ELSD is one of the most common detectors used with HPLC and is preferred to other detection methods for analyses such as carbohydrate, natural sugars and polymer analysis as well as being used for a wide range of other applications.
As a mass-detector, ELSD has advantages over some other methods of detection such as UV detection that require analytes to contain chromophores and it is unaffected by mobile-phase variations and gradient baseline shifting¹.
Operating Principle
The evaporative light scattering detector (ELSD), otherwise known as an Evaporative Mass Detector, is an aerosol-based HPLC detector suitable for the detection of non-volatile sample components in a volatile eluent. When the eluent (sample dissolved in mobile phase solvent) elutes from the HPLC column, it is converted to a fine spray, or nebulized, and heated to evaporate the mobile phase from the droplets formed.
Gas pressure and flow rate, sample concentration and the solvent used as the eluent are key factors affecting the particle size achieved by the nebulization and evaporation process. By optimising these parameters it should be possible to generate droplets of as uniform a size as possible to improve results. The purity of the gas (usually nitrogen) used is also key since it needs to be clean (oil-free), dry and inert, to avoid high background/noisy baseline in order to optimise detection, and available at flow rates of 0.5-5.0 litres per minute.
To gain maximum sensitivity, particle radius should typically measure in the region of 4-10µm, depending on the wavelength used in the detector. This requires low gas pressure (around 2 bar) with a constant purity and pressure maintained throughout analysis. Generally, gas flow rates are directly correlated to volatility of the solvent, with low flow rates required for less volatile solvents, and higher flows for more volatile solvents, with the eluent being completely volatile.
Key analyses
One key analysis that utilizes the strengths of ELSD is sugar analysis. One advantage that ELSD offers over other detection techniques such as IR for this analysis is that it allows gradient elution of sugars. Gradient elution uses a range of solvents with different polarities which change in composition, or ratio, in order to optimise elution of compounds. Monosaccharides including fructose, glucose, sucrose, maltose, lactose and raffinose have been studied in a variety of drinks and fruit samples, typically by using a mobile phase combination of water and acetonitrile with ELSD detection.
As well as sugar analysis, gradient elution with ELSD detection is also used in analyses such as amino acid and protein analysis and surfactant analysis. There are a number of established methods routinely employed in industry that rely on ELSD and therefore on nitrogen for the nebulizer gas. The Solaris Nitrogen generator produces gas at purities up to 99.5% and flow rates up to 10 LPM, meaning that one generators can provide ELSD gas supply for multiple ELSDs. With an optional compressor, it is possible to have a dedicated, stand-alone nitrogen generator supporting your ELSD analysis.
Solaris Nitrogen generator with stackable air compressor
- Success with Evaporative Light-Scattering Detection, Craig S. Young, Shimadzu Scientific Instruments Inc., Columbia, Maryland, USA and John W. Dolan, BASi Northwest Laboratory, McMinnville, Oregon, USA.
Ed Connor Dr.Sc. is a GC-MS Application Specialist, Peak Scientific, Inchinnan Business Park, Scotland, UK . Prior to joining Peak, Ed completed his Dr.Sc. at ETH Zurich in Switzerland using GC-MS to look at herbivore induced plant volatiles and their interaction with beneficial insects. He then joined the University of Zurich where his work focused primarily on volatile collection methods and analyses using GC-MS and GC-FID.