Chromatography Forum

Can anybody tell me how to separate isopropanol and benzene. Their boiling points are really close so i know that the separation is really tricky. I am using a 6890GC with a 5973 MS and an HP-5 column for the analysis and i get a large blob which indicates that both elute at the same time. I want to measure the benzene concentration accurately. Thanks in advance!

Are you stuck with the HP-5 column and what are the relative amounts of IPA and benzene?

Are you stuck with the HP-5 column and what are the relative amounts of IPA and benzene?

Unfortunately yes. But I am interested in knowing if any column does the job. Benzene concentration is about 20% by volume

Start from around 40 °C and go up slowly (~5 °C/min), with your stationary phase benzene will elute a bit after 2-propanol, propanol is basically unretained. Raise your split ratio and/or dilute the sample and you should be fine.

I have only run the polar alcohols on my VOA instruments so I don't have a feel for their peak shape on an HP-5.
As an example I have analyzed a wastewater sample containing 5% IPA with labeled benzene as a internal standard using vacuum distillation and GC/MS on a Vocol column. Peak shape for IPA was not great but usable.

If you must use the DB-5 column one step that might give you better separation and better peak shape would be to add a derivitization agent into the mix. Then, the alochol would not be an alcohol - and would give a better peak shape. Derivatives could include TMS or acyl groups. Pick derivitization on the composistion of the mix and on what derivative will give you adequate separation from benzene. Someone readign ghis discussion may have some experience - on this one I can only toss in an idea...

I get a large blob which indicates that both elute at the same time.

No !!! You inject each separately to confirm that.

I want to measure the benzene concentration accurately.

Can you use an m/e ion present in benzene but not in IPA to measure the benzene, and then don't care about coelution? Or do you need to measure IPA too?

A more-polar column will preferentially retain the IPA, so you could get chromatographic separation, which is the preferred approach. Trying to make an assay fit a specific in-house column can be a hindrance to good science.

If you're using the 5973 then you have no need to physically separate the compounds. Benzene's primary ion is 78; there is no 78 in IPA. If benzene is really 20% of your mixture you'll need to inject under high split conditions or dilute the sample. It would be much better if you could change to a more polar column, but we have gotten excellent separation of IPA and benzene on an equivalent phase (Restek RXi-5Sil MS). Start at a relatively low temp, say 35, and make sure your carrier is set to optimum velocity for your column.

Your large blob is probably indicative of you overloading the column. Increase your split rate - for your sample I'd inject <0.5 uL using a split flow of around 200.

If you're using the 5973 then you have no need to physically separate the compounds. Benzene's primary ion is 78; there is no 78 in IPA. If benzene is really 20% of your mixture you'll need to inject under high split conditions or dilute the sample. It would be much better if you could change to a more polar column, but we have gotten excellent separation of IPA and benzene on an equivalent phase (Restek RXi-5Sil MS). Start at a relatively low temp, say 35, and make sure your carrier is set to optimum velocity for your column.

Your large blob is probably indicative of you overloading the column. Increase your split rate - for your sample I'd inject <0.5 uL using a split flow of around 200.

works like a charm. Thanks!