Principles and Practice of Chromatography - The Basic Column Chromatograph > The Detector Output > Page 58

Figure 26 Different Types of Noise

Short term noise consists of baseline perturbations that have a frequency that is significantly higher than the eluted peak. Short term detector noise can be easily removed by appropriate noise filters without significantly affecting the profiles of the peaks. Its source is usually electronic, originating from either the detector sensor system or the amplifier. Long term noise consists of baseline perturbations that have a frequency that is similar to that of the eluted peak. This noise is the most significant as it is indiscernible from very small peaks in the chromatogram. Long term noise cannot be removed by electronic filtering without affecting the profiles of the eluted peaks. In figure 26, it is clear that the peak profile can easily be discerned above the high frequency noise but is lost in the long term noise. Long term noise usually arises from temperature, pressure or flow rate changes in the sensing cell. Drift are baseline perturbations that have a frequency that is large to that of an eluted peak. Drift is almost always due to either 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 trace at the bottom of figure 26.

The detector noise is defined as the maximum amplitude of the combined short– and long-term noise measured over a period of 15 minutes. The detector must be connected to a column and mobile phase passed through it during measurement. The detector noise (ND) is obtained by constructing parallel lines embracing the maximum excursions of the recorder trace over the defined time period as shown in figure 27.