Signal-to-Noise Calculation in OpenLab ChemStation

Answer

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 signal-to-noise ratio.

Tips: To report the signal-to-noise ratio in OpenLab CDS ChemStation Edition, see the article Reporting Signal-to-Noise 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 signal-to-noise ratio calculation;

• 6 Sigma: Noise is calculated using six times the standard deviation (6SD) of the linear regression. The data for the noise calculation is taken from a specific time interval in the current signal. If you defined multiple time intervals, the interval closest to the peak will be used.
• Peak-to-Peak (P2P) noise: Noise is calculated using the Peak-to-Peak formula that the ChemStation uses to calculate for USP and EP signal-to-noise ratio.
• ASTM: ASTM noise determination (ASTM E 685-93) is based on the standard practice for testing variable-wavelength photometric detectors used in liquid chromatography, as defined by the American Society for Testing and Materials.

  
  
  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.

Signal-to-Noise 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 Peak-to-Peak(P2P) formula. (Figure 2)

S/N is calculated using the formula:

• S/N (ุ6 Sigma) = H/h
• S/N (ุUSP) = 2 H/h

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 half-height 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. Signal-to-noise ratio

Signal-to-Noise Ratio Calculation according to the EP Definition:

For European Pharmacopoeia(EP), Noise is calculated using the Peak-to-Peak 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:

• S/N (EP) = 2 H/h

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 half-height 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.

1. In the sequence table, in the Sample Type column, select Blank for the blank sample.
2. In the same row, leave the Sample Location column empty (See Figure 3)

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 signal-to-noise ratio in your method: Improving and Reporting Signal-to-Noise Ratio in OpenLab CDS ChemStation e-learning course available from Agilent education.