Written and maintained by CASRAI Editorial Board
Last updated
An evaporative light scattering detector (ELSD) is a liquid chromatography detector that turns the column eluent into a fine aerosol, evaporates the mobile phase, and measures the light scattered by the dry solute particles that remain. Because the signal depends on the mass of the solute rather than on a chromophore, it is called a universal, mass-based detector. This page walks through the hardware stage by stage, the two main designs, how nebulizer choice relates to flow rate, and how drift tube temperature affects sensitivity. For a side-by-side with UV, DAD, RI, FLD and MS, see the HPLC detector selection comparison.
Who uses it: chromatographers and lab staff analysing compounds that absorb little or no UV light, such as phospholipids, carbohydrates, small peptides, nutraceuticals and synthetic polymers, and teams running drug discovery or high-throughput screening work.
What an evaporative light scattering detector is
An ELSD sits at the end of an HPLC or other liquid chromatography system. It needs no chromophore, fluorophore or electroactive group on the analyte, which is why it is used where a UV detector would see little. Its response is related to the mass of the solute reaching the optical cell. It can be used with gradient elution, which is one practical difference from a refractive index detector. For the surrounding technique, see the HPLC guide.
How an ELSD works: nebulizer, drift tube, optical cell
Detection happens in three stages: nebulization, vaporization and detection.
1. Nebulization
The eluent is forced through a narrow orifice with a stream of gas, which breaks it into droplets. A flow of nitrogen at a pressure of 2-4 bar is typically used. A secondary, focusing inert gas can sheathe the particles and centre them in the optical cell.
2. Vaporization in the drift tube
The aerosol passes through a heated tube where the mobile phase evaporates, leaving the less volatile solute as small dry particles. Larger droplets fall to the bottom of the drift tube and drain away, so mainly the smaller particles reach the evaporation zone. That allows a lower temperature and a better signal-to-noise ratio.
3. Detection
A light beam is directed at the dry particles and a detector measures the scattered light. The amount of sample is inferred from the amount of scatter (US patent 6,229,605 describes this four-step sequence: nebulize, evaporate, illuminate, detect).
Types: single-flow and split-flow designs
| Type | How it works | Key difference or trade-off |
|---|---|---|
| Single-flow | Nebulized effluent goes straight into the heated drift tube. | Described as not especially preferred for relatively volatile samples in relatively non-volatile mobile phases. Aqueous mobile phases need a higher evaporation temperature, which risks losing volatile sample and lowering sensitivity. |
| Split-flow | A nebulization chamber removes larger droplets first, so smaller droplets reach the drift tube, where the mobile phase is more easily evaporated. | Relatively non-volatile samples may be lost in the split. The split ratio depends on laboratory temperature, so run-to-run reproducibility can vary. |
Source: US patent 6,229,605, “Evaporative light scattering device”.
Matching the nebulizer to the flow rate
| Nebulizer class | Approximate flow range |
|---|---|
| Capillary | about 2-75 µL/min |
| Low HPLC | about 20 µL/min – 1 mL/min |
| HPLC | about 200 µL/min – 2.5 mL/min |
| High HPLC | about 1-5 mL/min |
Source: American Laboratory application note, “Optimizing the Sensitivity of an Evaporative Light Scattering Detector” (checked 2026-10-29). A mismatch matters: at a flow too high for the nebulizer, oversized particles form and need more heat to evaporate; at a flow too low, very small particles form.
How to choose and set up
- Match the nebulizer to the column flow rate using the ranges above.
- Use the minimum temperature required to evaporate the mobile phase. In the application note, the peak height for caffeine, which sublimes, at 30 °C was about ten times that at 50 °C, and the sensitivity for urea rose by more than tenfold when the temperature was lowered from 39 °C to 25 °C. The best temperature was found empirically for each compound.
- Consider the design. Split-flow suits samples that evaporate poorly in aqueous phases; single-flow avoids the loss of relatively non-volatile sample in a split (patent 6,229,605).
Common mistakes
- Running the drift tube hotter than needed. Semi-volatile analytes can be lost, as the caffeine and urea results show.
- Mismatching nebulizer and flow rate, which gives oversized or very small particles.
- Ignoring baseline noise sources. Unevaporated solvent, residue, sorbent particles and particulates in the mobile phase are a major determinant of sensitivity.
FAQ
What does an ELSD measure?
The light scattered by dry solute particles left after the mobile phase has evaporated; the response is related to the mass of the solute.
Can an ELSD run gradients?
Yes. The application note states gradient elution is possible, unlike with a refractive index detector, and reports detecting glucose and sucrose at the 5 ppm level with a gradient.
What gas does an ELSD use?
Nitrogen at a pressure of 2-4 bar is typically used for nebulization, according to the application note.
Sources
Date checked: 2026-10-29.
- American Laboratory, Optimizing the Sensitivity of an Evaporative Light Scattering Detector (checked 2026-10-29).
- US patent 6,229,605, Evaporative light scattering device (checked 2026-10-29).
Related reading: HPLC gradient elution design and transfer and SFC vs HPLC.
