Help - NP HPLC - what solvent can be used to replace hexane?

[harry2008]

Hi there,

I am wondering if somebody could give me some help. I am using NP HPLC to separate my samples. The mobile phase is consist of 90% hexane and 10% EtOH. And I also used this MP to dissolve my samples. But I found the results were very unstable. It was determined that was because hexane is too easy to evaporate. I was recommended using Toluene as MP, but I don't like its smell. I am wondering if I can use other solvent to replace hexane for a better result.

Many thanks,

harry

[JGK]

I don't do a great deal of Normal phase work but have you tried Iso-octane (slightly more polar, more viscous and 30Chigher boiling point.

If the volatility issue is with the samples, do you have a cooling capability on the sample tray? setting samples at a lower temp or 1 injection/vial may eliminate the problem.

[chromatographer1]

The simple answer already addressed is to chose a higher homolog of hexane. Isooctane (trimethylpentane) is a good choice, but decane, and dodecane could be used for your samples. Even tetradecane will stay liquid at room temperatures.

I don't think ethanol will have any problem mixing with any of these hydrocarbon solvents.

best wishes,

Rod

[harry2008]

Thanks for your quick reply. They're good ideas. However, I am not quite sure if the new MP can dissolve my polymers and I don't know either how long it will take to set up the new method. Just wondering if I keep using hexane MP but using more stable solvent (ie isooctane) to dissolve my samples, will it work ? Thanks again.

[chromatographer1]

Unless your lab is extremely warm you are correct about keeping your mobile phase the same and just changing your dissolution solvent.

Only you can determine if there is a problem with making changes but I suspect that going to a high homolog solvent will not affect your dissolution, in fact I would guess it might improve it. But try it and see.

best wishes,

Rod

[harry2008]

Rod,

Thanks so much for your help. I am trying to choose one solvent you suggested. Just wondering which solvent (i.e. isooctane, decane, or Toluene) has no UV absorbance at 320 nm. Thanks a lot!

harry

[chromatographer1]

The alkanes should all have very similar UV absorbance spectra. That would be the first choice over toluene, an aromatic hydrocarbon.

My first choice would be decane. It is the most similar to hexane of your choices and has a higher boiling point than isooctane or toluene.

best wishes,

Rod

[harry2008]

Thanks a lot, Rod! I just used Toluene, it showed me a big solvent peak, which probably covers my sample peak . I'll try decane as you suggested. Best regards, harry