Anal. Chem. 1990, 62, 2183-2188
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Membrane/Gas Chromatographic System for Automated Extraction and Determination of Trace Organics in Aqueous Samples Richard G. Melcher* and Paul L. Morabito T h e Dow Chemical Company, Analytical Sciences, 1602 Building, Midland, Michigan 48667
A new automated extraction analysfs system, which comblnes membrane cell technology wlth a pneumatlcally operated pressurlzed rotary gas chromatographlc lnjectlon valve (POPSI), was developed and tested. The system was evaluated by uslng chlorlnated aromatlc compounds and pesticides as test sample mlxtures. These compounds were successfully extracted and determlned In water and organlc/water type matrlces In the part-per-trllllon to part-per-bllllon range. Two modes of membrane operation were demonstrated: the continuowflow mode, whlch produces concentratlon factors of -3-5, and the stop-flow mode, whlch produces concentratlon factors of -50-200. The effects of addlng common organic solvents to the samples were examined and unexpected Increases In extractlon efflclency were observed. To reduce sample matrlx effects, an lntemal standard procedure was evaluated. The Internal standard, which was added to the sample, was extracted wlth the analytes of Interest to help compensate for matrix effects. The accuracy and preclslon were significantly improved over the use of an external standard analysis; however, the Internal standard analysis was unable to completely compensate when hlgh percent levels of organlc solvents were added to the sample. Membrane/POPSI technology has potential both for use in sophisticated laboratory systems for automated extraction/ analysis and also for simpllled, dedlcated systems for on-line analyzers.
INTRODUCTION In the determination of higher boiling trace organics in water, most methods consist of extracting the analytes into an organic solvent, followed by a Kuderna-Danish evaporative distillation concentration of the extract to achieve the necessary sensitivity. These multistep procedures are tedious and are prone to error and contamination. Although several techniques (1-3) have been developed for direct injection of aqueous samples, there is no selective preseparation of the analytes from the matrix components and the sensitivity obtained is not comparable to extraction/evaporation techniques. An automated extraction/injection system for gas chromatography would be valuable for both laboratory trace analysis and on-line GC applications. One approach for automated extraction and injection was proposed by Fogelquist et al. for eight halocarbons in seawater (4). Extraction was performed in a liquid-liquid segmented flow in a glass coil. The phases were separated with the aid of a hydrophobic microporous membrane. Nonporous membranes have also been found useful for the extraction of a wide range of compounds, particularly for samples containing high concentrations of dissolved solid and particulates often found in wastewater and process streams (5-9). This present paper describes a system based on the membrane extraction and
* T o whom inquiries should be addressed.
concentration of trace compounds and the automatic injection of the extract into a gas chromatograph (GC) using a POPS1 (pneumatically operated pressurized sample injector) technique. The first key parameter in the development of this system was designing a membrane cell that could be used with nonpolar extraction solvents. A previous study (6) has shown that a nonporous, tubular silicone rubber membrane can be used to advantage for interfacing aqueous samples directly to de tectors and analytical instruments. One of the first analytical applications of this type of membrane was described by Westover et al. (10). A tubular silicone rubber membrane wa, used to interface air and water samples directly to a mass spectrometer. In a different approach (11, 22) a flat silicone polycarbonate membrane was used to separate volatile organic compounds from an aqueous sample. The permeated compounds were purged from the membrane cell with an inert gas stream and injected into a gas chromatograph. Both of these approaches depend on volatization of the permeated compound and are best suited for volatile and moderately volatile compounds. The membrane system described in this present work interfaces the aqueous sample to a hexane extractant. Since the mechanism is similar to a liquid/liquid extraction (6), this technique can be used for compounds of moderate and low volatility. High resolution of complex samples can be obtained by injecting the extract from the membrane into a capillary gas chromatographic system. On-column injection is critical to obtain maximum sensitivity, particularly for the determination of trace levels. There are two references in the literature that describe automated small volume injection techniques using rotary valves. Steele and Vassilaros described a technique that utilizes an internal slot rotary valve to automate oncolumn injections (13). They successfully chromatographed seven compounds from a test mixture using that approach The valve unfortunately had to be cooled before each injection to prevent sample vaporization from occurring in the valve In a similar approach, Hopper used a 10-port Valco valve that used an external sample loop and wash loop to automate on-column injections (14). A high injection port temperature and an external wash loop were necessary to reduce peak broadening and sample carry-over. A laboratory of The now Chemical (Netherland) B.V. in Terneuzen utilized a rotatory valve to make split injections into a capillary column. In this present work, a procedure was developed to greatly increase sensitivity by making splitless, on-column injections and to reduce carry-over by increasing the carrier gas flow as a pulse, during injection.
