Chemistry 155 Introduction to Instrumental Analytical Chemistry

Chem 155 Unit 1 Page 26 of 3131.8.1 A Graphical Look at Detection Limit (C MIN )Consider the minimum detectable signal in thecontext of the confidence interval:S MIN = 3σ b + S bTo what confidence interval does 3σ correspond?Assume N=2 (two replicate measurements of S b )zσ/N 1/2 = 3σ z/N 1/2 = 3 z = 3*1 1/2 = 3z = 3 corresponds to the 99.7% confidence intervalAnother way of saying this (crudely) is that weconsider a signal ‘detected’ when it falls outside of theboundaries corresponding to the 99.7% confidenceinterval for the blank signal.Page 26 of 313