Chapter 6
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In Situ Derivatization-Supercritical Fluid Extraction Method for the Determination of Chlorophenoxy Acid Herbicides in Soil Samples 1
1
2
Viorica Lopez-Avila , Janet Benedicto , and Werner F. Beckert 1
Midwest Research Institute, California Operations, 555-C Clyde Avenue, Mountain View, CA 94043 National Exposure Research Laboratory, U.S. Environmental Protection Agency, 944 East Harmon Avenue, Las Vegas, NV 89119
2
This paper describes an in-situ derivatization/supercritical fluid extraction (SFE) procedure for 11 chlorophenoxy acid herbicides from a soil matrix. Soil samples (freshly spiked or spiked and weathered soils) are amended with pentafluorobenzyl bromide (PFBBr) and triethyl amine (TEA), prior to S F E , and are pressurized (static) with supercritical carbon dioxide at 400 atm/100°C for 60 min. During this time, the acids are converted to their corresponding P F B esters, which are very soluble in carbon dioxide, and thus, easily extracted from the soil matrix. During a 30-min dynamic extraction, the esters are collected in acetone or on a C -bonded silica trap and subsequently rinsed off the trap with acetone. The acetone extract is subjected to silica chromatography to remove the excess reagents and analyzed by gas chromatography with electron capture detection. Single-laboratory data and some results of a collaborative study are presented here. 18
Chlorophenoxy acid herbicides are of interest because of their widespread use in agriculture for weed control. Conventional methods for the analysis of chlorophenoxy acid herbicides involve extraction with organic solvents followed by derivatization with diazomethane and analysis by gas chromatography with electron capture detection. These procedures are time-consuming, use large volumes of organic solvents, and use diazomethane, which is explosive. Several approaches to the extraction of chlorophenoxy acid herbicides from soil samples with supercritical fluids were investigated by us and other researchers (1-4). In one approach, we investigated the feasibility of derivatizing the chlorophenoxy acid herbicides in situ with tximethylphenylammonium hydroxide (TMPA) or tetrabutyl ammonium hydroxide (TBA) and methyl iodide (MI). Our experiments, while failing to prove that the derivatization with T M P A was taking place during SFE, indicated that the presence of T M P A was necessary in order to recover these compounds by SFE. We then 0097-6156/96/0630-0063$15.00/0 © 1996 American Chemical Society In Herbicide Metabolites in Surface Water and Groundwater; Meyer, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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HERBICIDE METABOLITES IN SURFACE WATER AND GROUNDWATER
found that the derivatization reaction with T M P A took place in the injection port of the gas chromatograph. Since not all chlorophenoxy acid herbicides could be recovered satisfactorily, we investigated T B A / M I as derivatization agents, mainly because they have been successfully used by Hopper (5) to derivatize 2,4-D, 2,4,5-T, and 2,4-DB extracted from food samples. The extractions with T B A / M I were performed at 400 atm and 80°C, 15 min static followed by 15 min dynamic. We used 0.5 mL of a 25-percent solution of TBA in methanol and 0.5 mL of neat M I for each 2-g soil sample. The extracted material was collected in 2 mL methanol and analyzed by gas chromatography/mass spectrometry. Sand, clay, and topsoil samples were spiked at 50 and 250 μg/g with seven chlorophenoxy acids and extracted as mentioned above (spiked samples were used because of lack of certified reference materials). Overall, the average recovery for the three matrices and the two spike levels was 95.5 percent. The individual recoveries ranged from 54.2 to 128 percent (4). Although our recoveries from freshly spiked soil samples were acceptable and the procedure was fairly simple to perform (when compared with those recommended by E P A in Methods 8150B and 8151), we decided to investigate another SFE procedure since not all Method 8151 compounds could be recovered quantitatively from freshly spiked soil samples. In the second SFE approach, we investigated the extraction of the chlorophenoxy acids from soil samples with carbon dioxide modified with 10 percent methanol at 400 atm/80°C/30 min static followed by 30 min dynamic. Quantitative recoveries for most compounds were achieved when the soil sample (2 to 5 g) was amended with 1 mL glacial acetic acid prior to extraction. This second approach, which appeared quite promising as an alternative to the E P A Methods 8150B (which requires 60 mL acetone and 450 mL diethylether for a 50-g sample) and 8151 (which requires 900 mL methylene chloride for a 30-g sample), did not work with several commercial SFE systems. The acetic acid corroded the frits of the extraction vessels, and especially when multivessel systems were used, the acid was in contact with the vessel for prolonged periods of time causing severe corrosion. Therefore, a third approach was investigated. In this third approach, the chlorophenoxy acid herbicides were converted to their corresponding pentafluorobenzyl (PFB) esters. The derivatization reaction took place at 400 atm and 100°C by pressurizing the soil sample, containing the chlorophenoxy acids, pentafluorobenzylbromide (PFBBr), and triethylamine (TEA), with supercritical carbon dioxide for 60 min, followed by a 30-min dynamic extraction. Both freshly spiked and weathered samples were extracted by this procedure. In this paper we report on this latter in-situ derivatization/SFE procedure with PFBBr and T E A and report data from an interlaboratory study in which 12 laboratories (including our laboratory) extracted samples, prepared by us, following the SFE method described here. The SFE extracts were sent back to us for silica chromatography and analysis by gas chromatography with electron capture detection. The interlaboratory study was performed to evaluate the method with various operators using different commercially available SFE systems. Experimental Section Standards. Of the 11 chlorophenoxy acid herbicides used in the method development (Table I), compounds 1 through 6 and compound 11 were purchased from Chem Service, Inc. (West Chester, PA); compounds 7 and 8 from Crescent In Herbicide Metabolites in Surface Water and Groundwater; Meyer, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