EXPERIMENTAL SECTION Apparatus. Membrane Cell. The membrane used in this study was Dow Corning Silastic medical grade tubing, a seamless silicone rubber tubing designed for clinical and laboratory ap plications. Silicone rubber is chemically and mechanically stable and has a high permeation rate for a large variety of organic compounds. The membrane size used was 0.012 in. i.d. X 0.025 in. 0.d. (Dow Corning Catalog No. 602-105). The general design
0003-2700/90/0362-2 183$02.50/0 Q 1990 American Chemical Society
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Figure 1. Membrane extraction cell: (a) tubular slllwne rubber membrane: (b) glass call body; (c)membrane mne&. delivefy end: (d) membrane connection. supply end: (e)sample inlet and exit pats: (1) sample: (g)Sample pump: (h) extractant stopflow valve; (I)Internal slot injection valve; (i)connection to capillary column (see Figure 3).
Figwe 3. Injection and gas chromatographicsystem: (a) membrane system (see Figure 1); (b) extractant stopflow valve: (c) extractant pump: (d)ext-actant waste: (e)solenoid controller:(1)Jolendd%(g) 100 psi N, cf He: (h) valve actuata lines; (i) drarcoal trap; (i) check valves: ( k) flow controller; (I) 30 psi He: (m) injection valve; (n) internal slot sample b p ; ( 0 ) retention gap capillary covered with glass wool insulation: (p) Injection port; (q) 0.3-m retention gap: (r) capillary connector; (s) capillary column; (1) electron capture detector.
Figure 2. Adjustable membrane end ccnnectcf: (a) '/,,-In. stainless steel tubing: (b) Teflon tubing bushing; (c)Upchurch Fhpartlght fming: (d) 'I, in. column end lilting; (e)S w a w ' I , In. nut: (r) tubular silicone rubber membrane: (g)glass cell bc4y: (h)swelllshrlnk fit of membrane on tubing; (i) Teflon '1, in. ferrule; (i)sample flow. for the membrane cell and setup is shown in Figure 1. The cell is constructed of '/,-in. 0.d. Pyrex glass with 2 mm i.d. The side ports are used to connect the sample flow, while the membrane connections are made through the straight length of the cell tube. The membrane connections are critical to the success of the cell. Since the membrane swells to almost twice its width and length when solvents such as hexane and methylene chloride are used as extractants, permanent potting of the membrane ends is difficult. Permanent potting may be possible by prestretching the correct length of membrane while potting. The design for the adjustable membrane connection used for this study is shown in Figure 2. The connections on both ends of the cell are the same, except the stainless steel tubing (a) a t the delivery and is short (-4 cm), while the tubing a t the supply and is long enough (-30 cm) to extend into the cell body when the membrane is not swollen. For the cell (30 cm total length). a 16 cm length of membrane was used. To connect the membrane to the tubing, the end of the membrane is placed in xylene for -3 min. When it becomes swollen, -2 cm is carefully slipped over the tubing. The end of the tubing must be rounded and smooth to facilitate insertion and prevent tearing the membrane. When the xylene evaporates and the membrane shrinks, it forms a tight seal on the tubing. Other solvents, such as methylene chloride, could he used as a substitute if xylene causes an inference. After the connections are made and the cell is assembled, as shown in Figures 1 and 2. water is pumped through the sample inlet and the extractant, such as hexane, is pumped through the tubing and membrane. The Fingertight connector (Upchurch Scientific, Inc., Oak Harbor, WA) on the supply end (Figure 2) is loosened and the tubing pulled out slowly as the membrane swells. If the tubing is slightly too long, it will form a spiral in the cell; however, if the tubing swells to the point it doubles over, it may block the extractant flow and eventually pull loose. Care must be taken when connecting the membrane cell to the valves and system.
