Determination of acidic chlorinated pesticides in water - Springer Link

Determination of Acidic Chlorinated Pesticides in Water: Comparison of Official EPA Method 515.1 and Liquid-Solid Extraction HPLC/UV and HPLC/PBMS Analyses F. B r u n e r t / A. Berloni / P. Palma* Istituto di Scienze Chimiche, Universit~ di Urbino, Piazza Rinascimento, 6, 61029 Urbino, Italy

Key Words Gas chromatography Column liquid chromatography ECD, UV and MS detection EPA Method 515.1 Chlorinated acidic pesticides

Summary A comparison of the official EPA method 515.1 for determination of chlorinated acidic pesticides and a modification of it is illustrated. Extraction of the analytes from water and their determination and quantitation is by gas chromatography-electron capture detection (GC-ECD), liquid chromatography-UV detection and liquid chromatography-particle beam mass Spectrometry. Although HPLC-PBMS was found to be less sensitive than the GC-ECD method, it was, nevertheless, more sensitive than HPLC-UV. The modified tnethod is simpler, quicker and allows more accurate determination of pesticides in aqueous samples.

Introduction Monitoring the presence of pesticides in environmental Samples at very low concentration is becoming more and more important, due to their widespread use in agriculture. Their determination in aqueous samples requires the use of very accurate and sensitive analytical tools in order to ensure valid environmental control. Since the majority of modern pesticides are hydrophilic, their extraction from water samples is a crucial step, XVhichcould lead to significant loss of analyte. t Deceased July 31, 1996

Original 0009-5893/96/09 279-05 $ 03.00/0

In the official EPA method 515.1 for the determination of chlorinated acidic pesticides in water, analytes are extracted from the aqueous matrix by liquid-liquid extraction, converted to their methyl esters using diazomethane and determined by gas chromatographyelectron capture detection (GC-ECD) [1]. Liquid-liquid extraction (LLE) is no longer considered the best approach by many analysts since liquid-solid extraction (LSE) became a well established technique for extraction and preconcentration of organic compounds from aqueous matrices [2-4]. The introduction of a wide choice of sorbents by many manufacturers, the need for very small volumes of solvents and containment of costs favour LSE compared to LLE. Derivatization with diazomethane, in addition to being a cumbersome and a time-consuming procedure, introduces further sample loss, as demonstrated by the high relative standard deviation expected following EPA method 515.1. New developments in monitoring pesticides in water have been thoroughly investigated by Di Corcia et al. who elaborated a sensitive method based on LSE extraction and HPLC-UV and HPLC-ESMS quantitation [5-8]. In their experiments they exploited the possibility of extracting various classes of pesticides from aqueous samples using graphitized carbon black and the recoveries were compared with those obtained using other kinds of adsorbent. This approach is very simple and sensitive and allows direct HPLC analysis of the extract without further derivatization. In this work we propose a modification of the EPA method 515.1 in which the LLE step is substituted by LSE, using cartridges filled with Carbograph 1, a graphitized carbon black (GCB) [9-11]. This adsorbent has been extensively used for solid phase extraction of various classes of compound from water (see reference [3] and references therein). It is non-porous, non-polar with a homogeneous specific surface of about 100 m 2 g-1.

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TableI. Selectedpesticidesand SIMAcquisitionparameters, Pesticide

Class

CAS reg. Nr.

m/z

Acifluorfen Bentazon Chloramben 2,4-D 2,4-DB 3,5 Dichlorobenzoicacid Dicamba 4-Nitrophenol Dichlorprop Dinoseb Pentachlorophenol Picloram 2,4,5-T 2,4,5-TP

diphenylether thiadiazinone benzoic acid phenoxyacid phenoxyacid benzoic acid methoxybenzoicacid phenol phenoxyacid phenol phenol pyridinecarboxylica c i d phenoxyacid phenoxyacid

50594-66-6 25057-89-0 133-90-4 94-75-7 94-82--6 51-35-5 1918-00-9 100-02-7 120-36-5 88-85-7 87-86-5 1918-02-1 93-76-5 93-72-1

361 198 205,207, 163 162, 164 162, 164, 198 173, 190,192 173,220 65,109, 139 162, 164,234 163 266 196 196, 198 162, 196, 198

Some of the analytes extracted were derivatized with diazomethane followed by GC-ECD analysis, some were determined by HPLC-MS with a modified particle beam interface for more accurate, rapid and reproducible results. Our research group has developed a method for the analysis of acidic pesticides based on the use of a modified interface that allows maximization of signal response and reliability of results [17-18]. The use of a micronebulizer permits ~tL min -1 mobile phase flowrates. The scaled down solvent intake allows improved sensitivity especially with mobile phases of high water content, such as those used in reversed-phase liquid chromatography. In addition, the particle beam target zone inside the ion source has been coated with a Teflon FEP layer. In fact, it is well known that when acidic pesticides enter the ion source, they undergo decomposition due to the high temperature of the ionization chamber (> 200 ~ This inconveniently causes a reduction in sensitivity and an unwanted memory effect. Coating the ion source reduces adsorption and decomposition and enhances the gas phase conversion of analytes. The differences between GC-ECD and HPLC-MS methods are illustrated and discussed.

