Chapter 8
Watershed Monitoring in Sustainable Agriculture Studies 1
1
1
2
Downloaded via TUFTS UNIV on July 11, 2018 at 12:48:39 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
T. D.Spittler ,S. K.Brightman ,M. C.Humiston ,and D. R. Forney 1
Cornell Analytical Laboratories, New York State Agricultural Experiment Station, Cornell University, Geneva,NY14456 DuPont Agricultural Products, Chesapeake Farms, Chestertown,MD21620 2
To obtain maximum efficiency and sensitivity in pesticide residue studies of various cropping systems on multiple watersheds at Chesapeake Farms, a Dupont environmental research center, a series of triazines and acetanalides were first analyzed by gas-chromatography with electron capture detection (GC-ECD) at sensitivities selected to give the maximum number of direct readings with minimal dilutions (0.5-1.5 ppb). Samples having non-detects for specific compounds in the multi-residue series were individually reanalyzed by enzyme immunoassay at a sensitivity of 0.1 ppb. Two synthetic pyrethroids were also analyzed using similar GC-ECD parameters, and a separate flow injection liposomal immunoassay (FILIA) was developed for imazethapyr.
Maintaining the ability to effectively analyze a number of pesticides in a series of samples having varying matrices and extremes in concentration is a desirable goal, but an organizational challenge. In this study we undertook a significantfractionof the analytical burden in a four-year sustainable agriculture program being conducted at Chesapeake farms, a Dupont-owned environmental research center on the eastern shore of the Chesapeake Bay near Chestertown, MD. Chemicals were being applied in accordance with four corn/soybean/forage production protocols to both replicated plots and production-scale fields. From the former, pan- and suction lysimeter water samples were obtained; runoff and well samples were derived from the latter which were sited on four measured and monitored watersheds. Full information on the field portion of the study is contained elsewhere in this volume (1). Analytical Requirements and Strategy The pesticides applied in each of the four protocols are in Table I, along with the general class of compound to which each belongs. Dupont undertook the analyses
126
© 2000 American Chemical Society
Steinheimer et al.; Agrochemical Fate and Movement ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
127 of all sulfonyl urea herbicides at their own facilities, requiring that all water samples be split upon collection for shipment to the respective laboratories. Of the five classes of compounds assigned to our laboratories, the triazines and acetanalides were recognized as appropriate for a multi-residue scheme coupling solid phase extraction and gas-chromatography with electron capture detection (SPE/GCECD) we had developed in 1985 and used in a variety of studies since then (2-4). Table I. Pesticides Applied A Plots Atrazine - tz Cyanazine - tz Nicosufuron - su Esfenvalerate - py Tefluthrin - py Chlorethoxyfos - op
Β Plots Atrazine - tz Cyanazine Metribuzin - tz Metolachlor - ac Alachlor -ac Chlorimuron Ethyl - su Nicosulfuron - su Trifensulfuron Methyl - su
C Plots Atrazine - tz Chlorimuron Ethyl - su Nicosulfuron - su Trifensulfuron Methyl - su Tribenuron Methvl - su tz - triazine su - sulfonyl urea py - synthetic pyrethroid
D Plots Nicosulfuron - su Trifensulfuron Methyl - su Tribenuron Methyl - su Imazethapyr - im ac - acetanalide op - organo phosphate im - imidazolinone
Attaining maximum sensitivity for all pesticides in a multi-residue scheme can be important, but maintaining apex performance in the face of variable supply lots, fluctuating sample cleanliness, background/matrix effects and analyte concentration extremes is the primary consideration. The ability to function consistently at a sensitivity that yields a maximum number of direct readings with minimal dilutions for linear range, coupled with a default step for samples requiring additional sensitivity, significantly enhances efficiency. To this end we have found that enzyme linked immunosorbent assays (ELISA), available from a variety of vendors, can complement a multi-residue scheme in a manner not usually encountered. Frequent reference is made to the use of ELISA techniques for rapid screening of numerous samples followed by more specific or more accurate determinations by GC-mass spectrometry or related techniques (5,6). We have employed this strategy ourselves, frequently (7). When the sample load can have large, unpredictable variations in concentration, operating a GC detector at maximum sensitivity (or some other low level of quantitation, LOQ) requires that many samples be diluted and rerun to bring the signal within the linear dynamic range of the detector. ELISA is also capable of good sensitivity for those
Steinheimer et al.; Agrochemical Fate and Movement ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
128 compounds for which a system exists, but its response range is generally more narrow than for a chromatographic detector—ca. a factor of ten vs. two to three orders of magnitude. It was advantageous, therefore, to set our GC-ECD level of quantitation in the range where the maximum number of readable concentrations would occur, then redetermine those data points below the GC LOQ by ELISA. The ELISA LOQ is defined primarily by the system being utilized, unless preconcentration is employed (8). Esfenvalerate, tefluthrin and chlorethoxyos were all sensitive to election capture, also, and thus allowed further utilization of our GCECD set-up. The published methods for imazethapyr required complicated and expensive HPLC-MS equipment or extensive derivatization. We therefore developed a flow injection liposomal immunoassay (FILIA) that is described in another chapter of this volume (9). Experimental The collection of water samples had started over a year before the establishment of the analytical portions of the project. Samples had been stored at 0-4°C in glass. While we prefer to immediately freeze water samples in plastic bottles and maintain them at -10°C, or colder, a requirement included in the analytical protocol, the chilled samples represented a valuable data series if they could be established as still viable. Accordingly, a storage stability study was initiated wherein standard solutions were both frozen and held at 1°C for analyses at six-month to one-year intervals, until the age of the oldest samples run had been duplicated. This would not only give a comparison of individual compound stabilities at the two temperatures, it would give correction factors for storage intervals if they were found to be significant. Sample Preparation. Samples were thawed and up to 200 mL were filtered through Gelman A/E Glassfilters(1.0M), with the filtrate being returned to the original cleaned container, or to a new Nalgene polypropylene bottle if sample still remained in the original container. A portion of the sample in that 25% requiring pyrethroid analyses was decanted, but left unfiltered, because preliminary recovery work showed an unacceptable loss of esfenvalerate and tefluthrin on the glass filters. Sample volumes were occasionally small (
