Mixed Immunosorbent for Selective On-line Trace Enrichment and Liquid Chromatography of Phenylurea Herbicides in Environmental Waters A. Martin-Estebana, P. Fern�andeza, D. Stevensonb and C. C�amara*a a Departamento de Qu�ýmica Anal�ýtica, Facultad de Ciencias Qu�ýmicas, Universidad Complutense de Madrid, 28040 Madrid, Spain b Robens Institute, University of Surrey, Guildford, Surrey, UK GU2 5XH An immunosorbent containing antiisoproturon and antichlortoluron antibodies immobilised on aldehyde-activated silica was employed for the on-line preconcentration and liquid chromatography–diode array detection of several phenylureas.The efficiency of the coupling of the immunosorbent to the liquid chromatographic system and the characteristics of the immunosorbent were evaluated. The on-line system allowed the selective trace enrichment of chlortoluron, isoproturon, metobromuron, linuron and chlorbromuron at the 0.05–0.5 mg l21 level in ground and river waters and provided detection limits in the range 0.01–0.03 mg l21 by percolating only 10 ml of water sample.The proposed method was validated by analysing freeze-dried tap water samples with a high content in pesticides of different chemical functionalities. The results obtained were compared with the mean value obtained in an interlaboratory exercise. Keywords: Phenylurea herbicides; immunosorbent; liquid chromatography; environmental waters Solid-phase extraction (SPE) is a powerful tool for sample handling in the analysis of water samples for pesticides.Moreover, the application of on-line coupling of SPE to liquid chromatography (LC)1 has allowed the determination of a wide variety of pesticides, reaching the detection limits required for the quality control of drinking water (0.1 mg l21 in Europe for a single pesticide).2 However, the lack of selectivity of the sorbents typically used (C18, apolar copolymers) prevents similar detection limits being achieved in surface water owing to the broad peak obtained at the beginning of the chromatogram and to the noisy baseline, which shows the need to search for highly selective sorbents.Immunosorbents have been used for years in medicine3 but only recently for pesticide determinations. An immunosorbent consists of antibodies against a target compound (antigen), immobilised on an appropriate support material. Theoretically, it would permit the selective preconcentration of the antigen (or related compounds) when a sample is run through the immunosorbent and subsequently eluted free of co-extractives.Once the analytes have been eluted, they can be determined by chromatographic techniques. In recent years, antibodies against atrazine,4 simazine,5 isoproturon4,6 and chlortoluron5,7 have been immobilised on aldehyde-activated silica and antibodies against atrazine8,9 have been immobilised on diol-activated silica, and they have been successfully employed to preconcentrate these pesticides and also closely related compounds able to bind to the antibody from environmental waters.The use of immunosorbents for the preconcentration of triazines,10 phenylurea herbicides11 and carbofuran12 in plant material has also been reported. In a previous study,13 the complementary action of antiisoproturon and antichlortoluron antibodies for the preconcentration of various phenylurea herbicides from environmental waters was demonstrated.The antibodies were successfully immobilised on aldehyde-activated silica and the mixed immunosorbent allowed the selective preconcentration of several phenylureas, which were subsequently eluted in 1 ml of a simple phosphate-buffered saline–ethanol mixture at pH 2. Since the immunosorbent can be reused and is pressure resistant, the aim of this work was to develop an on-line system coupled to LC for the determination of several phenylurea herbicides in environmental waters.This work was focused on the efficiency of coupling of the immunosorbent to the LC system, and on the parameters affecting analyte binding and desorption. The analytes selected were chlortoluron, isoproturon, metobromuron, linuron and chlorbromuron. Experimental Apparatus Eluent delivery was provided by a ConstaMetric 4100 Series high pressure pump from Thermo Separation Products (Hemel Hempstead, Herts., UK) coupled with a SpectroMonitor 5000 photodiode-array detector from LDC Analytical (Riviera Beach, FL, USA).Stainless-steel precolumns (1 cm 3 4.6 mm id or 5 cm 3 4.6 mm id) were coupled to the loop of a Rheodyne (Cotati, CA, USA) Model 7725i injection valve in order to carry out the extraction and enrichment steps. The preconcentration pump was a Waters Model 590 from Millipore (Bedford, MA, USA). Reagents High-purity water from a Milli-Q system (Millipore) and RSgrade acetonitrile (Scharlau, Barcelona, Spain) were passed through a 0.45 mm nylon filter (Whatman, Maidstone, Kent, UK) before use.Chlortoluron, isoproturon, metobromuron, linuron, chlorbromuron and propanil were obtained from Riedel-de Ha�en (Hanover, Germany). Stock standard solutions (1000 mg l21) were prepared in acetonitrile and stored at 220 °C in the dark. Glacial acetic acid, disodium hydrogenorthophosphate, potassium dihydrogenorthophosphate, potassium chloride and sodium chloride were of analytical-reagent grade from Merck (Darmstadt, Germany) and Panreac (Barcelona, Spain).Phosphate-buffered saline (PBS) of pH 7.2–7.4 was prepared by adding 8.0 g of sodium chloride, 0.2 g of potassium chloride, 0.2 g of potassium dihydrogenorthophosphate and 2.9 g of disodium hydrogenorthophosphate to 1 l of Milli-Q-purified water. Preparation of the Mixed Immunosorbent Polyclonal antibodies against isoproturon and chlortoluron were raised in two different mature Suffolk sheep and collected Analyst, October 1997, Vol. 122 (1113–1117) 1113after a long period of immunization (66 weeks for antiisoproturon antibodies14 and 91 weeks for antichlortoluron antibodies15).The unpurified polyclonal antibodies against isoproturon and chlortoluron were separately immobilised on aldehyde-activated porous silica (particle size 90–130 mm, pore size 1000 Å) (Clifmar