Since the membrane cannot tolerate pressures much greater than 5-10 psi, appropriate tuhing and fittings must he used. Extraction System. The sample pump used was an FMI ,W Jr. (Fluid Metering, Inc., Oyster Bay, NY). As shown in Figure 1, the sample pump was connected downstream of the membrane cells so that the sample is pulled through. In this configuration, pump contamination would not be a factor in analysis. Sample flow rates varied from 1 to 4 mL/min for the 2 mm i.d. cell. Larger cells required high flow rates. A four-way Model 7040 LC valve (Rheodyne, Inc., Cotati, CA) was used as the stop-flow valve. The connections to the membrane and extractant pump, as shown in Figure I, allow extraction with either a continuous extractant flow or a stopped extractant flow mode. A Kratos Spectroflow 400 LC pump (AB1 Analytical, Ramsey, NJ) was used to pump the hexane extractant through the membrane. The flow rate was set a t 0.1 mL/min for all the experiments described. Automated Injection and Gas Chromatographic System. The automated injection and analysis system was composed of a Hewlett-Packard 5890 gas chromatograph. a four-port Valco intemal slot sampling valve, a valve controller, a Nelson Analytid 6ooo chromatqraphy data system, and assorted plumbing fixtures and gases. A diagram of the system is illustrated in Figure 3. A Valco (Valco Instruments Co., Inc., Houston, TX) internal slot sampling valve (A214UWPl) was mounted over the injection port of the gas chromatograph. Two different internal slot volumes were used, the 1- and 3-sL (SSAIWP.7) volumes. The valve controller was an in-house device that consisted of three Clippard solenoids that were individually activated from a contact closure. Nitrogen or helium a t 100psi was used to supply gas to the controller. This device supplies the gas pressure necessary to switch the Valco internal slot valve between the sampling and inject positions. In addition, as illustrated in Figure 3, a gas line was connected between one of the solenoids and the carrier flow system. At a predetermined time into the injection process, a high pressure gas stream was activated and flowed from the valve controller into the Valco valve aiding the injection. The high-pressure pulse duration was -1-2 s, which was immediately followed by normal carrier flow. A carrier gas system was built to accommodate the automated injection process. This system consists a purified helium or hydrogen gas flowing through a Porter flow controller (Model VCD-1000, Porter Instrument Company, Inc., Hatfield, PA), through a Nupro check valve (1psi) and through a small activated carbon trap. This carrier flow eventually enters the Valco valve
ANALYTICAL CHEMISTRY, VOL. 62, NO. 20, OCTOBER 15, 1990
Minutes
.
