Using TLC to Scout Flash Chromatography Solvents

TLC is the tool most used for normal-phase flash chromatography method development. For many chemists, a solvent system of hexane (or heptane) + ethyl acetate is the first, and sometimes only, solvent system evaluated. Though often useful, ethyl acetate may not always provide the optimal purification conditions.

Chromatographic solvents can be classified in various selectivity groups. The selectivity of a particular solvent can and likely will influence a compound’s retention vs. other molecules in the mixture. Performing two or three parallel TLC runs with different solvents will help identify which are optimal for a particular purification.

In normal-phase chromatography, paraffinic solvents  (like hexane and heptane) provide no selectivity and are used to dilute the polar solvent’s concentration. For solvent scouting, evaluating solvent mixtures from different selectivity classifications can quickly provide chemists with the right solvents for the purification (Flash Purification Consumables – A Guide to Cartridges & Method Development, page 25).

SolventSelectivity Group
EtherI
MethanolII
EthanolII
IsopropanolII
TetrahydrofuranIII
DichloromethaneV
AcetoneVIa
Ethyl acetateVIa
AcetonitrileVIb
TolueneVII
ChloroformVIII
Hexane
Heptane
Isooctane

Commonly used flash chromatography solvents.

As a way to highlight solvent choice impact on separation performance, three TLC plates with three solvents were developed for the separation of a 5-component mix. One plate was developed with 20% ethyl acetate in hexane (EtOAc is a class VI solvent), a second plate was developed using toluene (Class VII), and a third plate developed in dichloromethane (DCM) (Class V).

As seen by the data, hexane/ethyl acetate separates each compound but the resolution/selectivity between the spots is tight, which limits sample load quantity. The product of interest in this mix is the compound in the middle of the hexane/ethyl acetate separation (Rf=0.47). In toluene and DCM, the compound of interest is fully separated from all other compounds, but in toluene it elutes at an ideal spot (Rf=0.32) vs. an Rf of 0.7 in DCM.

TLC selectivity comparison
TLC selectivity comparison

Published by

Bob Bickler

Technical Specialist, Biotage

3 thoughts on “Using TLC to Scout Flash Chromatography Solvents”

  1. Thanks for your useful posts!
    Please ask: Can I use Normal phase TLC to develop reverse phase flash column chromatography ?

    1. Hi Nguyen,

      Thank you for your question regarding the use of normal-phase TLC for reversed-phase flash chromatography. Because the chemicals used (solvents and stationary phase) are so different depending on whether normal- or reversed-phase is used it is not possible to use normal-phase TLC for reversed-phase method development.

      Hope this answers your question,

      Bob

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