Associates, Guilford, UK) according to the following procedure. Disposable polypropylene separation columns (11.0 3 1.0 cm id; Lab M, No. D823), each provided with a polyethylene matrix support frit, were each packed with 0.5 g of aldehyde-activated silica and washed with 50 ml of PBS buffer.Next, 5 ml of PBS buffer were dispensed into each column followed by the addition of 100 ml of neat antiserum. The columns were then closed from both sides and left rolling on a rotamixer for 2 h at room temperature. Once again each column was washed with 10 ml of PBS buffer and 5 ml of 10 mm sodium cyanoborohydride (pH 6), made up with 1 m glycine buffer, was carefully added to the individual columns, which were then rotated overnight at room temperature. The next day, each column was washed with 10 ml of 0.3% HCl (pH 2) followed by 20 ml of PBS buffer.16 About 1 g of each immunosorbent was mixed to obtain an immunosorbent containing 2 g of dry solid phase and 200 ml of each antiserum.13 Stationary Phases and Columns The analytical column was a 25 cm 34.6 mm id column packed with Spherisorb 5 mm ODS2 from Symta (Madrid, Spain).The preconcentration step was carried out using a precolumn (1 cm 3 4.6 mm id) laboratory-packed with the styrene–divinylbenzene copolymer PRP-1 (Hamilton, Reno, NV, USA) or a precolumn (5 cm 3 4.6 mm id) filled manually with the mixed immunosorbent (approximately 0.35 g of dry material). Sample Preparation Ground and river water samples spiked with phenylureas at concentrations between 0.05 and 0.5 mg l21 were directly preconcentrated as explained below.Amounts of 0.5 g of freeze-dried tap water samples containing atrazine, simazine, carbaryl, propanil, linuron, fenamiphos and permethrin at concentration levels in the range 0.5–17.3 mg g21 were reconstituted by adding 0.5 l of 1023 mol l21 HCl according to the procedure recomended.17 Then 500 ml of the reconstituted water sample were diluted to 10 ml with PBS and preconcentrated on the immunosorbent or on ecolumns as explained below. Analytical Procedure Ground, river or reconstituted freeze-dried tap water samples (10 ml, pH 7) were filtered on a 0.45 mm nylon filter and then preconcentrated on the immunosorbent precolumn (also on the PRP-1 precolumn for the freeze-dried tap water samples) at a flow rate of 2 ml min21.After washing the precolumn with 10 ml of PBS, the valve was switched and the analytes were eluted and separated in the analytical column with the following elution gradient: from 65% A (0.005 mol l21 KH2PO4, pH 2 adjusted with glacial acetic acid) and 35% B (acetonitrile) to 25% A–75% B in 20 min and returning to the initial conditions in 10 min at a flow rate of 1 ml min21.The immunosorbent was conditioned before each analysis with 3 ml of PBS–ethanol (1 + 1) at pH 2 and 25 ml of PBS. The polymeric precolumn was conditioned with 10 ml of acetonitrile, 10 ml of water and finally with 10 ml of PBS. The immunosorbent was stored in PBS at 4 °C after use. Quantitative measurements of peak areas by LC–UV at 244 nm were carried out for all pesticides studied.Results and Discussion The pesticides selected in this study were chlortoluron, isoproturon, metobromuron, linuron and chlorbromuron, in order to cover an intermediate degree of polarity. Based on previous experiments,13 more polar analytes (e.g., methoxuron) were discarded because they were weakly retained and had very low breakthrough volumes, and more hydrophobic analytes (e.g., chloroxuron) were ruled out because they were retained by the organic matrix dissolved in the sample, hindering their interaction with the antibodies and giving rise to low recoveries.On-line Immunoextraction Coupled to Liquid Chromatography There are several problems caused by coupling the immunosorbent to the LC system. Previous experiments13 demonstrated that more than 50 ng per gram of silica for each phenylurea herbicide in a mixture of all of them produced overloading of the immunosorbent and competition among the analytes for the free binding sites.Also, the presence of a large amount of organic solvent (ethanol) prevents on-line elution of the analytes from the immunosorbent and their re-preconcentration in a second precolumn packed with C18 or apolar copolymers that is subsequently coupled on-line to the LC system.8,9,12 Pichon et al.18 reported the direct coupling of an immunosorbent with antiisoproturon antibodies immobilised on aldehyde- activated silica to an LC system by using a long precolumn (3 cm 3 4.6 mm id), the analytes being eluted with an acetonitrile gradient.Theoretically, the peaks on the chromatogram should be broader than those obtained by direct injection; however, no band broadening was detected despite the large size of the precolumn. This may be because the retention in the immunosorbent is based on antigen–antibody interactions, which are not comparable to the hydrophobic interactions in the C18 analytical columns. With this in mind and since our immunosorbent had a very low capacity, a long precolumn (5 cm 3 4.6 mm id) was coupled directly to the loop of the injection valve and after the preconcentration step the analytes were eluted with an acetonitrile –water gradient.Although no band broadening was detected, it was not possible to elute the retained analytes completely. Quantitative recoveries and no memory effects were only obtained when the pH the mobile phase was < 3 and the injection valve was kept in the inject position up to a proportion of acetonitrile of 50%.The chromatogram obtained at 244 nm after preconcentration of 10 ml of Milli-Q-purified water spiked with 0.25 mg l21 of each phenylurea (Fig. 1) shows that the direct coupling of a long precolumn to the LC system is successfully achieved. It should be noted that the downward slope of the baseline on the chromatogram is due to the use of acetic acid, which absorbs at 244 nm and the content of which decreases owing to the applied gradient. Capacity of the Immunosorbent The capacity of the immunosorbent depends on the total number of sites available to bind the antigen.The immunosorbent used