The MDL Number and Spiked Samples and Method Blanks

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The mdl number is a unique identification number that defines each reagent. It also identifies each reaction and variation within the reagent. This number can be used to track and maintain the reagent within the chemistry laboratory.

The mdl number

An mdl number is required to be calculated by most laboratories for all methods and instruments used to support the National Pollutant Discharge Elimination System (NPDES) program. The mdl number is generated according to the procedure specified in the latest revision of 40 CFR Part 136, Appendix B and is derived from a calculation of either spiked samples or method blanks.

Spiked Samples and Method Blanks

The mdl numbers require a minimum of seven low-level spiked samples and seven method blanks for one instrument. For a method that is performed regularly, this means that there are likely 16 spiked samples per instrument (2 per quarter over 2 years) and many more blanks.

For a method that is performed very rarely, there may be less than 7 spiked samples and more blanks. In this case, the laboratory may need to analyze additional samples for determining the initial MDL.

Does Revision 2 of the MDL procedure require fewer sample spikes and blanks than Revision 1.11?

No. The new MDL procedure can reduce the number of spiked samples and blanks that a laboratory will need to calculate an initial MDL by as much as half, depending on how much of the spiked sample and blanks have been available over the last 2 years. This can be particularly helpful for laboratory with new instrument that have a lot of sample spikes and blanks available to them, but do not need to analyze them frequently.

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