Anal. Chem. 1998, 70, 3498-3504
High-Speed Gas Extraction of Volatile and Semivolatile Organic Compounds from Aqueous Samples Carrie Leonard,† Hua-Fen Liu,‡ Stephen Brewer,‡ and Richard Sacks*,†
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, and Department of Chemistry, Eastern Michigan University, Ypsilanti, Michigan 48197
A device is described for high-speed gas extraction of volatile and semivolatile organic compounds from aqueous samples. High-speed extraction is achieved by the use of elevated temperatures. Large quantities of water vapor produced at elevated temperatures are managed with a reflux condenser, which efficiently removes water vapor without sample loss by returning the condensed water along with any cocondensed organic compounds to the extraction cell. The heated extraction cell uses a 1-2mL sample with an extraction gas flow of typically 30 mL/ min. Extraction temperatures as high as 95 °C can be used while maintaining a dew point temperature of ∼5 °C for the extraction gas and sample vapor leaving the device. Extraction profiles are obtained by connecting the device directly to a flame ionization detector. Extraction profiles for benzene show that quantitative recovery can be achieved in ∼30 s at an extraction temperature of 90 °C. Large increases in recovery at higher extraction temperatures are also demonstrated for tridecane and ethyl alcohol. For analytical studies, the device is interfaced to a commercial cryofocusing inlet system for highspeed gas chromatography. If all the extraction gas is trapped and injected into the separation column, detection limits for volatile organic compounds typically are in the 50-200 parts-per-trillion range. The emergence of high-speed methods using gas chromatography (GC)1-3 and mass spectrometry (MS)4,5 for the analysis of volatile and semivolatile organic compounds holds promise for dramatic increases in sample throughput and reductions in analysis costs. However, many environmental samples require extraction and preconcentration prior to analysis. These procedures often require several minutes to several tens of minutes and seriously compromise the effectiveness of high-speed analysis †
University of Michigan. Eastern Michigan University. (1) Sacks, R. D.; Smith, H. L.; Nowak, M. L. Anal. Chem. 1998, 70, 29A-37A. (2) Venkatramani, C. J.; Xu, J. Z.; Phillips, J. B. Anal. Chem. 1996, 68, 14861492. (3) Yan, X.; Carney, K. R.; Overton, E. B. J. Chromatogr. Sci. 1992, 30, 491196. (4) Wollnik, H.; Becker, R.; Gotz, H.; Kraft, A.; Jung, H.; Chen, C. C.; Vanysacker, P. G.; Jansen, H. G.; Snijders, H. M. J.; Leclercq, P. Q.; Cramers, C. A. Int. J. Mass Spectrom. Ion Processes 1994, 130, L7-L11. (5) Dagan, S.; Amirav, A. J. Am. Soc. Mass Spectrom. 1996, 7, 737-752. ‡
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methods. Gas extraction followed by adsorbent trapping (purge and trap) often is used for aqueous samples. Even with protracted extraction times, quantitative extraction may be difficult to achieve. Purge and trap or dynamic gas extraction was first described in 1962.6 Dynamic gas extraction coupled to capillary GC was first described in 1974.7 Today, it is commonly used for many applications including food,8,9 water,10 soil,11 and biological fluid12 analysis. There are several important limitations to dynamic gas extraction. To achieve low detection limits, large sample volumes and long purging times usually are used. Most procedures require 5-50 mL of sample. This can be a problem in some applications. Extraction times typically are in the 10-30-min range. This would significantly limit sample throughput when used with high-speed GC, high-speed GC/MS, or direct MS. Trapping can be done using a room-temperature sorbent trap, such as Tenax,13-15 or a cold trap.9,15,16 Sorbent traps often have long desorption times, on the order of 3 min, and can be subject to problems such as thermal degradation and memory effects.15 Capillary cold traps are easily blocked by large amounts of water vapor in the extraction gas.15 Dynamic gas extraction follows typical first-order kinetics,17 and dramatic improvements in extraction rates can be achieved by the use of elevated temperature and high extraction gas flow rates relative to the sample volume. However, increasing temperature and/or flow rate increases the quantity of water vapor, and the management of this water vapor is a significant problem. The use of a cold condenser or a Nafion membrane for water vapor management has been reported for use with gas extraction at elevated temperatures.15 While rapid extraction and efficient (6) Swinnerton, J.; Linnenborm, V.; Cheek, C. H. Anal. Chem. 1962, 34, 483. (7) Bellar, T. A.; Lichtenberg, J. J. J. Am. Water Works Assoc. 