in this study was prepared with unpurified polyclonal antibodies against isoproturon and chlortoluron and thus contained at least two different kinds of antibodies. This implies that the interaction (and therefore the capacity of the immunosorbent) of a given analyte with the immunosorbent will be different depending on whether it is alone or in a mixture of related compounds.To carry out this study, the immunosorbent was loaded with 10 ml of Milli-Q-purified water spiked with the herbicides to be tested at a concentration level within the range 0.025–12 mg l21 when each phenylurea was alone in the sample or within the 1114 Analyst, October 1997, Vol. 122range 0.025–4 mg l21 when they were all together in the mixture. Fig. 2 shows the binding curves obtained, which are plots of the amount of each herbicide loaded on the immunosorbent versus the concentration in the percolated sample when the herbicides were (A) alone or (B) in a mixture. The slopes obtained for each herbicide in the linear region [Fig. 2(A)] show that all the pesticides tested have the same affinity, while the capacity varies from 20 ng for isoproturon to > 50 ng for chlorbromuron. This may be because when unspecific antibodies are used, the differential recognition of similar compounds is based on molecular mass and/or hydrophobicity. 19 The binding curves for isoproturon and chlortoluron show two different linear regions, which may indicate the presence of different kinds of antibodies in the immunosorbent, as confirmed by the results in Fig. 2(B). It is clear that the binding curves for isoproturon and chlortoluron reach a plateau when the total amount of herbicides is high, which indicates the presence of specific antibodies against isoproturon and chlortoluron and it is possible to load around 5 ng for isoproturon and 10 ng for chlortoluron when they are in a mixture because the competition will take place among the other herbicides present in the sample.This is clearly illustrated by the shape of the metobromuron binding curve, which shows a decrease in the amount loaded when the concentration of herbicides is high and, as indicated above, recognition depends on the molecular mass and polarity and so the analytes compete for the free binding sites.In summary, the immunosorbent used in this study contained three different kinds of antibodies: one specific for isoproturon, another specific for chlortoluron and the third consisting of unspecific antibodies. The results of this study suggest that it should be possible to preconcentrate the phenylurea herbicides tested at concentrations < 0.5 mg l21 when 10 ml of sample are percolated. Although this capacity is very low, it should allow the determination of the selected analytes at the low levels present in real water samples, because of the high selectivity of the antigen–antibody interactions.Binding Flow Rate The binding flow rate was evaluated within the range 0.5–3 ml min21 by preconcentration of 10 ml of Milli-Q-purified water spiked with herbicides at 0.25 mg l21. The only effect detected was a decrease of 50% in the efficiency of preconcentration for metobromuron when the sample flow rate was increased from 2 to 3 ml min21.The recoveries of the other phenylureas were not affected by increasing the sample flow rate. In order to obtain quantitative recoveries for all the herbicides studied, a flow rate of 2 ml min21 was chosen as the optimum. Breakthrough Volumes The effect of sample volume was studied at a constant mass of 2.5 ng of each phenylurea using 10, 25 and 50 ml of Milli-Qpurified water fortified with 0.25, 0.1 and 0.05 mg l21 of each analyte, respectively. The results obtained show that the sample volume (within the range studied) did not affect the preconcentration of the selected pesticides, except for metobromuron, for which a 20% decrease in preconcentration efficiency was observed when the sample volume was 50 ml.Selectivity: Application to Different Matrices The efficiency of the immunosorbent for the selective preconcentration of chlortoluron, isoproturon, metobromuron, linuron and chlorbromuron from ground, river and freeze-dried tap water samples was evaluated.Fig. 3 shows the chromatograms obtained from on-line preconcentration on the immunosorbent of 10 ml of (A) ground water and (B) river water spiked at the 0.05 mg l21 level with each phenylurea. It clearly demonstrates the high degree of clean-up obtained with the proposed method, since the baselines in the two chromatograms are comparable and as clean as that obtained by preconcentrating Milli-Q-purified water (see Fig. 1). This should allow calibration graphs to be constructed for all analytes in Milli-Q-purified water and to be used for any kind of water. Freeze-dried tap water sample containing pesticides with different chemical functionalities [atrazine and simazine (tria- Fig. 1 LC–UV trace obtained at 244 nm after on-line immunoextraction of 10 ml of Milli-Q-purified water spiked with 0.25 mg l21 of each phenylurea herbicide. Peaks: 1 = chlortoluron, 2 = isoproturon; 3 = metobromuron; 4 = linuron and 5 = chlorbromuron.The asterisk indicates an impurity arising from the synthesis of the immunosorbent. For LC conditions, see Experimental. Fig. 2 Binding curves for chlortoluron (1), isoproturon (2), metobromuron (3), linuron (4) and chlorbromuron (5) after on-line immunoextraction of 10 ml of Milli-Q-purified water sample when the compounds are alone (A) or mixed (B). See text for details. Analyst, October 1997, Vol. 122 1115zines), carbaryl (carbamate), propanil (propioanilide), linuron (phenylurea), fenamiphos (organophosphorus) and permethrin (pyrethroid)] at concentrations within the range 0.5–17.3 mg g21 was also preconcentrated in order to demonstrate the high selectivity of the immunosorbent.Fig. 4 shows the chromatograms obtained after on-line preconcentration of 10 ml of diluted reconstituted tap water sample (see Experimental), (A) on a precolumn packed with the apolar copolymer PRP-1 and (B) on the immunosorbent.The chromatogram is cleaner after preconcentration on the