As Acetonitrile have more polarity than methanol then how it is more eluting strength than methanol? What is the exact chemistry behind this?
You are right. What I meant was this: at low concentrationsof acetonitrile or methanol, you generally see that less acetonitrile is needed, compared to methanol, in order to elute most compounds. At higher concentrations of organic modifier, which are needed for very lipophilic compounds, this rule is not so general. As an example, a gradient up to 100% acetonitril will not elute phosphatidylcholines within reasonable time from most C18 columns, but a gradient up to 100% methanol will.
Hi Shrikant,
it is true that for mixtures of water with less than 50% of acetonitril or methanol, acetonitril will provide higher elution strength. At these levels, the primary function of the organic modifier is to weaken hydrogen bonds, which acetonitril will do better than methanol (the hydroxyl group of methanol can actually participate in hydrogen bonding, and methanol is, as far as I know, generally regarded as more polar than acetonitril). At levels above 50%, the solvating properties of the pure organic solvent becomes important. Acetonitril is at these levels not generally a stronger eluent than methanol, it depends much on the compounds to be eluted.
Best wishes
Martin
But I have many times observed that ACN always elutes most of compounds than Methanaol...so please clarify it..
You are right. What I meant was this: at low concentrationsof acetonitrile or methanol, you generally see that less acetonitrile is needed, compared to methanol, in order to elute most compounds. At higher concentrations of organic modifier, which are needed for very lipophilic compounds, this rule is not so general. As an example, a gradient up to 100% acetonitril will not elute phosphatidylcholines within reasonable time from most C18 columns, but a gradient up to 100% methanol will.
In RP-HPLC, the lless polar a compound is, better will be its eluotropic power. It is at the hearth of the chromatographic process.
Hello Shrikant,
What you have observed is true. And, as Martin explained it is also not general. There are some compounds which elute in Methanol but not Acetonitrile. But the Acetonitrile will give better peak shape and also higher absorbance. We should also consider absorbance of the peak we are looking into. Please check this link which might be useful for you: https://www.shimadzu.com/an/hplc/support/lib/lctalk/35/35lab.html
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