Frequent Questions

What is the SW-846 position concerning the use of accuracy measurements determined from the matrix spike in place of the laboratory control sample?

Please provide the SW-846 position concerning Section 1.7.3.2.3 in V1M4 of the 2009 TNI standard which allows the use of accuracy measurements determined from the matrix spike to replace that of the laboratory control sample (LCS) in establishing analytical process control. The section states this is acceptable as long as the matrix spike (MS) meets LCS criteria. Is this acceptable practice and when should it apply (e.g., when one or more analytes fail recovery criteria in an LCS or only when an LCS is not performed)?

 

Under performance-based methodology, the official SW-846 position is the use of MS in place of the LCS as long as the acceptance criteria are as stringent as for the LCS, and it meets the needs of the project, it would be allowed, just as it is in the TNI standard.

There is a downside, however. Oftentimes, especially in metals, matrix spike amounts are not appropriate for the levels seen in the native sample and recoveries are not calculated. Therefore, if no LCS is available or if the LCS failed, there may be no accuracy check information at all on a particular parameter or several parameters from MS data alone. This coupled with the fact that MS is in a real matrix with matrix effects, and there can be sampling differences between the native sample aliquot and the spiked sample aliquot, getting the MS to come in under LCS criteria will be extra challenging, especially for multiple analytes. In addition to meeting the acceptance window criteria for the LCS, we would view the LCS acceptance criteria to also include the minimum number of analytes listed under 1.7.3.2.3.

We would view the note in the TNI standard to be used as an occasional "batch saver" wherein, for whatever reason, the LCS failed or was not available, but you have back-up accuracy data from the matrix spike, and use that instead to be able to report data for that analyte for that batch of samples. You would not want to rely on MS data instead of LCS data for accuracy checks on a routine basis, especially for multi-analyte methods. Both forms of quality control are needed on a routine basis.

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