immunosorbent than on the PRP-1 precolumn, because the antibodies immobilised are only able to retain phenylurea herbicides. However, in Fig. 4(B), two peaks are observed, one at the retention time of linuron and another which was identified as propanil by constructing its specific calibration graph in Milli-Q-purified water using the procedure described. Although propanil belongs to another family of pesticides (propioanilides), there is a clear similarity between the structures of propanil and linuron, as shown in Fig. 4(B), and the antibodies are not able to differentiate between compounds with this high degree of structural similarity. This would make it possible to increase the number of compounds that the immunosorbent is able to retain, such as anilines, which are transformation products from the phenylurea herbicides and some of which are included in the list of priority pollutants to be monitored in environmental waters in Europe.This interesting possibility is now under study. Analytical Performance and Method Validation Calibration graphs for ground and river water were constructed by applying the on-line immunoextraction procedure to 10 ml of spiked water within the range 0.05–0.5 mg l21 and the resulting correlation coefficients were satisfactory (r2 > 0.99). The RSD for the selected analytes at a concentration of 0.25 mg l21 using the full procedure and evaluating the peak areas was in the range 1–9% (n = 5), depending on the pesticide.The detection limits, which were calculated as three times the standard deviation of the lowest concentration solutions, were within the range 0.01–0.03 mg l21, depending on the phenylurea herbicide, although they could be lowered further by preconcentrating a larger sample volume. These low detection limits obtained with only 10 ml of sample are a remarkable result and are good enough to allow the fate and transport of phenylureas to be studied directly in environmental waters.Calibration graphs were also constructed for propanil and linuron within the range 0.05–0.5 mg l21 by the on-line immunoextraction of 10 ml of Milli-Q-purified water and were used for the determination of propanil and linuron in the freezedried tap water samples. The results (Table 1) were in good agreement with the mean values obtained in an interlaboratory exercise,17 in which only validated methods were used. Reusability of the Immunosorbent The immunosorbent used in this study was employed in the analysis of more than 50 samples, including dirty water samples such as river water, and no decrease in the capacity of the immunosorbent was detected.This long lifetime is due to the good stability of the antibodies immobilised on aldehyde- Fig. 3 LC–UV traces obtained at 244 nm after on-line immunoextraction of 10 ml of ground (A) and river (B) water spiked with 0.05 mg l21 of each phenylurea herbicide.Peak numbers as in Fig. 1. For LC conditions, see Experimental. Fig. 4 LC–UV traces obtained at 244 nm after on-line preconcentration of 10 ml of freeze-dried reconstituted tap water sample on a PRP-1 precolumn (A) and on the immunosorbent (B). For LC conditions, see Experimental. Table 1 Mean values ± standard deviations (mg g21) obtained in the determination of propanil and linuron in freeze-dried tap water samples Interlaboratory On-line Compound exercise* immunoextraction† Propanil 11.70 ± 1.31 11.12 ± 0.24 Linuron 5.47 ± 0.90 5.66 ± 0.36 * Data obtained from ref. 17. † Mean value of three independent determinations. 1116 Analyst, October 1997, Vol. 122activated silica and because the regeneration (25 ml of PBS) and storage were appropriate for the period tested. Conclusion The use of a precolumn containing a mixed immunosorbent with immobilised antiisoproturon and antichlortoluron antibodies allows the simultaneous on-line immunopreconcentration of several phenylurea herbicides from ground and river water.The high selectivity shown by the immunosorbent allows the determination of phenylurea herbicides at trace levels when only 10 ml of water sample are percolated. Moreover, the selectivity of the immunosorbent was clearly demonstrated by analysing freeze-dried tap water samples spiked with large amounts of pesticides with different chemical functionalities, and the immunosorbent only retained linuron (phenylurea herbicide) and propanil (propioanilide), which show a high degree of structural similarity.Although this immunosorbent is not specific for isoproturon and chlortoluron, it offers the advantage of recognizing the phenylurea family of herbicides and very closely related compounds, which may be of great interest for environmental analysis. This work represents yet another example of the great potential shown by the immunosorbents used to date for the preconcentration of pesticides and further development in this area should be undertaken.This work received financial support from the Standards, Measurements and Testing Programme under contract number MAT1-CT940001 and PB95-0366-C02-01. The authors thank Max Gorman for revision of the manuscript. References 1 Barcel�o, D., and Hennion, M. C., Anal. Chim. Acta, 1995, 318, 1. 2 EC Directive relating quality of water intended for human consumption (80/778/EC), Off.J. Eur. Communities, L229/11, 1980. 3 van Ginkel, L. A., Stephany, R. W., van Rossum, H. J., and Zoontjes, P. W., Trends Anal. Chem., 1992, 11, 294. 4 Pichon, V., Chen, L., Hennion, M. C., Daniel, R., Martel, A., Le Goffic, F., Abian, J., and Barcel�o, D., Anal. Chem., 1995, 67, 2451. 5 Pichon, V., Chen, L., Durand, N., Le Goffic, F., and Hennion, M. C., J. Chromatogr. A, 1996, 725, 107. 6 Shahtaheri, S. J., Kwasowski, P., and Stevenson, D., J. Chromatogr.A, submitted for publication. 7 Shahtaheri, S. J., Katmeh, M .F., Kwasowski, P., and Stevenson, D., J. Chromatogr. A, 1995, 697, 131. 8 Thomas, D. H., Beck-Westermeyer, M., and Hage, D. S., Anal. Chem., 1994, 66, 3823. 9 Rollag, J. G., Beck-Westermeyer, M., and Hage, D. S., Anal. Chem., 1996, 68, 3631. 10 Lawrence, J. Fenard, C., Hennion, M. C., Pichon, V., Le Goffic, F., and Durand, N., J. Chromatogr. A, 1996, 752, 147. 