Evaporative light scattering detectors (ELSD), also known as evaporative mass detectors, are suitable for detecting nonvolatile sample components in volatile eluents. Eluent streams are passed through a nebuliser into an evaporation chamber, where a solvent gets evaporated leaving behind a mist of small sample particles. The particles scatter a light beam, with the extent of light scattered being proportional to the quantity of sample particles present. Whenever several detectors are used, the ELSD gets fitted last since the output rises into the fume cupboard.
Since ELSDs happen to be among the most universal detectors for HPLC, their applications are very wide. Unlike refractive index detectors i.e. RI, gradient elution can be used with ELSD, unlike a refractive index detector which uses dissolved buffer salts in an eluent. Mostly ELSDs need supplies of clean, dry gas, preferably inert. While compressed air can be used sometimes, it is important to be cautious and remove the traces of water or oil, which can produce a noisy baseline. The clean, dry gas ought to be provided at a pressure of approximately 30 psi, with the capability of gas flow rates ranging between 0.5-5 L a minute. Since the detector features a solvent vapour exhaust, it should be used in a fume cupboard, or get vented to a fume cupboard using a flexible pipe about 8 cm in diameter. It is important to ensure that there are no back drafts down the pipe if using the exhaust pipe option. The gas will be partly used; as a drying gas for the evaporating chamber and to run the nebuliser. When using modern instruments, it is possible to use very low flow rates.
Nebulisation refers to the formation of fine dispersions of droplets by passing effluent flow using a venture that operates on inert gas or compressed air. The atomised mist formed then gets carried to the evaporation chamber. A similar process is utilised in aerosol powered TLC spray reagent atomisers.
ELSDs are not similar to laser light scattering detectors. In the evaporation Chambers of ELSDs, the solvent gets removed, leaving behind a sample of fine particles, which then get used to scatter light. In laser light scattering detectors however, there are molecules specifically used to scatter light, hence giving information regarding molecular size. Generally, laser light scattering detectors are way more expensive compared to ELSDs. The light source utilised often generates white light. When you look at old models, you realise that the white light is emitted from tungsten halide lamps, in comparison to modern instruments where the light gets omitted from LEDs.
Particle size is very crucial, since the kind of light scattering that occurs heavily depends on particle size. The majority of ELSD applications employ reflection and refraction of light. Such a reflection or refraction occurs when the diameter of a particle is similar to or larger than the wavelength of the used light source. When the particles have smaller diameters, Mie light scattering occurs, resulting in low levels of reflected light ash normal measuring angles. Rayleigh light scattering happens when the particle size is further reduced, making them act as point sources. The concentration of particle samples often determines the particle size achieved by the evaporation process and nebulization.