This Information Applies To: Agilent OpenLab ChemStation
Issue
This article describes the fundamentals of noise determination and calculation in Agilent OpenLab ChemStation to report the signaltonoise ratio.
Tips: To report the signaltonoise ratio in OpenLab CDS ChemStation Edition, see the article Reporting SignaltoNoise in OpenLab ChemStation.

Resolution
Noise Determination and Calculation
Noise can be determined from the data point values from a selected time range of a signal, and can be calculated for up to seven ranges of the signal. The ranges are specified as part of the system suitability settings in the reporting parameters. OpenLab ChemStation has the following options to calculate the noise for the signaltonoise ratio calculation;
Figure 1. Noise determination by the ASTM method
The ASTM noise is calculated as:
Where N_{ASTM} is the noise based on the ASTM method. An ASTM noise determination is not done if the selected time range is below one minute. See also OpenLab ChemStation Data Analysis Reference Manual for more details of Noise Calculation by the ASTM Method.
SignaltoNoise Calculation without reference signal (6 Sigma and USP):
The range closest to the peak is selected from the ranges as specified in the system suitability settings. The noise is calculated either using six times the standard deviation (ุ6 Sigma or 6SD) of the linear regression or using the PeaktoPeak(P2P) formula. (Figure 2)
S/N is calculated using the formula:
Where:
H – Height of the peak.
h – The absolute value of the largest noise fluctuation. It is calculated considering the value of the baseline after injection. It is observed over a distance equal to 20 times the width at halfheight of the peak in the chromatogram obtained with the reference solution, and situated equally around the place where this peak would be found.
Figure 2. Signaltonoise ratio
SignaltoNoise Ratio Calculation according to the EP Definition:
For European Pharmacopoeia(EP), Noise is calculated using the PeaktoPeak formula but it is taken from a blank reference signal over a range of 10 and +10 times W_{0.5} flanking each peak. The noise is calculated in the last blank run (sample type = Blank) which was acquired before the injection.
S/N is calculated using the formula:
Where:
H – Height of the peak corresponding to the component concerned in the chromatogram obtained with the prescribed reference solution.
h – The absolute value of the largest noise fluctuation. It is calculated considering the value of the baseline after injection or application of a blank. It is observed over a distance equal to 20 times the width at halfheight of the peak in the chromatogram obtained with the reference solution, and situated equally around the place where this peak would be found.
If you use the S/N (EP), you must inject the Blank sample to evaluate the reference signal for specific peaks as defined by European Pharmacopeia. You can specify the reference data file (reference signal) in the sequence table by choosing the sample type Blank for the corresponding samples.
Figure 3. Set Blank sample in the sequence table
If you use multiple reference files, the order of the files is essential. OpenLab ChemStation uses one reference file for all successive runs, until there is new reference file given in the sequence table. The reference file of a blank sample serves as its own reference.
Learn how to effectively improve the signaltonoise ratio in your method: Improving and Reporting SignaltoNoise Ratio in OpenLab CDS ChemStation elearning course available from Agilent education. 