There are no specific criteria set for the Limit of Quantitation (LOQ) and Limit of Detection (LOD) but guidance is available from specifications and pharmacopeias.

The noise is measured by running the instrument at maximum gain with no test being processed. The ripple generated is noise due to the instrument’s electronics, etc. The peak is measured relative to this noise and the ratio is calculated. This is known as the Signal to Noise Ratio (SNR).

Generally:

• LOD SNR should be greater than 2:1. Peak A is acceptable for LOD but not for quantitation;

• The LOD can be calculated if the standard deviation (SD) of the response (which is standard deviation of the blank) and the slope is determined:

  LOD = 3.3 x SD slope

• Similarly LOQ = 10 x SD slope

• The LOQ SNR should generally be above 10:1. Peak B is suitable for quantitation;

• Precision as a percentage relative standard deviation should be 5 - 10% at the limit for LOQ.