All about ELS detectors – part 002

I’ve already written a comprehensive post on evaporative light scattering detection (ELSD) and its advantages for flash chromatography separations. But there is always something left to add. This post is all about expanding and refreshing our knowledge on ELS detectors.

With Covid-19, we’re living through strange times. Writing the blog has given me some solace and I hope our blog posts are keeping you as excited about chromatography and chemistry as ever.

Well, one side of the current situation is that there have been a lot of cancellations and postponements due to the virus. Album release dates, the Olympics, the James Bond sequel. Did you know that there are already 26 films in the spy franchise? Only Godzilla has more! I was looking forward to the latest 007 thriller, so as a somewhat of a substitution, I’ve decided to offer a sequel post of my own.

Funnily enough, I’ve chosen to write the sequel about a spy in chromatography, instead of the British MI6. Yes, today I unveil the grand follow-up to “How to overcome limitations in flash chromatography with an ELSD”.

What’s left to say about evaporative light scattering (ELS) detectors after my last post? Well, a lot.

Like any good sequel, I’d like to give you a reminder of what ELSD is all about and how it works.

ELS detectors in chromatography systems measure the amount of light scattered by particles of solvent that have been dried through evaporation. I’ve enhanced the visual effects since my last post, so here is a graphical representation of the process:

ELSD, evaporative light scattering detectors, ELS detectors, flash chromatography, prep HPLC

As a reminder, in step 1, a nebulizer combines a gas flow of air or nitrogen with the column or cartridge effluent to produce an aerosol of tiny droplets. In step 2, the droplets are introduced into a drift tube where the solvent evaporates, leaving behind the target compound particulate. In step 3, light strikes the dried particles that leave the drift tube, the light photons are scattered and detected by a photodiode.

The resulting peak area (A) is related to the quantity of analyte in the column:

ELSD, ELS detectors, flash chromatography, prep HPLC

Where:

m = solute mass
a,b = constants that are governed by factors such as particle size, concentration and type of target substances, gas flow rate, mobile flow rate, drift tube temperature

The ELS detector is well suited to spying on compounds that can be missed by other spies, detectors such as UV and fluorescence.

As I’ve stated previously, an ELS detector as part of your chromatography unit is an excellent choice for the separation of compounds without a chromophoric group. Carbohdydrates, lipids, fats and polymers all fall in this category. I’ve also spent some time discussing the various advantages of ELS detectors, such as sensitivity, gradient compatibility and semi-quantitative use. But as something new, I’d like to point out some limitations of ELSD.

An ELS detector cannot detect highly volatile analytes, such as ethanol in wine. Generally, the compound of interest must be less volatile than the mobile phase. Even more, any modifier present in the mobile phase needs to be volatile. James Bond can be rather disruptive, and so can ELSD. This means, you should keep the sample amount transferred to an ELS detector, as a disruptive detector, to a minimum.

What solvents are suitable for use with ELSD? Well, mobile phases with lower boiling points tend to be more suitable for use with ELS detectors.

SolventBoiling point (°C)
Toluene111
Water100
Benzene80
Hexane69
Acetone56

But don’t be so quick to discount solvents with high-boiling points. Agent 007 can work with hot tempers and cooler heads; he just changes his strategy. There are also ways to deal with high-boiling-point solvents, such as DMS, DMF, toluene or water. For example, these mobile phases could be evaporated at high temperatures. But here, you would run the risk of destroying your target substances. Alternatively, the solvents can be nebulized into extremely tiny droplets, so you can evaporate such miniscule droplets even at room temperatures.

Of course, as I’ve suggested in my last post on UV detectors, probably the best way to ensure a fantastic purification is to equip yourself with a system that combines UV detectors and ELS detectors. James Bond performs best with a partner, and so would your ELS detector. Check out the post to see why.

And if you want to learn more about the other detectives, I mean, detectors, then check out our comprehensive Chromapedia volume 1: Detection Methods.

And that’s a wrap.

Till next time,

The Signature of Bart Denoulet at Bart's Blog