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Water: CWA Methods

Matrix Effects

Matrix effects that are consistently present in a sample may lead to increased false positive or false negative rates, low or high bias or poorer precision. Additional clean-up steps or other method modifications may mitigate matrix effects.

EPA has provided two tools to help users address their matrix problems:

  1. A recent regulatory amendment (40 CFR Part 136.6) describes the conditions under which users may modify approve methods without EPA review.
  2. EPA has updated the 1993 "Pumpkin Book" which describes specific steps users may consider in dealing with matrix effects.

Chapter 4, 5 and 6 of the Pumpkin Book (PDF) (53 pp, 204K) cover "How to Document a Matrix Interference", "Reports of Matrix Interferences," and "Solutions to Matrix Inteferences," respectively. Chapter 8 is titled, "When a Matrix Interference is Demonstrated" and tells what a discharger or laboratory is to do when a matrix interference is encountered.

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