Anal. Chem. 1994,66, 1339-1346
Membrane Extraction with a Sorbent Interface for Capillary Gas Chromatography Mln J. Yang, Susan Harms, Yu 2. Luo, and Janusz Pawllszyn’ The G u e b k Waterloo Center for Graduate Work in Chemistty and the Waterloo Center for Groundwater Research, University of Waterloo, Waterloo, Ontario, Canada N2L 3 6 1 A new analytical method, combining the hollow fiber membrane, cryofocusing, and thermal desorption technologies, has been developed to allow rapid routine analysis and long-term continuous monitoring of volatile organic compounds in various environmental matrices. This method of membrane extraction with a sorbent interface (MESI) is simple, effective, and solventfree and easy to automate. It minimizes the loss of analytes by interfacing the membrane extraction module directly to a capillary gas chromatograph(CC). Application of hydrophobic membranes prevents moisture from entering the carrier gas. The sensitivity of the system is significantly enhanced with the cryogenic sorbent trap because of a high sample throughput. Experimentally, the MESI system consists of a hollow fiber membrane module, a cryofocusing and thermal desorption sorbent interface, an isothermal capillary CC,and a computer. The membrane is in direct contact with a sample or its headspace. Analytes of interest diffuse across the membrane and are collected at the cryogenic trap. A heat pulse desorbs all collected analytes at the trap and produces a narrow concentration band at the front of the GC column. During the method development process, several designs of membrane modules were investigated, and their extraction performances were compared. At the optimum experimental conditions, the limit of detection for trichloroethene in water was 1pg/L, and a RSD below 3%was achieved using a flame ionization detector. The detection limit for the same analyte obtained with the conventional purge-and-trap followed by CC/MS is typically 0.2 pg/L. Parameters influencing the sensitivity and precision of the MESI system were studied. Quantitative extraction was performed to improve accuracy of the analysis without the need for internal standards. A number of environmental applications were demonstrated, which included analysis of VOC contaminants in clean water, wastewater, and laboratory air. The widespread presence of organic contaminants has led to growing public concern about the quality of the environment. Increased testing to meet this concern has created additional demand for the development of simple and efficient analytical methods to detect and quantify organic compounds in various environmental matrices. The technique for isolation of target analytes from a sample matrix can directly influence the accuracy, precision, and sensitivity of an analytical process. The current organic extraction methods, such as headspace sampling, purge and trap, liquid-liquid extraction, and solidphase extraction are costly, time consuming, and tedious and often incur loss of analytes. An ideal extraction system would be simple,rapid, highly selective, solvent-free, and independent 0003-2700/94/03651339$04.50/0 0 1994 American Chemical Society
of instrument design. It should also be adaptable to automation for convenienton-line analysisor monitoring. A new analytical technique, membrane extraction with a sorbent interface (MESI), has been developed which includes several of these characteristics. Membranes are used for a number of diverse applications such as microporous filtration, ultrafiltration, reverse osmosis, dialysis, electrodialysis, and gas separation. The selective nature of the membranes has made them particularly suitable for rapid analytical determination or continuous on-line monitoring. Along with the first analytical application of a membrane described by Hoch and K o k , 2 a significant amount of research has been devoted to interface air and water samples directly to a mass spectrometer (MS). In a membrane introduction MS,one side of the membrane is directly exposed to the vacuum of the ion source of the instrument. Exposing the other side of the membrane to an aqueous or gaseous sample results in permeation of organic compounds through the membrane wall followed by diffusion in gas phase to the ion source which provides parts per million (ppm) to parts per billion (ppb) sensitivity. Although, many early methods used supported membrane sheets, it has been found that the hollow fiber has a more useful geometry for analytical applications in its self-supporting tubular form. Since the first tubular silicone rubber application introduced by Westover et aL,3 most of the recent developments of membrane techniquesa have focused on the use of hollow fibers. Kotiaho et al.’ recently provided a review on the theories and applications of membrane introduction MS. Despite the three decades of development in analytical membrane technology, little information is available on its use in sample preparation for chromatography. For instruments without a vacuum, such as the gas chromatograph (GC), an inert gas must be used as a stripping phase to facilitate the solvent-free extraction process. Blanchard and Hardy* were among the first who used nitrogen as a stripping gas to transfer the permeated analytes from a surface of a flat membrane to a bed of activated charcoal. The compounds were later desorbedonto a GC for analysis. Stetter and Caog used a solid-state gas sensor to detect chlorinated (1) Warren, D. Anal. Chem. 1984, 56. 1529A. (2) Hoch, G.; Kok, B. Arch. Blochem. Bfophys. 1963, 102, 160. (3) Westover, L. B.; Tou, J. C.; Mark, J. H.Anal. Chrm. 1974, 46, 568-571. (4) Slivon, L. E.; Bancr, M.R.; Ho, J. S.;Buddc, W. L.Anal. Chem. 1991,63, 1335-1340. ( 5 ) Lapack, M.A.; Tou, J. C.; Enkc, C . G . Anal. Chem. 1990,62, 1265-1271. (6) Brodbelt, J. S.; Cooks, R. G.; Tou, J. C.; Kallos, G. J.; Dryzga, M.D. Anal. Chem. 1987,59, 454-458. ( 7 ) Kotiaho, T.; Lauritsen, F.R.;Choudhury, T.K.; Cooks, R. G.; Tsao, G. T. Anal. Chem. 1991, 63, 815A-883A. (8) Blanchard, R. D.; Hardy, J. K. Anol. Chem. 1984, 56, 1621-1624.