11 Lawrence, J. F., Menard, C., Hennion, M.C., Pichon, V., Le Goffic, F., and Durand, N., J. Chromatogr. A, 1996, 732, 277. 12 Rule, G. S., Mordehai, A. V., and Henion, J., Anal. Chem., 1994, 66, 230. 13 Mart�ýn-Esteban, A., Kwasowski, P., and Stevenson, D., Chromatographia, 1997, 45, 364. 14 Katmeh, M. F., Frost, G., Aherne, W., and Stevenson, D., Analyst, 1994, 119, 431. 15 Katmeh, M. F., Aherne, W., and Stevenson, D., Analyst, 1996, 121, 1699. 16 Katmeh, M. F., Thesis, Robens Institute, University of Surrey, Guidford, 1994. 17 Mart�ýn-Esteban, A., Fern�andez, P., C�amara, C., Kramer, G. N., and Maier, E. A., Int. J. Environ. Anal. Chem., in the press. 18 Pichon, V., Chen, L., and Hennion, M. C., Anal. Chim. Acta, 1995, 311, 429. 19 Price, C. P., and Newman, D. J., Principles and Practice of Immunoassay, Macmillan, London, 1991. Paper 7/02828H Received April 25, 1997 Accepted July 22, 1997 Analyst, October 1997, Vol. 122 1117 Mixed Immunosorbent for Selective On-line Trace Enrichment and Liquid Chromatography of Phenylurea Herbicides in Environmental Waters A.Martin-Estebana, P. Fern�andeza, D. Stevensonb and C. C�amara*a a Departamento de Qu�ýmica Anal�ýtica, Facultad de Ciencias Qu�ýmicas, Universidad Complutense de Madrid, 28040 Madrid, Spain b Robens Institute, University of Surrey, Guildford, Surrey, UK GU2 5XH An immunosorbent containing antiisoproturon and antichlortoluron antibodies immobilised on aldehyde-activated silica was employed for the on-line preconcentration and liquid chromatography–diode array detection of several phenylureas.The efficiency of the coupling of the immunosorbent to the liquid chromatographic system and the characteristics of the immunosorbent were evaluated. The on-line system allowed the selective trace enrichment of chlortoluron, isoproturon, metobromuron, linuron and chlorbromuron at the 0.05–0.5 mg l21 level in ground and river waters and provided detection limits in the range 0.01–0.03 mg l21 by percolating only 10 ml of water sample.The proposed method was validated by analysing freeze-dried tap water samples with a high content in pesticides of different chemical functionalities. The results obtained were compared with the mean value obtained in an interlaboratory exercise. Keywords: Phenylurea herbicides; immunosorbent; liquid chromatography; environmental waters Solid-phase extraction (SPE) is a powerful tool for sample handling in the analysis of water samples for pesticides. Moreover, the application of on-line coupling of SPE to liquid chromatography (LC)1 has allowed the determination of a wide variety of pesticides, reaching the detection limits required for the quality control of drinking water (0.1 mg l21 in Europe for a single pesticide).2 However, the lack of selectivity of the sorbents typically used (C18, apolar copolymers) prevents similar detection limits being achieved in surface water owing to the broad peak obtained at the beginning of the chromatogram and to the noisy baseline, which shows the need to search for highly selective sorbents.Immunosorbents have been used for years in medicine3 but only recently for pesticide determinations. An immunosorbent consists of antibodies against a target compound (antigen), immobilised on an appropriate support material. Theoretically, it would permit the selective preconcentration of the antigen (or related compounds) when a sample is run through the immunosorbent and subsequently eluted free of co-extractives. Once the analytes have been eluted, they can be determined by chromatographic techniques.In recent years, antibodies against atrazine,4 simazine,5 isoproturon4,6 and chlortoluron5,7 have been immobilised on aldehyde-activated silica and antibodies against atrazine8,9 have been immobilised on diol-activated silica, and they have been successfully employed to preconcentrate these pesticides and also closely related compounds able to bind to the antibody from environmental waters.The use of immunosorbents for the preconcentration of triazines,10 phenylurea herbicides11 and carbofuran12 in plant material has also been reported. In a previous study,13 the complementary action of antiisoproturon and antichlortoluron antibodies for the preconcentration of various phenylurea herbicides from environmental waters was demonstrated. The antibodies were successfully immobilised on aldehyde-activated silica and the mixed immunosorbent allowed the selective preconcentration of several phenylureas, which were subsequently eluted in 1 ml of a simple phosphate-buffered saline–ethanol mixture at pH 2.Since the immunosorbent can be reused and is pressure resistant, the aim of this work was to develop an on-line system coupled to LC for the determination of several phenylurea herbicides in environmental waters. This work was focused on the efficiency of coupling of the immunosorbent to the LC system, and on the parameters affecting analyte binding and desorption.The analytes selected were chlortoluron, isoproturon, metobromuron, linuron and chlorbromuron. Experimental Apparatus Eluent delivery was provided by a ConstaMetric 4100 Series high pressure pump from Thermo Separation Products (Hemel Hempstead, Herts., UK) coupled with a SpectroMonitor 5000 photodiode-array detector from LDC Analytical (Riviera Beach, FL, USA). Stainless-steel precolumns (1 cm 3 4.6 mm id or 5 cm 3 4.6 mm id) were coupled to the loop of a Rheodyne (Cotati, CA, USA) Model 7725i injection valve in order to carry out the extraction and enrichment steps.The preconcentration pump was a Waters Model 590 from Millipore (Bedford, MA, USA). Reagents High-purity water from a Milli-Q system (Millipore) and RSgrade acetonitrile (Scharlau, Barcelona, Spain) were passed through a 0.45 mm nylon filter (Whatman, Maidstone, Kent, UK) before use.Chlortoluron, isoproturon, metobromuron, linuron, chlorbromuron and propanil were obtained from Riedel-de Ha�en (Hanover, Germany). Stock standard solutions (1000 mg l21) were prepared in acetonitrile and stored at 220 °C in the dark. Glacial acetic acid, disodium