Chromatography Forum

Let's say I want to use RP for my prep but my sample really dislikes water and will show the beginnings of crashing out...

I would love to dissolve my sample in neat acetonitrile or something that loves my sample just as much - but the chromatogrpahy is a mess because solvent strength of my sample is much greater than that of my mobile phase...

I try increasing solvent strength of the mob phase but now my target molecule flies off the column...

etc etc....

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so what have people tried?
what is the compromise?

the ideal is same solvent composition/pH of sample and mobile phase, but in this case not viable.
so how much "strong" solvent can you get away with before the chromatography suffers?

I might add that we're talking 1ml injection volume on semi-prep (10ml i.d.)... 2ml or higher for prep (21mm i.d.)

I think what you're asking for is something Uwe Neue described in his paper "At-column dilution for improved loading in preparative chromatography" published in Chromatographia. There's a description on the Waters site, the link is below.

http://www.waters.com/webassets/cms/lib ... pNotes.pdf

Can you tell us more about your compound? Is it neutral or does it have an ionizable group? Any polar groups?

What is its log P, calculated water solubility?
http://www.vcclab.org/lab/alogps/start.html

Often times it can be tricky to dissolve solid directly in mostly aqueous starting mobile phase. Sometimes you can get around this by initially dissolving the material in a minimal amount of solvent "it likes" (THF, DMSO, MeOH, MeCN), then carefully dilute with starting mobile phase (dropwise, with constant stirring/shaking, sonnication can't hurt).

If your sample precipitates upon dilution with starting mobile phase, you definitely have good reason to suspect that it would also precipitate once loaded.