S/N: P2P vs ASTM, which aligns best to USP?

Question

How to Check the signal-to-noise ratio with Agilent OpenLab CDS (version 2.x) - Chromatography Software Articles - Chromatography Software Portal - Agilent Community Lists both Peak to Peak and ASTM as using the same formula for calculation S/N. What is the difference between these two parameters in the software? 
 Is one more aligned to USP <621> which also gives S/N ratio calculation as 2H/h? 
 
 Thanks

martin.adams · Answer

Hello, 
 
 I believe the Peak to Peak most closely matches the USP noise calculation. While both PtoP and ASTM noise use signal max - min over a time range ASTM breaks up the range into smaller segments. See the help for both calculations below. 
 
 Marty Adams

Illustration of peak-to-peak noise with drift 
 
 The drift is first calculated by determining the linear regression using all the data points in the time range of a peak. The linear regression line is subtracted from all data points within the time range to give the drift-corrected signal. 
 The peak-to-peak noise is then calculated using the formula: 
 
 N = I max - I min

where

N

Peak-to-peak noise

I max

Highest (maximum) I x value in the time range

I min

Lowest (minimum) I x value in the time range

I x

Intensity of the signal, corrected by the drift (drift is calculated using the LSQ formula)

For European Pharmacopoeia calculations the Peak-to-Peak noise is calculated using a blank reference signal over a range of -10 and +10 times W 50 flanking each peak. This region can be symmetrical to the signal of interest, or asymmetrical if required due to matrix signals.

Determination of noise from the chromatogram of a blank sample 
 
 Where 
 20 W 50 is the region corresponding to the 20 fold of W 50 . 
 h n is the maximum amplitude of the baseline noise in the 20-fold W 50 region.

Noise determination by the ASTM method 
 
 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. Based on the size of the time range, three different types of noise can be distinguished. Noise determination is based on peak-to-peak measurement within defined time ranges.

Cycle Time, t 
 Long-term noise , the maximum amplitude for all random variations of the detector signal of frequencies between 6 and 60 cycles per hour. Long-term noise is determined when the selected time range exceeds one hour. The time range for each cycle (dt) is set to 10 minutes which will give at least six cycles within the selected time range. 
 Short-term noise , the maximum amplitude for all random variations of the detector signal of a frequency greater than one cycle per minute. Short-term noise is determined for a selected time range between 10 and 60 minutes. The time range for each cycle (dt) is set to one minute which will give at least 10 cycles within the selected time range. 
 Very-short-term noise (not part of ASTM E 685-93), this term is introduced to describe the maximum amplitude for all random variations of the detector signal of a frequency greater than one cycle per 0.1 minute. 
 Very-short-term noise is determined for a selected time range between 1 and 10 minutes. The time range for each cycle (dt) is set to 0.1 minute which will give at least 10 cycles within the selected time range.

Number of Cycles, n 
 The number of cycles is calculated as:

where t is the cycle time and t tot is the total time over which the noise is calculated.

Peak-to-Peak Noise in Each Cycle 
 The drift is first calculated by determining the linear regression using all the data points in the time range. The linear regression line is subtracted from all data points within the time range to give the drift-corrected signal. The peak-to-peak noise is then calculated using the formula: 
 N = I max - I min 
 where N is the peak-to-peak noise, I max is the highest (maximum) intensity peak and I min is the lowest (minimum) intensity peak in the time range.

ASTM Noise 
 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. Depending on the range, if the selected time range is greater than, or equal to one minute, noise is determined using one of the ASTM methods previously described. At least seven data points per cycle are used in the calculation. The cycles in the automated noise determination are overlapped by 10 %.

S/N: P2P vs ASTM, which aligns best to USP?

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