Gas Chromatography Detectors - Detector Noise > Measurement of Detector Noise > Page 14
A combination of all three sources of noise is shown by the trace at the bottom of figure 4. The sensitivity of the detector should never be set above the level where the combined noise exceeds 2% of the F.S.D. (full scale deflection) of the recorder (if one is used), or appears as morethan2%F.S.D. of the computer simulation of the chromatogram.
Measurement of Detector Noise
Detector noise is defined as the maximum amplitude of the combined short– and long-term noise measured over a period of 10 minutes (the E19 committee recommends a period of 15 minutes). The detector is connected to a column and mobile phase must be passed through it during measurement. The detector noise is obtained by constructing parallel lines embracing the maximum excursions of the recorder trace over the defined time period (see figure 5). The distance between the parallel lines measured in millivolts is taken as the measured noise (vn), and the noise level (ND) is calculated in the following manner. where (A) is the attenuation factor and (B) is the amplification factor.
Figure 5 The Measurement of Detector Noise