hydrogenorthophosphate, potassium dihydrogenorthophosphate, potassium chloride and sodium chloride were of analytical-reagent grade from Merck (Darmstadt, Germany) and Panreac (Barcelona, Spain). Phosphate-buffered saline (PBS) of pH 7.2–7.4 was prepared by adding 8.0 g of sodium chloride, 0.2 g of potassium chloride, 0.2 g of potassium dihydrogenorthophosphate and 2.9 g of disodium hydrogenorthophosphate to 1 l of Milli-Q-purified water.Preparation of the Mixed Immunosorbent Polyclonal antibodies against isoproturon and chlortoluron were raised in two different mature Suffolk sheep and collected Analyst, October 1997, Vol. 122 (1113–1117) 1113after a long period of immunization (66 weeks for antiisoproturon antibodies14 and 91 weeks for antichlortoluron antibodies15).The unpurified polyclonal antibodies against isoproturon and chlortoluron were separately immobilised on aldehyde-activated porous silica (particle size 90–130 mm, pore size 1000 Å) (Clifmar Associates, Guilford, UK) according to the following procedure. Disposable polypropylene separation columns (11.0 3 1.0 cm id; Lab M, No. D823), each provided with a polyethylene matrix support frit, were each packed with 0.5 g of aldehyde-activated silica and washed with 50 ml of PBS buffer.Next, 5 ml of PBS buffer were dispensed into each column followed by the addition of 100 ml of neat antiserum. The columns were then closed from both sides and left rolling on a rotamixer for 2 h at room temperature. Once again each column was washed with 10 ml of PBS buffer and 5 ml of 10 mm sodium cyanoborohydride (pH 6), made up with 1 m glycine buffer, was carefully added to the individual columns, which were then rotated overnight at room temperature.The next day, each column was washed with 10 ml of 0.3% HCl (pH 2) followed by 20 ml of PBS buffer.16 About 1 g of each immunosorbent was mixed to obtain an immunosorbent containing 2 g of d00 ml of each antiserum.13 Stationary Phases and Columns The analytical column was a 25 cm 34.6 mm id column packed with Spherisorb 5 mm ODS2 from Symta (Madrid, Spain). The preconcentration step was carried out using a precolumn (1 cm 3 4.6 mm id) laboratory-packed with the styrene–divinylbenzene copolymer PRP-1 (Hamilton, Reno, NV, USA) or a precolumn (5 cm 3 4.6 mm id) filled manually with the mixed immunosorbent (approximately 0.35 g of dry material). Sample Preparation Ground and river water samples spiked with phenylureas at concentrations between 0.05 and 0.5 mg l21 were directly preconcentrated as explained below.Amounts of 0.5 g of freeze-dried tap water samples containing atrazine, simazine, carbaryl, propanil, linuron, fenamiphos and permethrin at concentration levels in the range 0.5–17.3 mg g21 were reconstituted by adding 0.5 l of 1023 mol l21 HCl according to the procedure recomended.17 Then 500 ml of the reconstituted water sample were diluted to 10 ml with PBS and preconcentrated on the immunosorbent or on the PRP-1 precolumns as explained below. Analytical Procedure Ground, river or reconstituted freeze-dried tap water samples (10 ml, pH 7) were filtered on a 0.45 mm nylon filter and then preconcentrated on the immunosorbent precolumn (also on the PRP-1 precolumn for the freeze-dried tap water samples) at a flow rate of 2 ml min21.After washing the precolumn with 10 ml of PBS, the valve was switched and the analytes were eluted and separated in the analytical column with the following elution gradient: from 65% A (0.005 mol l21 KH2PO4, pH 2 adjusted with glacial acetic acid) and 35% B (acetonitrile) to 25% A–75% B in 20 min and returning to the initial conditions in 10 min at a flow rate of 1 ml min21.The immunosorbent was conditioned before each analysis with 3 ml of PBS–ethanol (1 + 1) at pH 2 and 25 ml of PBS. The polymeric precolumn was conditioned with 10 ml of acetonitrile, 10 ml of water and finally with 10 ml of PBS. The immunosorbent was stored in PBS at 4 °C after use. Quantitative measurements of peak areas by LC–UV at 244 nm were carried out for all pesticides studied.Results and Discussion The pesticides selected in this study were chlortoluron, isoproturon, metobromuron, linuron and chlorbromuron, in order to cover an intermediate degree of polarity. Based on previous experiments,13 more polar analytes (e.g., methoxuron) were discarded because they were weakly retained and had very low breakthrough volumes, and more hydrophobic analytes (e.g., chloroxuron) were ruled out because they were retained by the organic matrix dissolved in the sample, hindering their interaction with the antibodies and giving rise to low recoveries.On-line Immunoextraction Coupled to Liquid Chromatography There are several problems caused by coupling the immunosorbent to the LC system. Previous experiments13 demonstrated that more than 50 ng per gram of silica for each phenylurea herbicide in a mixture of all of them produced overloading of the immunosorbent and competition among the analytes for the free binding sites.Also, the presence of a large amount of organic solvent (ethanol) prevents on-line elution of the analytes from the immunosorbent and their re-preconcentration in a second precolumn packed with C18 or apolar copolymers that is subsequently coupled on-line to the LC system.8,9,12 Pichon et al.18 reported the direct coupling of an immunosorbent with antiisoproturon antibodies immobilised on aldehyde- activated silica to an LC system by using a long precolumn (3 cm 3 4.6 mm id), the analytes being eluted with an acetonitrile gradient.Theoretically, the peaks on the chromatogram should be broader than those obtained by direct injection; however, no band broadening was detected despite the large size of the precolumn. This may be because the retention in the immunosorbent is based on antigen–antibody interactions, which are not comparable to the hydrophobic interactions in the C18 analytical columns. With this in mind and since our immunosorbent had a very low capacity, a long precolumn (5 cm 3 4.6 mm id) was coupled directly to the loop of the injection valve and after the preconcentration step the