Tuesday, April 06, 2010
For a previous discussion on the meaning of detection and quantification see Detection - What is it and What Does it Mean for Photography?.
Your Drinking Water
A drinking water laboratory tests water samples to determine what is in them. The ultimate objective is to ensure the safety and reliability of drinking water supplies. But every analytical measurement has error. To control error, laboratories must include check samples to distinguish the known from the unknown. Sometimes the quantity of a substance is too low for the method to distinguish from a sample with nothing in it (a blank). The lowest threshold at which a method can determine the presence of a substance versus its absence is called the detection limit. In other words, detection occurs when a test method confirms the presence of a substance (analyte).
Detection and Quantification
A method detection limit (aka MDL in United States and Lc in most other countries) represents the detection capability of a method including all analytical and preparatory steps. While meaningful in a statistical context, an analytical result at or slightly above detection is not quantified and by itself cannot generally be used to make decisions about the quantity of analyte present in the analyzed sample other than the fact that it is present. The IUPAC (International Union of Pure and Applied Chemistry) term for detection is the Critical Value (Lc).
Quantification is when the method establishes that the amount of the substance is sufficiently large that noise is a small fraction enabling scientific, legal, and process decisions. A quantified result is one that can be assigned a number with a known precision and accuracy while a detected result is one where it is only possible to state that a substance exists at some quantity above zero but the exact number cannot be known.
A quantified result is one with an acceptably low error so that a decision can be made about the quantity of analyte. Reporting Limit (RL), Minimum Level (ML), Detection Limit for Reporting (DLR), FNQS (False Negative Quality Control Sample), and Practical Quantitation Limit (PQL) are some of the terms used to describe the minimum concentration necessary to make a quantified decsion. The IUPAC term for quantification is the Limit of Quantification (Lq). Another term, Limit of Detection (LOD) is an IUPAC quantity between the Lc and Lq that minimizes false negatives.
Measurement Error and False Decisions
All analytical results include measurement error so that any result has a specified probability of being either falsely positive or falsely negative. An substance detected when it is not present is a false positive; a substance not detected when it is present is a false negative. The detection limit (MDL/Lc) is a statistic that is defined to limit the odds of a false positive to 1% while a quantification limit (FNQS, ML, DLR, Lq) is generally defined to limit the odds of a false negative to 5%. Thus a measurement that barely exceeds the MDL could be in error 1 time out of 100 and a measurement that is non-detect at the quantification level could be in error 5 times out of 100. In the former case, the the analyzed substance would be falsely concluded as being present and in the latter it would be falsely concluded as being absent.
In my next blog, I'll show how to calculate detection limits in a way that avoids taking staff out of routine production.