Environ. Sci. Technol. 1996, 30, 2071-2077
On-Line Preconcentration and Gas Chromatographic Determination of N-Methylcarbamates and Their Degradation Products in Aqueous Samples EVARISTO BALLESTEROS, MERCEDES GALLEGO, AND MIGUEL VALCARCEL* Department of Analytical Chemistry, Faculty of Sciences, E-14004 Co´rdoba, Spain
A continuous liquid-solid extraction system coupled to a gas chromatograph for the preconcentration and determination of N-methylcarbamates and their corresponding phenols in water is reported. Various sorbent materials were assessed, of which XAD-2 provided the best results in the preconcentration of propoxur, carbofuran, carbaryl, 2-isopropoxyphenol, 3-hydroxycarbofuran, and 1-naphthol. The proposed method is highly sensitive to trace levels of the analytes; it features detection limits between 0.7 and 1 µg/L, which are substantially lower than those afforded by flame ionization detection. Changes in three river water samples containing only carbamate pesticides or phenols and pesticides plus their hydrolysis products (phenols) at the microgram per liter level were studied under natural conditions over a 6-week period; the rate of degradation was lower for the N-methylcarbamates than for their respective phenols. The proposed continuous system was evaluated for environmental samples without detectable residues of pesticides.
Introduction Analyses of organic materials in environmental samples frequently entails concentrating the sample to improve the sensitivity of the analytical method and the separation of matrix. The sample preparation typically involves tedious, time-consuming operations that are the source of much of the imprecision and inaccuracy of the overall analysis (1). In order to increase the sensitivity, the enrichment of trace compounds of interest on suitable sorbents has been used prior to application of an appropriate chromatographic technique (2). Sorbents employed in liquid-solid extraction (LSE) are generally similar to those used in column liquid chromatography. Sorbents such as activated carbon, alumina, silica gel, magnesium silicate (Florisil), and * Corresponding author fax: 34-57-218606.
S0013-936X(95)00883-2 CCC: $12.00
1996 American Chemical Society
chemical bonded silica phases and polymers (e.g., styrenedivinylbenzene copolymers) such as XAD-2 and PRP-1 have been employed for this purpose (3). Graphitized carbon black has been used for the LSE of polar (4) and acid pesticides from water samples (5). The latter required several steps to preconcentrate the acid pesticides that include evaporation of the extract (ca. 6 mL) in order to obtain a final volume of 100 µL; in addition, detection limits were lowered (from 0.1 to 1 µg/L) by using a new ion source, larger volume injections, and liquid chromatography/mass spectrometry detection (5). Flow techniques have been employed for the continuous extraction of pesticides in combination with a liquid chromatograph and UV (6), diode-array (7), or mass spectrometric detection (8). A gas chromatograph is more difficult to couple to a continuous system than is a liquid chromatograph (HPLC); as an obvious result, there is scantier literature on the former type of coupling. Various interfaces have been devised for introducing the organic extract, containing the analytes, from a continuous-flow system into the gas chromatograph; most are highly complex and require an on-column injector. Pesticides have been determined by this technique using an oncolumn interface (9) or injection valve with no alteration of the instrument (10, 11). Liquid-solid extraction is no doubt one of the preconcentration techniques most frequently employed in combination with liquid chromatographic instruments; there are currently many offline LSE instruments on the market, but eluates continue to be transferred manually to the chromatograph (3). Brinkman et al. (12) reported various continuous systems based on LSE coupled on-line to gas chromatographic instruments; the main disadvantage of this coupling is the excessive number of injection valves required. These authors recently proposed an on-line LSE-gas chromatography-atomic emission detection system using an oncolumn interface that requires three chromatographic columns and four injection valves for the determination of organophosphorous pesticides (13). The ensuing method is highly sensitive to trace levels of pesticides in water (detection limits of 1-30 ng/L are obtained for 10-100-mL samples). The processing time per sample (2.2 mL) is 51 min. Accurate, sensitive, analytical methods are required for reliable environmental control analyses. Gas chromatography is commonly used for the determination of pesticides. Unfortunately, only some carbamates can be analyzed directly because they are especially thermolabile and decompose to their corresponding phenols. N-Methylcarbamates and their metabolites (phenols) are biologically active or toxic, so analyses for both types of compound are very interesting (14). The aim of this work was to develop a straightforward continuous system for the preconcentration of N-methylcarbamates and their phenols from waters and its coupling to a gas chromatograph equipped with a conventional detector (FID) in order to avoid labourintensive manual operations that detract from accuracy, precision and sampling frequency and facilitate application to environmental samples containing low concentrations of both types of pollutants. A kinetic study on primary and
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FIGURE 1. Flow manifold for the preconcentration of N-methylcarbamates and their phenols in water samples. (A) on-line and (B) off-line mode. IV, injection valve; MC, mixing coil; S, tube stopcock; IP, injection port; W, waste. Sample, nitrogen, and eluent flow rate: 3.5, 3.5, and 0.10 mL/min, respectively.