1974, 66, 739744. (8) Colemann, W. M. J. Chromatogr. Sci. 1992, 30, 159-163. (9) Badings, H. T.; de Jong, C. J. High Resolut. Chromatogr. 1985, 8, 755-763. (10) Lesage, S.; Brown, S. Anal. Chem. 1994, 66, 572-575. (11) Askari, M. D. F.; Maskarinec, M. P.; Smith, S. M.; Beam, P. M.; Travis, C. C. Anal. Chem. 1996, 68, 3431-3433. (12) Ashley, D. L.; Bonin, M. A.; Cardinali, F. L.; McCraw, J. M.; Holler, J. S.; Needham L. L.; Patterson, D. G. Anal. Chem. 1992, 64, 1201-1209. (13) Curvers, J.; Noy, T.; Cramers, C.; Rijks, J. J. Chromatogr. 1984, 289, 171182. (14) Lepine, L.; Archambault, J. F. Anal. Chem. 1992, 64, 810-814. (15) Noij, T.; van Es, A.; Cramers, C.; Rijks, J.; Dooper, R. J. High Resolut. Chromatogr. 1987, 10, 60-66. (16) Pankow, J. F. J. High Resolut. Chromatogr. 1987, 10, 409-410. (17) Lin, D. P.; Falkenberg, C.; Payne, D. A.; Thakkar, J.; Tang, C.; Elly, C. Anal. Chem. 1993, 65, 999-1002. S0003-2700(98)00153-X CCC: $15.00
© 1998 American Chemical Society Published on Web 07/03/1998
Figure 1. Diagram of the high-speed gas extraction device: S, sample cell; B, heated block; H, heating cartridge; G, extraction gas inlet; I, sample inlet; V, valve; P vacuum pump; R, reflux tower; C, condenser; F, support cylinder.
water management were reported, loss of polar and higher boiling point sample components was significant. This report describes a very efficient dynamic gas extraction device that combines high-temperature extraction with reflux water condensation in order to achieve rapid extraction with efficient water management. A cooled reflux tower is located directly above the heated extraction cell, and organic compounds that cocondense with water are returned to the heated cell. This eliminates sample loss. Exhaustive extraction of BTEX compounds (benzene, toluene, xylene, ethylbenzene) is achieved in ∼30 s at an extraction temperature of 90 °C. Device design and performance are considered. Extraction rate-time profiles for several compounds are used to illustrate the advantages of the device. Finally, the device is combined with a high-speed GC instrument in order to achieve analysis cycle times of ∼1 min. EXPERIMENTAL SECTION Apparatus. The extraction cell was designed to accommodate a sample volume of 0.5-2.0 mL and use extraction gas flow rates of 10-50 mL/min. The shape of the cell was chosen to facilitate efficient rinsing between samples with minimal sample carry-over. Because of the small sample volume and cell volume, a relatively small diameter (0.83-mm i.d.) extraction gas delivery tube was used. This results in smaller bubbles of extraction gas, which reduces the probability of violent agitation of the sample liquid. The high-speed gas extraction device was designed and constructed in house. A diagram of the device is shown in Figure 1. The device consists of two distinct thermal zones. The hot zone consists of a stainless steel block B, heating cartridges H, the sample cell S, and a sample injection port I. The injection port uses a septum in a 3.2-mm tube fitting for sample injection. The sample injection tube is 0.83-mm i.d. and 55 mm long. The total heating capacity of the heating cartridges is 250 W. The sample cell has a diameter of 9.5 mm and a volume of 2.0 mL. Nitrogen extraction gas is provided from a pressurized source G. The bottom of the cell is machined at an angle to facilitate draining and rinsing without sample carry-over. Sample is drained through valve V by means of vacuum pump P. The waste liquid is recovered in a trap located prior to the vacuum pump. All
components in the hot zone are stainless steel except for the cylindrical aluminum housing F. The space between the housing and the heated block is packed with cotton fiber insulation. The cold zone contains a reflux tower R, which is cooled by a water-cooled condenser C. The tower inner diameter is 6.3 mm, and the length of the cooled portion is ∼41 mm. The stainless steel tube used for the tower passes through the aluminum housing for the heated zone and is pressed into the top of the extraction cell. The total volume of the cell and tower tube is ∼4.6 mL. Water is circulated through the condenser by a submersible pump (Little Giant Pump Co., Oklahoma City, OK, model 12E-38N), which is placed in an ice bath. Analytical data were obtained with a Varian 3500 GC (Varian, Walnut Creek, CA) equipped with a commercial high-speed inlet system (Chromatofast Inc., Ann Arbor, MI, model L). An 8-mlong, 0.25-mm-i.d. nonpolar dimethylpolysiloxane (DB-1) column (J&W Scientific, Folsom, CA) was used. The stationary-phase film thickness is 0.25 µm. The oven temperature was 65 °C. A flame ionization detector was used at a temperature of 200 °C. Purified