Detection of Antibiotic Residual in Soil.

Samples of agricultural soil were among the first matrices where antibiotics were first discovered. Over the past 70 years, naturally produced antibiotics were more or less systematically identified by their mode of resistance and chemical properties. Synthetic and semisynthetic compounds with antibiotic properties were also identified and produced. Multiple classes and hundreds of different antibiotic compounds have been used for treatment of infections, prevention of disease, and to enhance feed conversion efficiency in livestock production. Substantial quantities of natural and synthetic antibiotics are introduced annually to soils either through either direct application or through indirect application of livestock manure containing metabolized and un-metabolized chemicals. Soils are difficult matrices for consistent and accurate low-level detection of antibiotics. In comparison to other matrices, the highly variable composition of a soil matrix presents a significant challenge in measuring individual compounds at environmentally relevant levels. The unique properties of many antibiotics, together with their generally polar, water soluble nature increases the difficulty in separating individual compounds from interferences. Solvent extraction, using relatively polar solvent combinations, typically provides the best quantitative separation of most antibiotics from most soil matrices. Unfortunately, these mixtures also co-extract naturally occurring compounds that interfere with even the most advanced detection technologies. Liquid chromatography with tandem mass spectrometry provides a robust and sensitive detection technique, but only when the interferences in detection are removed. While many extract cleanup procedures have been developed and published, complete removal of interferences while quantitatively recovering all compounds is seldom possible. The use of internal standards can help offset the effects of interferences, though the use of isotope dilution is limited to a few antibiotics. For these reasons, development of low-cost, routine, sensitive and robust multi-residue detection methods provides an ongoing challenge in characterizing the quantitative occurrence of antibiotic residues in the soil environment.