to 0 and to infinity.
lim uopt ) 6.9Dg,of2/r
(5)
lim uopt ) 2.1Dg,o f2/r
(6)
kf0
kf∞
In Figure 2, the values of f2 (A), Dg,o (B), and their product (C) are plotted vs po for air with an inlet pressure of 101 kPa (1.0 atm) and a Dg,atm value of 0.09. This is close to the values for BTEX (benzene, toluene, ethylbenzene, xylene) compounds in air at ∼30 °C.26 Note that both plots A and C are scaled along the left vertical axis. For low outlet pressure and atmospheric inlet pressure, which are the conditions used in this study, P is large, and eq 3 can be approximated by eq 7. Thus, f2 varies nearly linearly with
f2 ) 3/2P ) 3po/2patm
(7)
outlet pressure for low po values. This is seen in Figure 2. Also for large P and atmospheric inlet pressure, the product Dg,of2 is given by eq 8. For a retention factor value of 2.0 and using values of 0.09 cm2/s for Dg,atm and 0.0125 cm for r, the optimal carrier gas velocity is 30 cm/s. (25) James, A.; Martin, A. Biochem. J. 1952, 50, 679. (26) Fuller, E.; Schettler, P.; Giddings, J. J. Ind. Eng. Chem. 1966, 58, 19.
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Dg,of2 ) (Dg,atmpatm/po)(3po/2 patm) ) (3/2)Dg,atm (8) The important point in eq 8 is that constraining pi to atmospheric pressure results in a smaller increase in uopt than can be achieved by the use of subatmospheric inlet pressure. Also note that since both pi and po are constant, the value of uopt is independent of column length. The unique nature of eq 8 is the result of using atmospheric pressure at the column inlet. The implications of using air as carrier gas are clear from the Golay plots shown in Figure 3 for a 6-m-long, 0.25-mm-i.d. column (a) and a 12-m-long, 0.25-mm-i.d. column (b). The solid-line plots are for vacuum outlet operation (po ) 0.3 psia), and the brokenline plots are for atmospheric pressure outlet operation. In all cases, the contributions to band broadening from extracolumn sources and from resistance to mass transport in the stationary phase have been neglected. A retention factor of 2.0 and a Dg,atm value of 0.09 cm2/s were assumed. The vertical tick marks on the vacuum outlet plots in Figure 3 indicate the actual average carrier gas velocity values obtained for the two column lengths with atmospheric pressure air at the column inlet. The regions to the left of these lines on the vacuum outlet plots correspond to inlet pressure values of less than 1 atm, while the regions to the right are for inlet pressures greater than 1 atm. The rapid loss in efficiency for large average carrier gas velocity values, even for the vacuum outlet case, is due to the relatively small Dg values in air and the relatively large viscosity of air. Thus, the shapes of the plots in Figure 3 are unique for air as carrier gas. Note that the loss in efficiency with increasing average linear carrier gas velocity is smaller for shorter columns and for wider bore columns.19,20 These Golay plots point out a serious drawback of air as carrier gas for HSGC. Most HSGC work has been done with H2 as carrier gas because the combination of lower viscosity and larger binary diffusion coefficient values results in a much flatter right-hand flank of the Golay plots. The result is that a much higher average linear carrier gas velocity value can be used with minimal loss in column efficiency. With air as carrier gas, it is important to operate columns with linear velocity values closer to uopt despite greater retention times. For the 6-m-long column under vacuum outlet conditions and atmospheric pressure air as carrier gas, the average carrier gas velocity is 133 cm/s, and plate height is 0.048 cm. For the atmospheric pressure outlet case with the same carrier gas velocity, the plate height is 0.091 cm. For the 12-m-long column under vacuum outlet conditions, the average carrier gas velocity is reduced to 67 cm/s. This value is closer to the optimal value with the result that plate height is reduced to ∼0.027 cm. Under vacuum outlet conditions with atmospheric pressure air at the column inlet, the 6-m-long column generates ∼12 500 plates and the 12-m-long column generates ∼44 000 plates. The constraints of using carrier gas at a pressure of 1 atm amplify the need for high-efficiency columns. With conventional GC, the column length can be increased to generate more resolving power while maintaining relatively constant average linear carrier gas velocity, by simply increasing the inlet pressure. In theory, a 2-fold increase in column length results in a ∼41% (21/2) increase in resolving power and a 2-fold increase in retention times if average carrier gas velocity is kept constant. By constrain5202 Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
Figure 3. Golay plots for vacuum outlet operation (solid lines) and atmospheric pressure outlet operation (broken lines) with air as carrier gas using a 6-m-long, 0.25-mm-i.d. (a) and a 12-m-long, 0.25-mmi.d. column (b). The outlet pressure is 2.1 kPa. The vertical tick marks on the vacuum outlet plots correspond to an inlet pressure of 101 kPa (1.0 atm).