analytes were eluted with an acetonitrile –water gradient.Although no band broadening was detected, it was not possible to elute the retained analytes completely. Quantitative recoveries and no memory effects were only obtained when the pH the mobile phase was < 3 and the injection valve was kept in the inject position up to a proportion of acetonitrile of 50%.The chromatogram obtained at 244 nm after preconcentration of 10 ml of Milli-Q-purified water spiked with 0.25 mg l21 of each phenylurea (Fig. 1) shows that the direct coupling of a long precolumn to the LC system is successfully achieved. It should be noted that the downward slope of the baseline on the chromatogram is due to the use of acetic acid, which absorbs at 244 nm and the content of which decreases owing to the applied gradient.Capacity of the Immunosorbent The capacity of the immunosorbent depends on the total number of sites available to bind the antigen. The immunosorbent used in this study was prepared with unpurified polyclonal antibodies against isoproturon and chlortoluron and thus contained at least two different kinds of antibodies. This implies that the interaction (and therefore the capacity of the immunosorbent) of a given analyte with the immunosorbent will be different depending on whether it is alone or in a mixture of related compounds.To carry out this study, the immunosorbent was loaded with 10 ml of Milli-Q-purified water spiked with the herbicides to be tested at a concentration level within the range 0.025–12 mg l21 when each phenylurea was alone in the sample or within the 1114 Analyst, October 1997, Vol. 122range 0.025–4 mg l21 when they were all together in the mixture. Fig. 2 shows the binding curves obtained, which are plots of the amount of each herbicide loaded on the immunosorbent versus the concentration in the percolated sample when the herbicides were (A) alone or (B) in a mixture.The slopes obtained for each herbicide in the linear region [Fig. 2(A)] show that all the pesticides tested have the same affinity, while the capacity varies from 20 ng for isoproturon to > 50 ng for chlorbromuron. This may be because when unspecific antibodies are used, the differential recognition of similar compounds is based on molecular mass and/or hydrophobicity. 19 The binding curves for isoproturon and chlortoluron show two different linear regions, which may indicate the presence of different kinds of antibodies in the immunosorbent, as confirmed by the results in Fig. 2(B). It is clear that the binding curves for isoproturon and chlortoluron reach a plateau when the total amount of herbicides is high, which indicates the presence of specific antibodies against isoproturon and chlortoluron and it is possible to load around 5 ng for isoproturon and 10 ng for chlortoluron when they are in a mixture because the competition will take place among the other herbicides present in the sample.This is clearly illustrated by the shape of the metobromuron binding curve, which shows a decrease in the amount loaded when the concentration of herbicides is high and, as indicated above, recognition depends on the molecular mass and polarity and so the analytes compete for the free binding sites.In summary, the immunosorbent used in this study contained three different kinds of antibodies: one specific for isoproturon, another specific for chlortoluron and the third consisting of unspecific antibodies. The results of this study suggest that it should be possible to preconcentrate the phenylurea herbicides tested at concentrations < 0.5 mg l21 when 10 ml of sample are percolated. Although this capacity is very low, it should allow the determination of the selected analytes at the low levels present in real water samples, because of the high selectivity of the antigen–antibody interactions.Binding Flow Rate The binding flow rate was evaluated within the range 0.5–3 ml min21 by preconcentration of 10 ml of Milli-Q-purified water spiked with herbicides at 0.25 mg l21. The only effect detected was a decrease of 50% in the efficiency of preconcentration for metobromuron when the sample flow rate was increased from 2 to 3 ml min21.The recoveries of the other phenylureas were not affected by increasing the sample flow rate. In order to obtain quantitative recoveries for all the herbicides studied, a flow rate of 2 ml min21 was chosen as the optimum. Breakthrough Volumes The effect of sample volume was studied at a constant mass of 2.5 ng of each phenylurea using 10, 25 and 50 ml of Milli-Qpurified water fortified with 0.25, 0.1 and 0.05 mg l21 of each analyte, respectively. The results obtained show that the sample volume (within the range studied) did not affect the preconcentration of the selected pesticides, except for metobromuron, for which a 20% decrease in preconcentration efficiency was observed when the sample volume was 50 ml. Selectivity: Application to Different Matrices The efficiency of the immunosorbent for the selective preconcentration of chlortoluron, isoproturon, metobromuron, linuron and chlorbromuron from ground, river and freeze-dried tap water samples was evaluated.Fig. 3 shows the chromatograms obtained from on-line preconcentration on the immunosorbent of 10 ml of (A) ground water and (B) river water spiked at the 0.05 mg l21 level with each phenylurea. It clearly demonstrates the high degree of clean-up obtained with the proposed method, since the baselines in the two chromatograms are comparable and as clean as that obtained by preconcentrating Milli-Q-purified water (see Fig. 1). This should allow calibration graphs to be constructed for all analytes in Milli-Q-purified water and to be used for any kind of water.Freeze-dried tap water sample containing pesticides with different chemical functionalities [atrazine and simazine (tria- Fig. 1 LC–UV trace obtained at 244 nm after on-line immunoextraction of 10 ml of Milli-Q-purified water spiked with 0.25 mg l21 of each phenylurea herbicide. Peaks: 1 = chlortoluron, 2 = isoproturon; 3 = metobromuron; 4 = linuron and 5 = chlorbromuron.The asterisk indicates