ultimate degradation of N-methylcarbamates was also conducted.
Experimental Section Materials. Ethyl acetate, ethanol, methanol, acetone and n-hexane, all HPLC-grade, were purchased from Romil Chemicals (Loughborough, U.K.). N-Methylcarbamates and their phenols were obtained from Dr. Ehrenstorfer (Augsburg, Germany). Heptadecanoic acid methyl ester was supplied by Sigma Chemical Co. (St. Louis, MO). Darco 20-40 activated carbon, XAD-2 styrene-divinylbenzene, and polygosyl-bonded silica reversed-phase sorbent with octadecyl functional groups (RP-C18), 40-63 µm, were purchased from Aldrich-Chemie (Steinheim, Germany), Serva Feinbiochemica (Heidelberg, Germany), and Sigma, respectively. Standard stock solutions containing 2 g/L propoxur, carbofuran, carbaryl, 2-isopropoxyphenol, 3-hydroxycarbofuran, and 1-naphthol were prepared in 99.9% acetone and stored in PTFE bottles at 4 °C. Appropriate volumes of these stock solutions were diluted with Milli-Q water to prepare more dilute solutions containing a few micrograms per liter of each pesticide or phenol at pH 3. Ethyl acetate containing 20 mg/L heptadecanoic acid methyl ester (internal standard) was used as the eluent. Liquid-Solid Extractor and Interface Unit. The flow manifold used comprised a Gilson Minipuls-2 pump furnished with poly(vinyl chloride) and Solvaflex tubes for water samples and ethanol (for cleanup), respectively, a Rheodyne 5041 injection valve, PTFE tubing for coils (0.5 mm i.d.), and a displacement bottle for pumping ethyl acetate. Laboratory-made adsorption columns packed with XAD-2 styrene-divinylbenzene copolymer, activated carbon and RP-C18 were also employed. Sorbent columns were made from poly(tetrafluoroethylene) capillaries (3 mm i.d.), and small cotton wool plugs were placed above and below the sorbent bed to prevent material losses. The sorbent column was conditioned by passing ethanol (1 min) and Milli-Q water (1
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min) at a flow rate of 3.5 mL/min. The columns remained usable for at least 2 months with no change in their properties. The interface unit between the continuous flow system module and gas chromatograph was an injection valve (Knauer 633200) similar to that used for coupling an extraction unit to a gas chromatograph (15); the 5-µL loop of the injection valve (inner volume 2.5 µL) and the shortest possible connecting coil (2.5 µL) were constructed from PTFE tubing. The valve was connected to the instrument via a 10 cm × 0.3 mm i.d. stainless steel tube with a needle soldered on one end that allowed direct fitting to the injection port by insertion into the septum. The carrier gas (nitrogen) inlet was split into two that were directly connected to one of the valve ports and the chromatograph injection port. The inlet was shut by a stopcock so that the instrument could be used for manual injections by allowing the nitrogen stream to travel its normal pathway through the instrument. Chromatographic Analysis. A gas chromatograph Hewlett-Packard Model 5890-A equipped with a flame ionization detector and an integrator Hewlett-Packard Model 3392-A was used. Chromatographic assays were performed on a cross-linked poly(dimethylsiloxane) fusedsilica column (15 m × 0.5 mm i.d., 3.0 µm film thickness) supplied by Hewlett-Packard (HP-1). Nitrogen was used as carrier gas at a flow rate of 15 mL/min. Sample injection was done in the splitless mode. Temperature program: injector and detector at 160 °C; oven at 100 °C for 2 min, 5 °C/min ramp to 200 °C, and hold the final temperature for 3 min. Injected volumes of 2 or 5 µL for off-line and on-line analyses, respectively, were used. Extraction Procedure. A continuous liquid-solid extraction system for the isolation-preconcentration of pesticides and phenols from water samples was employed (Figure 1). First, 50 mL of aqueous sample or standard solution containing 5-60 µg/L N-methylcarbamates and their corresponding phenols at pH 3 was continuously introduced into the system at 3.5 mL/min and propelled
