Evaporative light scattering detection (ELSD) has become widely used in the field of HPLC. For non-UV absorbing compounds, ELSD is a primary choice since the principle of detection does not rely on the optical properties of the solute.
Even if a compound does contain UV active components, ELSD can offer benefits in terms of uniformity of response for non-volatile solutes since the response does not vary as a function of extinction coefficient. Refractive index (RI) detection is also used in HPLC but is not compatible with gradient elution.
A perfect example of the benefits of ELSD as a preferred detector is illustrated in Fig. 1. Polyethylene glycols (PEGs) are low molecular weight polymers of the general formula H(OCH2CH2)nOH which are widely used as excipients or drug delivery agents in the pharmaceutical industry and additives in cosmetics and home care products. Oligomeric separation of low MW PEGs by gradient reversed phase HPLC is widely used to verify the composition of the polymer. PEG has no UV chromophore and, as a gradient elution is required, RI detection is not a viable alternative.
The ELSD responds to all compounds which are less volatile than the mobile phase. At higher ELSD operating temperatures, semi-volatile analytes may evaporate along with the eluent making detection difficult or even impossible. By operating the ELSD at very low temperatures (or even ambient temperature), losses of semi-volatile sample components can be minimised, preserving sample integrity and offering maximum sensitivity. Fig. 2 shows a separation of Parabens which are synthetic preservatives frequently used in cosmetics and personal care products and pharmaceutical products. These low molecular weight compounds are relatively volatile but can easily be detected by operating the ELSD at very low temperature, in this case 30°C.
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Polymer Laboratories is based inChurch Stretton, Shropshire, UK. www.polymerlabs.com
