Anal. Chem. 1989, 61, 935-939
935
Extraction and Isolation of Triazine Herbicides from Water and Vegetables by a Double Trap Tandem System Massimo Battista, Antonio D i Corcia,* and Marcello Marchetti Dipartimento di Chimica, Universitci “La Sapienza” di Roma, Piazza Aldo Mor0 5, 00185 Roma, Italy
The ablllty of a two-trap tandem system, one contalnlng a nonspecific adsorbing rnaterlal, such as graphltlred carbon black (Carbopack B), and the other one fllled wlth a sulfonlc acld type slllca-based cation exchanger (SCX), In extracting and Isolating basic compounds from real matrices was evaluated by applylng thls device to the determlnatlon of trlarlne residues In water and vegetables. After percolatlon through the Carbopack column (extractlon cartridge) of water samples or sultably prepared vegetable extracts, the two traps were connected In series, a methylene chlorlde-methanol mixture was allowed to flow along them, and triazines dlsplaced from the extractlon cartrldge were selectlvely readsorbed via salt fonnatlon on the strong acld exchanger column (Isolation cartrldge). After the column was washed, the analytes were removed from the Isolation cartridge by 0.7 mL of aqueous methanol contalnlng 70 mmol/L KCI. After the Internal standard was added, the flnal solution was dlrectly InJectedInto the “high-performance” liquid chromatographlc apparatus, whlch was operated lsocratlcally In the reversephase mode wlth UV detectlon at 220 nm. The analytlcal recoveries of eight trlarlnes from the two matrices considered ranged between 95 % and 100 % The llmlts of sensltlvlty of thls method for trlarlnes were set at 10 ng/g of vegetable materlal and 10 ng/L of water by sampllng 100 mL of It.
procedural losses. Moreover, these procedures are not highly specific as they are derived from multiresidue purification methods. Recently, a concentration factor higher than lo00 and total recovery of simazine and atrazine present at 50 ng/L in water volumes of about 1 L were accomplished by the use of a small cartridge containing Carbopack B, which is a well-known example of graphitized carbon black (GCB) (12). Very recently, triazine isolation from soil extracts (13) has been effectively accomplished by exploiting the particular affinity that these compounds, which are slightly basic in nature, have for a strong acid exchanger. The object of this work was that of evaluating the ability of a two-trap tandem system, consisting of one cartridge filled with a nonspecific adsorbing material, such as Carbopack B, and the other one containing a sulfonic acid type silica-based cation exchanger (SCX), to realize in a simple and rapid way the simultaneous extraction and isolation of basic compounds, such as triazines, from both water samples and acetonitrilewater extracts of green vegetables. With the SCX cartridge suitably sized and a proper eluant system selected, direct injection of the eluate into an HPLC column operating in the reversed-phase mode was made possible, thus eliminating the lengthy and critical solvent removal step without significant loss of the sensitivity of the method.
Triazine derivatives are popular herbicides introduced about 30 years ago and applied to a variety of crops including green vegetables. As a result, herbicide residues may contaminate crops and also wells and streams due to spills, spraying, or runoff. Organic solvent extraction from vegetables ensures quantitative recoveries of pesticide residues ( 1 ) . At the analysis limit, however, the final determination by capillary gas chromatography or high-performance liquid chromatography (HPLC) of triazines in uncleaned vegetable extracts is precluded because of interferences from the great number of coextracted plant materials. Extraction with an organic solvent ( 2 , 3 )or adsorption on suitable materials ( 4 , 5 ) have been recommended for the isolation of triazines from water. In the analysis of environmental waters largely contaminated by other, unknown organic compounds, a rapid and simple cleanup step prior to chromatographic determination is desirable as it could allow quantitative measurements of the analytes without the necessity of good separation from interfering compounds. In any case, a selective isolation procedure following extraction from water can provide added evidence for compound identity. Cleanup procedures quoted in the literature (6-11) for triazine herbicides from extracts of environmental samples do not encourage their inclusion in an analytical scheme because they require repeated manipulation of the sample and therefore they are tedious, time-consuming, and prone to