through the sorbent (XAD-2) column located inside the loop of the injection valve (IV). N-Methylcarbamates and phenols were retained, and the sample matrix was sent to waste (W1). Then, the sorbent column was dried for 3 min with a strean of nitrogen introduced via the same sample tube. Second, IV was switched and the eluent, ethyl acetate containing 20 mg/L internal standard, was passed through the sorbent column at 0.10 mL/min for 1 min. The eluate, containing the analytes, was homogenized in a mixing coil (MC) of 75 cm × 0.5 mm i.d. located below the column. For on-line analysis (A), IV was switched again, and the N2 stream transported the eluate (100 µL) to the valve interface; the loop contents (5 µL) were injected into the nitrogen carrier gas 30 s after IV was switched and transferred to the chromatograph port. For off-line analysis (B), the eluate (100 µL) was collected in glass vials, and 2-µL aliquots were manually injected into the chromatograph by means of a syringe.
Results and Discussion In order to minimize hydrolysis of N-methylcarbamates (propoxur, carbaryl, and carbofuran) during the solution vaporization process to their respective phenols, the chromatographic conditions were optimized. For this purpose, 2 µL of a standard solution containing 20 mg/L of each N-methylcarbamate in ethyl acetate was manually injected into the injection port at variable temperatures (100-260 °C). A temperature of 160 °C was selected as it allowed N-methylcarbamates to be vaporized (their boiling points range from 85 to 150 °C) without their phenols being formedsat least not at detectable concentrations. A similar temperature (160 °C) for the flame ionization detector and 200 °C at the most for the oven were also selected. Under these chromatographic conditions, mixtures of pesticides and their phenols could be efficiently resolved. Selection of the Sorbent and Eluent. Three sorbent materials (activated carbon, RP-C18, and XAD-2), used routinely in conventional solid-phase extraction of pollutants, were assayed for the preconcentration of Nmethylcarbamates and their phenols. This study was carried out using the same amount of sorbent (80 mg), packed as described under the Experimental Section. The aqueous standard solution (sample), containing 20 mg/L propoxur, carbofuran, carbaryl, 2-isopropoxyphenol, 3-hydroxycarbofuran, and 1-naphthol at pH 4, was passed through the column at 1 mL/min. In order to determine the sorbent capacity of the different materials, 1-mL fractions of the sample were collected on passage through the sorbent column (unadsorbed fraction); simultaneously, an identical volume of original sample (total fraction) was taken. Both fractions were extracted with 1 mL of ethyl acetate, and the extracts were dried over anhydrous sodium sulfate. Finally, 2-µL aliquots of the extracts were injected into the chromatograph for analysis. The difference between the chromatographic signals obtained gave the sorption efficiency of the sorbent material. Activated carbon and XAD-2 exhibited a sorption efficiency close to 100% (no chromatographic peak was produced by the effluent collected from the sorbent column); on the other hand, the efficiency of the RP-C18 sorbent was only ca. 40%. The optimal amount of sorbent was determined for both materials (XAD-2 and activated carbon) by using various columns (3 mm i.d.) loaded with variable amounts of sorbent (20-100 mg). For this purpose, calibration graphs were run for each column by using aqueous standard