Experimental Reagents The pesticides considered here are listed in Table I. Some of them were from Riedel-De Ha6n (Hannover, Germany), while others were kindly supplied by Alltech (Alltech Associates Inc. Deerfield, IL, USA). The Dialzald kit and all the reagents needed for the preparation of diazomethane were from Aldrich (Aldrich Chemical Company Inc. Milwaukee, WI, USA). Trifluoroacetic acid was from Sigma Scientific (St. Louis, MO, USA). All solvents were supplied by J. T. Baker (Deventer, NL). Reagent grade water was from a Milli-Q water purification system (Millipore Corp., Bedford, MA, USA).

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Extraction Equipment and Procedure The extraction cartridge was made using a polypropylene tube 6.5 • 1.4 cm i.d. packed with 250 mg Carbograph 1 80-100 mesh and kept in place by two polyethylene frits (20 ~tm pore size) (Alhech). Prior to any other operation, the GCB was sequentially washed with 5 mL of a mixture of dichloromethane-methanol: 80-20 to remove possible contaminants, and 2 mL pure methanol. The water sample was prepared as follows: 1 L reagent grade water was spiked with 100 pL of a methanolic solution containing 250 mg L -1 of each pesticide. After vigorous shaking, the water was forced through the cartridge at 10 mL min-1 using a vacuum apparatus placed below the trap. After the sample had passed through, the adsorbent was dried in a gentle stream of nitrogen. The compounds were desorbed by 6 mL of a solution of dichloromethane-methanol 60:40 made alkaline with 0.016 M KOH. The extract was then acidified with 300 pL of a 5 % v/v solution of trifluoroacetic acid (TFA) in methanol. The use of an alcoholic solution of TFA instead of an aqueous one is the necessity to avoid high boiling point solvents. In addition, at the concentration chosen, the solubility limit in water is exceeded for 2,4-DB, 2,4,5-T, Acifluorfen, Dinoseb and pentachlorophenol. The extract was then dried under a stream of nitrogen at 40 ~ Before HPLC analysis, the pesticides were dissolved in 100 pL methanol acidified with 5 % v/v TFA. For GC analysis, the residue was dissolved in 0.5 mL of a solution of diazomethane in tert-butyl methyl ether (MTBE) and reacted for at least 30 min. The final solution was appropriately diluted so that the sample amount injected was ~ 250 pg. The standard solution was prepared as follows: 100 ~tL standard stock solution (250 mg L -1 each pesticide in methanol) were added to 6 mL extracting mixture. The following steps were carried out as described for the water sample preparation.

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Tableii. Recoverypercentage • RSD obtained with LSE followedby HPLC-UV,GC-ECD,HPLC-PBMScompared to those expected followingEPA method 515.1. Compound

HPLC-UV

GC-ECD

HPLC-PBMS

EPA Method 515.1

Acifluorfen Bentazon Chloramben 2,4-D 2,4-DB Dicamba 3,5 Dichlorobenzoic acid Dichlorprop Dinoseb 4-Nitrophenol Pentachlorophenol Picloram 2,4,5-T 2,4,5-TP

100+ 2 97+ 3 93 + 11 101+ 3 105 -+13 100 • 2 91 -+ 2 100-+ 3 95• 3 97 • 2 85 • 5 100 • 4 102• 3 102• 5

98+ 3 97_+ 3 97 • 9 94• 4 97 _+14 103 • 3 n.d. 93 • 3 91+ 4 n.d. 98 • 10 90 5:8 90• 7 93• 6

106+ 11 90_+ 8 85 + 15 79+ 6 91 • 16 90-+ 3 87 + 4 106 • 3 93• 5 93 + 11 75 • 14 89 5:10 92+ 6 82• 7

121 _+16 120_+i7 111 • 14 131+26 87 • 13 135-+32 102 -+16 107 + 20 42• 131 • 24 130 _+31 91 + 16 117• 134+31

GC Equipment

Particle Beam-MS Analysis

A Hewlett-Packard 5890 gas chromatograph equipped with an electron capture detector was used (HewlettPackard, Palo Alto, CA, USA). The column was a SPB 5 (Supelco Inc., Bellefonte, PA, USA) 30 m x 0.25 mm i.d., 25/.tm film thickness, according to EPA method 515.1. Carrier gas was helium at a linear velocity of 30 cm sec -1. Temperature program was: 60 ~ to 80 ~ at 4 ~ rain -1, then up to 300 ~ at a rate of 6 ~ min -1. Injection volume: l~tL in splitless mode. Internal standard: 1,2,3trichlorobenzene (Aldrich Chemical Co).

