If you are using Empower3 you do not need a custom field to do this calculation. It is part of the system suitability calculation options.
See attachement on how to set this up.
thanks for your answer
please see the attached
What I forgot to mention was that you need to specify which injection is the blank. This is done in the sampleset where you have a new column called blank. If you specify more than one blank injection Empower will use the average noise values from the blank injections. Remember that the blank injections must come before the samples and standards in the sampleset.
So step by step here is what you need to do :
1. Mark the blank injections in the sample set.
2. Create the processing method with the system suitability set to :
3. Specify that you want to calculate detector noise and drift. It does not matter what noise interval you choose, since Empower will calculate the noise for each peak from the blank injection in the region where the peak elutes.
4. Process you sampleset.
Empower will then calculate the signal to noise ratio s/n using the noise from the blank injection but the signal (peak height) from the actual chromatogram. It will calculate the s/n for every integrated peak in the chromatogram.
I have update the presentation to reflect what you need to do.
I hope this helps.