hydrogen at an average linear velocity of 200 cm/s was used as carrier gas. The inlet system uses cryofocusing to collect (integrate) sample and inject it as a narrow plug into a capillary separation column. The inlet uses a vacuum pump to pull the atmospheric-pressure gas exiting from the reflux tower through a capillary metal trap tube, which was cooled to at least -80 C. Flow rate into the inlet is 1.6 mL/min. The end of the capillary sampling tube from the inlet system was positioned by a connector ∼5 mm above the end of the reflux tower. Sample is collected in the trap tube for a software-selected time. After sample collection, the trap tube is pressurized with carrier gas, and the tube is resistively heated by a capacitive discharge power supply. This injects a sample vapor plug ∼10 ms in width into the separation column.18 An electrometer with a time constant of ∼5 ms was built in house. A Gateway 2000 4DX2-66V PC (Gateway 2000, Sioux Falls, SD) with an A/D board (Computer Boards Inc., Mansfield, MA, model CIO-DAS-1600) and Labtech Notebook software (Laboratory Technologies Inc., Wilmington, MA) was used to control data acquisition. Procedures. A long-needle syringe was used to introduce 1.0-mL samples into the preheated extraction cell. Prepurified nitrogen extraction gas flowed continuously. Flow rate was measured by a Hastings flowmeter (Teledyne-Hastings Corp, Hampton, VA, model PR4-AJ). Sample cell and condenser temperatures were measured using a thermocouple (Omega Engineering, Stamford, CT, model 199A type J). After completion of an experiment, the drain valve was opened and the sample drained. Several milliliters of distilled water were injected through the condenser tower to rinse the cell between runs. The quantity of water vapor exiting the reflux tower was measured by connecting a 6-mm-i.d., 80-mm-long polyethylene tube containing calcium sulfate to the top of the tower. To collect sufficient water for accurate mass determination, water vapor was collected for 30 min for each experiment. A balance with an accuracy of 0.1 mg was used to weigh the tube before and after water collection. (18) Klemp, M. A.; Akard, M. L.; Sacks, R. D. Anal. Chem. 1993, 65, 25162521.
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Extraction rate-time profiles and associated recovery data were obtained by connecting the gas extraction device directly to the flame ionization detector (FID) of a Varian model 3700 GC (Varian, Walnut Creek, CA) using a 75-cm length of 0.53-mm-i.d. deactivated fused silica tubing. The entire extraction gas sample passed through the detector. A nitrogen extraction gas flow rate of 10 mL/min was used. A small positive pressure in the extraction cell was required due to the restriction of the transport tube. Significantly higher flow rates resulted in unstable flame operation. Samples were prepared in the ppm range by spiking water with reagent-grade volatile organic compounds (VOCs). Numerical integration of the extraction profiles was used to obtain recovery-time plots. For extractions that were not complete in the measurement time, the total peak area was estimated by the extrapolating the falling portion of the extraction profile to the baseline. The extrapolation assumed logarithmic decay of the falling portion of the profile. It was also assumed that the condenser tower had a negligible effect on the shape of the extraction profile at the concentrations used. Analytical data were obtained using 1.0-mL water samples containing trace concentrations of reagent-grade organic compounds. Stock solutions of 1000 ppm were prepared in ethyl alcohol. Serial dilutions of stock solutions with distilled water were used to generate analytical curves. Sample carry-over was determined by analyzing a distilled water blank after the analysis of a high-concentration sample. Extractions were carried out using a cell temperature of 82 °C and a gas flow rate of 30 mL/ min. The cryofocusing inlet system was switched into the sampling mode just prior to injection of the 1.0-mL sample into the heated extraction cell. Sample vapor was collected by the inlet system for 30 s following sample injection. The inlet system was then switched into a purge mode, which abruptly terminates vapor collection. The metal trap tube was then rapidly heated by the current pulse from a capacitive discharge power supply. RESULTS AND DISCUSSION Design Features and Performance. Insight into the design requirements and operating conditions for a high-speed gas extraction system is gained from eq 1 for sample recovery,13 where
R ) 1 - exp[-Vp/(KVs + Vg)]
(1)
Figure 2. Sample temperature vs time plots for initial cell temperatures of 50 (A), 60 (B), 75 (C), and 90 °C (D).
