Estimation of relative standard deviation related to limit of detection and limit of quantitation

Abstract

Limit of detection (LOD) is usually determined by calculation of the standard deviation of measurements of blank samples. In several technical reports, LOD is defined as the concentration at the relative standard deviation (RSD) 30% (or other values) for repeated measurements. Similarly, limit of quantitation (LOQ) is also defined at the RSD level 10%. When a laboratory starts to use a method, the method verification should be carried out in advance to guarantee this method can be implemented correctly and the method characteristics (eg. LOD and LOQ) can be satisfied in this lab. But in routine test, this verification is not conducted every time and the measurement result might be effected by several factors. Then the verification results can vary under different test conditions. In this article the critical value of relative standard deviation is presented, when the RSD from routine test of blank samples of n times larger than the critical value it can be concluded that this measurement system cannot meet the requirement of LOD/LOQ that defined by the methods related with RSD. This method shows a quick check whether the routine measurement can satisfy the defined LOD/LOQ.