Detector Noise Level

GC detector noise is defined as the trace resulting from random changes in detector output in the absence of solute vapor. Detector noise is important and determines the ultimate detector sensitivity, or minimum detectable concentration. Noise can be divided into three types, short term noise, long term noise and drift. These noise sources combine to give the total noise of the detector. The different types of noise are depicted in figure 60.

Figure 60. Different Types of Detector Noise

Short- term noise

Sort-term noise is the term given to baseline perturbations that have a frequency that is significantly higher than those of the eluted peaks. It is not often a serious problem as short-term noise can be easily removed by appropriate noise filters without significantly affecting the profiles of the peaks. Its source is usually electronic.

Long-Term Noise

Long-term noise consists of baseline perturbations that have a frequency that is similar to that of the eluted peak. Long-term noise is the most deleterious 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 60, it is seen 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 that occur in the detector sensor-cell.

Drift

Drift is a baseline perturbation having a frequency that is significantly larger than that of the eluted peak. Drift is caused by changes in ambient temperature, carrier gas flow-rate and/or column-bleed. It follows that baseline drift can become very significant at high temperatures. In general, the detector sensitivity should not be set above the level where the combined noise exceeds 2% of the full scale deflection (F.S.D.) of the computer simulation of the chromatogram (or the recorder if one is used). Detector noise is measured as the maximum amplitude of the combined short and long term noise taken over a period of about fifteen minutes.