Chemistry Forum

Could anyone tell me why a mixed solvent system can be used in chromatography, and what is its advantage compare to single solvent?

With a mixture you can catch different molecules

ChenBeier wrote: ↑Tue Oct 26, 2021 10:51 am With a mixture you can catch different molecules

Is that the only advantage?

ChenBeier is absolutely correct - with a solvent mixture you are able to collect a large array of different molecules.
I'm not sure exactly what else you're question entails, but I've tried to focus in only on information pertaining to what you originally posted: why mixed solvents can be used in chromatography, and the advantages of using mixed solvents compared to a single solvent in chromatography.

• Since organic compounds are often not sufficiently soluble in water to permit oxidation in completely aqueous systems, there have been recent uses of mixed solvent systems (such as acetone and water or alcohol and water) in which the oxidant and reductant are mutually soluble.
• The use of certain mixed solvents in chromatography can prevent over-oxidization.
• A suitable mixed solvent system can also be used depending upon the temperature to be employed and the reaction desired.
• If using high polarity solvents, there are many more compounds that can be used to be dissolved into the solvent (polar, non-polar, very polar, etc.).
• The lower the polarity of the compounds being used in chromatography, the more time they spend in the mobile phase (i.e. "moving" along with the solvent). Meaning that the high polarity compounds spend more time "stuck" in the stationary phase. These "phases" are known as the 'Partitioning Effect'.
• The eluting strength of a solvent is primarily related to how strongly it adsorbs onto the adsorbent and because typical adsorbents are highly polar; thus, eluting strength increases with solvent polarity.
• Mixtures of solvents are employed to achieve optimum separation by chromatography.
• When using solvent mixtures, it should be kept in mind that addition of only a minor amount of a polar solvent can result in a large increase in the eluting power of the mixture.

Noteworthy Points:
• In general, the adsorptivity of compounds increases with increased polarity (i.e. the more polar the compound then the stronger it binds to the adsorbent).
• The eluting power of solvents increases with polarity. Therefore, low polarity compounds can be eluted with low polarity solvents, while higher polarity compounds require solvents of higher polarity.
• The stronger a compound is bound to the adsorbent, the slower it moves up the chromatography plate.
• Non-polar compounds move up the plate most rapidly (higher Rf value), whereas polar substances travel up the chromatography plate slowly or not at all (lower Rf value). In general, low polarity compounds have higher Rf values than higher polarity compounds.