Soil and Sediments: Pesticides Content Sampling, Sample Preparation, and Preservation

For pesticide residue determination, field sampling and analysis of agricultural soils and sediments requires the consideration of several seemingly interrelated factors. The mechanisms by which pesticides are adsorbed, absorbed, and desorbed from soils are not well understood for most families of chemicals or for most soils. In addition, soil properties are both spatially and temporally variable, so that the question of sampling methodology becomes important in order to insure that the samples taken are statistically representative of the larger population being examined. Thus, the investigator must weigh the attributes and limitations of each and then choose the best overall compromise among sample size and numbers, analyte specificity, and experimental designs. Following proper collection, the laboratory analyst must then choose among several options for selective extraction techniques for removal of the pesticide analytes of interest. For most applications, one of the following will be used: a classical approach employing a suitable solvent or cosolvent mixture in either a batch shake-out or Soxhlet extraction apparatus. Other more recently developed options include the use of extractant solvents or cosolvent mixtures held under elevated pressures and temperatures (pressurized fluid extraction (PFE)) or pure substances amended with cosolvent modifiers and held above their respective critical points (CPs) (supercritical fluid extraction (SFE)). Another option is the use of ultrasonic energy (sonication) to facilitate extraction. Each of these approaches relies upon an input of sufficient kinetic energy from an externally applied source to the sample slurried with extractant. As the binding energies responsible for the retention of pesticides on soil are overcome, sorbed solutes are released into the solution phase. Studies which address pesticide fate issues often also require determination of soil metabolite and degradate compounds along with the registered parent chemical. For many families of insecticides and herbicides, these same extraction protocols can be used to quantitatively remove the major daughter compounds from soils without requiring an additional extraction step.