The Method Detection Limit (MDL) is defined as “the minimum concentration of a substance that can be measured and reported with a 99% confidence that the analyte concentration is greater than zero” (40CFR sec. 136 Appendix B). The MDL is used for various reasons in an analytical laboratory, with the primary reason being it is used to determine the reporting limits for each analyte that a laboratory analyzes for. Reporting Limits (RL’s) are generally 3-5 times the MDL. On occasion a laboratory can report lower than the calculated MDL, however these results are always flagged as estimates and cannot not always be considered legally defensible data. MDL studies are also required as part of the process of maintaining certifications.
A MDL is initially established when a method is set up, a new piece of equipment is brought online or if there is a significant change in equipment or location (i.e. moving to a new location, significant maintenance or replacement of major parts). MDL’s are confirmed on a regular basis according to method specifications (generally on an annual basis but for some parameters, such as anions, it is done twice a year). MDL studies are performed on a regular basis to assure that there is no loss of sensitivity on the equipment due to wear and tear or the constantly changing environmental conditions.
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