Chromatography Forum

Imh, if you are fuzzy, no wonder that I am confused.

There is 2 ways to calculate the S/N
you will need:
data point,
excel
calculate average of both data point for your mean. write =aver(all data here) enter
calculate standard deviation for both data point in excel write =stdev(all data point) enter
graph the data vs the time, (time domain) for the graphing approached.
1)We will compare for example two data points, the WL-60-2-RuNaCl-6.5 against the blank WL-60-2-NaCl-6.5.
The signal is the mean of the sample minus the mean of the blank. The noise will be the standard deviation, peak to peak divided by 6.
The mathematic approached will be as follow:
S/N:[ mean signal – mean of blank] / [peak-to-peak/6]=
for example;
Mean signal – Mean of blank= 1087.264- 770.7772= 316.4866

Peak-to-peak/6= max – min= (1342- 941)/6= 66.83333

S/N:[ mean signal – mean of blank] / [peak-to-peak/6]= 4.73546

2)We can also calculate it by looking at the graph 1 which gives us approximately the same result:
Peak-to-peak/6= (1342- 1276)/6= 11
The Signal will be calculated by : Mean signal – mean of blank=
[(1342-1276)/2]-[(1009-974)/2]= 33- 17.5= 15.5

S/N:[ mean signal – mean of blank] / [peak-to-peak/6]=15.5-11= 4.5


hope this helps!!!

Ruth:

Nice try, but this is not what the regulatory agencies are calling S/N.

Uwe, whether you measure the signal from the middle or top of the noise band, signal-to-noise has always meant to me the height of the peak (signal) divided by the noise band (peak to peak). I poked around on the web today (don't have any books with me) and could find nothing that defined noise as 2xsignal/noise.

One might logically use 4x signal/noise = signal /(noise/4), since in most error discussions in chromatography we use 1-sigma values, and noise/4 ~ 1 sigma. But half the noise signal (or 2x height) doesn't seem logical at all. So my puzzlement is how this got into the EP regulations in the first place -- there must be a logical reason.

Finally, in terms of measuring signal, the more I think about it the more I'm convinced that the middle of the noise band to the top of the peak is correct. There is error in picking the top of the peak, but the error of being low (signal - noise error) vs being high (signal + noise error) should be equal, shouldn't it? If that's the case, measuring signal from the top of the noise band to the top of the peak would, on the average, undercalculate the peak height.

I completely share the concerns of John Dolan on this. Yes, I agree that the peak height should be determined from the center of the noise height. BUT, why the heck should the peak-to-peak noise be divided by 2 (or, as the equation indicates, the peak height multiplied by 2 (which makes the equation look even stranger when you first read it!))?? Peak-to-peak noise (including any sigma adjustment) is just that, and shouldn't be divided by 2. Why should things suddenly change and, as John has asked, who got the EP to even consider this?

My 2 cents - Wilhad Reuter