Measurement

Measurement of Detector Noise

The detector noise is defined as the maximum amplitude of the combined short – and long-term noise measured over a period of 10 minutes. The detector is connected to the column and mobile phase passed through it during measurement. Parallel lines embracing the maximum excursions of the recorder trace are constructed over the defined time period, as shown in figure 14.

Figure 14. The Measurement of Detector Noise

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.

(7)

Detector Sensitivity or the Minimum Detectable Concentration

Detector sensitivity or minimum detectable concentration (MDC) is the minimum concentration of solute passing through the detector that can be unambiguously discriminated from noise. The size of the signal that will make it distinctly apparent from the noise (the signal–to–noise ratio) is generally accepted as two (this criteria has been adopted for defining detector sensitivity). Thus for a concentration sensitive detector, the detector sensitivity (XD) is given by

(8)

(Rc) and (ND) being determined in the manner previously described.

The sensitivity of a detector is not the minimum mass that can be detected. This would be the system mass sensitivity, which would also depend on the characteristics of the apparatus as well as the detector and, in particular, the type of column employed.