Contaminants in aged pesticide grade dichloromethane interfering in herbicide residue analysis and a method for their removal

A procedure developed for analyses of persistant herbicide residues in soil samples was found to give poor recoveries and extra peaks in gas chromatograms from fortified samples after initial studies had shown acceptable recoveries and no extra peaks. The problem was also found in a "solvent only" sample fortified with prometryn and metribuzin. These two herbicides were then shown to be altered by merely mixing with an aged pesticide grade dichloromethane being used for extractions. Contaminant(s), apparently formed in aged solvent to cause the extra peaks, can be removed by extracting dichloromethane, immediately before it is used, with 10% sodium carbonate solution. Chromatograms are presented to illustrate the effects of the contaminant(s) and their removal by such extraction with recoveries of 0 to 50-percent raised to above 75 percent for the two herbicides when clean solvent is used for extractions. With one of the oldest bottles of dichloromethane in stock for 8 years, the odor of dichlormethane was masked by a pungent acidic odor, which was removed by the sodium carbonate extraction. The contaminant(s) may arise with time in the pesticide grade solvent because of lack or degradation of an additive present in some other grades of this solvent. A literature review for data to explain the contaminants revealed numerous inhibitors, stabilizers and preservatives added at varying levels in many grades of dichloromethane but little information available on how these additives function, or whether they or their altered products may interfere in analyses. The data herein reinforce the report that careful evaluation of dichloromethane for additives or degradation must be made before using this solvent for extractions in trace organic analyses.