Evaporative light scattering detection (ELSD) has become well established in the field of liquid chromatography (LC), writes Jeanie Watkins.
The first detectors were developed in the 1970s and soon became commercially available when Applied Chromatography Systems Ltd. (ACS, Macclesfield, Cheshire, UK) launched its EMD 750/14. Following the success of this instrument several other companies have produced ELSD detectors. Over the years, improvements in detector design have heralded increased sensitivity and allowed operation at significantly lower gas flow rates.
Three specific processes are involved in ELSD: nebulisation, evaporation and light scattering detection. Nebulisation occurs when the mobile phase is combined with an inert gas and passed through a fine needle to form a plume of aerosol droplets. The aerosol then passes through a heated chamber where the solvent is evaporated. The resulting solute particles pass through an optical chamber causing scattering of a collimated incident light beam. The magnitude of the scattered light, measured using a photodetector, is proportional to the solute concentration.
Aerosol formation and detection are dependent on a number of factors which include sample concentration and volatility, the physical properties of the mobile phase and the form of light scattering involved (which is dependent on the size of the solute particles). It is apparent that all these factors make it unlikely that ELS detection could ever be defined as true mass detection, although it is often referred to as such.
However in the majority of application areas the ELSD provides a reliable measure of concentration and, where specific quantification is required, calibration using standards at known concentrations is appropriate.
Analysis of samples using an ELSD detector only requires that the solute components are less volatile than the mobile phase, and that the mobile phase is volatile. There is no requirement that samples contain a chromophore, be fluorogenic, that the method is isocratic or that a low UV absorbance mobile phase is used.
ELSD detectors are therefore suitable for a wide range of separations and techniques, including HPLC, GPC, LC-CC, and more. For samples that have poor or do not contain UV chromophores, or which require gradient elution, the ELSD is the detector of choice. Application areas include lipids, carbohydrates, pharmaceuticals, nutraceuticals and polymers.
MS and ELSD applications share the same chromatographic requirement that the eluent must be volatile. The two detection methods are therefore compatible and complementary, with the ELSD providing concentration information and the MS providing molecular weight and composition data. ELSD is ideal as a split flow concentration detector in LC-MS applications, and can also be used in isolation for offline method development, avoiding dead time on expensive MS systems.
ELSD is suitable for a wide range of application areas where the only requirement is that the compounds for detection are less volatile than mobile phase. Detection is not dependent on functional groups or optical properties, and therefore ELSD can be used as an indicator of mass response for a wide range of LC applications.
ENQUIRY No 56A
Jennie Watkins is a chromatography technical specialist with Polymer Laboratories Ltd, Church Stretton, Shropshire, UK. www.polymerlabs.com