ing the inlet pressure, it is not possible to maintain a constant average carrier gas velocity, and the same 2-fold increase in column length results in a 3-4-fold increase in retention times. For this study, the inlet pressure was constrained to exactly 101 kPa. In a field-portable instrument using ambient air as carrier gas, barometric pressure changes would change the inlet pressure and produce small but significant changes in retention times. However, these changes in inlet pressure would not be great enough to produce significant changes in column performance. Figure 4 shows chromatograms of the test mixture using a 6-m TFS column (a) and a 12-m TFS column (b). Both chromatograms were obtained using atmospheric pressure air as carrier gas with an outlet pressure of 0.3 psia. The oven temperature was set at 30 °C. Peak labels correspond to those in Table 1. The large variation in peak size is due to the large variation in detector response for the target compounds. For both chromatograms, good peak shapes are observed with relatively little asymmetry. While the use of the longer column results in a large increase in separation time, the resolving power of the longer column is substantially greater with the result that peaks 2 and 9 are completely separated with the longer column, and a resolution of 1.3 is obtained for peaks 7 and 8. The failure of this column to separate p-xylene (peak 12) and m-xylene (peak 13) is not surprising. Also note that chlorobenzene (peak 10) and ethylbenzene (peak 11) are not separated with the TFS column. Comparison of Column Efficiencies. To compare efficiencies of the different columns for the target compound mixture, plots were generated of the ratio of peak width to retention time
Figure 4. Chromatograms of the 18-component test mixture using a 6-m-long TFS (a) and a 12-m-long TFS column (b) with atmospheric pressure air as carrier gas. The outlet pressure was 2.1 kPa. The oven temperature was set at 30 °C. Peak numbers correspond to label numbers in Table 1.
Figure 5. Plots of the ratio of peak width (full width at half-height) to retention time vs retention time for the test mixture using the 6-m PMS (A, ]), the 6-m CPS (B, 9), the 6-m DMS (C, 4), the 6-m TFS (D, /), and the 12-m TFS column (E, b).
