Why do so many of the QC operations (MS, MSD, LCS, blanks, etc.) have to be run "once for every 20 samples?"
The 1 per 20 (5%) frequency is a typical value that has been used in many EPA programs for many years. The Office of Resource Conservation and Recovery (ORCR) also recognizes that other frequencies may be appropriate under certain circumstances. For example, in the case of a long-term monitoring project involving a small number of analyses of a sample matrix that does not change, it may not be necessary to prove that the method applies to the matrix each time that samples are collected and analyzed. Therefore, the MS/MSD analyses may be run less frequently than 1 every 20 samples. To that end, ORCR recommends that, if another frequency for the QC analyses is chosen, it be clearly documented in a sampling and analysis plan that is reviewed and approved by the relevant regulatory authority.