An Evaporative Light Scattering Detector (ELSD), otherwise known as an Evaporative Mass Detector, is suitable for the detection of non-volatile sample components in a volatile eluent. The eluent stream passes through a nebuliser into an evaporation chamber, where the solvent is evaporated to leave a mist of tiny sample particles. These scatter a light beam, and the extent of the light scattering is proportional to the amount of sample present. If more than one detector is to be used, clearly the ELSD must be fitted last because the output goes up the fume cupboard!
Applications of an ELSD are very wide because it is one of the most universal detectors available to HPLC. Unlike Refractive Index (RI) detectors, an ELSD can be used with Gradient Elution, but an RI detector can be used with dissolved buffer salts in the eluent, whereas an ELSD cannot.
An ELSD requires a supply of clean dry (preferably inert) gas. Sometimes compressed air is used, but care must be taken to remove traces of oil or water, which will give a very noisy baseline. The gas must be provided at a pressure of about 30psi (2Bar), and be capable of gas flow rates in the region of 0.5 – 5 litres per minute. Because there is a solvent vapour exhaust from the detector, it must either be used in a fume cupboard, or vented to a fume cupboard via a flexible hose of approximately 8cm diameter. If the exhaust pipe option is used, it is very important that there are no back drafts down the tube. The gas will be used partly to run the nebuliser, and partly as a drying gas in the evaporating chamber. On more modern instruments, quite low flow rates can be used.
Nebulisation is the formation of a fine dispersion of droplets by passing the eluent flow through a venturi operated with compressed air or inert gas. This atomised mist is then carried through to the evaporation chamber. (A similar process is used in the aerosol-powered TLC spray reagent atomisers).
Evaporative Light Scattering Detectors are not the same as Laser Light Scattering Detectors. In the Evaporation Chamber of the ELSD, the solvent is removed, leaving the sample as very fine particles, and it is these particles which scatter the light. In a Laser Light Scattering detector it is the molecules which scatter the light, and hence this can give information about molecular size. An ELSD typically costs about £9K, as opposed to nearer £50K for a Laser Light Scattering detector!
The light source used usually generates white light. In older models, this is from a tungsten halide lamp, but in more modern instruments, an LED is used.
Particle size is very important, because the type of light scattering which occurs is particle size-dependent. For most ELSD applications we are looking at Reflection and Refraction of the light. This occurs when the particle diameter is equal to or greater than the wavelength of the light source used. At smaller particle diameters, Mie light scattering occurs, and this gives low levels of deflected light at the normal measuring angles. If the particles were smaller still, they act as point sources and Rayleigh light scattering occurs. The particle size achieved by the nebulisation and evaporation process is dependent upon the concentration of the sample, the gas pressure and flow rate, the eluent flow rate, and the solvent used as eluent. Hence it is necessary to optimise these parameters, and to specify them in a method. The aim is to generate droplets of a specific size, and with as narrow a size distribution as possible.
Sensitivity is at its maximum when the particle radius is about 4x the wavelength of the light used, and under normal working conditions, this means that we are looking for particles in the region of 4-10u.To achieve this, a gas pressure of about 30psi is required, and this should be maintained as constantly as possible. Gas flow rates should be in the region 0.5 - 5 litres per minute, and eluent flow rates should be of the order of 0.1-2ml/min, depending on the eluent. As a general rule, less volatile solvents require slower flow rates than more volatile eluents. To achieve a desired droplet size, certain parameters can be adjusted, and these include the evaporation chamber temperature, the eluent flow rate, and the gas flow rate, all of which are adjustable on the detector.
Evaporation Chamber temperature. The available range will usually be between about 30-200oC. Newer models operate at lower temperatures than older models. The eluent must be completely evaporated, but to avoid possible sample degradation, the lowest temperature which achieves complete evaporation should be used. The higher the flow rate, the higher the temperature that will be required. Hence if a high boiling eluent is used (such as for some forms of GPC), it is best to use a low flow rate. If this is not possible with the HPLC (or GPC) column being used, the flow rate can be reduced using a flow splitter, but this does reduce sensitivity.Once the optimum temperature has been established, sensitivity is optimised by varying the gas flow rate to the nebuliser. High flow rates cause tiny particles to be formed, which give rise to Mie scattering and low sensitivity. So the flow rate is started high and then reduced until the optimum particle size is achieved, as evidenced by the greatest sensitivity.
Eluent choice is dictated by the needs of the separation. However it is worth stressing that the eluent must be totally volatile. Non-volatile components such as buffer salts will dry in the evaporator and because of their high abundance (relative to the sample) will not be completely purged through by the gas stream, and will collect in the detector. Their removal will require disassembly of the instrument, and the removal and cleaning of the evaporation chamber.
Maintenance is limited to replacement of the light source, and cleaning of the evaporation chamber.
Problems arising with an ELSD are usually manifest in a noisy baseline. This can have several causes:
- Evaporation Temperature too low: the solvent is not completely evaporated.
Evaporation Temperature too high: the solvent is boiling in the nebuliser.
Air or nitrogen not clean: remove traces of water or oil with a filter.
Gas flow too low: poor nebulisation of the eluent.
- Involatile material in the eluent: replace any buffer salt with a volatile one (such as ammonium acetate) and ensure that the eluent is particle free.