Figure 4. Membrane extraction profiles: (a) (- -) hexachlorobenzene; (b) (- - -) tetrachlorobenzene; (c) (.-) trichlorobenzene; (d) (-) di-
chlorobenzene. and supplies the column with flow. In addition, the high-pressure gas stream described earlier is connected to this carrier gas system by a second Nupro check valve. A Hewlett-Packard 5890 gas chromatograph was used. The column was channeled through an injection port and connected directly to the Valco valve. The small piece of column exposed outside the gas chromatograph (-1 in.), along the injection port tip, was wrapped with aluminum foil. The heat escaping from the injection port sufficiently heated the small piece of column outside the gas chromatograph. To prevent the valve temperature from rising, a thick layer of glass wool was placed between the Valco valve and the gas chromatograph. Two column configurations were evaluated. One configuration consisted of a 0.3 m X 0.53 mm i.d. capillary retention gap column connected to the valve. The analytical column was then connected to the retention gap using a Hewlett-Packard capillary metal column connector (connector, 5061-5801;ferrule, 5061-5807)inside the column oven. The second column configuration involved connecting the analytical column directly to the valve. In the external standard analysis study the analytical column used was a 15 mm X 0.53 mm i.d. capillary column coated with 1.5 pm of DB-5 (J&W): injection port temperature, 200 "C; electron capture detector temperature, 355 "C; He or H2 carrier gas flow at 6 mL/min with 30 mL/min N2 make-up. The initial oven temperature and program rate varied depending on the compounds to be analyzed. In general, the initial oven temperature started in the range of 40-70 "C for a hold time of 3 min. The temperature program rate was -10-12 "C/min to a final temperature of -230 "C. In the internal standard analysis study, a 1 m X 0.53 mm i.d. deactivated megabore retention gap (Restek Corp., Bellefonte, PA) connected to a 15 m X 0.32 mm i.d. analytical column (J&W Scientific, Inc., Folson, CA) coated with 0.1 pm of DB-5 was used. H2 carrier gas flow rate was 3 mL/min with 50 mL/min N2 make-up. Initial temperature was 70 "C for 1 min and then programmed to 100 "C at 30 "C/min. After 1 min at 100 O C the temperature was programmed to 230 "C at 30 "C/min. The Nelson 6000 Data System (Nelson Analytical, Cupertino, CA) was used to control all timed events (opening and closing of contact closures) and to collect all data. The Nelson analog to digital (A/D) converter box was connected to the gas chromatographs for data collection purposes. In addition, the contact closures available on the back of the A/D box were connected to the valve controller. A method file was programmed into the Nelson System which closes and opens the contact closures at predetermined times.
OPERATION OF SYSTEM Sample is pumped through the cell at a flow rate of 1-3 mL/min for a t least 10 min. Sample extract (from the membrane cell) is continuously flowing through the Valco valve, keeping the internal slot always filled. Meanwhile, carrier gas is flowing, in another port on the Valco valve,
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maintaining column flow. When analysis is desired, the start button on the gas chromatograph is pushed. This, in turn, starts the Nelson system preprogrammed method which then triggers the valve controller to rotate valves and turns on the high-pressure gas for 2 s. The stop-flow valve, Figure 1, allows extraction of the sample stream either with a continuous extractant flow or, when the valve is activated, in a extractant stop-flow mode. In both cases, the permeation through the membrane is identical; however, in the extractant stop-flow mode the permeate is diluted by a smaller volume of extractant. For both situations, the concentration factor (CF)is experimentally determined by dividing the response from a membrane injection by the response of a standard injection equivalent to the concentration in the aqueous sample before extraction. In the continuous extractant flow mode, the actual concentration in the extract is dependent on the total permeation of the compound through a specific membrane and the flow rate of the extractant concentration in extract
permeation (ng/min)
flow rate( m L / min)
ng/mL
In the extractant stop-flow mode, the valve is turned so that no flow is going through the membrane. The concentration factor is dependent on the permeation rate of the compound per unit internal volume of the membrane and the time in stop-flow mode concentration in extract = permeation (ng/min)/x min membrane volume (mL) = ng/mL The timing is critical when running in the stop-flow mode. After the extractant flow resumes, the plug of extractant containing the concentrated analyte flows into the internal sample slot of the rotary injection valve. The switching of the rotary valve must be coordinated to optimize precision and the sensitivity. The injection system injects 1 or 3 yL, while the total volume in the membrane is -25 pL. At an extractant flow rate of 100 yL/min, there is a 6- to 7-s leeway on each side of the injection midpoint.