R ) 1 - exp(-Vp/Vg)
(3)
and for low-solubility, high-vapor-pressure analytes (relatively small, nonpolar molecules). For this limiting case, high recovery can be achieved if Vp . Vg. From the standpoint of extraction cell design, small headspace volume is desirable. Since the total volume of extraction gas is equal to the product of volumetric flow rate F and extraction time t, eq 3 can be recast as eq 4 using
R ) 1 - exp(-Ft/Vg)
(4)
more instrumental parameters. Thus, the use of high flow rates of extraction gas can greatly reduce extraction time. Also note that, for this limiting case, recovery is not significantly influenced by the value of K, and thus sample temperature is relatively unimportant. For the other limiting case, KVs . Vg and eq 1 reduces to eq 5. By combining eqs 2 and 5 and replacing Vp with Ft, eq 6 is
R ) 1 - exp(-Vp/KVs)
(5)
R ) 1 - exp(-FtP°γ/CPtVs)
(6)
Vp is the total volume of extraction gas, Vs is the sample volume, Vg is the headspace volume of the cell, and K is the distribution ratio of the analyte between the gas and liquid phases. Here, recovery is defined as the fraction of the total analyte initially present in the cell that exits the reflux tower in the vapor phase. Equation 1 neglects any sample losses during water removal, trapping, and subsequent desorption of the sample. The distribution ratio is described by eq 2. Here C is a constant, Pt is the
obtained. Again it is clear that small sample size and high extraction gas flow rates favor fast extraction with high recovery. The effects of temperature on recovery for a fixed extraction time can be found from the effect of temperature on P° through the integrated form of the Clausius-Clapeyron equation19
K ) CPt/(P°γ)
where ∆Hv is the heat of vaporization of the analyte, R is the gas constant, Te is the extraction temperature, and A is an integration constant. For many organic compounds, the ratio of ∆Hv to boiling point Tb is nearly constant (Trouton’s rule).20 By combining this fact with eq 7, and noting that P° is equal to 1 atm at
(2)
total headspace pressure (usually 1 atm), P° is the vapor pressure of pure analyte at the extraction temperature, and γ is the analyte activity coefficient in water. Two limiting cases are of interest. If Vg . KVs, then eq 1 can be approximated by eq 3. This case occurs for very small samples 3500 Analytical Chemistry, Vol. 70, No. 16, August 15, 1998
ln P° ) A - ∆Hv/RTe
(7)
(19) Karger, B. L.; Snyder, L. R.; Horvath, C. An Introduction to Separation Science; Wiley: New York, 1973; pp 21, 68. (20) Atkins, P. W. Physical Chemistry, 5th ed.; Freeman: New York, 1994; p 134.
