There are three steps in the detection of an eluent using an evaporative light scattering detector:

1- Nebulization: The eluent from the column is mixed with an inert gas and passed through a narrow orifice to generate a homogeneous mist. This mist contains droplets of mobile phase and the compound of interest.

2- Evaporation of the Mobile Phase: The nebulized eluent is passed through a heated drift tube to evaporate the mobile phase.

3- Detection: The stream of solid particles enters a flow cell which includes a light source and a photomultiplier. The intensity of the light scattered by the particles is directly related to the mass of the eluted compound.

Essentially all compounds can be detected. Detection is based on a universal property of all analytes and does not require the presence of a chromophoric group, an electroactive group, etc.

The detector response is directly related to the mass of the eluted compound. Accurate quantitative analytical data can be obtained for unidentified compounds.

Gradient mobile phases can be used to separate the sample. Since the mobile phase is removed from the eluent before detection, a gradient can be used to optimize the separation. With ELS detection, a broad range of mobile phase modifiers such as AcONH₄, AcOH, HCOOH, TFA, NH₄CO₃, HFBA, and N(C₂H₅)₃ can be readily used to separate complex samples.

In an ELS detector, the mobile phase is evaporated from the nebulized eluent by passing it through a heated tube. The temperature of this tube is perhaps the most critical parameter in optimizing the detection, if the temperature is too high, thermally labile compounds in the sample may decompose and will not be detected. In addition, lower ELSD operating temperature leads to the formation of larger particles, which provide a greater signal than smaller particles.

SEDEX detectors can evaporate high boiling solvents at low temperatures. As an example, the Model 75 can easily evaporate a mobile phase consisting of 100 % H₂O at less than 40°C!

Our innovative design provides performance that is superior to that obtained from systems which require a special low temperature accessory! In the top figure, the importance of low temperature evaporation is clearly demonstrated. At 39°C, a 1 µg sample of urea is barely detected, but the intensity increases by a factor of greater than 12 when the mobile phase is evaporated at 25°C, and the signal from 100 ng is clearly useful.