Method 6020A states to run a post-digestion spike if the matrix spike (MS)/matrix spike duplicate (MSD) recoveries are unacceptable and if the post-digestion spike fails a serial dilution is to be performed. Is the serial dilution to be performed on the sample or is the post-digestion spiked sample to be serially diluted?
The purpose of the post-digestion spike is to determine if the unacceptable MS/MSD results are due to the digestion part of the procedure or the analysis part of the procedure. An acceptable post-digestion spike tells you the unacceptable MS/MSD results were caused by the digestion (this is very typical for analytes like antimony in soil samples). An unacceptable post-digestion spike tells you the problem may be with the analysis stage due to matrix interferences. By doing a dilution, matrix interferences can be reduced. If the dilution test passes, it indicates that there is a matrix interference during analysis that is responsible for the unacceptable MS/MSD results. However, in order to do a meaningful dilution test, enough analyte present must be present in the undiluted sample such that the diluted results are still quantifiably detected. That is more likely if a spiked sample is diluted, which in this case is the post-digestion spike. 6020A Section 9.11.2 dilution test is usually meant to be run on the post-digest spike. There are situations where there are sufficient analytes in the native sample digest that running the diluted native sample digest would give meaningful data. Diluting that digest instead of a post-digestion spiked sample would answer whether matrix interferences are responsible for unacceptable MS/MSD results.
Method 6020B, which was recently released with SW-846 Update V, has been revised to better clarify the purpose of the dilution test and the post-digest matrix spike test.