In Method 8151A, should we spike field samples (matrix spikes) and the laboratory control samples (LCS) with the herbicide acids, or can we use the methyl esters?
You must use the form of the analyte that matches the form that is present in the samples, in other words, the parent acids, not the methyl esters. This is because the procedures in Method 8151A include both the derivatization and the determinative analysis. Therefore, spiking the QC aliquots with the methyl esters will not provide any information on the efficiency of the derivatization itself. If you are having problems with the recoveries of the acids, then spiking with the methyl esters and analyzing the samples again may be a useful diagnostic tool to see if the problem lies in the derivatization or only in the determinative steps. However, this would only be done infrequently, when the results for the parent acids suggested a problem.