EXPERIMENTAL SECTION Reagents. Authentic triazine derivatives were obtained from Supelco (Bellefonte, PA). They are as follows: 2-chloro-4,6bis(ethy1amino)-s-triazine (simazine); 2,4-bis(ethylamino)-6(methy1thio)-s-triazine (simetryn); 2-chloro-4-(ethylamino)-6(isopropy1amino)-s-triazine(atrazine); 2,4-bis(isopropylamino)6-methoxy-s-triazine (prometon); 2-(isopropylamino)-4-(ethylamino)-6-(methylthio)-s-triazine (ametryn); 2-chloro-4,6-bis(isopropy1amino)-s-triazine (propazine); 2,4-bis(isopropylamino)6-(methylthio)+triazine (prometryn); 2-(ethylamino)-4-(tertbuty1amino)-&(methylthio)-s-triazine (terbutryn). Approximately, the pK, of chlorotriazines is equal to 2 while that of the other triazines considered is 4 (14). A standard solution was prepared by dissolving 100 mg/L of each herbicide in acetone. This solution was further diluted to obtain a working standard solution of 1 mg/L. For HPLC, distilled water was further purified by passing it through a Norganic cartridge (Millipore, Bedford, MA). Acetonitrile was of HPLC grade from Carlo Erba (Milan, Italy). All other solvents were of analytical-reagent grade (Carlo Erba) and were used as supplied. Apparatus. Both Carbopack and the silica-based cation exchanger had a particle size range between 37 and 74 pm; 150 mg of both Carbopack B and SCX were packed in polypropylene tubes, 6 cm X 1 cm i.d. Polyethylene frits (20 pm pore size) were located above and below the two sorbent beds. Connection between the two traps was realized by a plastic adapter. All the materials cited above were kindly supplied by Supelco. The sample reservoir had a narrow opening at the bottom that fitted into the cartridge down to few centimeters from the top of the Carbopack bed. Liquids were forced to pass through the cartridges by use of vacuum from a water pump. Before use, the cation exchanger material was converted from the Na form to the H form by washing it with 2 mL of 0.12 mol/L HCl in methanol at a flow
.
* To whom correspondence should be addressed.
0003-2700/89/0361-0935$01.50/0 0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
rate of about 1mL/min, followed by 2 mL of methanol and 2 mL of acetonitrile. Procedure. With Water Samples. When necessary, real water samples were then filtered through Whatman GF/C glass-fiber pads (pore sized 10 pm) to remove suspended sediments that caused the cartridges to plug. Artificially polluted water samples were prepared by adding known volumes of the working standard solution and agitating the water for 1 min to ensure complete solution of the herbicides. The organics from the water samples were then adsorbed onto the Carbopack surface by passing the sample through the trap at a flow rate of about 40 mL/min. Then, the sample reservoir was disconnected and 2 mL of distilled water was percolated through the Carbopack cartridge at about 3-4 mL/min. The major part of water was removed from the cartridge by drawing room air through it by vacuum for 2 min. Residual water was eliminated by slowly passing 250 pL of methanol. After this, the Carbopack cartridge was connected to the SCX one, which, just before connection, was partially filled with 2 mL of acetonitrile. Five milliliters of methylene chloride-acetonitrile (60:40, by vol) was passed through the Carbopack at a flow rate of about 1.5 mL/min to desorb triazines which were again adsorbed on passing through the SCX column. The two cartridges were then disconnected and the strong acid exchanger was washed with 2 mL of acetonitrile. After eliminating residual amounts of acetonitrile by drawing air, 1 mL of 0.7 mol/L KCl in water was passed through the SCX bed drop by drop. After the trap was air-dried for 1 min, triazines were eluted by applying to the top of the SCX trap 700 pL of 70 mmol/L KCl in methanol-water (9010, by vol). To the 450-pL final solution collected, 100 pL of 3,4-dimethylaniline (internal standard) at a concentration of 1 mg/L in water was added and 50 pL of this mixture was injected into the HPLC apparatus. With Vegetable Samples. Samples were obtained in area retail markets. The leaves were washed and air-dried and 4 g of leaf was weighed out for each sample. Samples were spiked with known volumes of the 1mg/L working herbicide standard solution and allowed to dry at room temperature. Next, the leaves were blended for 10 min with 40 mL of 40% water in acetonitrile by using a homogenizer (Kinematic, Zurich). Thereafter, the homogenizate was transferred to a vacuum filtration apparatus and filtered through Whatman GF/C glass filters and the crude plant material was washed with 100 mL of water. The washing and the original extract were combined and further diluted with 340 mL of water t o decrease the acetonitrile content down to 5%. Then 120 mL of this solution was applied to the top of the Carbopack cartridge and henceforth the same procedure as for water samples was followed. HPLC Apparatus. A liquid chromatograph Model 5000 (Varian, Walnut Creek, CA) equipped with a Rheodyne Model 7125 having a 50-pL loop and with a Model 2050 UV detector (Varian) was used. A 25 cm X 4.6 mm i.d. column filled with 5 pm (average particle size) of LC-1&DB reversed-phase packing (Supelco)was used. The mobile phase was acetonitrilephosphate buffer (10 mmol/L; pH 6.7) (38:62, by vol) and the flow rate was 2 mL/min. Triazines were monitored with the detector set at 220 nm. The concentrations of the herbicides in water and vegetable samples were calculated by measuring the peak area ratios of each herbicide and the internal standard and comparing them with those obtained with a standard solution. The latter was prepared by adding an appropriate volume of the herbicide working standard solution to 450 p L of the mixture used for elution of triazines from the SCX column. To this solution was added 100 pL of the solution containing the internal standard.