solutions containing between 10 and 200 µg/L carbamates and phenols at pH 4 (sample volume, 50 mL). From the results, it was concluded that (a) no signicant difference was observed by varying the amount of either sorbents and (b) the sensitivity (slope of the calibration graph) was ca. 10 times higher for the XAD-2 sorbent than for activated carbon. This result is consistent with previous reports (16), where XAD-2 provided better recoveries than activated carbon for similar solutes at trace levels in analytical procedures requiring solute desorption owing to their irreversible sorption on carbon. An XAD-2 sorbent column (30 mm × 3 mm i.d.) containing 80 mg of material was thus selected. Various organic solvents (viz., ethyl acetate, acetone, methanol, ethanol, and n-hexane) were assayed as eluents for N-methylcarbamates and phenols retained on XAD-2. In these experiments, 50 mL of an aqueous sample containing 20 µg/L N-methylcarbamates and their phenols was passed through the sorbent column at 3.5 mL/min. The column was then dried with a stream of N2 and the eluent passed at 1 mL/min. Finally, several eluate fractions (ca. 0.5 mL) were collected in glass vials containing anhydrous sodium sulfate, and that yielding the highest signal was selected for delivery of the analytical results. Ethyl acetate proved to be the most effective eluent for these organic compounds, and methanol and n-hexane were the least. Ethanol and acetone were less efficient than ethyl acetate (8 and 10 times, respectively). Therefore, ethyl acetate was selected as eluent for N-methylcarbamates and phenols. Optimization of the Working Conditions. Carbamate insecticides hydrolyze under alkaline conditions; the hydrolysis rate is dependent on the structure of the compound and the hydrolysis conditions. Generally, mild alkali at room temperature is sufficient to cause hydrolysis (14, 17). Therefore, the effect of pH on the hydrolysis of carbamates and on their sorption and that of their phenols was studied by using a continuous system similar to that depicted in Figure 1 and off-line analyses (mode B). For this purpose, aqueous samples containing 20 µg/L N-methylcarbamates at a variable pH (adjusted with dilute HNO3 or NaOH) were introduced into the system. As can be seen in Figure 2a, the signals for N-methylcarbamates decreased with increasing pH above 4 as a result of the hydrolysis; simultaneously, peaks for the hydrolysis products (phenols) started to appear at pH 4.5. Above pH 12, all carbamates were virtually completely hydrolyzed, and above pH 11.5 or pH 12.8, the analytical signals for 1-naphthol and 3-hydroxycarbofuran or 2-isopropoxyphenol (hydrolysis products) also decreased through decreased sorption on the sorbent column. This effect was confirmed by analyzing aqueous samples containing 20 µg/L 2-isopropoxyphenol, 3-hydroxycarbofuran, and 1-naphthol at a variable pH. As can be seen in Figure 2b, retention of phenols on XAD-2 sorbent remained virtually constant up to pH 11, above which the signal started to decrease. A sample pH of 3 was selected, where N-methylcarbamates were not hydrolyzed and pesticides and phenols could be simultaneously determined. The ionic strength of the water samples, adjusted with potassium nitrate, did not affect the signal up to 1.5 M. Using an internal standard corrected the chromatographic signals obtained by manual injection of 2 µL (offline mode) or automatic injection of 5 µL (on-line mode) because it allowed a relative area (the ratio of analyte peak
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N-methylcarbamates and their phenols was studied between 0.05 and 0.5 mL/min for 1 min. The chromatographic signals increased with increasing flow rate up to 0.09 mL/ min. Above 0.14 mL/min, the analytical signal decreased because eluted analytes dispersed in higher volumes of eluent. An eluent flow rate of 0.10 mL/min through the sorbent column for 1 min (corresponding to an eluent volume of 100 µL) was selected as optimal as it ensured complete elution of the analytes as confirmed by a second elution step without sampling, in which no analyte was detected. For on-line analysis (Figure 1, mode A), the flow rate of the chromatographic carrier gas was optimized by varying it between 10 and 20 mL/min in order to minimize adsorption of the analytes in the loop of interface valve and the tube connecting it to the injection port as well as to improve chromatographic resolution of the peaks. An overall flow rate of 15 mL/min (flow through interface valve and injection port, 10.8 and 4.2 mL/min, respectively) was selected as optimal because it resulted in no adsorption or distorted peaks.