Table 1. Reflux Tower Efficiency at Different Sample Temperatures and Purge Gas Flow Rates purge gas flow rate (cm3/min)
sample temp (°C)
expected (mg)
exptl (mg)
% diff
dew point (°C)
mass water in gas (mg/L)
21 24 26 36 37 38 38 38 66
95 90 60 95 90 70 25 45 90
4.0 4.6 5.0 6.8 7.1 7.2 7.2 7.3 8.4
3.1 4.1 5.9 8.1 7.8 6.6 7.0 6.8 7.0
-23.4 -10.3 18.1 18.7 9.8 -7.9 -3.0 -6.3 -16.6
0.1 2.4 6.6 6.6 5.4 2.8 3.6 3.0 1.3
4.9 5.7 7.5 7.6 7.0 5.9 6.2 6.0 5.3
temperature Tb, eq 8 is obtained where P° is expressed in Torr
log P° ) 7.7 - 4.8(Tb/Te)
(8)
and the temperature is in kelvin. The constants 7.7 and 4.8 contain the gas constant, the Trouton’s rule constant, the log of 760, and the conversion to base 10. The important point in eq 8 is that the analyte vapor pressure depends on the ratio of the analyte boiling point to the extraction temperature. For extraction temperatures near the analyte boiling point, increasing extraction temperature should have relatively little effect on recovery. For extractions from water, the extraction temperature is limited to the boiling point of water. For analytes with boiling points significantly less than the boiling point of water, very fast extractions should be possible. For high-boiling-point analytes, increasing extraction temperature should dramatically increase recovery or shorten extraction time. While gas extraction at elevated temperatures has been used, water vapor management has been a problem. The use of a lowtemperature condenser or a Nafion membrane can remove the bulk of the water vapor from the extraction gas. However, polar analytes are also removed.15 Memory effects also have been noted with these water removal techniques. The system described in this report uses a reflux condenser so that polar or high-boilingpoint analytes, which cocondense with the water vapor, are returned to the extraction cell for continued extraction. To achieve high efficiency, a relatively small sample (1.0 mL) is injected by syringe into the preheated extraction cell. The cell is designed to have relatively large thermal mass so that injection of the room-temperature sample results in minimal cooling of the cell. Figure 2 shows temperature-time profiles obtained from direct thermocouple measurement of the sample temperature. Initial cell temperatures of 50 (A), 60 (B), 75 (C), and 90 °C (D) were used. Immediately after sample injection, the temperature falls due to cooling by the room-temperature sample. The extraction gas flow rate was 10 mL/min., and the gas bubbles achieve efficient mixing. A minimum temperature is reached ∼2 s after sample injection, and this is followed by a relatively rapid recovery of the cell temperature. After ∼15 s, the sample temperature has risen ∼90% of the interval between the initial room-temperature value and the initial cell temperature. Since extraction gas is continuously flowing into the cell, extraction begins immediately following sample injection. During this initial sample-heating interval, extraction conditions change rapidly. The reflux tower was designed to provide efficient water removal while contributing minimal extra headspace volume to
Figure 3. Extraction profiles (a) and recovery plots (b) for benzene extracted at temperatures of 24 (A), 59 (B), 70 (C), and 92 °C (D).
the cell. Extraction gas flow through the tower is laminar, and the rate of condensation at the cold tower wall is limited by diffusion of water vapor in the extraction gas. The average diffusion time td is given by the Einstein equation for planar diffusion,19
td ) r2/4D
(9)
where D is the binary diffusion coefficient for water in the extraction gas and r is the tower radius. Thus, the use of a smaller tower radius should improve mass transport and improve water management. However, if the radius is too small, droplets formed during condensation could block the tower and be expelled by the extraction gas. This could result in sample loss and contamination of the inlet system interface line. For a tower radius of 0.32 cm, and using a value of 0.24 cm2/s for D at a tower wall temperature of 5 °C,21 the average diffusion time is found to be ∼0.1 s. At an extraction gas flow rate of 30 (21) Weast, R. C., Ed. CRC Handbook of Chemistry & Physics, 49th ed.; CRC Press: Cleveland, 1968; pp F47, D109.
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Figure 4. Extraction profiles (a) and recovery plots (b) for ethyl alcohol extracted at temperatures of 20 (A), 55 (B), and 75 °C (C).
Figure 5. Extraction profiles (a) and recovery plots (b) for tridecane extracted at temperatures of 20 (A), 55 (B), and 75 °C (C).
