Defining Lower Limits of Quantitation - A Discussion of S/N, Reproducibility and Detector Technology

When analyzing an unknown sample, the analytical chemist’s task is to provide an estimate of the concentration of an analyte with acceptable uncertainty (accuracy and precision). Very often, due to the time required to perform a good statistical assessment of instrument performance, the signal-to-noise ratio (S/N) (the ratio of a peak’s height to the variability in the background signal) is used as a way of estimating the lowest quantifiable amount (the lower limit of quantitation LOQ) for the analyte. The LOD (limit of detection) and LOQ (limit of quantitation) are often defined as the concentrations which yield a measure peak with S/N of 3 and 10 respectively. However, as will be described, this method can be misleading and should not be used when evaluating instrument performance, and should particularly be avoided when comparing one instrument model to another.