Unstable s/n
Posted: Sun Feb 04, 2018 10:07 pm
by Asia
Hello,
I have a problem with my method on LC-MS. I injected three times the same solution of stanards. I integrated peaks and I found out that RSDs for peak area and intensity are low while RSD for signal to noise ratio is high.
Can you explain me it? For me it's strange event. What should I do? I checked and on the diffrent apparatus situation was the same
Re: Unstable s/n
Posted: Sun Feb 04, 2018 10:58 pm
by HPLC chemist
There could be 'many' factors but pH is the most common. In short you may have several ionic forms without a pH buffer, thus protonation (usually 0.1% formic acid) will reduce them and give you a stable S/N ratio.
Re: Unstable s/n
Posted: Tue Feb 06, 2018 1:24 pm
by lmh
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.