FIGURE 2. Influence of pH on the hydrolysis of carbamates (a) and sorption of phenols (b) (1) carbaryl, (2) carbofuran, (3) propoxur, (4) 1-naphthol, (5) 2-isopropoxyphenol, and (6) 3-hydroxycarbofuran. Analyte concentration: 20 µg/L; relative area: ratio of analyte area to internal standard area (20 mg/L heptadecanoic acid methyl ester).
area to internal standard peak area) to be used; in addition, it increased the precision. Two organic compounds (phenanthrene and heptadecanoic acid methyl ester) were assayed as internal standards added to the eluent (ethyl acetate). Heptadecanoic acid methyl ester was chosen because, unlike phenanthrene, it was not retained by the sorbent. The influence of the sample flow rate on the chromatographic signals was examined over the range 0.5-4.0 mL/ min. The signals remained constant throughout the interval studied, so a flow rate of 3.5 mL/min was chosen in order to ensure a high throughput. The same flow rate for nitrogen was used to dry the sorbent column before elution and to transport eluted pesticides and phenols. The effect of the eluent flow rate (volume of eluent) on the elution of
Analytical Performance. The performance and reliability of the proposed on-line LSE-gas chromatographic system (Figure 1) were assessed by determining the sensitivity (slope of the calibration graph), analyte detectability, linearity range, and relative standard deviation for the three N-methylcarbamates and their phenols in water samples spiked at microgram per liter level. Table 1 gives the equations for the standard curves (correlation coefficients ranged from 0.996 to 0.999) obtained by plotting the analyte-to-internal standard peak area ratio against the analyte concentration as well as other analytical figures of merit obtained by processing 50 mL of sample. The linear range was similar for all N-methylcarbamates and their phenols (5-60 µg/L). Detection limits (for sample volumes of 50 mL), defined as the minimum concentrations providing a chromatographic signal three times higher than background noise, and relative standard deviations (obtained by measuring 11 samples containing 20 µg/L of each compund) are also listed in Table 1. On the assumption of 100% extraction recovery for all compounds, the preconcentration factor was calculated as the ratio between the slopes of the calibration graphs run by using the flow system depicted in Figure 1 (on-line mode) and those obtained by manual injection of standards containing between 2 and 40 mg/L of pesticides and their phenols in ethyl acetate. The mean values of these factors (for all pollutans) were 110, 241, and 502 for sample volumes of 10, 25, and 50 mL, respectively; higher preconcentration factors could probably be obtained with sample aliquots larger than 50 mL. The sensitivity of the off-line method can be increased by introducing an evaporation step using
TABLE 1
Analytical Figures of Merit of the Determination of N-Methylcarbamates and Phenols by On-Line LSE-GC
a
compound
regression eqa
detection limit (µg/L)
rsdb (%)
preconcn factorc
propoxur carbofuran carbaryl 2-isopropoxyphenol 3-hydroxycarbofuran 1-naphthol
Y ) 0.0014 + 0.0153X Y ) 0.0100 + 0.0150X Y ) -0.0050 + 0.0246X Y ) -0.0059 + 0.0232X Y ) 0.0131 + 0.0201X Y ) -0.0084 + 0.0240X
0.9 0.9 0.8 1 0.7 1
3.1 3.0 3.7 2.9 3.6 3.4
490 485 520 525 495 500
Y ) relative area, X ) concentration, in µg/L. b Relative standard deviation (n ) 11). c Preconcentration factor for a sample volume of 50 mL.
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TABLE 2
Percent Recovery of N-Methylcarbamates and Phenols Spiked to Aqueous Samplesa % R, river water
% R, pond water
% R, waste water
compound
20 µg/Lb
40 µg/Lb
20 µg/Lb
40 µg/Lb
20 µg/Lb
40 µg/Lb
propoxur carbofuran carbaryl 2-isopropoxyphenol 3-hydroxycarbofuran 1-naphthol
96.0 ( 3.4 103.2 ( 2.1 97.8 ( 3.6 95.5 ( 2.9 100.8 ( 3.1 94.0 ( 3.0
103.4 ( 1.9 98.6 ( 3.1 98.4 ( 1.9 100.8 ( 3.1 97.2 ( 2.4 96.0 ( 1.5
98.5 ( 2.8 101.6 ( 3.2 100.8 ( 3.4 102.2 ( 2.1 94.2 ( 1.9 102.3 ( 2.5
102.1 ( 1.7 103.3 ( 2.6 98.8 ( 3.6 96.8 ( 1.6 101.4 ( 2.1 98.5 ( 3.4
103.5 ( 2.8 94.5 ( 2.3 103.5 ( 3.2 103.0 ( 2.5 97.8 ( 1.8 98.0 ( 3.0
101.3 ( 1.8 102.5 ( 1.7 96.5 ( 1.9 94.8 ( 2.5 103.4 ( 2.7 102.3 ( 3.6
a
% R, percent recovery ( standard deviation (n ) 3).
