Comparison of various HPLC detectors:

In general, detectors based on absorbance measurements are insensitive to temperature variations of the sample. The major advantage of these detectors is their low cost and high sensitivity for many chemical and biological compounds of interest. Comparatively speaking, a variable wavelength detector offers a range of wavelengths, 190 to 600 nm which permits choice of wavelength depending on the nature of solute. It is also probable to select a wavelength which can suppress the absorption of an interfering solute or the mobile phase. However, their noise levels are greater and hence they are less sensitive.

Simultaneous monitoring of radiation at many wavelengths and the acquisition of data may present three dimensional chromatograms which may be stored. In case of single fixed wavelength detector, it is not probable to go back and look for other information at other wavelengths. Therefore, with diode array it is possible to extract data at other wavelengths from the memory. Comparison of absorption spectra along with spectra in a user generated library frequently gives positive identification of sample components. It is now possible to evaluate a peak for purity by software data manipulation rather than by iteration and refinement of the chromatographic separation.

Fluorescence is a means of increasing selectivity and sensitivity of HPLC analysis. Certainly, selectivity is enhanced because all compounds do not fluoresce at the absorbing radiation. Although many fluorescent derivatives can be prepared but that puts limitation on their use. Typical fluorescing compounds are polynuclear aromatics, steroids, plant pigments, vitamins, alkaloids, aflatoxins and porphyrins. Sensitivity is improved because the fluorescing signal is measured against a low background assuming that the mobile phase does not fluoresce. In general, a fluorescence detector is 100 to 1000 times more sensitive than absorbance detector and is approximately 1ng/mL for strongly fluorescing compounds. Though it is a powerful tool for specific applications of selective detection of trace components but it is not meant for general use.