6. LOPEZ-AVILA ET AL.
In Situ Derivatization-SFE Method
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Table I. Chlorophenoxy Acid Herbicides Investigated in This Study
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Compound No,
Compound Name
CAS Registry No.
Abbreviation
1 2 3 4 5 6 7 8 9 10 11
2-(4-chloro-2-methylphenoxy)propanoicacid 3,6-dichloro-2-methoxybenzoic acid 4-chloro-2-methylphenoxyacetic acid 2-(2,4-dichlorophenoxy )propanoic acid 2,4-dichlorophenoxyacetic acid 2-(2,4,5-trichlorophenoxypropionic acid 2,4,5-trichlorophenoxyacetic acid 4-(4-chloro-2-methylphenoxy)butanoicacid 4-(2,4-dichlorophenoxy )butanoic acid 4-amino-3,5,6-trichloro-2-pyridine carboxylic acid 5-(2-chloro-4(trifluoromethyl)phenoxy)-2nitrobenzoic acid
7085-19-0 1918-00-9 94-74-6 120-36-5 94-75-7 93-72-1 93-76-5 94-81-5 94-82-6 1918-02-01 62476-59-9
MCPP Dicamba MCPA Dichlorprop 2,4-D 2,4,5-TP 2,4,5-T MCPB 2,4-DB Picloram Acifluorfen
SU IS
3,4-dichlorophenoxyacetic acid 4,4' -dibromooctafluorobipheny 1
588-22-7 10386-84-2
3,4-D
a
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The CAS Registry No. is that for the sodium salt of acifluorfen.
Chemical Co. (Hauppauge, NY), and compounds 9 and 10 from Ultra Scientific, Inc. (North Kingstown, RI). 3,4-D (purity 96 percent) was purchased from Aldrich Chemical Co. (Milwaukee, WI). A l l compounds were used as received without further purification (their purities were stated to be at least 99 percent). Stock solutions of the individual acids were prepared in acetone at 10 mg/mL and kept at 4 °C in the dark. A spiking solution of the chlorophenoxy acids was made by combining portions of the individual stock solutions and diluting them to 10 to 20 μg/mL with acetone. 4,4 -Dibromooctafluorobiphenyl, used as internal standard (IS), was purchased from Ultra Scientific as a solution in methanol at 250 μg/mL. ,
Reagents. PFBBr and TEA (purity > 99 percent) were purchased from Sigma Chemical Co. (St. Louis, MO) and Chem Service, respectively. A fresh 25-percent PFBBr solution in acetone was prepared weekly by dissolving 2.5 g of PFBBr in acetone. A 10-mL volumetric flask was used for this purpose; 2.5 g PFBBr was weighed directly in the volumetric flask and acetone was added to bring volume to 10 mL. The volumetric flask was kept in a refrigerator at 4°C. SFC/SFE grade carbon dioxide with a helium head pressure of 2,000 psi was obtained from Air Products and Chemicals, Inc. (Allentown, PA). Soil Samples. One soil sample used in this study, obtained from the Sandoz Crop Protection Division (Gilroy, CA), was a clay loam (34 percent sand, 35 percent silt, 31 percent clay; pH 7.4; moisture content 10.6 percent; organic carbon content 1.8 percent). Three soil samples, identified as RT-801, RT-802, and RT-803, were sandy loam and clay loam samples. The physico-chemical properties of the soil samples are listed in Table II.
In Herbicide Metabolites in Surface Water and Groundwater; Meyer, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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HERBICIDE METABOLITES IN SURFACE WATER AND GROUNDWATER
Table II. Physico-Chemical Properties of the Soil Samples Prepared by RT Corporation Parameter
Units
Moisture Chloride, soluble* Fluoride, soluble* Sulfate, soluble* Ammonia as N , extract Nitrogen, total Kjeldahl Nitrate as N , soluble* Phosphorus, total Phosphorus, extractable pH, saturated phase Cation exchange capacity Carbonate, total Sulfur, total Neutralization potential Total organic carbon Sand, 2.00 to 0.062 mm Silt, 0.062 to 0.002 mm Clay, < 0.002 mm
% mg/kg mg/kg mg/kg mg/kg % mg/kg % mg/kg units meq/100g % as CaC0 % % as CaC0 mg/kg % % %
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5
c
d
RT-801
3
3
3.0 5.0 0.5 72 2.3 0.05 6.4 0.02 5.0 8.1 3.8 2.95
