Big problem with reproducibility.

Chromatography Forum: GC Archives: Big problem with reproducibility.
Top of pagePrevious messageNext messageBottom of pageLink to this message  By David Ewans on Tuesday, January 21, 2003 - 06:44 am:

Hello,

I'm working in a enviromental company ,we use preconcentration technic solid phase extraction ( SPE).
We use the C-18 phase as a solid phase. We measure pesticides in drinking water (Lindane, Hexachlorobenzene, Heptachlor, p-p DDT, Methoxychlor). Our problem is in poor reproducibility of the same level of concentration from one SPE column to other SPE column. I realy know, that my GC work properly, because when I measured one sample from the same SPE column e.g. five times reproducibility, is excellent(RSD15%).
Our SPE procedure is:
Column conditioning: 2column volume methanol, then 2 column volume dist.water
Sample application: Slowly aspirate 1000 ml drinking water through the column.
Column washing: 1 ml dist. water, dry column under vacuum
Elution: 1 ml hexane
I think possible reason is:
Not equal particle size of C18 sorbent.
I do not understand about this, because producer of C 18 SPE column is quaranteeing excellent reproducibility from SPE column to other SPE column.
Thank you very much for your advice.
Your sincerely
David Ewans


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, January 23, 2003 - 08:09 am:

Dale,
Not meaning to be rude, but 15% RSD on replicate injections seems a bit high to me. I used to work in an environmental lab and usually our RSDs were much smaller if I remember right. Also, you didn't tell us what the SPE column to column RSD was either.

Regards
Mark


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, January 23, 2003 - 10:55 am:

I agree. We are shooting for les than 3% RSD on replicate injections of the same standard. If you are getting 15 % on replicate injections, I would think there is something wrong with your GC.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By B.Buglio on Thursday, January 23, 2003 - 02:40 pm:

How are you injecting? An autoinjector should be
giving you a c.v of 1% or less.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Monday, January 27, 2003 - 02:33 am:

how about use of a internal standard???


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