Detector Noise

Detector noise is the term given to any perturbation on the detector output that is not related to an eluted solute. It is a fundamental property of the detecting system and determines the ultimate sensitivity or minimum detectable concentration that can be achieved. Detector noise has been arbitrarily divided into three types, 'short term noise', 'long term noise' and 'drift' all three of which are depicted in figure 13.

Short Term Noise

Short term noise consists of baseline perturbations that have a frequency that is significantly higher than that of the eluted peak. Short term detector noise can be easily removed by appropriate noise filters without affecting the profiles of the peaks. Its source is usually electronic.

Figure 13 Different Types of Detector Noise

Long Term Noise

Long term noise is perturbations that have a frequency that is similar to that of the eluted peak. Long term noise is the most damaging as it is indiscernible from very small peaks in the chromatogram. Long term noise cannot be removed by electronic filtering. In figure 13, the peak profile can be discerned from the high frequency noise, but not from the long term noise. Long term noise arises from temperature, pressure and flow rate changes in the sensing cell or irregular column bleed. Long term noise ultimately determines the limits of detector sensitivity or the minimum detectable concentration.

Drift

Drift results from baseline perturbations that have a frequency much smaller than that of the eluted peak. Drift can be due to changes in ambient temperature, changes in mobile flow rate, or column bleed in GC; in LC drift can be due to pressure changes, flow rate changes or variations in solvent composition. A combination of all three sources of noise is shown by the lowest trace in figure 13.