mL/min, the gas residence time in the tower is ∼2.6 s. Since residence time is more than 25-fold greater than average diffusion time, efficient condensation should be achieved, and the dew point temperature of the gas leaving the tower should be nearly equal to the tower temperature. This also suggests that most of the condensation occurs near the base of the reflux tower. Table 1 lists the weight of water collected on a desiccant and the associated dew point temperature for various combinations of extraction cell temperature and gas flow rate. Water vapor was collected for 30 min. The expected values in the table are based on saturation vapor pressure values for water21 at the measured temperature of the gas exiting the tower and the measured flow rate of extraction gas. For all cases, there is reasonable agreement between the water mass recovered and the expected value, indicating that the extraction gas leaving the tower has a dew point temperature of ∼5 °C. Thus, efficient water vapor management has been achieved. For a 1.0-min extraction at a 30 mL/ min flow rate, ∼200 µg of water vapor will leave the device. This value is nearly independent of extraction cell temperature. For a 1.0-min extraction at 95 °C without water removal, the amount of water vapor in the extraction gas would be ∼15 mg. This is ∼75fold more than with the reflux tower present. Extraction Profiles. Extraction profiles for individual compounds were obtained by directly connecting the extraction device to the FID. The transport time from the top of the condenser tower to the FID was 1.0 s. The pressure drop across the fused silica was ∼1.2 Torr. For the device described here, the total volume of the extraction cell and the reflux tower is 4.6 mL. The sample size was 1.0 mL, and thus the headspace volume was 3.6 mL. Figure 3a shows extraction profiles for 22 ppm benzene at four extraction cell temperatures. Plots A-D correspond to cell temperatures of 24, 59, 70, and 92 °C, respectively. A relatively low extraction gas flow rate of 10 mL/min was used in order to
obtain more detailed information regarding the rising edge of extraction profiles and to obtain more stable FID operation. Note that, at this flow rate, Vp/Vg is 2.8, and from eq 3, the recovery approaches 95% for a 1-min extraction of high-vapor pressure, lowpolarity solutes. Numerical integration of the extraction profiles gives the recovery vs time plots shown in Figure 3b. At 24 °C, the peak in the extraction profile occurs ∼15 s after sample injection. This corresponds to the passage of ∼2.5 mL of extraction gas. As the cell temperature is increased, the peak is shifted to slightly earlier times. Noting the sample temperature vs time curves in Figure 2, the sample temperature is increasing steadily during this time interval. Most significantly, the extraction profile becomes much sharper at high sample temperatures. The boiling point of benzene is ∼80 °C, and rapid extraction is expected at 92 °C. The recovery plots show that, at 92 °C, nearly quantitative recovery is achieved in ∼30 s. As the sample temperature decreases, the time required for quantitative extraction increases steadily, and at 24 °C, ∼90% recovery is obtained in ∼100 s. Note that, even at this lowest temperature, extraction is quite rapid because of the small sample size and the relatively small headspace volume (see eq 1). For analytical work, the extraction gas flow rate was increased to 30 mL/min. This results in the shifting of the extraction profiles and recovery plots to shorter times, thus further enhancing the high-speed performance of the device. Alcohols usually are considered difficult to extract because of their strong interactions with water. Noij et al.15 reported zero recovery for 1-nonanol extracted at elevated temperatures using a cold condenser or a Nafion membrane for water vapor management. Figure 4a shows extraction profiles for 20 ppm samples of ethyl alcohol using the high-speed extraction device, and (b) shows the corresponding estimated recovery plots. The extraction temperatures were 20 (A), 55 (B), and 75 °C (C). At 20 °C, a
3502 Analytical Chemistry, Vol. 70, No. 16, August 15, 1998
Table 2. Analytical Results for Gas Extraction of Selected VOCs log-log plot
a
no.
compound
bp (°C)
LOD (k ) 3) (ppb)
% RSDa (n ) 4)
slope
R2
1 2 3 4 5 6 7 8 9
hexane benzene heptane toluene octane ethylbenzene m-xylene o-xylene nonane
69 80 98 111 125 136 138 143 151
1.2 0.82 1.8 1.2 2.9 2.3 2.3 2.7 7.5
14 6.8 11 7.7 4.8 8.9 8.6 7.7 3.0
1.09 1.03 1.05 0.99 0.92 1.03 1.06 1.06 0.75
0.9975 0.9991 0.9994 0.9968 0.9924 0.9987 0.9992 0.9994 0.9579
Relative standard deviations for a concentration of 1 ppm.