b
Concentration added.
study focused on the degradation kinetics of a mixture containing 19 organophosphorous and organonitrogen pesticides over a 6-month period in different water types and under varying conditions (17). One of the major results was a great difference in degradation behavior between river water and filtered river water due to the influence of particulates (through adsorption). No reference seems to exist, however, to the degradation kinetics of N-methylcarbamates in parallel to their hydrolysis products (phenols) in river water. The present study was aimed at monitoring the degradation kinetics of three N-methylcarbamates and their phenols in river water in order to obtain their chemical degradation curves and half-lives.
FIGURE 3. Gas chromatogram obtained in the analysis of pond water: (a) unspiked and (b) spiked with 20 µg/L N-methylcarbamates and phenols. (1) 2-isopropoxyphenol, (2) 3-hydroxycarbofuran, (3) 1-naphthol, (4) propoxur, (5) carbofuran, (6) carbaryl, and (IS) internal standard.
a nitrogen stream to reduce the overall eluted volume (100 µL) to ca. 25 µL; as a result, the detection limit is lowered by a factor of about 4. Analysis of Water Samples Spiked with Pesticides and Phenols. The proposed on-line preconcentration method was applied to the determination of N-methylcarbamates and their phenols in different types of water, namely: river water, pond water, and wastewater. Samples were passed through 0.45-µm filters (Micron Separations Inc., 45 mm diameter, Westboro, MA) to remove particulates and adjusted to pH 3 with diluted HNO3 before analysis. Aliquots of 50 mL were passed through the sorbent column as described under Extraction Procedure. Because none of the pesticides or phenols was detected, the samples were spiked with a few micrograms per liter of both types of compound. Figure 3 shows the chromatograms obtained for unspiked pond water (a) and spiked with 20 µg/L of each pollutant (b). Table 2 lists the average recoveries obtained under the optimal working conditions for river water, pond water, and wastewater spiked with 20 and 40 µg/L pesticides and phenols; each sample was analyzed in triplicate (n ) 3). Recoveries ranged from 94.0 to 103.5%. Degradation of N-Methylcarbamates and Phenols Spiked to River Water. There is a wealth of literature on pesticide metabolism, most of which deals with the biochemical transformation rather than with environmental products however (14). Pesticides can undergo three general types of degradation processes in the aquatic environment, namely, chemical (hydrolysis), biological, and photochemical. The half-life of carbamates in waters has been estimated by several authors (18-20). The most recent
Three samples were used for the experiments, namely, river water from the Guadalquivir (a river near Co´rdoba), which was spiked (20 µg/L) with N-methylcarbamates (carbaryl, propoxur, and carbofuran) or phenols (1naphthol, 2-isopropoxyphenol, and 3-hydroxycarbofuran), and with a mixture of the six pollutants at identical concentrations. The samples (10 L) were maintained at their natural pH (7.5), unfiltered, in a closed 25-L PTFE bottle under natural sunlight. The temperature ranged from 10 to 30 °C (environmental conditions). Aliquots of 50 mL were periodically taken from the samples, filtered, adjusted to pH 3, and analyzed using the proposed method. The container volume was not adjusted to its initial value by adding the same amount of river water, as in other experiments (17), in order to overcome the dilution effect on the concentrations of pesticides or phenols. In addition, because the initial volume (10 L) was reduced to about 8 L, the volume of air (25-L bottle) remained virtually constant. The bottles were capped to avoid volatization and contamination. The experiments enabled the influence of the following factors to be determined: (1) sudden temperature changes; (2) photodegradation due to exposure to direct sunlight, which increased in the presence of humic acids (photosensitizers) (21); and (3) adsorption onto particulates (spiked river samples were not filtered). Adsorption led to faster degradation or increased half-lives. No comparison between filtered and unfiltered water, sterile and nonsterile water, was made as it was previously done in many reported studies on pesticide degradation (17). The degradation curves obtained over a 6-week period for N-methylcarbamates (a), phenols (b), and the mixture of both (c) are shown in Figure 4. An overall 50 mL of water was initially sampled each hour, then each day, and finally once a week; duplicate measurements were made in all cases. The chemical degradation of the pesticides and their phenols fitted a first-order degradation curve, Ct ) C0e-Kt, where C0 and Ct are the initial concentration and that at time t, respectively, and K is the rate constant. The half-
