An analyst sets up a routine reverse-phase run for a new reference compound, watches the UV trace at 280 nm, and sees nothing but a gently drifting baseline. The sample was loaded, the pressure trace looks normal, and the column is fine. Switching to 214 nm produces a few weak bumps that are hard to integrate. The instrument is not broken. The molecule simply does not absorb ultraviolet light well enough to be seen, and the lab needs a different way to detect it.
That situation is the reason evaporative light scattering detection (ELSD) and charged aerosol detection (CAD) exist. This article explains what they measure, what they cost the analyst in convenience, and how to judge a purity number produced by one of them.
Why UV detection has gaps
UV detectors respond to chromophores, parts of a molecule that absorb at the chosen wavelength. In peptide work two wavelengths do most of the work:
- Around 280 nm, absorbance comes mainly from aromatic side chains, especially tryptophan and tyrosine. Sequences without them are close to invisible here.
- Around 210–220 nm, the amide bonds of the backbone absorb. Every peptide has these, so low-wavelength detection is the default for purity methods, as described in how reverse-phase HPLC measures peptide purity.
The trouble starts with molecules that have few or no amide bonds and no aromatic groups. Many sugars, lipids, some counter-ions and inorganic salts, and a range of small non-peptide compounds fall into this group. For them, a UV trace can hide the main component, its impurities, or both.
How the two detectors work
ELSD and CAD share their first two stages. The column effluent is sprayed into a fine mist by a nebulizer, then carried through a heated evaporation tube where the mobile phase turns to vapor. Anything not volatile enough to evaporate is left as tiny dry particles. The detector then measures those particles, so the signal depends on how much nonvolatile material is present rather than on whether it absorbs light.
The final stage is where they differ:
- ELSD passes the particle stream through a light beam and measures the scattered light with a photodetector.
- CAD mixes the particles with a stream of ionized nitrogen gas. Charge transfers onto the particles, and the detector measures that charge directly.
Because the charge a particle picks up depends less steeply on its size than the light it scatters, CAD usually gives a response that is more uniform between different compounds and holds over a wider concentration range. It generally has better sensitivity as well. The trade-off is instrument cost.
Practical limits to plan around
Neither detector is a drop-in replacement for UV. Before building a method, work through these constraints:
| Constraint | What it means in practice |
|---|---|
| Nonlinear response | Signal does not rise in straight proportion to amount; ELSD in particular follows a curved, roughly power-law relationship. Quantitation needs a calibration curve, and raw area percentages carry less meaning than they do under UV. |
| Volatile mobile phase only | Nonvolatile buffers such as phosphate leave residue and a high background. Use volatile additives such as formic acid or ammonium formate instead. |
| Volatile analytes are lost | A compound that evaporates with the solvent never forms particles and gives no signal. |
| Gradient-dependent baseline | Nebulization efficiency changes with organic content, so response shifts across a gradient. Some systems compensate with a second pump delivering an inverse gradient so the detector sees constant composition. |
| Sensitivity | For a compound that absorbs well, UV is usually more sensitive and more linear. |
When a mass-sensitive detector is the right choice
There are three common cases. The first is the obvious one: the analyte has no useful chromophore, so there is nothing to compare against. Choosing between ELSD and CAD becomes the only decision.
The second is a mixture of species with very different absorbance. If one component carries a tryptophan and another has no aromatic residue at all, UV area percentages will badly distort their ratio because each responds with a different strength per unit mass. A mass-sensitive detector gives a much closer picture of the true proportions. This is one of the reasons purity values from different laboratories or methods can disagree, a topic covered in why suppliers report different peptide purity.
The third is the most easily missed: looking for what UV cannot see. A UV chromatogram only shows absorbing species. Running ELSD or CAD alongside it can reveal a nonabsorbing contaminant, such as a leftover reagent, an additive or excess counter-ion, that the UV trace skipped over entirely.
What these detectors cannot tell you
Neither ELSD nor CAD identifies anything. A peak means only that some nonvolatile material reached the detector at that retention time. Establishing what it is still requires mass spectrometry or comparison with a characterized reference standard.
They also do not make UV obsolete for ordinary peptides. Where the backbone absorbs well at low wavelength, UV remains cheaper, more sensitive, linear over a useful range and far better understood. The mass-sensitive detectors are a complement for specific problems.
Reading a purity figure with the detector in mind
A purity percentage is only meaningful alongside the detector that produced it. Two results that both read 98% are not directly comparable if one came from UV at 214 nm and the other from ELSD, because the two detectors weigh each component differently. Neither number converts neatly into the other.
When reviewing any analytical report, look for:
- The detector type, and for UV, the wavelength.
- Whether the compound actually absorbs at that wavelength.
- For ELSD or CAD, whether quantitation used a calibration curve or raw area percentage.
- The mobile phase additives, which should be volatile for any aerosol-based detector.
If a document credits a UV purity figure to a molecule with no chromophore, that is worth a follow-up question. For the basics of reading the trace itself, see how to read an HPLC chromatogram.
Questions
Is CAD always better than ELSD?
CAD usually offers more uniform response, wider dynamic range and better sensitivity, but it costs more. ELSD remains a reasonable choice for many qualitative and semi-quantitative jobs.
Can a phosphate-buffered method be moved straight to ELSD?
No. Phosphate does not evaporate, so it produces particles and a high background. The method must be redeveloped with volatile additives.
Does a mass-sensitive detector give equal response for every compound?
Not exactly. Response is more uniform than with UV, especially for CAD, but it still varies with volatility, particle formation and mobile phase composition.
Do these detectors confirm a compound’s identity?
No. They show that nonvolatile material eluted at a given time. Identity needs mass spectrometry or a matched reference standard.
Research use only. All products supplied by Battle Born Peptides are laboratory reference materials for in-vitro research and analytical use by qualified professionals. They are not drugs, foods, dietary supplements, cosmetics or medical devices; they are not approved by the FDA or any other regulator for use in humans or animals; and they are not intended to diagnose, treat, cure, mitigate or prevent any disease, or to affect the structure or any function of the body of humans or animals. Nothing in this article is preparation, handling or dosing guidance. See our full research-use terms.