Gas Chromatography Detectors - Detector Noise > Page 13

 

Detector noise is 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. Detector noise has been divided into three types, 'short term noise', 'long term noise' and 'drift' all three of which are depicted in figure 4.

 

Short term noiseresults from baseline perturbations that have frequencies significantly higher than those of an eluted peak. Short term noise is not a serious problem as it is easily removed by appropriate noise filters without significantly affecting the profiles of the peaks. Its source usually originates from either the detector sensor system or the amplifier.

 

Long term noiseresults from baseline perturbations that have frequencies similar to those of an eluted peak. This type of noise is the most damaging as it can not be differentiated from very small peaks. Long term noise cannot be removed by electronic filtering without affecting the profiles of the eluted peaks. It is clear from figure 4 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. and is largely controlled by detector cell design and it is this noise that ultimately limits the detector sensitivity or the minimum detectable concentration.

 

Drift results from baseline perturbations that have a frequencies that are significantly larger than those of the eluted peak. Drift is almost always due to either changes in ambient temperature, changes in mobile flow rate, detector cell pressure or column bleed in GC. As a consequence, certain detectors have very significant baseline drift at high column temperatures. Drift is easily constrained by choosing operating parameters that are within detector and column specifications.