Procedures for Detection and Quantitation

WORKING DRAFT FOR DISCUSSION

Blank Subtraction Issue

Question to the FACDQ

Should the Technical Work Group (TWG) continue to examine candidate statistical procedures for LD and LC that require the use of blank subtraction? Does the FACDQ want to consider a detection limit procedure that would require the complete revision of some or all 40 CFR 136 analytical methods?

Context

The Technical Work Group is examining several candidate statistical procedures for the determination of LC and LD. One of these procedures, the one advocated by the Water Research Centre (WRC), requires that the analytical methods that generate the data used to calculate LC and LD use blank subtraction. Using blank subtraction has certain advantages and disadvantages. The use of blank subtraction is a technical issue with policy implications. The TWG is requesting input from the FACDQ on this issue.

Advantages

The concept of subtracting the measured concentration of the method blank from the measured concentration of each sample is recognized as an appropriate procedure to minimize false positives by the IUPAC and ISO. The WRC developed its LC and LD procedures with blank subtraction as an integral part of the procedure. Given the need that the LC and LD procedures have the acceptance of the scientific community, using procedures recognized by IUPAC and ISO would give the LC and LD an important legitimacy. Further, if blank correction were not at least considered the WRC procedure would have to be dropped as a candidate procedure, which could be unfortunate if it were in fact superior to the other candidates.

Disadvantages

The USEPA does not currently allow for blank subtraction and laboratories are not set up for blank correction. To incorporate the blank correction issue into current TWG activities would require a great deal of work, both conceptually and logistically. Conceptually, there is more than one way to incorporate blank substitutes (e.g. using one blank, all blanks in a single batch, using the last X number of blanks from the last several batches, etc.). It seems unlikely that the TWG would be able to resolve this issue and then incorporate that resolution into a study design for LC and LD. Further, laboratories are not currently set up to perform blank subtraction. If a pilot study were performed, it would require a significant amount of additional planning and effort to prepare laboratories to perform blank subtraction. Finally, if the FACDQ were to ultimately recommend the WRC procedure, it would also have to recommend the use of blank subtraction. This change would go above and beyond the change in the calculation of LD and LC. It is unclear how this would be implemented from a regulatory perspective without re-writing all of the USEPA's existing Clean Water Act-approved analytical methods.

Top of Page