How to reduce noise and remove ghost peaks

Introduction

Sometimes the chromatograms contain curves with a noisy baseline. The noise can be caused by several factors, for example a dirty flow cell, air bubbles, electrical noise, dirty buffers, etc. The amount of noise can usually be reduced by taking proper precautions, for example filtration of buffers and instrument maintenance.

You can also use the smoothing function to reduce or remove background noise from a selected curve. Smoothing is always a compromise between noise removal and preservation of peak shape.

How to smooth a curve

The table below describes how to select a smoothing function and smooth a curve:

Step

Action

1

Select Operations:Smooth.

Result: The Smooth dialog box is displayed.

2

Select the curve to be smoothed and its target destination.

3

Select the Filter type to be applied. The options are:

  • Moving average. Use this if you have noise along most of the curve. It affects peak height but not retention. There is little effect on the peak area.

  • Autoregressive. Use this if you have periodic noise along the whole curve. It affects peak height and retention, although this has little effect on the peak area.

  • Median. Use this if there is only one or a few noise spikes, for example caused by air bubbles, or if the noise is confined to only a small part of the curve. It can flatten peaks and affect peaks areas slightly, but does not affect retention.

  • Savitzky-Golay. Use this to calculate the smoothing and differentiation of data by a least squares technique.

4

Select an appropriate smoothing parameter value from Light to Hard for the selected filter in the Filter Parameters field. Use the slider, or insert a value manually in the text field. The smoothing effect increases with increasing parameter values.

  • Click OK.

Tip: Start with a low parameter value, for example the default value, and increase it until the best result is achieved. A useful strategy is to increase the parameter value by the default value for each try.

Note: By default, smoothed curves are given the suffix SMTH. The default curve name can be changed as needed.

2005-06-15