RESULTS AND DISCUSSION Sorption of weak bases on strong acid exchangers under strictly anhydrous conditions has been established to occur via salt formation or hydrogen bond (15, 16). Moreover, it was found that the nature of the organic solvent plays an important role on the retention of solutes (17). In this respect, for the double trap system under discussion, we evaluated the ability of some selected solvents or mixtures of solvents to readily elute triazines from the extraction column (Carbopack cartridge) and, at the same time, to unaffect their readsorption
Table I. Recovery of Ametryn and Propazine from Both the Carbopack and the SCX Cartridges by Passing through Various Eluant Systems recovery,” % CH2Clz/MeOH (60:40, v/v) Carbopack SCX ametryn
99
propazine
100
97 65
CHZC12 Carbopack SCX 84 87
CH&&/CH&N (60:40, v/v) Carbopack SCX
82
98
86
99
96 96
Mean values obtained from triplicate measurements. on the isolation column (SCX cartridge). For these experiments, ametryn and propazine, which, in terms of basic strength, are representative of the entire group of triazines considered, were applied directly to the top of the Carbopack cartridge from a methanolic solution. Results are shown in Table I. As one can read, a methylene chloride-methanol solvent mixture succeeded in readily eluting triazines from the Carbopack column, but in this mixture propazine was scarcely retained by the strong acid exchanger material. This effect was traced to the presence of methanol, which can be rather strongly bound via hydrogen bond to the acidic adsorption sites of the exchanger and thus it competes for adsorption with propazine, which is about 100 times less basic than ametryn. (No adsorption of chlorotriazines, that is simazine, atrazine, and propazine, was found to occur on the SCX material when they were dissolved in methanol.) On the other hand, the use of pure methylene chloride as eluant met the requirement of ready readsorption of triazine on the second trap, but this solvent showed some difficulties in displacing the compounds considered from the Carbopack surface, probably because of a weak specific interaction of triazines, with some active centers contaminating the surface framework of this sorbent. The object of rapid removal of triazines from the Carbopack surface followed by their complete readsorption on the SCX column was attained by adding acetonitrile to methylene chloride. Acetonitrile is sufficiently polar to displace triazines from the active centers of the Carbopack surface, while its low capacity of forming hydrogen bond makes it unable to compete with triazines for adsorption on the strong acid exchanger. Initially, when triazines were added to water and this solution was carried through the procedure, a severe loss of chlorotriazines was observed. This effect was traced to the presence of a residual amount of water still remaining in the Carbopack trap even after 10-min of purging by drawing room air by vacuum. This water, being removed together with analytes by the acetonitrile/methylene chloride mixture, can compete with the less basic triazines for sorption on the active sites of the exchanger. As a matter of fact, it was observed that 10% water added to acetonitrile caused the retention of chlorotriazines on the SCX column to be dramatically decreased. This unwelcome effect was eliminated by removing water from the extraction column with methanol, before passing the chosen solvent mixture for triazine desorption. Furthermore, the precaution was taken to partially fill the SCX containing tube with acetonitrile before joining it with the Carbopack trap, so that residual amounts of water or methanol could be sufficiently diluted with acetonitrile to unaffect adsorption of the analytes on the SCX column. With respect to the solvent extraction-solvent reduction methods, one of the advantages offered by the sorbent trap preconcentration technique is that a small amount of solvent or solvent mixture is needed to elute trapped organics. If this desorbing system is to some extent compatible with the mobile phase for HPLC analysis, namely the two liquid phases exhibit
ANALYTICAL CHEMISTRY, VOL. 61,NO. 9,MAY 1, 1989 1.0-
,c Ci
Table 11. Recovery of E i g h t T r i a z i n e s Spiked in 300 mL of T a p Water
.
recovery, %
-8.