Ana&tlcalChemistry, Voi. 66,No. 8, Aprll 15, 1994 1339
Membrane Extraction Module
total orpnic cons.
time binary inisstion slgnal from computer
Cryofocusing Sorbent Interface total organic conc.
time coolant
time
C. Headspace Separation Column
tactor signal to computer
time
Figure 1. Schematicshowlngthe major components of the membrane extractlon sorbent Interface (MESI) system.
hydrocarbons in the helium purge flow from the interior of multiple hollow fibers. A membrane system to perform exhaustive extraction on water samples was developed using microporous polypropylene and nonporous silicone rubber hollow fibers."J The conditions under which quantitative extraction occurred were studied extensively through mathematical modeling of the system." A hollow fiber membrane module was constructed in conjunction with a multiplex GC system to monitor volatile organic compounds (VOCs) in water.12 Most recently, a combined technique of hollow fiber membrane and high-density carbon dioxide extraction has been developed for analysis of semivolatile organic compounds in water.13 Based on the previous studies,lG13theuseof hollow fiber membranes to extract organic analytes from aqueous samples for GC analysis is a simple, inexpensive, and solventfree alternative for sample preparation. The membrane interface can also be easily adapted to automation for convenient on-line analysis or monitoring. In this paper, the various components of the MESI system are described. In order to determine the optimum experimental conditions for the system, several membrane module configurations are investigated and compared. A few applications of the new method are demonstrated.
EXPERIMENTAL SECTION Apparatus and Reagents. Figure 1 depicts a MESI system which consists of four parts: (1) the membrane extraction module, (2) the cryofocusing and thermal desorption sorbent interface, (3) the GC, and (4) the computer process control and data acquisition center. (9) Stetter, J. R.; Cao, 2.Anal. Chem. 1990, 62, 182. (10) Pratt, K. F.; Pawliszyn, J. Anal. Chem. 1992, 64, 2107-21 10. (11) Pratt, K. F.; Pawliszyn, J. Anal. Chem. 1992, 64, 2101-2106. (12) Yang, M. J.; Pawliszyn, J. Anal. Chem. 1993,65, 1758-1763. ( 1 3 ) Yang, M . J . ; Pawliszyn, J. Anal. Chem. 1993, 65, 2538-2541.
1340 Analytical Chemistry, Vol. 66, No. 8, April 15, 1994
Figure2. Differentconfigurationsof hollow flber membraneextraction module for analysis of volatlle organlc compounds.
The membrane module designs, shown in Figure 2, were investigated for different applications. Each membrane module consisted of a piece of hollow silicone fiber (Dow Corning Canada Inc., Mississauga, ON) encased in either a sample container, typically a 40-mL glass vial, or a 10-cm glass capillary tube. The fiber's inner diameter was 305 pm and its wall thickness was 165 pm, while the typical active length ranged from 6 to 10 cm. The flow-over and flowthrough module fabrication were previously described.'OJ* A syringe pump (Sage Instruments, Cambridge, MA) was used for these configurations to provide consistent flow of a sample through the membrane module. The other modules consisted simply of two pieces of a 24-gauge stainless steel needle (Hamilton Co., Reno, NV) and a piece of the hollow fiber membrane. To connect the membrane to the needle, one end of the silicone fiber was first submerged in toluene for 10 s, and after the fiber had swollen, 1 cm of the stainless steel needle was inserted into the hollow fiber. When toluene evaporated, the membrane shrank and a tight seal formed between the membrane and the needle wall. The carrier gas was routed, through the membrane before entering the GC column. The exterior of the membrane was exposed to a sample or the headspace of the sample. The cryofocusing and thermal desorption sorbent interface consisted of a cryogenic well, a sorbent tube, an electrical heating coil, and a solid-state relay. The cryogenic well consisted of a Styrofoam container, 100 g of solid C02, which was placed on top of the GC's on-column injection port. A DB-5 column (30.0 m X 0.53 mm, with a stationary phase thickness of 1.5 m, J&W Scientific, Folsom, CA) was positioned along the well axis, passed out the bottom, and
-
-
protruded into the GC injection port. A 2-cm section of the column, close to the inlet, was considered as the sorbent tube in this study and covered by solid C02 and cooled to --78 OC. The carrier gas line leading to the injector port was disconnected and reconnected to the gas inlet of the membrane module using a l / d n . stainless steel union (Swagelok Canada Ltd., Niagara Falls, ON) and 0.8-mm graphite-vespel ferrules (Supelco Inc., Bellefonte, PA). The gas outlet of the membrane module was directly connected to the head of the sorbent tube with a 0.8-mm capillary column connector (Supelco Inc.). The electric thermal desorption device used in this MESI method was very similar to the one discussed in a recent paper.12 A piece of Ni-Cr wire (20% Cr, 104.1 Mm in diameter, and 1.12 m in length, 124 fl total resistance, Johnson Matthey Metals Ltd.) was closely coiled around the sorbent tube inside the cryogenic well, so that the heating coil covered the entire length of the capillary column exposed to solid C02. A solid-state relay switched on and off the electrical power supply to the heatingcoil according to a computer signal. A Variac was used to regulate the ac power to a consistent 26-V level before it was delivered to the heating coil. When electric current passed through the coil, heat was generated and analytes were desorbed. When the electrical power was turned off, the coil cooled very rapidly to the sublimation point of solid C02. A Varian 3500 GC (Varian Canada Inc., Georgetown, ON) equipped with a flame ionization detector (FID) and a modified on-column injector was used. Chromatographic separations were done isothermally with a column temperature of 70 "C, an injector temperature of 70 OC, and a detector temperature of 280 "C. Temperature-programmed GC analysis can also be run for samples containing unresolved components. Ultra-high-purity grade nitrogen was used as the carrier gas, at a flow rate of 5.0 mL/min. Finally, a microcomputer controlled the overall MESI system process. A FORTRAN program was developed to collect and store GC detector signals for a selected analysis time, to control the desorption pulse duration, and to regulate the trapping time. To record chromatograms, the GC detector signals were first fed through an instrumentation amplifier with a built-in variablegain switch. Theoutput of the amplifier was connected to a 12-bit analog-to-digital signal converter on a Lab Master DMA data acquisition board (Scientific Solutions Inc., Solon, OH). The signal sampling frequency was set at 4 Hz. Trichloroethene (TCEY) used for standard aqueous sample preparation was of spectrophotometric grade (Aldrich, Milwaukee, WI). Deionized water was used in all aqueous sample preparation. Procedure. The MESI analysis procedure depended on the choice of the membrane module configuration (Figure 2). In the static configuration, 25 mL of a water sample was initially placed in a 40-mL glass vial which was sealed with a rubber septum. The carrier gas passed through the center core of the hollow fiber membrane, via the stainless steel needles, while the membrane was completely submerged into the water sample inside the vial. The stirred configuration, which facilitated the mass-transfer process and improved extraction efficiency, was developed based on the static module. The headspace membrane module was similar to the stirred
configuration such that the membrane was positioned in the headspace of the sample rather than being submersed in it. The dynamic configuration was the same as the static one with the exception that the sample was continuously replaced in the vial at a constant flow rate with the syringe pump, so that analyte concentration remained constant during extraction. In the flow-over configuration, the carrier gas was routed through the center core of the membrane while the water sample was introduced into and removed from the glass tube, allowing it to flow around the exterior of the membrane fiber. Lastly, the flow-through configuration was the reverse version of the flow-over configuration. The water sample was pumped at a constant rate through the center core of the hollow membrane with the syringe pump. The carrier gas from the GC was routed through the glass tube of the module and flowed around the exterior of the membrane. In general, the membrane was used as a phase separator between the sample and the carrier gas. The gas carrying extracts from the membrane module was routed directly into the GC column, where cryofocusing and separation took place. As shown in Figure 1, the total organic concentration in the carrier gas exiting the membrane module is at a constant elevated level. When the organic analytes reached the cold column section in the cryogenic well, they slowed and focused into a narrow band on the stationary phase of the column. After a certain time, an electrical pulse was applied to thermally desorb the analyte into the carrier gas stream for GC analysis. The concentration profile of the organic analytes just after the sorbent interface is also illustrated in Figure 1. The optimal trapping time and desorption pulse duration were determined for the MESI system. To determine the maximum cooling time before TCEY began to break through the trap, a 500 pg/L TCEY water sample was pumped through the flow-through membrane module at a flow rate of 0.34 mL/min, and the component peakintensity was measured for different trapping periods. The optimal heating pulse duration, needed for complete desorption of analytes from the cold trap, was also determined. The static module was used to expose the membrane to the TCEY solution for 1 min, and then the membrane was transferred to another vial containing only deionized water. The trapping time was 10 min. Two electrical pulses were then applied 1 min apart, with progressive duration time starting at 0.25 s. Two component peaks, 1 min apart, were observed in the chromatogram. This experiment was repeated, increasing the pulse duration, until the second component peak disappeared. The final tested pulse in this series gave the minimum duration to completely desorb the analytes from the cryogenic trap. Once the optimum trapping and heating times were obtained, they were incorporated into the computer program that controlled the MESI system. During an analysis, the computer sent a sequence of binary injection signal including two pulses of preset duration (Figure 1) to the solid-state relay. The first pulse at time 0 clears the cryogenic trap. The second one, after a trapping period, which could be a preset default value or be set manually, was sent to desorb all analytes into the carrier stream for GC analysis. The second pulse also started the program's real-time GC detector signal collection and display on the computer monitor. When the Analytical Chemistry, Vol. 66, No. 8, April 15, 1994
1341
Table 1. GC Peak Intenrfty Obtained with Varlour Membrane Modules lor Mlterent Concentratlono of Trlchloroelhene In Water
peak intensity' 10 50 100 500 pg/L PgIL rg/L CCglL CCglL 1
module type static0 stirreda headspace' dynamicb flow-overc flow-throughc
nil nil nil nil nil 53
nil nil nil 2120 nil 305
lo00 MIL
Table 2. Extraction Proflie' for Trlohkroothone In Water Obtalnd with the Headapace Membrane Module Contalnlng VartaMe Amounts of Sample
RSDd
1755 2280 11560 21600 5180 10400 58100 100100 4230 9920 46200 103400 4270 12090 58600 124900 973 2320 9920 21700 1649 3870 16660 36100
(%)
12.8 3.0 2.0 4.0 5.8 2.6
a The trap ing time was 17 min. The trapping time was 17 min; the sample &w rate was 0.33 mL/min. The tra ing time was 5 min; the sample ump rate was 0.14 mllmin. *!is refers to the scatter in four repEcates at the concentration of 100pgIL. e Arbitrary units.
