The Evaporative Light Scattering Detector The evaporative light scattering detector, as its name implies, utilizes a spray that continuously atomizes the column eluent into small droplets. These droplets are allowed to evaporate, leaving the solutes as fine particulate matter suspended in the atomizing gas. Figure 48. A Diagram of the Evaporative Light Scattering Detector
The atomizing gas can be air or, if necessary, an inert gas. The suspended particles pass through a light beam and the scattered light viewed at 45o to the incident light beam by means of a pair of optical fibers. The scattered light transmitted through the fibers is sensed by a photomultiplier and the output electronically processed and passed either to a computer data acquisition system or to a potentiometric recorder. A diagram of the light scattering detector is shown in figure 48. Theoretically, the detector responds to all solutes that are not volatile and as the light dispersion is largely Raleigh scattering, the response should be proportional to the mass of solute present; as a consequence, it is sometimes referred to as the mass detector. For a linear response, the droplet size must be carefully controlled as it also determines the particle size of the dried solutes
The Evaporative Light Scattering Detector Figure 49 The Sensor of a Commercial Evaporative Light Scattering Detector
Detector sensitivity is claimed to be between 10 and 20 ng of solute. However, in these terms it is difficult to compare with other detectors. The great advantage of the detector, however, is its catholic response and that its output is linearly related to the mass of solute present. However, the magnitude of the response does vary widely between different substances.
A diagram of the sensor of the evaporative light scattering detector manufactured by Polymer Laboratories is shown in figure 49. The eluent is atomized in a stream of nitrogen and the finely divided spray passes down a heated chamber during which time the solvent is evaporated. The removal of the solvent produces a stream of particles which then pass through a collimated beam of light. The scattered light at an angle to the incident light is focused onto a photomultiplier tube and the output is processed in an appropriate manner electronically.
The Evaporative Light Scattering Detector Figure 50. The Separation of Some Lipid Class Materials Monitored by a Evaporative Light Scattering Detector
The device is fairly compact and relatively simple to operate. An example of the results obtained when used for monitoring a general lipid class analysis is shown in figure 50. The minimum detectable mass estimated from this chromatogram appeared to be about 10 ng of solute. To some extent, this detector provides a replacement for the transport detector as it detects all substances irrespective of their optical or electrical properties.
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