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Procedures for Detection and Quantitation
US EPA Minimum Level (ML) in Clean Water Act Programs
The ML is the lowest concentration that gives a recognizable signal and acceptable calibration point.
Formula is 10*s or 3.18*MDL (for seven replicates), rounded to the number nearest to (1, 2, or 5) x 10^n, where n is zero or an integer. MDL is the method detection limit that is specified at 40 CFR part 136 appendix B, and s is the standard deviation of the dataset.
The ML definition in EPA mercury Method 1631 (Oct 9, 2001, 66 FR 51517) is:
Section 17.12 Minimum Level (ML)-The lowest level at which the entire analytical system must give a recognizable signal and acceptable calibration point for the analyte. It is equivalent to the concentration of the lowest calibration standard, assuming that all method-specified sample weights, volumes, and cleanup procedures have been employed. The ML is calculated by multiplying the [seven replicate MDL] MDL by 3.18 and rounding the result to the number nearest to (1, 2, or 5) x 10 n, where n is an integer (See Section 1.5).
Section 1.5 The detection limit and minimum level of quantitation in this Method usually are dependent on the level of interferences rather than instrument limitations. The method detection limit (MDL; 40 CFR 136, Appendix B) for Hg has been determined to be 0.2 ng/L when no interferences are present. The minimum level of quantitation (ML) has been established as 0.5 ng/L. An MDL as low as 0.05 ng/L can be achieved for low Hg samples by using a larger sample volume, a lower BrCl level (0.2%), and extra caution in sample handling.