analysis was finished, the chromatogram was stored in ASCII file format for future referral. This cycle of trapping and analysis was repeated automatically for continuous monitoring.
RESULTS AND DISCUSSION Optimization. The MESI process involves extraction, adsorption, and desorption. In order to discover the full potential of the system, the optimum conditions must be determined for every step of the process. The optimum parameters for membrane extraction of VOCs from water were previously studied.lOJ1 For the MESI method, several membrane module designs (Figure 2) were investigated. The choice of a membrane module and its extraction efficiency depend on the application and the sample matrix. In order to compare the performance of the various designs, each module was used to analyze the same set of samples, consisting of water with TCEY in various concentration levels. Table 1 summarizes the test results. Under the experimental parameters described in the table, most tests demonstrated a relative standard deviation (RSD) of less than 5% with the exception of the static (Figure 2A) and the flow-over modules (Figure 2E). The peak intensities presented in Table 1 are the averages of the results obtained from the analyses of four replicates. Digital signals with no analog gain and an analog gain of 10 were multiplied by factors of 100 and 10, respectively, to normalize with the signals having an analog gain of 100. The peak intensities were proportional to analyte concentrations for all membrane modules. It is expected that the proposed method has a linear range of at least 4 orders of magnitude since the FID used in the method has a linear range of 6 orders of magnitude. However, we only tested for analyte concentrations ranging from 1 to 1000 p g per liter because our current data acquisition system has a limited dynamic range which truncates the high signal peaks resulting from the analysis of a high-concentration sample. The flowthrough moduleshowed the best sensitivity, being able todetect TCEY in the 1 pg/L aqueous solution. Limit of detection below 1 pg/L could be achieved with the MESI system by using a longer trapping time, higher sample pump rate, or longer membrane. A simple and durable membrane module is desired. The static, stirred, and headspace modules (Figure 2A-C) do not need pump and sample conduit lines. Thus, the designs are simpler and less fragile. Although the static configuration 1342 Analytical Chemlstry, Vol. 66,No. 8, April 15, 1994
Time (min)
1mL at25OC
2mL at25OC
10 20 30 40 50 60 70 80 90 100
500 856 400 200 288 150 100 60 37 18
586 1279 146 307 303 392 395 294 151 50
peak intensity 3mL 1OmL at25OC at25OC 670 1519 1237
865 480 464 174 800 70 20
839 2047b 2047b 2047b 1860 1530 1100 747 370 60
2mL at40 OC 582 1446 485 309 168 61 50 nil nil nil
a The f i i e s in this table represent the trichloroethene peak intensity (in arbitrary units) in chromatograms obtained from sequential extractions from the same sam le with a trapping time of 5 min. This peak intensity exceeded t i e dynamic range of the data acquisition board.
had relatively poor precision, it is simple and sturdy enough to be used as a sniffer for rapid, qualitative testing directly in a well, a river, or any aqueous effluent stream. As the figures in Table 1 indicate, the sensitivity and precision of the system greatly increased with the addition of a stirring bar. Stirring the solution improves extraction efficiency by increasing the mass transport of the analyte. A change of stirring speeds, however, was found to have very little effect in overall system sensitivity. The analytes can also be effectively extracted from the headspace of a ~ a m p l e . l The ~ * ~advantages ~ of the headspace method are from the easy attainment of analytes which are free from its matrix. The headspace membrane module was proven to be a simple, efficient, reproducible, and most versatile configuration. The headspace sampling technique extended the MESI applications to analysis of volatiles in more complex samples that contain solid or high molecular weight materials such as soil and sludge. For the analysis of TCEY in water, an RSD of 2.0% and a limit of detection at low microgram per liter range were obtained. It was observed that exhaustive extraction could be obtained using the headspace membrane module, likewise for the static and stirred modules. Typically, for 2 mL of a 50 pg/L TCEY solution in a closed vial at 25 OC, 100 min was required for quantitative extraction. Table 2 summarizes the extraction profiles for TCEY in water using the headspace MESI system. Each profile was obtained by performing a series of adsorption and desorption cycles on the same 50 pg/L TCEY solution in a closed vial with a trapping time of 5 min. The TCEY peak intensity in the chromatograms varied with time. Based on the results shown in Table 2, the amount of TCEY in the gaseous phase increased in the first 20 min and decreased thereafter regardless of the aqueous sample volume. This indicates that, with efficient stirring, the mass transfer of analyte from water to gas is negligible compared to that from gas through the membrane. In other words, the diffusion through the membrane is the rate-determining step in the (14) Poole, C. F.; Schuette, S. A. HRC CC, J. High Resolut. Chromatogr. Chromatogr. Commun. 1983, 6, 526-549. (15) Zhang, Z.; Pawliszyn, J. Anal. Chem. 1993.65, 1843-1852.
