normal phase LC-MS additives

[hfredricks]

Hi everyone, we've been using a normal phase method in our lab for LCMS for years;

a gradient of 100% [20% IPA in hexane] to 50:50 [20% IPA in hexane] : [10% water in IPA] - both eluents have 0.1% formic acid and 0.04% of (25% NH3 aq)

now we are moving to new hplc columns (long story), they are a little less retentive and so changing eluent A to be 10% IPA in hexane instead of 20% IPA in hexane seems to work well... except that [i]sometimes[/i] the ammonium formate crashes out of solution and leaves a mess of white floccy ppt at the bottom of the bottle. Thats bad enough but it only happens sometimes... I don't get it. how can I get over this? I know I'm pushing the solubility of NH4OH in so much hexane. Would adding more acid work? is it hopeless? both the formate and ammonium are essential in the method for ionization / adduct formation so I don't want to lose them...

thanks, Helen

[sepscientologist]

How about if you double the ammonium formate in B and take it out of A? You may introduce some mixing noise
but it may be worth a try.

[hfredricks]

hey that's an idea! I'll try it - so long as there is enough NH4 to adduct an early eluting compound we should be ok. thanks!

[KDF]

Another approach is to get rid of the water in the hexane side and just use ammonium formate salt and formic acid in IPA to make the buffer. Taking the buffer conc from 0.1% to 0.05% may help.

Also consider ammonium acetate as a more soluble alternative. This is soluble to about 8% in MeOH ; don't know about IPA.

In the long term try SFC [supercritical fluid chromatography] and get rid of the hexane.

[hfredricks]

thanks KDF, some more good ideas to go try...