by
lmh » Tue Feb 06, 2018 1:24 pm
I think this is expected. If your peaks I good and your method is behaving, then the area is a measure of how much you injected, and since you are injecting the same amount every time, the value is the same every time. The error is small.
The S/N ratio depends both on "S", which you know is good and constant, but also on "N", the noise. Almost by definition, noise is unpredictable and noisy. It's a small signal with a large relative variation. Exactly how reliable it is, and what it looks like, depends on the particular approach your software uses to assess noise (I remember Xcalibur had a number of integration algorithms, and their assessments of S/N ratio of the same peak could differ by orders of magnitude). If the noise in the baseline varies 2-fold (which you won't even notice in the context of a nice big peak), the S/N ratio is going to vary 2-fold.