Liquid Chromatography - The UV Detector 2

(L) is the path length of the cell,

(c) is the concentration of the solute,

(k) is the molar extinction coefficient of the solute for the

specific wavelength of the UV light.

or,

where (k') is the molar extinction coefficient of the solute.

Differentiating,

It is seen that there are two factors that control the detector sensitivity, the magnitude of the extinction coefficient of the solute being detected (which will depend on the wavelength of the UV light that is used) and the path length of the light passing through the cell. Thus, although the minimum detectable concentration can be changed by selecting a light source of different wavelength, the cell length can not be increased indefinitely to provide higher sensitivity as long cells will provide excessive peak dispersion with consequent loss of column resolution. It follows, that the optimum detector cell design involves the determination of the cell length that will provide the maximum sensitivity and at the same time constrain detector dispersion to a minimum so that there is minimum loss in resolution.