The Performance of the Lightscattering Detector

The evaporative light-scattering detector has two major advantages over many other liquid chromatography detectors. Firstly, like all transport detectors, its function is almost completely independent of the solvent used for chromatographic development, with the one proviso that all the solvents used must be sufficiently volatile. This provides a wide range of solvent choice, allowing unique solvents to be used that would be impossible with other types of detectors. Its second advantage is its catholic response, which is similar to that of the refractive index detector. Moreover, as opposed to the refractive index detector, the evaporative light-scattering detector readily tolerates gradient elution development.

However, there are also certain disadvantages to this type of detector and certain precautions that need to be taken in its operation. One safeguard is to use a 0.45 |jm filter in line with the nebulizing gas supply to remove any dust particles that may get caught up in the gas flow. Foreign particles in the nebulizing flow will contribute noise to the system and, as a consequence, reduce the sensitivity or increase the minimum detectable concentration. In addition, the nebulizer and drift tube will need to be cleaned regularly to remove accumulated sample deposits. This should be carried out every few weeks: failing to do this will not only result in significantly increased noise, but also adversely affect analytical reproduci-bility.

Occasionally the central jet of the nebulizer carrying the column eluent will become blocked, particularly if high solute concentrations or sticky solutions are nebulized. A blocked nebulizer tube will result in increased back-pressure and, if another detector is employed prior to the evaporative light-scattering detector, then the increased pressure can burst the sensor cell. A pressure sensor should be placed prior to the nebulizer so the back-pressure can be continuously monitored. If this pressure suddenly increases above the normal operating pressure, then the nebulizer will need to be disassembled and cleaned. A relief valve fitted behind the nebulizer will also protect any other detector that is being used from damage.

The nonlinear response of the evaporative light-scattering detector is a more serious problem as it renders quantitative analysis more involved. Furthermore, as the response varies between different solutes, calibration curves must be produced for each substance that is to be determined. The results are usually curve-fitted to an appropriate polynomial or power function which can then be used to modify the peak height or peak area measurements obtained in the actual analysis.

In general, the response of the detector can be fitted to the equation:

where y is the detector response, c is the concentration of solute in the eluent, a and b are constants.

Consequently, the curve relating log y against log c will be linear and the slope will provide the value of b. In practice, b is usually found to be less than 2, which is the value it would be if only Raleigh scattering was taking place.

The two main disadvantages to the evaporative light-scattering detector are its relatively poor sensitivity (or high minimum detectable concentration) and its nonlinear response to the concentration of solute. There are a number of different commercial detectors of this type available and the consensus of opinion is that the sensitivity (or minimum detectable concentration) is similar to that of the refractive index detector, i.e. about 3 x 10~6 g mL~\ This sensitivity compares unfavourably with that of the fixed-wavelength UV detector, c. 5x10~8gmL_1, the fluorescence detector, c. 1x10_9gmL_1 and that of the modified moving ribbon transport detector, c. 8x10~8gmL_1. Nevertheless, the sensitivity of 3x10~6gmL~1 is quite practical for use in liquid chromatography and, due to its near universal response and its solvent independency, the detector is popular for lipid analysis and for other materials that do not fluoresence or have UV chromatophores.

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