15 ng/L simazine
simetryn atrazine prometon ametryn
m6.
propazine
.4.
prometryn terbutryn a
.2.
,J -35
m 4 0
m45
a 5 0
-55
-60
m65
m 7 0
volume, rnl
Figure 1. Elution curves by frontal chromatography on the SCX column of terbutryn dissolved in various methanol/water mixtures: (+) MeOH 17 mmol/L KCI; (A)MeOH/H,O (9O:lO) 17 mmol/L KCI; (W) MeOHIH,O (80:20) 17 mmol/L KCI; (0)MeOH/H,O (70:30) 17 mrnol/L KCI; (0) MeOH/H,O (9O:lO) 70 mrnol/L KCI.
+
937
+
+
+
+
a similar eluotropic strength on the stationary phase adopted, the extract could be directly injected into the chromatographic apparatus obtaining the important advantage of eliminating the solvent removal step, which is in some cases extremely critical. Very recently (13),desorption of triazines from SCX cartridge was achieved by using methanol saturated with KC1, the potassium ion being very effective in displacing from the exchange sites the complex BH', where B indicates a generic weak base. However, some peak broadening for the most rapidly eluted triazines occurred even with injection of small amounts of the methanolic solution cited above into the HPLC column. With the view of eliminatingthe solventiremoval step without affecting the sensitivity of the method, the ability of various water-methanol mixtures, obtained by varying both the water and KC1 contents, was evaluated to satisfy the demands of the minimum amount needed to desorb triazines from the SCX cartridge and maximum amount injectable into the HPLC apparatus. To this purpose, experiments of frontal chromatography of terbutryn, which among the triazines considered has the lowest mobility on the SCX column, were performed. Results are depicted in Figure 1. As can be seen, at the lowest salt concentration considered, which corresponds to that of a saturated methanolic solution, moderate amounts of water added to methanol resulted in only a slight increase in the retention of terbutryn. This behavior may be explained considering that the decrease of the affinity of the solute for a water-containing mobile phase is compensated by an increase in the actual concentration of K+, as water decreases the tendency of inorganic salts to form ion pairs in anhydrous solvents. At higher water percentages, the mobility of terbutryn on the SCX column was severely weakened, as expected considering the lipophilic character of the solute considered,which is able to establish hydrophobic interactions with the material supporting the ion-exchange site. A t a given water percentage, the increase in KC1 concentration had the effect of progressively decreasing the retention of the triazine derivative on the SCX column to the point that a 10/90 water-methanol solution containing 70 mmol/L KC1 was remarkably more effective than KC1-saturated methanol in removing triazines from the exchanger surface. The loading capacity of the double trap device for water samples containing the triazine herbicides studied was assessed under stressed conditions, that is by processing at high flow
96.3 98.2 99.4 98.6 97.3 96.4 95.0 95.7
f 3.95O
* 3.46
f 3.46 f 3.35 f 3.09 f 3.37 f 3.65 f 3.71
2000 ng/L 97.7 99.8 99.8 100.3 99.4 97.7 96.5 97.2
f f f f f f f f
1.66 1.75 1.68 1.47 1.63 1.73 1.58 1.66
Standard deviation calculated from seven determinations.