headspace experiment. This is because a rapid equilibrium between aqueous and vapor phases can be achieved by stirring the aqueous sample to continuously generate a fresh liquid surface, and the diffusion of the analyte in the vapor phase is 4 orders of magnitude higher than in the aqueous phase.” For the same reasons, a larger sample volume produced a greater component peak intensity proportionally. On the other hand, the exhaustive extraction time decreased as the extraction temperature increased. For 2 mLof a 50 pg/LTCEY solution in a closed vial at 40 OC, 70 min was required for quantitative extraction, which is similar to the time required for quantitative extraction of the 2-mL sample using the flowthrough module at a pump rate of 2 mL/h. This is because the diffusion coefficient in the membrane and headspace concentration increase with temperature. Increase of temperature is also an effective approach to release analytes from the matrix.16 In addition, the decrease in quantitative extraction time can be accomplished by using a thinner or longer membrane. The headspace module can be used for both qualitative and quantitative analysis of VOCs in aqueous, gaseous, and solid matrices. The dynamic module (Figure 2D) has an added feature of allowing the sample to flow through the membrane module at a fixed rate. This is important when the sample needs to be conditioned before it comes into the extraction module. For instance, a hot solution needs to be cooled down before membrane extraction. It was found that changes in thesample flow rate through the dynamic module had little effect on sensitivity. There was only a slight improvement in peak intensity using greater flow rates. So far, the dynamic module had the highest signal response because the analyte concentration in the glass vial was kept at a constant level whereas the analyte concentration diminished in the other modules during the analysis. The flow-through module design (Figure 2F) is based on previous work to develop a membrane extraction interface for VOC extraction from water.lOJ2 It also requires a pump for consistent sample delivery. Exhaustive extraction of VOCs wasobtained at a low pump rate of 0.02 mL/min. Under this pump rate, the quantitative extraction results are independent of temperature using the flow-through module. As a result, extraction with this module was very reproducible with a 2.6% RSD and could be used for quantitative analysis. However, a carry-over problem was observed when a sample with high analyte concentration (over 1 mg/L) had been in contact with the membrane. Flushing allows the carrier gas to remove all the carry-over analytes from the membrane. For the analysis of 1 mg/L TCEY in water, a 5-min flushing time was needed before the next sample could be analyzed without obtaining a component peak from the previous sample. Longer flushing times would be needed after extraction from samples with higher analyte concentrations. The flow-over module (Figure 2E) is an alternative configuration of the same apparatus. This module can handle a higher aqueous sample flow rate than the flow-through module. Under the same experimental conditions, however, the extraction efficiency of the flow-over module was approximately half of the value obtained with the flow-through
module, because of the lower aqueous contacting surface to volume ratio. Unlike the dynamic module, changing the sample flow rate for the flow-through and flow-over modules had direct effects on sensitivity, hence the following discussion. If the column connections and the membrane-needle junctions are not properly sealed, excess moisture from the leaky joints could accumulate a t the cold trap and cause poor performance of the column and detector. This risk of leakage could be reduced by using a column connector instead of a stainless steel union fitting to connect the stainless steel needle from the membrane module to the head of the capillary column. An epoxy glue would be needed to seal the junction between the needle and the column connector. The sorbent interface for the MESI system has two functions: adsorption and desorption. The adsorption process can be accomplished by using various sorption techniques which commonly involve solid-phasesorbents such as charcoal, polymers, and ion-exchangeresins.14 However, the desorption process for the sorption techniques requires solvents or a highflow-rate gas for sample elution from the sorbent cartridge; hence, these techniques are not ideal to directly interface to a GC. On the other hand, cryogenic coolants can be used to increase the adsorption capacity of the stationary phase on a capillary GC column and focus concentration bands to improve chromatographic separation efficiency. The effect of cryofocusing on the relationship between the distribution constant and the capacity factor of the solute has been analyzed the~retically.’~Cryofocusing can be performed in two ways, depending on where the focusing is taking place. For oncolumn cryofocusing, analytes are trapped as a narrow band on the column in the GC oven. For external cryofocusing, the components are trapped as a narrow band outside the oven compartment. Theinitial method development for membrane extraction of VOCs from water used on-columncryofocusing.10 It required a long cooling period to bring the temperature of the entire oven to -30 OC in each analysis cycle and temperature-programming separation. In order to enable the MESI system to perform rapid routine analysis or continuous monitoring, external cryofocusing was applied together with isothermal separation. The effect of the amount of cryogen placed in the well on system performance was investigated using the static membrane module and a 100 pg/L solution of TCEY, The amount of cryogen was measured by its level from the bottom of the well. The results showed that varying the level of solid C02, from 1.5 to 6 cm, had virtually no effect on the component peak intensity because the amount of analyte extracted was quantitatively trapped at thesorbent tube during each run. When the cryogen level was below 1.5 cm, the peak intensity decreased as the analyte breakthrough had occurred. When the cryogen level was above 6 cm, the peak intensity decreased because a section of the capillary column just before the heating coil was also cooled and thereby trapped some of the analyte that could not be desorbed by the heating pulse. For subsequent experiments, a cryogen level ranging from 1.5 to 6 cm was ensured before each run. The thermal desorption device used in the MESI system has been proven to be fast, accurate, and reproducible.12 Because of the small thermal mass of the silica capillary
(16) Langenfeld, J. J.; Hawthorne, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1993, 65, 338-344.