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TABLE 3
Rate Constants and Half-Lives, t1/2 (ln K/2), for the Pollutants Studied compound carbaryl propoxur carbofuran 1-naphthol 2-isopropoxyphenol 3-hydroxycarbofuran a
K (h-1) 10-3
6.33 × 1.76 × 10-3 2.14 × 10-3 1.10 × 10-2 5.73 × 10-3 3.91 × 10-3
ti (h)a
t1/2 (h)
10 30 80 10 15 25
109.5 394 324 63 121 177
Starting degradation time.
obtained by spiking them separately (Figure 4a,b). 1-Naphthol appears to be more stable in the presence of carbaryl due to the contribution of its hydrolysis (see Figure 4a). All degradation curves could be modeled using a first-order expression (the best model). Degradation of the six pollutants increased with increasing K. Based on the results of Table 3, the rate of degradation was lower for the N-methylcarbamates than for their respective phenols; also, the latter exhibited shorter half-lives.
Conclusions The proposed preconcentration module-gas chromatograph system provides an effective means for the determination of trace levels of N-methylcarbamates and their phenols in water samples by use of a conventional detector (FID). The on-line coupled assembly is quite simple and affordable (no alteration of the instrument is required). The system offers all the inherent advantages of automatic methods (viz., low sample and reagent consumption; minimal manipulation and contact with the reagents; accurate, reproducible results; and a high throughput). The use of an FID can only be justified for validating the proposed extraction method because other detectors such as mass spectrometric ones are more specific. The ensuing method can be used for the analytical control of environmental pollutants (carbamates and phenols) at the microgram per liter level in water samples, with analyte recoveries from spiked samples close to 100%. In addition, from kinetic studies it has been shown that the phenols are degraded more rapidly than their corresponding N-methylcarbamates in river water; no total degradation of propoxur or carbofuran was observed over the 6-week period studied however. FIGURE 4. Degradation curves for N-methylcarbamates (a), phenols (b), and the mixture of pesticides and phenols (c) spiked at 20 µg/L to river water (10 L) under natural conditions (pH 7.5; ambient temperature 10-30 °C; sunlit, closed 25-L PTFE bottles). (1) carbaryl, (2) propoxur, (3) carbofuran, (4) 1-naphthol, (5) 2-isopropoxyphenol, and (6) 3-hydroxycarbofuran. The inset shows the real and simulated curve (from the K and C0 values obtained in the fit) for carbaryl and 2-isopropoxyphenol degradation.
life, t1/2, designates the time at which the pollutant concentration was equal to one-half the initial concentration (t1/2 ) ln 2/K). As can be seen in Figure 4a, carbaryl remained stable for a shorter time (10 h) than propoxur (30 h) and carbofuran (80 h); also, the rate constant for the first is the highest (Table 3). The hydrolysis of the pesticides was accompanied by the appearance of their corresponding phenols (see Figure 4a, bottom). The phenols started to be degradated before the pesticides, except for 1-naphthol (Table 3). The degradation curves for the mixture of carbamates and phenols (Figure 4c) were similar to those
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Acknowledgments The Spanish CICyT is acknowledged for financial support awarded in the form of Grant PB94-0450.
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Received for review November 27, 1995. Revised manuscript received February 12, 1996. Accepted February 15, 1996.X ES9508838 X
Abstract published in Advance ACS Abstracts, April 15, 1996.
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