EXPERIMENTS Before the total system was built, preliminary experiments were run to determine the extraction profile. A membrane cell was connected to a sample pump and an extractant pump. The system used a hexane extractant and an aqueous sample containing trace levels of 10 halogenated benzene and naphthalene compounds. These compounds were pumped through the membrane cell for approximately 40 min while the hexane extractant (flow rate = 0.2 mL/min) was collected in a vial periodically and analyzed by GC/electron capture detector. A plot of four representative compounds is shown in Figure 4. The general characteristics of the curves show a rapid attainment of equilibrium, a level steady state, and a rapid removal of the compounds from the membrane after the sample is removed. The injection technique (without membrane) was evaluated by injecting standards consisting of di-, tri-, tetra-, and pentachlorobenzene in hexane. Linearity, percent sample carry-over, and system injection precision studies were performed to evaluate the automated injection. The standards were pumped directly into the valve for all studies and all quantitation was based on an external standards analysis. After the injection system was evaluated, the membrane extraction cell was connected and the total system was evaluated. The same compounds used to test the injection system were used in this evaluation. The performance of the total system was
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evaluated for water samples and for mixtures of water and vnrious organic solvents. The extraction process can be affected by the matrix composition, temperature, and other system parameters. To study means of compensating these potential effects, RELDAN (Trademark of DowElanco) insecticide was used as an internal standard for the determination of chlorpyrifos a t the 10 ppb level in aqueous solutions. All spikes were prepared by adding microliter quantities of a chlorpyrifos stock solution to 1 L total volume of' either Milli-Q water or mixtures of Mill-$ water with percent levels of various organic solvents added. After the solution was mixed and allowed to equilibrate a t room temperature for several minutes, microliter quantities of RELIMN insecticide (internal standard) stock solution was added to the sample. The concentrations of chlorpyrifos and REI )PIAN insecticide for all spikes prepared were 10.2 and 41 4 ppb, respectively.
RESULTS AND DISCUSSION The permeation rate of a compound is a product of its diffusion rate in the membrane and its solubility in the niernhrane. In order to obtain good extraction, the compound must have a high partition coefficient from the sample into the membrane. Silicone rubber membranes have a solubility parameter close to hexane. In addition, the membrane is swollen with the hexane extractant. The process can be viewed very much like a liquid/liquid extraction from a large volume of sample into a small volume of hexane. A better understanding of the dynamics of the membrane extraction can be obtained from the extraction profile shown in Figure 4. It takes approximately 6-8 min to come to a steady state. This litne depends on the flow rate of the sample and extractant, on the diffusion rate of the analyte through the membrane, and on the wall thickness of the membrane. After the sample is removed from the cell by flushing with water, it takes approximately 10-12 min to return to baseline. In the stopflow mode, the total compound permeated (area under the curve) will be concentrated in the extractant encapsulated in the membrane. The permeation rate will be constant unless very long stop flow periods are used so that equilibrium is reached during the stop-flow period. Injection System. The injection system was studied first without the membrane by evaluating the linearity, percent carry-over, and precision. System linearity was evaluated by injecting standards over a range of 8-250 ppb for di-, tri-, and t,etrachlorobenzene and over a range of 1-50 ppb for pentachlorobenzene. All compounds exhibited linear behavior over their respective concentration ranges. For the internal standard analysis study chlorpyrifos was injected over a range of' 10 255 ppb but was linear only over the range of 10-200 ppb. System injection precision for both the external and internal standard techniques was evaluated by injecting a standard repeatedly. The relative standard deviation (RSD) I anged from 2.06 to 3.51% for the various compounds injected. Percent carry-over was evaluated by injecting a concentrated standard followed by an injection of hexane. The carry over was shown to he related to the boiling point of the r~ompound. Dichlorobenzene showed 0% carry-over, trichlorobenzene 1.71% , tetrachlorobenzene 2.7770, pentachlorohenzene 3.41 % , and chlorpyrifos 4.0% The effect of t kit. elevated injection pulse technique was evaluated by removing the high-pressure gas line and using only column flow t I) inject the sample. The percent carry-over using this approach varied over the range of 5 1 0 % showing the advantage of using the pulse injection for reducing carry-over. Membrane/Injection System. After the evaluation of the injection precision, the membrane extraction system was wnnected as shown in Figure 1. Figure 5 is an example of' p i t extxaction and analysis of di-, tri , tetra , and pentaI
Minutes Flgure 5. Extraction and analysis of test mixture from water: (1) dichlorobenzene (44 ppb); (2)trichlorobenzene 39 ppb); (3) tetrachlorobenzene (30ppb): (4) pentachlorobenzene (4 ppb). GC conditions: Ti, 40 O C for 3 min; rate A, 15 ' C h i n to 100 O C ; rate B, 6 OC/min to 220 O C ; injector, 220 O C ; detector, 355 O C ; flow, 6 mLlmin of He; column, 15 m X 0.53 mm i.d. with 1.5 pm of DB-5.
