ELSD

If you are looking at purchasing an ELSD, you most likely have an analyte without chromaphoric properties.  ELS detectors will detect any analyte that is not volatile at the temperatures required to volatilize the mobile phase. An ELSD will be more sensitive then a UV detector for compounds without chromaphores, but may be less sensitive then UV for chromaphoric analytes that are semi-volatile. 

LT-ELSD Principles of operation

1. NEBULIZATION
- the eluent from the column is mixed with an inert gas and passed through a narrow orfice to generate a homogeneous mist. this mist contains droplets of mobile phase and the compound of interest.

2. EVAPORATION of the MOBILE PHASE
- The nebulizd 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

Benefits

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.

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 phase 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 , N(C2H5)3 can be readily used to seperate complex samples.

Why low temp. evaporation is critical in ELS detection

In an ELS detector , mobile phase is evaporated from the neubulized 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 temp. leads to the formation of larger particles, which provide a greater signal than smaller particles.

SEDEX detectors can evaporate high boiling solvents at low temp . As an example Model 75 can easily evaporate a mobile phase consisting of 100% water at less than 40'C



[ Sensitivity ]

Temperature : Increase temperature a little bit will increase sensitive too. of course your sample should be non-vol.
Gain : Sedex 75 the optimum is generally between 8 and 10 (but it will depend on the background of your mobile phase).

     
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Source from... : SEDERE , chromatography forum