Figure 6. High-speed chromatogram of VOCs extracted at a temperature of 82 °C. See Table 2 for peak identification.
nearly flat, low-amplitude extraction profile is obtained. Less than 50% recovery is achieved in 100 s. While some improvement is observed at 55 °C, the extraction is still quite slow, and excessive time would be required to achieve high recovery. When the extraction temperature is increased to 75 °C, a large increase in extraction rate is observed, and nearly quantitative extraction is obtained in ∼100 s. Extraction profiles for tridecane, at a concentration of 75 ppm, are shown in Figure 5. For plots A-C, the initial cell temperatures were 20, 55, and 75 °C, respectively. Again, the extraction gas flow rate was 10 mL/min. The boiling point of tridecane is 234 °C, and relatively slow extraction is expected at 20 °C. The extraction profile peaks are much wider than those observed for benzene, and the peak values occur significantly later. Again, the peaks occur earlier at the higher temperatures. Extraction is much faster at 75 °C, although the increase in extraction rate is not as dramatic as that observed for the lower boiling compounds. This is partially due to the effects of the condenser. At high concentrations, compounds with higher boiling points are removed by the condenser, returned to the sample, and then repurged. This causes a distortion of the extraction profile, which makes it difficult to extrapolate the curve for recovery calculations. Analytical Evaluation. High-speed GC analysis of a ninecomponent VOC mixture was performed with a commercially available cryofocusing GC inlet system designed for high-speed operation. This system is convenient since the organic vapors in the extraction gas can be collected directly from the atmospheric pressure gas exiting from the reflux tower. Since the gas flow
Figure 7. Analytical curves (log-log) for selected VOCs using the high-speed gas extraction device. See Table 2 for component identification and for a statistical analysis of the plots.
rate into the inlet is ∼1.6 mL/min, only ∼5.3% of the extraction gas is collected at the 30 mL/min flow rates used here. Figure 6 shows a typical chromatogram for the mixture. The extraction was performed at 82 °C. The peak numbers correspond with the compound numbers in Table 2. The concentration of each sample component was 1.0 ppm. Note that the analysis is complete with adequate resolution of all components in just over 16 s. No significant distortion of peak shapes is observed. Figure 7 shows log-log plots of peak height versus concentration for some of the sample mixture components. The numbers on the plot correspond to the peak numbers in Figure 6. Table 2 summarizes analytical data from the plots. The slopes of the log-log plots are within the range 0.9-1.1, indicating good overall linearity of the method, and correlation coefficients are greater than 0.99. The exceptions to this are nonane and octane, which show deviations from linearity at concentrations greater than 5 ppm due to the effects of the condenser. Relative standard deviations (RSDs) for four replicate determinations typically are in the (5-15% range, which is typical of conventional purge and trap techniques. Sample carry-over following a wash with 1 mL of distilled water was less than 1% for most analytes. Limits of Analytical Chemistry, Vol. 70, No. 16, August 15, 1998
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detection (LODs) at a signal-to-noise ratio of 3 are in the low-ppb range. Other work has shown that the inlet system can be operated with sample gas flow rates greater than 30 mL/min. Modifications for this purpose are in progress. The result will be a nearly 19-fold (30 mL/1.6 mL) improvement in detection limits with values in the ppt range. For benzene, a detection limit of less than 50 ppt should be achieved. CONCLUSIONS The high-speed gas extraction device described in this report is both efficient and convenient. Since sample loss by cocondensation is nearly eliminated, even very polar compounds can be readily extracted from water at elevated temperatures. The use of syringe injection and a computer-operated drain valve will allow for sample introduction and rinsing with an autoinjector. Complete automation of the system should be straightforward. For many purgeable VOCs, a 30-s extraction is nearly quantitative at temperatures approaching the boiling point of water. This, coupled with high-speed GC should provide complete analysis cycle times of ∼1 min. The high efficiency of the reflux condenser allows operation of the device at temperatures approaching the boiling point of
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water and the use of longer extraction times. This may provide exciting opportunities for the gas extraction of higher molecular weight organic compounds. This is not practical with conventional extraction devices. However, the influence of cocondensation with water of high boiling point and highly hydrophilic sample components is not clear, and further studies are suggested. If the rate of cocondensation is concentration dependent, nonlinear analytical curves may result. This may explain the slope of the log-log plot for nonane in Figure 7. ACKNOWLEDGMENT The authors thank Varian for the analytical instrumentation used in these experiments. We are grateful to Chromatofast Inc. and Heather Smith for helpful discussions and technical support. We also acknowledge Eastern Michigan University researchers Uma Balasubramanian and Christa Hughes for preliminary work done with the gas extraction system.
Received for review February 10, 1998. Accepted May 28, 1998. AC980153T
