by
vmu » Wed Apr 27, 2022 6:26 pm
adam wrote:
USP defines signal-to-noise ratio as 2 x Signal/Noise
It is not 2 x Signal/Noise. Is is Signal/Noise, wherein the Signal is H, and the Noise is h/2, i.e. S/N = H/(h/2). This definition of noise is just one of the several possible definitions.
The choice of the definitions for noise and for S/N, as well as the choice of the limits for S/N (S/N > 2, S/N > 3, S/N > 10, etc), are just a matter of agreement between the users of these definitions and limits for some purposes. The values of S/N (with any definitions) are correlated with LOQ and LOD. However, the actual LOD is a matter of confident discrimination of the useful signal from the useless noise for qualitative analysis. The required level of confidence may vary in various applications.
The actual LOQ is a matter of precision of the signal (the signal here is usually the peak area) at low amounts of the analyte. The minimum required precision may vary in various applications.
The limit S/N = 10 (with any definition of noise) is in no way a universal indicator that the signal and/or the analyte quantity correspond to the real LOQ that complies with the requirements to the method precision.