(17) Brettell, T. A,; Grob, R. L. Am. Lnb. 1985, 17, 5C-66.
Analytical Chemistty, Vol. 66, No. 8, April 15, 1994
1343
column, it returns to the cryogenic trapping temperature very rapidly after the desorption heat pulse for subsequent cryofocusing and analyte injections to the GC. The cycle of cryofocusing and desorption can be repeated periodically without carryover effects. The amount of heat generated by the heating coil determines the analyte desorption efficiency from the cryogenic trap. Insufficient heating would lead to an incomplete desorption of analytes, whereas excess heating would damage the stationary phase in the capillary column. The amount of heat generated can be controlled by adjusting the level of the power supplied and the heating pulse duration. When an electrical potential, V, is applied to a conductor with resistance, R, for t seconds, the amount of thermal energy released, AQ,can be expressed as
7.OE+4
,
I
I
2.OE+4 1.OE+4
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Pump Rate (mL/mh) Figure 3. Effect of pump rate on the component peak intensity in the analysis of a 500 pg/L trichloroethene aqueous sample using the fiowthrough membrane module. The solid line representsthebest logarithmic curve fit to the data points with a correlation coefficient of 0.992.
AQ = pt/R In this study, the electrical potential was set arbitrarily at 26 V, the resistance of the given heating coil was 124 Q,and the heating pulse duration was 4 s. If all the electrical energy was converted to thermal energy, the amount of heat gained by the coil would be 21.8 J. Because an unknown amount of this thermal energy goes to sublimation of the solid C02 in the cold trap, the resistivity of the coil is temperature dependent, and the heat capacity of the capillary column is unknown; the actual desorption temperature can only be measured using a thermal couple. Although eq 1 cannot be used theoretically to predict the actual desorption temperature during the experiment, nevertheless, it indicates the rough relationship between the various experimental parameters and the desorption energy. Under the conditions stated above, the temperature between the column and the heating coil was measured up to 250 OC during a 4-s desorption pulse. A 4-s heating pulse duration was found to be appropriate for this MESI system as determined by the procedure outlined in the Experimental Section. This pulse duration can provide complete analyte desorption from the cryogenic trap without damaging the capillary column, weakening the column, or causing fractures. A pulse duration of 10 s burned the polyimide coating of the capillary column. As a result, the 4-s pulse duration was used in all subsequent MESI experiments. A shorter desorption pulse may be desired to produce sharper detector peaks with faster heating. This requires a greater electrical potential or lower resistance in order to generate the same amount of thermal energy required to raise temperature sufficiently and lead to complete desorption (eq 1). To prevent damage to the fused silica column during the high-temperature desorption, a high-temperature aluminumclad capillary column with 0.1-pm film of bonded methylsilicone (Quadrex, New Haven, CT) can be used as the sorbent tube instead.'* The current can be passed directly through the cladding of the tubing producing more efficient heating. Effects on Sensitivity. The sensitivity of the MESI system depends on both the sample throughput and the membrane extraction efficiency. For the flow-through module (Figure 2F), the sample throughput is determined by the liquid sample pump rate and the cryotrapping time. Although quantitative (18) Pawliszyn, J.; Liu, S. Anal. Chem. 1987, 59, 1475-1478.