Table I. Membrane Extraction of Chlorinated Aromatic Compounds (Precision Data)
compound 1,2-dichlorobenzene 1,2,4-trichlorobenzene 1,2,4,4-tet,rachlorobenzene pentachlorw benzene
extraction/injection spike concn in injection only concn, extract, 70RSD" ppb ppb CFb % RSD 2.44
44
251
5.7
3.91
3.51
39
226
5.8
3.15
2.53
30
169
5.7
9.30
2.06
4.0
13.4
3.6
a 70 KSD,percent relative standard deviation. "CF, tion factor (enrichment).
13.3
concentra-
chlorobenzene in water at the parts-per-billion levels. The extraction was performed in the continuous flow mode. The initial baseline upset in the chromatograph is an effect of the high-pressure N2 pulse used to push the sample out of the sampling valve. This extraction was performed several times at the same concentration level to evaluate precision and the data are listed in Table 1. The decrease in precision over the direct injection study, also shown in Table I, is attributed to the membrane extraction. Continuous Extraction vs Stop-Flow Extraction. The permeation rate of the analyte through the membrane is the same for both the continuous and the stop-flow extraction. For the continuous flow mode, the actual concentration injected is inversely proportional to the flow rate. High flow rates can be used as a dilution scheme for concentrated samples and low flow rates can be used to concentrate trace samples. Although a flow rate of 0.1 mL/min for the experiments was described, flow rates as low as 0.02 mL/min have been used successfully. However, lower flow rates would become a problem due to the transport of the extract to the sample loop and pump limitations. The stop-flow technique can be used to greatly increase the (.oncentration factor o f an extraction and can be performed
ANALYTICAL CHEMISTRY, VOL. 62, NO. 20, OCTOBER 15, 1990
Table 11. Extractant Stop-Flow Concentration Factors
compound dichlorobenzene trichlorobenzene tetrachlorobenzene pentachlorobenzene chlorpyrifos lindane heptachlor aldrin dieldrin endrin tribromoohenol a
stop flow time, min 20 20 20 20
15
CF" 89
460 100
103 169
25 5
227 4
800 40
58 53
20
20 20 20 20 15
estd detection limits, pptr
50 50
84 76 79
70 40
207
150
1
Table 111. Matrix Effect on Membrane Extraction
sample matrix water 5% acetone 25% acetone 50% acetone 5% methanol 25% methanol 50% methanol 75% methanol 50% 2-propanol 20% MEK
continuous flow/concentration factors' di tri tetra penta 4.7 4.4 6.8
7.5 4.9
5.0 4.3 2.1
5.1 11.6
4.0 4.9 7.7 10.4 4.4 4.7
3.4
2.1
4.6
3.4 5.8 8.5
3.8
3.6
2.4 6.9 12.2
2.4 6.4
7.6 9.3 4.3 4.9
10.2
3.2 2.9
2.8 1.7 5.4 10.9
'di, 1,2-dichlorobenzene; tri, 1,2,4-trichlorobenzene; tetra,
CF = concentration factor (enrichment).