rate (30-40 mL/min) increasing volumes of a largely contaminated surface water specimen, such as that of the Tevere River, spiked with 100 ng/L of each herbicide. Recovery data showed that no loss of herbicides occurred by increasing the water volume passed through the extraction cartridge from 100 to 1500 mL. Evidently, the combination of the small particle size and high surface area of Carbopack ensures rapid adsorption of the dissolved herbicides even when rapid flow rates are employed. Moreover, the results obtained demonstrate once more that the GCB surface is particularly suitable for trapping organics from water and that the efficiency of the second trap, that is the SCX column, is unaffected by the sample type analyzed. Strong cation exchangers have been proposed for selective extraction of basic compounds from aqueous samples (18,19), on condition that strongly retained inorganic cations, such as calcium, are removed from water by a chemical pretreatment. For the purpose of comparison, the extraction efficiency for triazines of a SCX cartridge was evaluated by directly percolating through it distilled water spiked with triazines and containing 0.1 g/L NaCl to simulate a pretreated actual water sample. Breakthrough volumes for simazine and atrazine of only about 10-13 mL per 100 mg of the SCX material were measured. Moreover, adsorbed chlorotriazines were partially eluted from the exchanger bed by passing through it acetonitrile. Likely, the adsorption of very weak basic compounds from water containing even moderate amounts of inorganic salts occurs mainly on nonspecific sites of the material surface supporting the exchanger. Therefore, under these conditions, the selective mechanism of adsorption of an ion exchanger is not displayed. From this point of view, in the device developed by us the role played by the cartridge filled with a nonspecific adsorbent is that of preserving the peculiar adsorption characteristics of an ion exchanger so as its singular features can be extended to selective extraction from real water samples of weak basic compounds. Aliquots of a tap water specimen were supplemented with the eight triazine herbicides considered at the levels of 15 and 2000 ng/L of individual concentrations and analyzed seven times, as described under the Experimental Section. Typical quantitative results, reported in Table 11, show that the recovery efficiency by this procedure was independent upon the triazine concentrations, thus demonstrating the absence of any adverse effect of both irreversible adsorption by the materials composing the two traps and saturation of the two sorbents. Compared to solvent extraction, one important advantage of the sorbent trap technique is that sampling and extraction can be simultaneously accomplished at the sampling site by using newly available submersible instrumentation (20) and the trap returned to the laboratory for drying and elution. In addition to avoid water collection, thereby eliminating most contamination and handling problems, the isolation by sorption of analytes from the matrix can eliminate degradation
938
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
I
l 2 3 2
is
I
1
1
I
time (min)
Figure 2. Chromatogram of extract obtained from 1 L of Tevere River water (June 1987) contaminated by simazine (peak 1, 4.4 ng/L), atrazine (peak 2, 32.8 ng/L), and prometryn (peak 3, 68.5 ng/L): internal standard ( i s ) 3,4dimethylanine (0.22 pg/mL).
by bacteria, as observed for hydrocarbons in stored water samples (21). On the other hand, it may occw that the original composition of an extract is altered owing to a prolonged contact with the sorbent which may catalyze some decomposition reactions (22). The effect of storage for the eight triazines was evaluated by passing through the Carbopack cartridge 250 mL of a Tevere River water sample supplemented with 100 ng/L of each herbicide. After most of the water was removed by drawing air through the cartridge by vacuum, the trap was stored for 3 weeks, taking no particular precaution. Thereafter, the water sample extract was carried out through the remaining part of the procedure. Recovery data obtained for five such experiments showed that no significant adverse effect took place during the 3-week storage. Figure 2 shows a typical chromatogram obtained by this procedure for an actual water sample. As to vegetables, the efficiency of aqueous acetonitrile in extracting triazines was assessed as the water percentage was varied. With respect to acetonitrile, which is very effective in extracting triazines from vegetable tissues, no decrease in the extraction efficiency was noted even by using a solution consisting of 40% water in acetonitrile. At higher water percentages, a progressive loss of triazines, especially of the less polar ones, namely prometryn and terbutryn, occurred. Direct application of the acetonitrilewater (6040) solution to the top of the Carbopack cartridge was precluded, as triazines dissolved in this solution relatively rich in acetonitrile solution had very low breakthrough volumes. This problem
0
I 8
1
I IC
I
1 24
t i m e (min) Flgure 3. Chromatogram of extract obtained from a spinach sample spiked with triazines at 50 ng/g: (1) simazine; (2) symetryn; (3) atrazine; (4) prometon; (5) ametryn; (6) propazine; (7) prometryn; (8) terbutryn. Internal standard ( i s ) was 3.4-dimethylaniline(0.22 pg/mL).
