How to overcome limitations in flash chromatography with an ELSD

Chromatography systems are typically equipped with a UV detector or with both a UV and an evaporative light scattering detector (ELSD). If you have ever wondered if ELSD can be beneficial to your chromatographic experiments, then you are in luck because this is exactly what this blog post is about.

Summer time is BBQ time! Last weekend, we hosted a large garden party and my wife decided to buy some extra picnic ware for the occasion. She came back from the store with a bamboo set. She was very pleased that the dishware was sturdy and more environmentally friendly than plastic. But at the end of the night, we were in for an unpleasant surprise. These particular bamboo dishes were hand-wash only. Ah, after a long night of grilling and chilling, washing so many dishes by hand was not at the top of our wish list. But we managed, all the while discussing some first-world problems.

Why can’t we put all kinds of materials in the dishwasher? Why can’t we wash all sorts of fabrics in the washing machine? Restrictions in daily life are so frustrating, but they can be even more so in the laboratory. Nearly every method and instrument faces limitations.

This is why I can’t help but get excited when a technology evolves enough to overcome some of these confines. A great example is the ELSD.

Flash chromatography systems equipped with ELSD enable the analysis of “difficult” samples, such as carbohydrates, lipids, ginkgolides, essential oils and natural products, which cannot be purified or collected using traditional UV detection alone.

Detection of α and β-santalol present in sandalwood perfectly illustrates this point (see below). Whereas UV fails to detect presence of the compound, the ELS detector awards the chemist with easily distinguishable peaks.

But how can an ELSD detect more types of compounds in a chromatographic run compared to a UV detector alone?

Well, the detector functions based on a simple three-step principle:

Step 1 – Nebulization
The column eluent mixes with a gas flow to form a dispersion of droplets and is thereby nebulized.

Step 2 – Mobile Phase Evaporation
The mobile phase is evaporated.

Step 3 – Detection
Dried analyte particles pass through a light beam in a flow cell. The particles scatter light which is collected by a photodiode. The amount of light scattering is related to the mass of the compounds of interest.

You might be aware that traditionally most flash chromatography equipment use a UV detector only. So, it is worth taking the time to point out a few major differences between ELS and UV detection.

The ELSD detects any non-volatile compound, independent of its nature whereas a UV detector only registers compounds which absorb UV light.

Compounds detected by an ELSD generate a near equivalent response for similar quantities.  UV responses strongly depend on extinction coefficients. In most cases, these coefficients do not reflect the real mass ratio of compounds in the sample.

As solvents get evaporated in an ELSD detector, there is little to no gradient baseline shifting. Furthermore, solvents can be used which are not compatible with UV detection due to their UV cut-off values.

These differences between ELSD and traditional detectors like UV can bring major advantages to your application:

Because the ELSD detects non-chromophoric molecules, this detector can help you see more impurities than when using a UV detector alone.

ELSD provides a more accurate indication of relative quantities compared to UV.

Chromatography systems equipped with an ELSD enable you to choose UV absorbing solvents which can give specific polarities and separations.

The ELSD simplifies fraction collection, as there is no need to collect everything and less fractions need to be handled during downstream processing.

Chromatography instruments with ELSD enable maximal recovery by minimizing sample loss.

For all you carbohydrate enthusiasts out there, check out the example below of how you can use an ELSD to detect acarbose, a carbohydrate used in the treatment of diabetic patients. This compound can be nicely separated from the other sugar compounds present in the sample. Without ELSD, detection of acarbose would be difficult to impossible.

Graph showing detection and separation of acarbose using an ELSD

All in all, ELSD can be incredibly beneficial for standard applications, even in the presence of UV visible compounds. This is because the ELSD response can give a better indication of the quantitative composition of your sample. ELSD remains especially useful for “difficult” samples where non-chromophoric compounds are present and where UV gives only a slight or no response. The ELSD detector will reveal all non-volatile molecules that are present in the sample.

So, when you are equipped with both ELSD and UV detectors, you can work with all the pieces of whatever puzzle you are trying to solve.

Now I only need to find a dryer that I could throw my leather jacket into!

Do you have experience with ELSD? I can’t wait to hear your opinion, so feel free to leave me a comment below.

Until next time,

The Signature of Bart Denoulet at Bart's Blog
38 Shares