Section 5.12 of Method 8151A states that standards may be purchased as certified solutions from commercial vendors. Does this mean that we can use commercially available standards of the methyl esters?
No, except in very specific circumstances. Method 8151A involves the derivatization of the target herbicides in the sample extracts. As a result, the calibration standards undergo the same derivatization process as the samples (although it is not necessary to derivatize them in the same batch as the samples). Therefore, if the samples are to be derivatized, then the standards must be prepared from the target herbicides and derivatized to their esters as well, using the same procedures.