The fluorescent detector (another detector in frequent use in LC) has a relatively high sensitivity (compared with the UV detector). When light is absorbed by a molecule, a transition to a higher electronic state takes place and this absorption is highly specific for the molecules concerned. If the excess energy is not dissipated rapidly by collision with other molecules or by some other means, the electron will return to the ground state with the emission of light at a lower frequency and the substance is said to fluoresce. As some energy is always lost before emission occurs the wavelength of the fluorescent light is always greater than the incident light [9]. This detector has high specificity and consequently high sensitivity, which arises from the very low background signal (noise level) that is inherent with detectors having specific response. Its linear dynamic range is little greater than two orders of magnitude.
One of the recently developed LC detectors is the light-scattering detector which, although responds to almost all involatile solutes, has relatively poor sensitivity (similar to the refractive index detector) and a non-linear response, which can be difficult to compensate electronically. This detector does, however, provide complete flexibility in the choice of solvent (assuming they are volatile) a property that is similar to the transport detectors.
Thin layer chromatography (TLC) has survived despite the efficacy of other chromatography techniques due to its low cost and its capacity for multiple separations. In addition, as a result of its development characteristics, it provides quite unique selectivity often difficult to obtain from other techniques. However, it is mostly used for qualitative or semi quantitative analytical work. One problem is the quantitative estimation of the thin layer spot. Estimation has improved with the use of high performance plates and better methods of coating, but accurate spot measurement is still a serious problem unless expensive scanning devices are employed. Using fluorescing derivatives, the spot can be scanned against a low noise background but the accuracy obtainable from TLC does not compare with that of other chromatography techniques.