1,2,3,4-tetrachlorobenzene; penta, pentachlorobenzene. 5
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Flgure 6. 20-min extractant stop-flow concentration of pesticides in water: (1) lindane (293 pptr); (2) heptachlor (185 pptr); (3) aldrin (176 pptr); (4) dleldrln (158 pptr); (5) endrin (297 pptr). GC conditions: T,, 130 OC for 5 min; rate, 10 'C/min; TF, 250 OC; injector, 220 O C ; detector, 355 O C ; flow, 6 mL/min of He; column, 15 m X 0.53 mm i.d. with 1.5 bm of DB-5.
with general-use LC pumps. Table I1 shows the great increase in sensitivity that can be obtained by using the stop-flow extraction technique. In this mode, the sample flow continues through the membrane cell while the extractant flow stops. By stopping the extractant flow in the membrane and continuing the sample flow outside the membrane, more analyte will diffuse into the extractant, resulting in a higher concentration factor as compared to the continuous flow mode. Parts-per-trillion levels of test compounds were determined in 20-min stop-flow experiments. Longer stopflow extractions would further increase the concentration factor. Concentration factors (CF) are increased from approximately 4 (for the chlorobenzene series), using the continuous mode, to 89-227 using the stop-flow mode. Figure 6 shows a chromatogram for a series of pesticides determined in water using a 20-min stop-flow concentration. Matrix Effects on Membrane Extraction. The initial work involved analysis of primarily aqueous samples; however, since process waste streams can contain high concentrations of water soluble organics, the matrix effect was studied. The matrix effect on membrane extraction was evaluated by analyzing samples prepared from water containing various amounts of methanol, acetone, 2-propanol, or methyl ethyl ketone. It was expected that the concentration factors would decrease as the amount of organic modifier was increased because the solubility of the analytes would increase in the
sample matrix. Table I11 shows some unpredictable behavior. For methanol, concentration factors increased in the 0% to 25% range and then decreased. For acetone the concentration factors continued to increase. For 20% MEK the effect was even greater; however, the system did not stabilize and visual changes in the membrane were observed. Several theories have been proposed but the cause has not been resolved a t this time. The order of the effect appears to correlate with the solubility parameter of the organic modifier (methanol, 14.61; 2-propanol, 11.49; acetone, 9.70) when compared to hexane, 7.27. These data show that the membrane/POPSI system could be used for mixed process waste streams containing large amounts of water-soluble solvents if calibration is made in a similar matrix or if an internal standard is used. Solvents could also be added to water samples to reduce the adsorption of compound on surfaces and particulates and to even out matrix effects of variable samples. The solubility of hexane extractant in water is relatively low, but if the sample contains large amounts of organic solvents, the solubility of hexane in the sample stream would increase. No data were obtained to determine if this effect is reflected in the data in Table 111. Internal Standard Precision and Accuracy Study. As long as the matrix remains relatively constant, the relative standard deviation appears to be within the range of 3-13% RSD for parts-per-billion levels. Depending on the analysis requirements, this precision is generally acceptable for trace analysis that incorporates an external standard analysis. However, as with most liquid-liquid extraction techniques, an internal standard procedure or sample spike recovery check is necessary for a widely variable sample matrix. Ideally the best internal standard would be a compound that has very similar physical and chemical properties to the analyte of interest. An isotopically labeled analogue is the preferred choice; however, this is not compatible with the analytical detection technique used. A compound with a similar chemical structure was the next choice. For chlorpyrifos analysis, RELDAN insecticide, which is the methyl ester form of chlorpyrifos (ethyl ester), was chosen. The substitution difference on the ester linkage was not expected to lead to a significant difference in the extraction and analysis properties between the two compounds. Except for slightly different retention times, analysis of the two comopunds over the range of 25-200 ppb in hexane revealed similar chromatographic behavior. The gas chromatograph was calibrated by injecting a series of standards that varied in chlorpyrifos concentration from 10 to 255 ppb, while the concentration of RELDAN insecticide in each standard was held constant. The relative standard deviation for the 10.2 ppb standard was 0.5% while the percent difference from actual concentration was 9.2%. The
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Table IV. Determination of Chlorpyrifos (10.2 ppb) in Various Matrices Using an Internal Standard and External Standard matrix
no
direct injection water 5% MeOH 20% acetone 15% MEK
12 16 10 20 14
internal std ~ , ~ p p% b RSD' 11.2 12.8 16.6 16.8 25.2
0.53 2.74 3.94 3.20 20
external std % RSD
x,ppb
10.2d 10.2O 24.1 14.4 167
7.0 10.6 63 38 38
(In,number runs. b ~ mean , value for n runs. % RSD, percent relative standard deviation. Value assuming external calibration was at the same concentration as the sample. eValue assuming calibration was done by membrane extraction of standard solution.