A Hewlett-Packard 5989A quadrupole mass spectrometer equipped with a modified Hewlett-Packard 59980B particle beam interface was used in this work. The original nebulizer was replaced with a laboratorymade micronebulizer as described elsewhere [16].

HPLC Equipment Liquid chromatography was carried out with a Kontron Instrument 420 dual pump binary gradient conventional H P L C system (Kontron Instruments, Milano, Italy). For this work, a 25 cm m i c r o - H P L C column was packed in our laboratory from a 1/16 in. o.d. 250 ~tm i.d. p o l y e t h e r e t h e r k e t o n e ( P E E K ) tubing (Alltech). The packing material was C18 reversed phase, 5 lain from Phase Sep (Queensferry, UK). Such a column can provide about 15.000 theoretical plates at 1 laL rain -1 flow rate [12-14]. In order to achieve the micro-flow rates needed to operate with such a column, a laboratorymade splitter was placed between the pumps and the injector [15]. A motor-assisted solvent mixer was placed between the pumps and the splitter. In this way a rapid and accurate delivery of the solvent gradient was ensured. The solvent gradient was from 100 % H 2 0 + 0.05 % T F A to 80 % CH3CN + 0.025 % TFA in 50 min at 2 ~tL min -1. The injector was a Valco (Houston, TX, USA) equipped with 0.5 laL internal loop. The detector was a U V Spectra 100 variable wavelength detector (Spectra-Physics, San Jos6, CA, USA). The micro detection cell was prepared from 30 ~tm i. d. fused silica tubing connected to the end of the column. The polyimide coating was r e m o v e d from the region in the path of the light beam. Original

The nebulizing gas was helium 5.6 purity grade supplied by S O L (Milano, Italy) at a flow rate of 0.1 L min -1. On account of the Teflon layer, the source t e m p e r a t u r e was set at 200 ~ while the analyzer was set at 120 ~ The mass spectrometer was operating in SIM m o d e during acquisition, as reported in Table I. Dwell time of 90 ms was calculated considering a mean of ~ 10 acquisition samples for each H P L C peak. Peak area values were calculated by automatic integration. The electron energy was set at 70 eV in positive ion mode.

Results and Discussion The selected pesticides were analyzed and quantified according to the procedures described in the experimental section. In all cases the extraction step is performed using LSE instead of LLE. The quantitations obtained with GC-ECD, H P L C - U V and H P L C - P B M S were compared with those expected following EPA m e t h o d 515.1. The results obtained are summarized in Table I1. As can be clearly seen, the percentage recovery for each pesticide is less scattered and the relative standard deviation, calculated on the average of five analyses, is much lower when the modified m e t h o d is performed. Less satisfactory results obtained with EPA m e t h o d 515.1 can be explained in light of two fundamental considerations. First, L L E can lead to loss of analyte, due to the complexity of the procedure which is carried out in several steps. Second, derivatization represents a further source of nonreproduce .ability, being d e p e n d e n t on the yield of the reaction.

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Time (min) Figure 1 Chromatograms by HPLC-UV (a) and GC-ECD (b) separation of pesticides extracted from 1 L spiked water. 1 = Internal standard; 2 = Dicamba; 3 = Dichlorprop; 4 = 2,4-D~$ = Pentachlorophertol; 6 = 2,4,5-TP; 7 = Chloramben; 8 = 2,4,5-T; 9 = 2,4-DB; 10 = Dinoseb; 11 --- Bentazone; 12 = Picloram; 13 = Acifluorfen; 14 = 4-Nitrophenol; I5 = 3,5-Dichlorobenzoic acid.

