Water as a Diluent

[Jana P]

Hi

I was wondering if anyone would be able to highlight some of the pros and cons for using water as a diluent for GC analysis.

I am currently working with a method which uses water as the diluent for my samples/stds etc and would like to be able to have a bit more confidence as using water diluent.

Thank you all

[Don_Hilton]

Water has a tremendous expansion volume in the inlet. Unless you keep the injected volume of samle down and/or the inlet pressure high, the sample can flash back into the inlet plumbing, resulting in poor quantitation and contamination of the instrument.

Various suppliers of chromatographic supplies have calculators for expansion volume. I would suggest you look at one of the web sites (Restek comes to mind - and I usually hit that one - just because I remember it.) and compute the expansion volume of what you intend to inject. Also look at other solvents that will disolve your analyte - and give less expansion volume.

Water will not hurt the methyl silicone columns - but stuff that disolves in water like salts and strong acids and bases can damage columns. I think there remains some debate about water on carbowax type columns.

I would suspect that the only pro for water - as far as the GC method goes is that it disolves your sample. If you are trying to share the sample with an LC method. Beware! Buffers are most likely to be a probem.

I've made injections in water - but prefer to avoid it if I can.

[Jana P]

Thank you for the advice

I shall have a look at the Restek site etc. as you suggest and look at what else might be an option for a diluent.

[RCBarata]

Can't you use something more like an alcohol (methanol comes to mind) instead of water?

[Jana P]

Methanol and ethanol would both be ideal, aside from the fact that they are what i am trying to monitor.

Any other solvents that come to mind?

[Don_Hilton]

What are you trying to monitor the methanol and ethanol in? And, at what concentration range are you expecting to find it?

[Consumer Products Guy]

We assay for alcohol using water as a diluent. We inject 0.5 ul, 30m x 0.53mm -624 type column, see <USP611>. As stated above, one needs a small injection volume due to large expansion of water in the inlet.

[zokitano]

A headspace extraction could be a decent choice if you're trying to analyze alcohols in water solution. You just need the HS instrumentation to perform it.
Other techniques that come to my mind (aside from direct injection and headspace) are solid-phase microextraction and liquid-phase microextraction (depends on what are the concentrations of the alcohols in your sample). They are suitable for determination of low conc of analytes in samples.

Regards