vs retention time. The value of this ratio is proportional to the square root of the height equivalent to a theoretical plate for the column and thus for any column should be relatively constant over the limited range of retention values considered in this study. Figure 5 shows these plots for the PMS (A), CPS (B), DMS (C), and TFS (D) 6-m-long columns. Plot E is the 12-m-long TFS column. The straight lines are linear regression plots for each column. Of the columns investigated, the PMS was the poorest performer, producing peak width-to-retention time ratios of ∼0.1. This is about twice as large as for the CPS column and 2-3 times larger than the other columns. In addition, the variation from compound to compound is much larger with the PMS column. These ratios are very reproducible with shot-to-shot variations for any given compound typically less than (0.01. Corresponding values for the CPS column (plot B) are significantly better for most of the target compounds. Deviations from the regression line are also significantly smaller for this column. The DMS and
TFS columns were the best performers, producing peak widthto-retention time ratios in the range 0.02-0.03 for all of the target compounds. The DMS column produces slightly narrower peaks than the TFS column for retention times less than ∼80 s. The corresponding plate height for the DMS column is ∼0.05 cm. This compares very well with the value of 0.048 predicted from the Golay equation for a retention factor value of 2.0 and neglecting band broadening from the stationary phase. The results in Figure 5 suggest that neglecting band broadening from the stationary phase is appropriate for the DMS and TFS columns but not for the other columns considered in this study. The relatively large variations in the peak width-to-retention time ratios for the different target compounds observed for the PMS and CPS columns is probably the result of differences in the liquidphase diffusion coefficients for the compounds in these stationary phases. The relatively poor efficiency observed here for the PMS stationary phase is based on studies with a single column, and further studies with other PMS columns are needed. Retention Stability. Week-long retention stability studies were conducted in order to evaluate stationary-phase durability with prolonged exposure to air. Each column was operated for five 8-h workdays with air as the carrier gas. The PEG and the nonbonded CAS columns are quite susceptible to air degradation and were maintained overnight at room temperature with a low flow of hydrogen. Since the CPS and TFP were the most promising polar phases on the basis of efficiency studies, these columns were maintained in air overnight to provide a more rigorous stability evaluation. Because of the very poor efficiency observed with the PMS column, stability studies were not conducted. During each workday, ∼30 total sample injections were made, and retention data were collected for all components in the test mixture over a temperature range from 30 to 60 °C. Three injections were made for each set of conditions. Some injections for each column also contained dodecane and tridecane, and column temperatures up to 130 °C were used to more rapidly elute these compounds. In no case was a column heated over 130 °C while operated in air. Retention stability was evaluated through plots of the log of the retention factor vs reciprocal temperature (van’t Hoff plots). Retention factor values were obtained from measured retention times and calculated column holdup times. Figure 6 shows plots from the first workday (solid lines) and the last workday (broken lines) for the PEG (a) and CAS (b) columns. Plots labeled A-D are for 2-butanone, trichloroethylene, toluene, and o-xylene, respectively. Error bars ((σ for three chromatograms) are shown for each point, but in most cases, they are too small to be discernible in the figure. Substantial decreases in slope and significantly more curvature are observed for the PEG column after prolonged exposure to air. For the CAS column, the slopes are relatively unchanged after operation in air, but the intercept values are significantly reduced. On the basis of these changes, air degradation was most severe for the PEG column. These results indicate that neither of these polar columns would be useful with air as carrier gas. Figure 7 shows similar plots for the CPS column (a) and the TFP column (b). Again, error bars based on (σ values for three chromatograms are shown for each point but are too small to be easily discernible in the figure. These columns performed very well and showed no significant slope changes after 40 continuous Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
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Figure 6. Van’t Hoff plots at the start (solid lines) and at the end (broken lines) of a week-long study with air as carrier gas for the PEG (a) and the CAS column (b). Plots labeled A-D are for 2-butanone, trichloroethylene, toluene, and o-xylene, respectively.