1344 Analytical Chemistty, Vol. 66, No. 8, Aprll 15, 1994
recovery of the analyte was no longer obtained at a high pump rate, a significant gain in the system sensitivity was obtained due to the increased sample throughput. Figure 3 illustrates the relationship between the component peak intensity and the sample introduction rate for the analysis of a 500 pg/L TCEY aqueous solution. As the sample introduction rate increased, the peak intensity became less sensitive to the pump speed. If the pump rate were set too high, however, excess water pressure would cause leakage at the membrane and stainless steel needle junction. A pump rate of 0.34 mL/min is recommended to ensure that the sensitivity and precision were representative of the system's potential and to avoid possible water leakage due to excess pressure. The cryofocusing technique can be used for trace enrichment to improve the sensitivity of a chromatographic system because it allows the accumulation of trace analytes prior to the analysis. Hence, the sensitivity of the system is directly related to the accumulation period or the cryotrapping time. Likewise, the sensitivity of the MESI system also depends on the cryotrapping time. A linear relationship between the component peak intensity and the trapping time was found when a 500 pg/L TCEY aqueous sample was detected using the flow-through membrane module at a pump rate of 0.34 mL/min. The line representing a least squares fit to the data point has a slopeof 7 180,an intercept of 77.6, and a correlation coefficient of 0.994. This was expected since the longer the trapping time, the greater the amount of the analyte would be collected at the cold trap during extraction and subsequently desorbed into the GC column. Thus, a longer trapping time can lead to a higher sensitivity of the system. However, the trapping time can never be longer than the breakthrough time of the target analyte in the trap. For instance, toluene with a freezing point of -95 OC displayed a breakthrough time at 30 min in the solid C02 trap. Using lower temperature, thicker stationary phase of the column, or longer traps, the breakthrough time increases but the peaks broaden. If quantitative extraction were not obtained, the precision and sensitivity of the MESI system would be affected by the variation of temperature. This can be explained by temperature dependence of the mass-transfer process. The temperature dependence was demonstrated by extracting TCEY from a 500 pg/L water solution using the static membrane module (Figure 2A). As shown in Figure 4, the component
1.5E+4
T 1.3E+4
h l.lE+4 4
f
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Figure 4. Effect of temperature on the component peak intensity for the analysis of a 500 l g / L trichioroethene aqueous sample using the statlc membrane module. The solid line represents the best thirddegree polynomial curve fii to the data points with a correlation coefficient of 0.976.
peak intensity increased with increasing temperature. For a temperaturevariation of 10 OC around the room temperature, the analysis results change by about 1 1%. In other words, the extraction temperature must be precisely maintained in order to obtain reproducible results for the MESI system in the nonquantitative mode. Applications. The MESI system was used for VOC analysis or monitoring of various environmental matrices. For a clean water matrix, the flow-through module can be used for both qualitative and quantitative analyses. Under a high sample flow rate, a long trapping time, and a sensitive detector such as an ion trap mass spectrometer, the limit of detection can be at the nanograms per liter 1 e ~ e l . lThis ~ can be compared with the method 524.2, outlined by the United States Environmental Protection Agency (EPA), which suggests the use of the conventional purge and trap followed by GC/MS analysis for measurement of purgeable organic compounds in water. The EPA method detection limits are compound and instrument dependent and vary from approximately 0.02 to 0.35pg per liter. A common group of organic contaminants in groundwater samples is BTEX, which consists of benzene, toluene, ethylbenzene, and three xylene isomers. The chromatogram in Figure 5 was obtained from a sample of 100 pg/L BTEX in water using the flow-through module with a 5-min trapping time and a sample flow rate of 0.34 mL/min. The entire process from an untreated water sample to obtaining the final chromatogram took 15 min, compared with the purgeand-trap method which typically requires much longer time. For wastewater, sludge, and solid matrices, the MESI system can be also be run with a relatively low sensitivity and some level of selectivity using the headspace module. The vapor pressure of theanalytes in the headspacecan be regulated to a suitable level by controlling the temperature of the sample vial. The trapping time can be reduced for high-concentration samples. Changing the type of membrane can alter the selectivity of the extraction module. The chromatogram in Figure 6 represents the analysis result of a run-down rain sample collected from a busy parking lot. The headspace MESI system was used. The trapping time was 30 min and the sample volume was 40 mL. The BTEX contents in the rain sample were residues of gasoline. Although the rain (19) Potter, D. W.; Pawliszyn, J. J . Chromatogr. 1992, 625, 241-255.
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Time (min) Figure 5. Chromatogramobtained from the analysis of a clean water sample containing about 100 pg/L each of benzene (peak l),toluene (peak 2),ethylbenzene (peak 3), m and pxyiene (peaks 4 and 5), and &xylene (peak 6).
