enlarge the chromatogram to calculate noise ?

[Newchromatographer]

Hi ,

I know that most books state that calculating S/N may varies according to the method that is been used .I would like to calculate the noise of my system by printing the chromatograme and draw parallel lines between the upper level of the noise and the down level .The proplem is that i found it difficult to zoom the chromatograme because i see the noise go down far which i cann't see it .ALL the values then shown bu ( - ) do i need to include this in calculation .

I am using LC solution Shimadzue .Do you think comapring just the peak height at each detector wavelength will be enough to judge which wavelength give better signal and so on according to that i choose my wavelength .It would be good to know how to show only noise in chromatogram with example if applicable

Thanks

[HPLCCONSULT]

Will your software allow you to enlarge (zoom) the baseline on the screen so you can see active noise and the 'Y' scale for a period of several minutes ? If so, just eye across the axis and estimate the noise level. This should provide you with a rough noise value in mV's (mAU's) to then compare to sample peaks.

[Newchromatographer]

Thanks for the help ,,, more question please :

1- when i calculate the top was ( -0.11 mV ) and the below ( - 0.13mV ) SO do i need to include the minus ( - ) or neglect it .

2-when three peak eluted to each other say ( A,B and C ) do i need to calculate signal to noise for each one or just calculate it once for all peaks ( because the same baseline for all and all are eluted in the same line no up or down line like what happen in gradient because my seperation is isocratic )

I know it simple but confusing a bit

[HPLCCONSULT]

To determine the average noise, you can subtract the two signal levels obtained. In your example 0.13 - 0.11 mV's is 0.02 mV's. Your baseline noise level average is 0.02 mV's at whatever signal wavelength and settings you are using. *I like to reference a time and drift value, if any, too.

To have value, Signal to Noise measurements should reference against something. You will want to measure EACH peak in your chromatogram and compare the value obtained (mV's) to the average noise value. The ratio obtained will be your peak S/N ratio (determine for each one). *When working at the limits of detection or quantitation, these values are very important as we have minimum spec's for them which vary, depending on the specific regulation (do not worry about this now).