hours of operation in dry air. Although not shown here, stability studies also were conducted for the nonpolar DMS column with results comparable to the TFS column in Figure 7. Table 2 summarizes retention time data from the first day and the last day of the stability study for the four polar phases considered in Figures 6 and 7. Data are not shown for the trichloroethylene with the CPS column since nearly complete coelution with 2-butanone occurred. The CPS and TFS columns show excellent retention time stability. In most cases, retention time variation over the week-long study is less than 100 ms. The nonbonded CAS column degraded more rapidly with decreases in retention time ranging from ∼180 ms to over 2 s. The PEG column was particularly susceptible to air degradation and showed a retention time decrease of nearly 15 s for o-xylene. Selectivity. For high-speed separations, peak capacity is sacrificed for speed. To make this tradeoff attractive, available peak capacity must be used with greater efficiency. This requires that greater attention be paid to column selectivity. Figure 8 shows chromatograms of the 18-component test mixture for the DMS column (a), the PMS column (b), and the CPS column (c). All columns were 6.0 m in length. Refer to Figure 4a for a comparable TFS chromatogram. Chromatograms are not shown for the PEG and CAS columns because of their poor retention time stability. All chromatograms were obtained at an oven temperature of 30 °C. Most striking in these chromatograms are the very broad peaks observed with the PMS column and to a lesser extent with 5204
Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
Figure 7. Van’t Hoff plots at the start (solid lines) and at the end (broken lines) of a week-long study with air as carrier gas for the CPS (a) and the TFS column (b). Plots labeled A-D are for 2-butanone, trichloroethylene, toluene, and o-xylene, respectively. Table 2. Retention Time Values at the Start and End of a One-Week Stability Study retention times (s) CPS start
end
TFS start
2-butanone 19.71 19.78 16.52 trichloroethylene 13.61 toluene 32.84 32.93 21.09 o-xylene 82.51 82.62 47.73
PEG
CAS
end
start
end
start
end
16.57 13.61 21.06 46.54
15.77 24.13 29.88 67.33
13.88 20.11 24.53 52.84
17.02 13.28 21.07 42.78
16.61 13.10 20.50 40.65
the CPS column. Several coelutions are observed with the DMS column, in particular peak pairs 1-4 (isopropyl alcohol, acetone) and 11-16 (ethylbenzene, nonane). Peak pair 12-13 (p-xylene, m-xylene) cannot be separated on any of the columns tested. While peaks 14 and 15 (o-xylene, styrene) show excessive overlap, adequate resolution should be obtained with a 12-m-long DMS column. While all of the polar phases tested produce dramatically different retention patterns, only the TFS and possibly the CPS columns have sufficiently high efficiency and stability in air to be considered as polar complements to the DMS column. Note that components 1 and 4 are completely resolved with the TFS column, and 14 and 15 have a resolution of ∼1.5 with the 6-m-long TFS column. Neglecting the p-xylene and m-xylene isomers, the 12m-long TFS column will separate all compounds in ∼250 s except components 10 and 11 (chlorobenzene and ethylbenzene). With
Figure 8. Chromatograms of the test mixture using the DMS (a), the PMS (b), and the CPS column (c) with atmospheric pressure air as carrier gas. All columns were 6 m long, and the outlet pressure was 2.1 kPa. The oven temperature was set at 30 °C.
the DMS column, chlorobenzene is easily separated but ethylbenzene coelutes with nonane (peak 16). CONCLUSIONS The constraint of using atmospheric pressure at the column inlet results in a larger increase in separation time with increasing
column length than is usually the case for conventional GC, where the inlet pressure can adjusted to control separation time. For 6-m-long, 0.25-mm-i.d. thin-film columns, separation times of 1-2 min can be achieved for the set of target compounds with a resolving power of ∼12 500 theoretical plates. For 12-m-long columns, the separation time increases to 4-5 min with a resolving power of ∼44 000 plates. Microbore columns (i.d. e 0.1 mm) would be attractive for use with atmospheric pressure air as carrier gas because of their higher optimal carrier gas velocity values and their greater efficiencies. However, volumetric flow rates are much lower that with the 0.25-mm-i.d. columns. This results in a large increase in extracolumn band broadening with the PID detector used for these studies. The retention stability studies reported here suggest that both nonpolar and polar columns can be used with air as carrier gas for the low-temperature separation of VOCs. In particular, the trifluropropyl polysiloxane phase showed very good retention stability and efficiency close to that observed with the nonpolar dimethyl polysiloxane phase. Clearly, studies conducted over a longer term and at higher temperatures are needed for the development of robust portable instruments using both nonpolar and polar columns. While the selectivities of the dimethyl polysiloxane and the trifluropropyl polysiloxane columns are somewhat complimentary, neither column alone, even using 12-m lengths, will resolve all compounds in the 18-component test mixture. In fact, chlorobenzene is not resolved on either column. Future work will involve combining these two columns in tandem (series) to obtain unique selectivities, which will provide a means to better resolve complex mixtures of interest in air monitoring applications. ACKNOWLEDGMENT The authors gratefully acknowledge the National Institute of Occupational Safety and Health (NIOSH) for financial support of this work.
Received for review May 27, 1999. Accepted September 6, 1999. AC990573Y
Analytical Chemistry, Vol. 71, No. 22, November 15, 1999
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