The LSE approach eliminates the risks of inaccuracy linked to L L E but, when G C - E C D analysis is performed, the derivatization step is still necessary, The good recoveries and low relative standard deviations ratify these considerations. Best results are achieved when LSE is followed by H P L C - U V quantitation, The percentage recoveries are comparable or better than those obtained by G C - E C D while the relative standard deviations are lower in all cases, with the only exception of Chloramben. Since sample preparation is the same in both cases, the differences in recoveries have to be ascribed to the derivatization process which involves a certain level of inaccuracy in the results. Figure la, b shows the chromatograms obtained with H P L C - U V (a) and G C - E C D (b),

282

Recoveries and relative standard deviation obtained by HPLC-PBMS are comparable with those by HPLC-UV, as illustrated in Table II. Figure 2a shows the reconstructed ion profile relative to a chromatographic separation. For this experiment, 100/aL of a 50 mg L -1 solution was used to spike 1 L of water. The amount of each pesticide injected was ~ 25 ng. It is well established that mass spectrometric detection allows identification of unresolved chromatographic peaks. In our case, since 4-nitrophenol and 3,5-dichlorobenzoic acid are coeluted, their H P L C - U V quantitation has been performed separately. The differences in the mass spectra can be exploited to separate the coeluted compounds by independent integration of the characteristic ion profiles when using the mass

Chromatographia Vol.43, No, 5/6, September 1996

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(a) Reconstructed ion profile from HPLC-PBMS separation of extract from 1 L spiked water; (b) Ion profiles obtained from coeluted compounds: (a) 4-Nitrophenol; (b) 3,5-Diehlorobenzoic acid.

Tablelll. DetectionlimitsinngL-l. Compound Acifluorfen l~entazon Chloramben 2,4-D 2,4-DB Ibicamba !3,5 Dichlorobenzoic acid j bichlorprop i 13inoseb 4"Nitrophenol Peatachlorophenol Picloram 12,4,5-T

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5.2 0,7 2.6 5.0 0.7 4.5 0.7 1.5 1.5 1.5 0.7 7.2 4.5 1.5

spectrometer, operating in SIM mode, instead of the U V detector, as shown in Figure 2b. The use of the mass spectrometer was restricted to the H P L C separation thanks to its superior performance over G C - E C D in the analysis of chlorinated pesticides, Detection limits were evaluated and the data obtained are summarized in Table III. The outline of the results obtained from these experiments shows that, for these classes of compounds, L S E combined with GC-ECD, H P L C - U V or H P L C - P B M S quantitation is more rapid and accurate than that performed following EPA m e t h o d 515.1. F u r t h e r m o r e , the possibility of using a sophisticated detector, such as the mass spectrometer, permits unequivocal identification and quantitation of target and unexpected compounds.


283

References [1]

[2] [3] [4] [5] [6] [7] [8] [9] [10]

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Methods for the Determination of Organic Compounds in Drinking Water. Method 515.1. Revision 4.0. Environmental Monitoring Systems Laboratory - Office of Research and Development - Unites States Environmental Protection Agency, Cincinnati, Ohio. A. S. Y Chau, B. K. Afgan, in "Analysis of Pesticides in Water" CRC Press, Boca Raton, FL, 1982. R. C Mc Crae, J. D. Fischer, K. W. Kuntz, Water Poll. Res. Can. 211, 67 (1985). J. Namiesnik, T. Goreki, M. Biziuk, Anal. Chim. Acta 237, 1 (1993). A. Di Corcia, M. Marchetti, Anal. Chem. 63, 580 (1991). A. Di Corcia, M. Marchetti, Environ. Sci. Technol. 26, 66 (1992). A. Di Corcia, R. Samperi, A. Marcomini, S. Stelluto, Anal. Chem. 65, 907 (1993). C. CrescenzL A. Di Corcia, S. Marchese, R. Samperi, Anal. Chem. 6"/, 1968 (1995). E Bruner, G. Crescentini, E Mangani, R. Petty, Anal. Chem. 55, 793 (1983). E Mangani, G. Crescentini, P. Palma, E Bruner, J. Chromatog. 452, 527 (1988).

[11] E Mangani, G. Crescentini, E Bruner, Anal. Chem. 53, 1627 (1981). [12] G. CrescentinL E Bruner, E Mangani, G. Yafeng, Anal. Chem. 60, 1659 (t988). [13] A. Cappiello, P. Palma, E Mangani, Chromatographia 32, 389 (1991). [14] G. Crescentini, A. R. Mastrogiacomo, J. Microcolumn Sep. 3, 539 (1991). [15] A. Berloni, A. Cappiello, G. Famiglini, P. Palma, Chroma" tographia 39, 279, (1994). [16] A. Cappiello, E Bruner, Anal. Chem. 65, 1281 (1993). [17] A. Cappiello, G. Famiglini, E Brunet, Anal. Chem. 66, 1416 (1994). [18] A. Cappiello, G. Famiglini, P. Palma, A. Berloni, E Brunet, Environ. Sci. Technol. 29, 2295 (1995).


Received: Mar 12, 1996 Revised manuscript received: May 20, 1996 Accepted: Jun 4, 1996

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