Method 6010C does have a clear recommendation for the frequency of analysis of the Interference Check Samples (ICS). Our lab had been analyzing at the beginning of each run, but an auditor had mentioned that this was inadequate. What is the recommended frequency when you do not apply corrections, but use the ICS to show absence of interference?
EPA Method 6010C provides different guidance for analysis of interference check solutions depending on if interelement corrections are applied.
Sections 4.2.8, 4.2.9, and 10.1.5.6 discuss procedures for when interelement corrections are applied. Section 10.1.5.6 of Method 6010C recommends verifying the interference correction routine by analyzing interference check standards after instrument optimization and before analyzing samples. Section 4.2.9 recommends verifying interelement corrections daily with interference check solutions but does not specify when during the run. Section 4.2.8 explains how to identify if a correction routine is not operating properly and recommends analyzing interference check solutions more than once to confirm a correction factor change has occurred.
When interelement corrections are not used, Section 184.108.40.206 recommends verifying the absence of interferences by analyzing an interference check solution on a continuing basis. The method does not specify the definition of “continuing basis”; however, the industry standard is generally to bracket sample results with good QC, so it seems appropriate to at least analyze the interference check solution at the beginning of the batch and again at the end of the batch. Of course, some instruments are more drift-prone and more frequent interference check solution analyses may have to be performed.