an impurity arising from the synthesis of the immunosorbent. For LC conditions, see Experimental. Fig. 2 Binding curves for chlortoluron (1), isoproturon (2), metobromuron (3), linuron (4) and chlorbromuron (5) after on-line immunoextraction of 10 ml of Milli-Q-purified water sample when the compounds are alone (A) or mixed (B). See text for details. Analyst, October 1997, Vol. 122 1115zines), carbaryl (carbamate), propanil (propioanilide), linuron (phenylurea), fenamiphos (organophosphorus) and permethrin (pyrethroid)] at concentrations within the range 0.5–17.3 mg g21 was also preconcentrated in order to demonstrate the high selectivity of the immunosorbent.Fig. 4 shows the chromatograms obtained after on-line preconcentration of 10 ml of diluted reconstituted tap water sample (see Experimental), (A) on a precolumn packed with the apolar copolymer PRP-1 and (B) on the immunosorbent. The chromatogram is cleaner after preconcentration on the immunosorbent than on the PRP-1 precolumn, because the antibodies immobilised are only able to retain phenylurea herbicides.However, in Fig. 4(B), two peaks are observed, one at the retention time of linuron and another which was identified as propanil by constructing its specific calibration graph in Milli-Q-purified water using the procedure described. Although propanil belongs to another family of pesticides (propioanilides), there is a clear similarity between the structures of propanil and linuron, as shown in Fig. 4(B), and the antibodies are not able to differentiate between compounds with this high degree of structural similarity. This would make it possible to increase the number of compounds that the immunosorbent is able to retain, such as anilines, which are transformation products from the phenylurea herbicides and some of which are included in the list of priority pollutants to be monitored in environmental waters in Europe.This interesting possibility is now under study. Analytical Performance and Method Validation Calibration graphs for ground and river water were constructed by applying the on-line immunoextraction procedure to 10 ml of spiked water within the range 0.05–0.5 mg l21 and the resulting correlation coefficients were satisfactory (r2 > 0.99). The RSD for the selected analytes at a concentration of 0.25 mg l21 using the full procedure and evaluating the peak areas was in the range 1–9% (n = 5), depending on the pesticide.The detection limits, which were calculated as three times the standard deviation of the lowest concentration solutions, were within the range 0.01–0.03 mg l21, depending on the phenylurea herbicide, although they could be lowered further by preconcentrating a larger sample volume. These low detection limits obtained with only 10 ml of sample are a remarkable result and are good enough to allow the fate and transport of phenylureas to be studied directly in environmental waters.Calibration graphs were also constructed for propanil and linuron within the range 0.05–0.5 mg l21 by the on-line immunoextraction of 10 ml of Milli-Q-purified water and were used for the determination of propanil and linuron in the freezedried tap water samples. The results (Table 1) were in good agreement with the mean values obtained in an interlaboratory exercise,17 in which only validated methods were used.Reusability of the Immunosorbent The immunosorbent used in this study was employed in the analysis of more than 50 samples, including dirty water samples such as river water, and no decrease in the capacity of the immunosorbent was detected. This long lifetime is due to the good stability of the antibodies immobilised on aldehyde- Fig. 3 LC–UV traces obtained at 244 nm after on-line immunoextraction of 10 ml of ground (A) and river (B) water spiked with 0.05 mg l21 of each phenylurea herbicide.Peak numbers as in Fig. 1. For LC conditions, see Experimental. Fig. 4 LC–UV traces obtained at 244 nm after on-line preconcentration of 10 ml of freeze-dried reconstituted tap water sample on a PRP-1 precolumn (A) and on the immunosorbent (B). For LC conditions, see Experimental. Table 1 Mean values ± standard deviations (mg g21) obtained in the determination of propanil and linuron in freeze-dried tap water samples Interlaboratory On-line Compound exercise* immunoextraction† Propanil 11.70 ± 1.31 11.12 ± 0.24 Linuron 5.47 ± 0.90 5.66 ± 0.36 * Data obtained from ref. 17. † Mean value of three independent determinations. 1116 Analyst, October 1997, Vol. 122activated silica and because the regeneration (25 ml of PBS) and storage were appropriate for the period tested. Conclusion The use of a precolumn containing a mixed immunosorbent with immobilised antiisoproturon and antichlortoluron antibodies allows the simultaneous on-line immunopreconcentration of several phenylurea herbicides from ground and river water.The high selectivity shown by the immunosorbent allows the determination of phenylurea herbicides at trace levels when only 10 ml of water sample are percolated. Moreover, the selectivity of the immunosorbent was clearly demonstrated by analysing freeze-dried tap water samples spiked with large amounts of pesticides with different chemical functionalities, and the immunosorbent only retained linuron (phenylurea herbicide) and propanil (propioanilide), which show a high degree of structural similarity.Although this immunosorbent is not specific for isoproturon and chlortoluron, it offers the advantage of recognizing the phenylurea family of herbicides and very closely related compounds, which may be of great interest for environmental analysis. This work represents yet another example of the great potential shown by the immunosorbents used to date for the preconcentration of pesticides and further development in this area should be undertaken. This work received financial support from the Standards, Measurements and Testing Programme under contract number MAT1-CT940001 and PB95-0366-C02-01. 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Paper 7/02828H Received April 25, 1997 Accepted July 22, 1997 Analyst, October 1997, V