Why you should add an ELSD to your flash chromatography system

ELSD, or evaporative light scattering detector, is an accessory item available for many commercially available flash chromatography systems. It is typically used to detect compounds that are UV transparent such as carbohydrates by using heat, nitrogen, and either an LED or laser light source to detect the resulting dried compound particles. Its practicality as a flash chromatography detector goes beyond detecting UV transparent compounds and is why you should consider adding an ELSD to your flash system.

What other application areas does the ELSD address? Well, it can effectively detect molecules whose UV absorption is masked by the solvents used in the purification process. There are several solvents that absorb UV within the same wavelength range as many organic compounds, presenting a common challenge for chromatography.

One of the most popular flash chromatography solvents is ethyl acetate (EtOAc), typically used with either hexane or heptane. Unlike saturated hydrocarbon solvents, ethyl acetate is not UV transparent. It absorbs UV within the range of 198 to 252 nm, with a peak absorption at 220 nm. Figure 1.

Figure 1. EtOAc UV spectrum.

Many synthetic compounds absorb UV most strongly below 254 nm making them a challenge to detect when using EtOAc as a chromatography solvent.

Consider, for instance, a 2-step imide synthesis. In the first synthesis, a classic Diels-Alder reaction of maleic anhydride and 1,3-cyclohexadiene was performed creating bicyclo[2.2.2]oct-5-ene-2,3-dicarboxylic anhydride, Figure 2.

Figure 2. Diels-Alder reaction.

The synthesis was performed in acetonitrile allowing the reaction mixture to be purified using reversed phase flash chromatography with a water/methanol gradient. Since the reactants and final product contain at least one double bond and the mobile phase solvents are essentially UV transparent, they should all be easy to detect using UV as depicted in Figure 3.

Figure 3. Diels-Alder purification with UV detection.

As it turned out, however, the product peak had relatively low molar absorptivity giving an indication of low synthetic yield. Since the product is a solid when dry, the reaction mixture was a good candidate for purification with an ELSD.

When purified using flash chromatography with ELSD, the response was far greater than UV enabling more of it to be collected, Figure 4.

Figure 4. Diels-Alder purification with ELSD and UV detection.

After the ELSD-triggered product fractions were dried, they were dissolved in a dichloromethane/acetonitrile mix and reacted with cyclohexylamine to create 2-cyclohexyl-3a,4,7,7a-tetrahydro-1H-4,7-ethanoisoindole-1,3(2H)-dione, Figure 5.

Figure 5. Imide reaction.

This reaction created a yellowish slurry which was found to be methanol soluble. Reversed phase flash failed to provide a suitable purification so normal phase was used with a heptane/ethyl acetate gradient. As discussed above, we know EtOAc absorbs UV at wavelengths below 252 nm and with the product containing a single double bond, UV detection alone would likely fail to adequately detect the product in this mobile phase, so the ELSD was needed to detect the eluting compounds, Figure 6.

Figure 6. Imide purification using ELSD and UV detection.

As the data shows, only the ELSD provided adequate detection of the reaction product. So, think about what this data is saying: incorporating an ELSD alongside your system’s UV detector enhances the likelihood of detecting all compounds undergoing purification.

Equipment used for this post include:

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