normal phase (hexane) eluents with a silica column ???

[hfredricks]

Hi guys,
I'm in a pinch, I'm trying to prep-out some polar compounds using a brand new silica column (Zorbax Sil) and the retention of my polar cpds is all over the place - I was aware of potential problems with water + silica causing changes in retention (though I can't find any literature except a few forum posts), the first run was perfect, subsequent runs show more & more retention of the polar stuff..... My gradient eluents are ~20% IPA in hexane going to ~10% water in IPA (max 65% B!) so there's effectively a gradient of water from zero to ~7%... I really have to make this work in a short space of time (the boss has been making optimistic promises again) so I was going to try adding 1% water to eluent A to get at least consistent retention - any other suggestions?
thanks
Helen

[Uwe Neue]

I do not know the answer, but check on the miscibility of IPA with 10% water with hexane. If OK, try to keep the ratio of water to IPA constant.

[hfredricks]

thanks for the speedy reply Uwe,
yup, the eluents are miscible, at least in the proportions that we use them, its our tried & true gradient that we normally use on a diol column. So if I'm keeping IPA/water constant and my eluent B is ~10% water in IPA then that would be ~2% water in my ~20% IPA in hexane eluent A... I hope that will dissolve! the 1% took some persuasion in the sonic bath. Helen

[Uwe Neue]

If the "1% took some persuasion in the ultrasonic bath", you have a possible clue. Water does accumulate on all polar packings, much worse on silica than on a diol, for example. So you are likely to form a water layer on the surface, and this is the cause for your change in retention. If my 2% trick is not possible, you just need to make many, many blank runs until the column is in a happy equilibrium. Also watch the consistency of preequilibration conditions.

[hfredricks]

thanks Uwe, here's a thought - how about an isocratic method? it looks like I could run isocratically with a total of ~6% water (~ 50% IPA and the rest hexane). then the water content will be constant... Helen

[Uwe Neue]

events in isocratic methods are often more comprehensible and controllable than in gradient methods.

[bhuvfe]

Maybe you can try this column for your analytes.

http://www.microsolvtech.com/hplc/silica-c.asp

If the separation is ok for your targets you will not have a lot of change in retention times....

[hfredricks]

that's an interesting looking column, not heard of a type-C silica before, just out of interest, does anyone know what silica type a Zorbax-sil is?

[Uwe Neue]

Gee....

I would not bother with undefined nonsense such as C-silica. I did not even suggest to you to switch to a Waters silica, although I should have since Waters is paying my salary...

[hfredricks]

well, whatever the possibilities I'm stuck with the column I've got and an isocratic method is working out - retention times are consistent... at least consistently moving 30 seconds faster, but its settling down now... phew.

thanks for all your help!

[bhuvfe]

Gee....

I would not bother with undefined nonsense such as C-silica. I did not even suggest to you to switch to a Waters silica, although I should have since Waters is paying my salary...

Uwe,

Just for your information. Microsolv is not paying my salary and I'm just happy about that kind silica for my analytes.

Can you enlighten me about what you mean with "undefined nonsense"?