was eliminated by lowering the acetonitrile percentage down to 5% by water addition. The extraction efficiency of the Carbopack cartridge at increasing vegetable extract volumes sampled was evaluated. When the extract volume applied to the top of the Carbopack column was twice as large as that reported under Procedure, some loss of propazine and prometon was observed. Among the triazines studied, propazine and prometon have the highest mobility on the Carbopack column. Likely, under the conditions mentioned above, the carbon surface is near saturation and displacement of the two triazines by coextracted plant substances takes place. Under the experimental conditions chosen, typical quantitative results obtained for vegetable samples of various origin spiked with triazine herbicides at 50 ng/g of individual concentration, which is half the maximum residue limit adopted in many European countries, are shown in Table 111. Figure 3 shows a typical chromatogram of a vegetable sample spiked with herbicides obtained by this procedure. The limits of detection (signal to noise ratio equal to 3), under the chromatographic conditions chosen, were estimated to be within the range between 0.13 and 0.66 ng of herbicide injected, the minimum and maximum figures referring respectively to the first and latter triazines eluted from the HPLC column. This means that by sampling only 100 mL of a water sample, levels of contamination of triazines lower
Anal. Chem. 1989, 6 1 , 939-945
Table 111. Recovery of Triazines at 50 ng per Gram of Various Vegetables
lettuce simazine simetryn atrazine prometon ametryn propazine prometryn terbutryn
98.4 97.3 97.8 99.0 96.3 97.8 95.0 95.3
recovery,” % spinach chicory 97.6 97.0 98.3 98.8 97.2 96.9 95.7 96.2
98.4 99.6 98.0 98.3 96.9 97.3 96.5 95.1
939
LITERATURE CITED Ambrus, A.; Lantos, J.; Visi, E.; Csatlos, I.; Sarvari, L. J . Assoc. Off. Anal. Chem. 1981, 6 4 , 733-742. Khan, S. U.; Marriage, P. B. J . Agric. Food Chem. 1877, 2 5 ,
endive
kale
97.9 98.3 97.5 99.1 98.1 98.0 96.3 96.6
97.5 99.4 97.0 97.6 98.5 97.4 95.8 95.9
Mean values calculate from four determinations.
1408-141 2. Muir, D. C. G.; Baker, B. F. J . Agric. Food Chem. 1878, 18, 111-116. Popl, M.; Voznakova, 2 . ; Tatar, V. J . Chromatogr. Sci. 1983, 2 , 39-42. Mangani, F.; Bruner, F. Chromatographia 1883, 17, 377-380. Binner, R. Tagungsber.-Akad. Landwirtschaftswiss. D . D . R . 1981, 187-1 92. Xu, Y.; Lorenz, W.; Pfister, G.; Bahadir, M.; Korte, F. Fresenlus’ 2. Anal. Chem. 1986, 3 2 5 , 377-380. Bailey, R.; Lebel, G. L.; Manners, T. G.; Renault, C. M. 8th Eastern Canada Workshop on Pesticide Residue Analysis, Ottawa, May 1976. Lawrence, J. F.; McLecd, H. A. J . Assoc. Off. Anal. Chem. 1877, 60, 979-986. Roseboom, H.; Herbold, H. A. J . Chromatogr. 1980, 202, 431-438. Lee, H. B.; Chau, A. S. Y. J . Assoc. Off. Anal. Chem. 1883, 66, 1322-1326. Di Corcla, A.; Marchetti, M.; Samperi, R. J . Chromatogr. 1887, 405, 357-363. Battista, M.; Di Corcia, A.; Marchetti, M. J . Chromatogr. 1988, 4 5 4 , 233-242. Weber, J. B. Spectrochlm. Acta, Part A 1867, 23A, 456-462. Gordon, J. E. J . Chromatogr. 1865, 18, 542-555. Funasaka, W.; Hanai, T.; Fujimura, K.; Ando, T. J . Chromatogr. 1872, 7 2 , 187-191. Funasaka, W.; Hanai, T.; Matsumoto, T.; Fujimura, K.; Ando, T. J . Chromatogr. 1974, 88, 87-97. Nielen, M. W. F.; Frei, A. W.; Brinkman, U. A. Th. J . Chromatogr. 1984, 317, 557-567. Kaczvinsky, J. R.; Saitoh, K.; Fritz, J. S. Anal. Chern. 1883, 5 5 , 1210-1 215. Green, D. R.; Stull, J. K.; Heesen, T. C. Mar. Pollut. Bull. 1988, 17, 324-329. Green, D. R.; Le Pape, D. Anal. Chem. 1987, 5 9 , 699-703. Di Corcia, A.; Liberatori, A.; Marchetti, M.; Samperi, R. Proceedings of the Workshop “Organic Micropollutants in the Aquatic Environment”, heid in Berlin, Oct 1986; pp 103-116.