average percent difference for the other standards (30-51 ppb) ranged from 2 to 3%. The high bias at the lower concentration range may be attributed to the fact that the limit of detection was approached. The calibration factors obtained were used to quantitate the concentration of chlorpyrifos for all of the extraction experiments. The extractions were performed by spiking a liter of water containing various amounts of methanol, acetone, or MEK with 10.2 ppb chlorpyrifos. Just before extraction, a known amount of RELDAN insecticide was added to the sample as an internal standard. A chromatogram of the membrane extract is shown in Figure 7 and the results are shown in Table IV. In all cases the precision was greatly improved by using the internal standard calculation. In the case of the extractions, a positive bias was observed in all cases. This bias was introduced by preparing the calibration curve by direct injection and assuming the extraction concentration factor for chlorpyrifos and RELDAN insecticide were identical. A more accurate calibration could have been obtained by preparing calibration standards in water and using the combined extraction/injection procedure. The bias is more pronounced for some solvent mixtures indicating a possible small polarity difference in the two compounds. These results point out the importance of selecting the internal standard. The internal standard must have not only the appropriate chromatographic properties but also a similar partition coefficient to that of the analyte. Even though a positive bias was observed when RELDAN insecticide was used as the internal standard, the accuracy and precision were used as the internal standard, the accuracy and precision were significantly improved over the use of external standard calculations. Additional improvement would also be obtained by preparing the running calibration mixtures in a matrix similar to that of the sample. If a mass spectrometric detector was used along with isotopically labeled analogues as internal standards, the bias due to matrix effects would be eliminated. Two different column configurations were evaluated. In the external standard analysis study, the analytical column (0.53 mm i.d.) was directly connected to the injection valve. A small section of the column was outside of the oven. Even though this small section was kept warm, this did contribute to the to slight peak fronting (see Figure 5). In the internal standard analysis study, a retention gap column (1m X 0.53 mm i.d.) was connected to the valve. The analytical column
2
I
I
!
1
2
3
-- -4
5
6
7
8
Minutes Flgure 7. Determination of chlorpyrifos using internal standard: (1) Reldan (internal standard 41.4 ppb), (2) chlorpyrifos (10.2 ppb); sample matrix, 20% acetone/80% water. GC conditions, see text. was then connected to the retention gap with a butt connector inside the gas chromatographs oven. This uncoated inlet eliminated the fronting peak problem experienced in the external standard study. Furthermore, by connecting the retention gap column to the injection valve, different diameter columns can more easily be connected by using the appropriate reducing press-fit connectors. This technology has potential both for use in sophisticated laboratory systems for automated extraction/analysis and also for simplified dedicated systems for on-line analyzers. Initial applications for this technology are being evaluated in the automated analysis of trace constituents in wastewater streams.
LITERATURE CITED (1) (2) (3) (4)
(5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
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RECEIVED for review March 13,1990. Accepted June 29,1990.