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Flgure 6. Chromatogram obtained from the analysis of a rundown rain sample collected from a busy parking lot; the components include benzene (peak 11, toluene (peak 21, ethylbenzene (peak 3), m and pxylene (peaks 4 and 5), and &xylene (peak 6).
sample contained suspended particles, no filtering was required prior to the analysis. For a wastewater sample with high analyte concentration, a short trapping time may be used for qualitative detection or monitoring. Quantitative headspace analysis, however, requires a careful and extensive calibration. A measured amount of sample must be placed in a closed vial and the extraction temperature held constant. Calibration can also be accomplished using exhaustive extraction conditions. For an air matrix, the MESI system can be used as a simple and effective VOC monitoring station. Unlike the common methods for air analysis, the MESI system eliminates the need for a sorbent cartridge, organic solvents, and a drying process. This was demonstrated by continuously monitoring laboratory air. The laboratory tested was used for analytical sample preparation and had a floor area of about 78 m2. A membrane probe, which consisted of the headspace membrane module without the sample vial, was set up near the center of the laboratory. The exterior of the membrane was exposed directly to the air while the carrier gas flowed continuously through the center core of the membrane. Figure 7 shows a set of selected chromatograms illustrating the solvent contents Anai’ical
Chemistry, Voi. 88, No. 8, April 15, 1994
1345
analysis requires the addition of a programmable autosampler. An autosampler designed for flow injection analysis meets the requirement for this application. Water samples are collected in a 50-mL sample bottle and sealed with a septum. The sampling arm of the autosampler consists of a needle, which pierces the septum and draws the sample from the bottle, the sample passes through the flow-through membrane module at a constant rate with an in-line fluid pump. The computer program can also be modified to control the autosampler.
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Figure 7. Selected chromatograms showlng the volatile organic compounds In the laboratory air at various times of the day using the monitoring mode of the membrane extraction wkh sorbent Interface system. The major pollutants Include methanol or ethanol (peak l), acetone (peak 2), propanol (peak 3), butan-241 (peak 4), l , l , l trlchloroethane(peak 5), benzene (peak e), and toluene (peak 7). The sample collection time for each analysis cycle was 10 mln.
in the laboratory air at different times during 1 day. These VOCs were identified by comparing their retention times with those measured in a series of standard sample analyses. All chromatograms were plotted on the same scale, so that the relative height of a given peak corresponds to the relative concentration level of the component in the laboratory air. For instance, the methanol or ethanol contents represented by peak 1 stayed at the same level throughout the day. Acetone (peak 2) and propanol (peak 3), the commonly used rinsing solvents in the laboratory, had variable concentrations in the air, depending on their use. An intense toluene peak (peak 7) appeared at 1:30 p.m. because it was used to clean GC inserts in the room at that time. It took more than 2 h for the toluene to be purged from the laboratory air. Also, a trace of benzene (peak 6 ) was detected near the end of the day. The monitoring process was controlled automatically using a microcomputer. At the beginning of an analysis cycle, a 4-s electrical pulse was sent to the coil heater to clear the previous accumulation in the cold trap. A 10-min sample collection period was started immediately after the pulse. At the end of the period, another 4-s electrical pulse was sent to the coil heater to desorb the sample analytes. At the same time, the computer began recording of the GC detector signal, and a chromatogram of a preset length was stored on the hard disk of the microcomputer. For continuous monitoring, this analysis cycle can be repeated immediately. Quantitative air analysis, however, requires careful and extensive calibration. This can be done by placing the membrane inside a defined air channel, in which a fan is used to control the air flow around the membrane. The MESI method can be used for automated, continuous air or water stream monitoring as described above. However, automating the MESI system for repetitive water sample 1348 An8l~lcaiChem/s?ty,Vol. 66, No. 8, April 15, 1994
CONCLUSION The MESI method presented in this paper is limited to the analysis of VOCs at trace level from various environmental samples. For extraction of semivolatile organics from water, a high-pressure membrane module developed previously is needed.13 For samples with high analyte concentrations, the signal may be truncated due to the limited dynamic range in the current data acquisition system. The membrane extraction technique may also present a carryover problem. The MESI approach is an alternative to a membranemultiplex method developed previously.12 Both approaches allow a large sample throughput and thus enhance the system sensitivity. The multiplex technique does not require cryofocusing, thus making it more field portable. However, correlation noise, which associates with the deconvolution mechanism, is included in the analysis result and limits the signal-to-noise ratio of the chromatogram. Also, the multiplex analysis produces derivative component peaks due to the risen baseline, making it difficult to integrate. The MESI system, which cryogenically traps and thermally desorbs the analytes, is a simpler approach for trace enrichment and produces a better chromatogram with greater signal-to-noise ratio compared to the multiplex approach. For the analysis of wastewater or high-concentration samples, the MESI method allows easy control of the sample throughput by simply setting the trapping time whereas the multiplex method requires modification of the computer program. Furthermore, the MESI system allows GC temperature programming for better separation of a complex sample mixture and multiplex GC analysis can only be run in the isothermal mode. Purge and trap is commonly used for analysis of VOCs in water, but it requires expensive instruments and the system cannot easily be adapted to on-line monitoring. The transfer of analytes desorbed from porous polymer cartridges onto a capillary column is frequently found problematic because the required flow rate for the desorption is approximately 30 mL/ min while capillary column flow rates are typically much lower. Other difficulties include determining the completeness of purging and preventing breakthrough of the analyte. The MESI method is a simple solution to the above problems. ACKNOWLEDGMENT This work was funded by the Natural Sciences and Engineering Research Council of Canada. Received for review October 1, 1993. Accepted January 24, 1993." a
Abstract published in Advance ACS Absrrocrs, March 1, 1994.