than 10 parts per trillion can be measured, while the limit of sensitivity of this method for triazine residues in vegetables can be set a t about 10 ng/g of vegetable. The reusability of both the Carbopack and SCX cartridges WBS estimated by carrying out repeated extractions of the eight triazines from aliquots of water and vegetable extracts. After each extraction, the Carbopack bed was restored with 3 mL of methylene chloride, followed by 2 mL of methanol and 2 mL of water, while 4 mL of 0.12 mol/L HC1 in methanol, 2 mL of methanol, and 1 mL of acetonitrile were passed sequentially through the SCX bed to restore it. After six such water extractions, recovery of the analytes considered did not decrease significantly. Vice versa, after three runs with vegetable extracts, the Carbopack B cartridge partially failed to quantitatively extract triazines. Registry No. H20,7732-18-5; simazine, 122-34-9; simetryn, 1014-70-6; atrazine, 1912-24-9; prometon, 1610-18-0; ametryn, 834-12-8; propazine, 139-40-2; prometryn, 7287-19-6; terbutryn,
RECEIVED for review July 19, 1988. Accepted December
886-50-0.
1988.
20,
Electrodialytic Membrane Suppressor for Ion Chromatography Douglas L. Strong and Purnendu K. Dasgupta* Department of Chemistry and Biochemistry, Texas Tech Uniuersity, Lubbock, Texas 79409-1061
A dual membrane helical eiectrodialytlc suppressor Is described. A platinum-wire-filled tube made of Naflon perfluorosulfonate membrane, inserted in another perfluorosulfonate membrane tube, is colied into a helix. The helical assembly Is inserted wlthln an outer jacket packed with granular conductlve carbon. An alkaline eluent, e.g., NaOH or Na,CO,, flows in the annular channel between the two membranes and pure water flows through the Inner membrane and the outer jacket, countercurrent to the eluent flow. A dc voltage (typlcally 3-8 V) Is applied across the carbon bed and the platinum wire. Na’ in the eluent mlgrates to the cathode compartment resulting In water as the suppressed effluent and NaOH as the catholyte effluent. The dual membrane design prevents direct electrode contact with the eluent; bubble-Induced noise In the suppressed eluent due to any residual gas Is ellmlnated or mlnimlzed with a microporous gas-permeabie membrane tube or by applying sufflcient back pressure to the detector exit. Up to 500 pequiv of NaOH/mln can be quantitatively suppressed with a membrane length of 50 cm and a band dispersion of 106 pL (20-pL sample). With typical eluents, the system permits detectlon llmlts In the low-parts-per-blliion level for most common anIons.
The advent of ion chromatography (IC) irrevocably changed the way anionic analysis is performed (1). Today conductometric IC flourishes both in the chemically suppressed form as originally introduced and in a single column version (2). Research in this laboratory has largely centered on suppressed IC, in particular on the use of membrane devices (3-10). In suppressed IC, the introduction of membrane-based suppressors to replace packed-column devices was a major event (11), permitting continuous and temporally invariant performance. For hydrodynamically well designed membrane suppressors, the attainable exchange capacity depends on the rate of ion transport through the membrane (3, 12, 13). Further, quantitatively exchanging high eluent concentrations requires proportionately high regenerant concentrations. The transmembrane passage of an ion similarly charged to the matrix of the ion exchange membrane (the “forbidden” ion) is prevented solely by the Donnan potential. This barrier is hardly absolute; with the thin membranes used in present suppressors, undesirable penetration of the forbidden regenerant counterion (e.g., sulfate from a dilute H2S04regenerant) occurs significantly at practical regenerant concentrations (14). To minimize regenerant penetration, lower regenerant concentrations may be used at higher flow rates. However, such a practice consumes too much liquid with a proportional
0003-2700/89/0361-0939$01.50/00 1989 American Chemical Society
