Elutropic series

[JA]

I have been working on a strange application involving the analysis of a glycolipid on C18 using a mixture of chloroform with 5-10% methanol. I would associate an eluent of this nature with a bare silica phase but nevertheless found it would allow some retention of the analyte in question, a disubstituted trehalose derivative, on the ODS phase.

Can anyone please indicate where chloroform lies in the elutropic series for C18 or even better provide some sort of comprehensive table?

Has anyone seen any eluent/column combinations as seemingly peculiar as this? In my experience chloroform and C18 were mutually exclusive but I don't know why.

[Uwe Neue]

C18's are generally not free from silanols. It is not impossible even likely that your method works because of residual silanols. This depends on your C18, and how it is prepared. Other things could be a factor as well, but you would know (embedded polar groups, polar endcapping etc.)

[tom jupille]

I would concur with Uwe that the "stationary phase" is probably residual silanols. The less likely possibility is that you have a methanol-poor layer on the packing surface, and are doing effectively reversed-phase liquid-liquid partition.

A quick-and-dirty check is to look at the effect of methanol concentration. If increasing methanol decreases retention, the mechanism is, by definition, "normal phase".

[JA]

A quick-and-dirty check is to look at the effect of methanol concentration. If increasing methanol decreases retention, the mechanism is, by definition, "normal phase".

At 10% MeOH the compound flies off the column with k<1. At 5% MeOH I gave up after waiting 30 minutes (k>10). Do you change your opinion of the mechanism if I add that it is derivatised as a diester with chain length >20?

[juddc]

It sounds like normal phase to me and the nature of the molecule you describe sounds consitent with that kind of retention mechanism under those conditions. I'd guess that the analyte-column interaction is largely based upon the hydrophilic trehalose and the silanols rather than the hydrophobic tails and the C18.

I do have some polarity index info from Lloyd Snyder's book and have a crude little excel spreadsheet that I use to determine rough isoelutropic mixtures which seems to work well enough for me. I can share if you wish. Please feel free to drop me an email, though I don't think that info would be relevant in this case as it doesn't seem that you have a "pure" RP mode separation.

Best of luck,

Chris