Need your suggestion- GC

[m.banerjee]

I am working as a Research Scientist at a nanotechnology company. We work on application of membranes in ethanol industry.

In one of our projects, i am stuck at this point. I am trying to analyse a mixture of organics by GC. This mixture possibly contains various alcohols, acids, aldehydes, ketones, esters, ethers etc.

Just to identify the retention time of various compounds on our coulmn, i am diluting each standard with ethanol and injecting it separately.

I am not able to get a peak for the following compounds as they tend to react with ethanol:
1. Acids (Acetic acid, propionic acid, butyric acid, isobutyric acid etc.)
2. Iso amyl alcohol
3. Phenylethanol
4. Diacetyl

I have also tried to dilute these with Acetonitrile and Acetone. Everytime it results in multiple peaks due side reactions.
I tried DMSO, but it doesn't elute in the method i am using. It requires a temp of 200 deg C.

The method i am using is: 50C for 25mins
50-150C at 20C/min
Hold of 10min at 150 deg C

The column i am using is DB-1 column (capillary: 60m), the stationary phase is Dimethyl Polysiloxane.

Can anyone help me out?

[chromatographer1]

Acids tend not to chromatograph well on a methyl silicone column

You should be able to see isoamyl alcohol and phenylethanol.

Diacetyl is volatile and may be hidden by your ethanol peak.

What is the matrix you are injecting within which you are looking for these impurities? Is it ethanol? or water?

What levels are you trying to see?

What detector are you using?

best wishes,

Rod

[m.banerjee]

Thanks for the reply!

I am trying to analysis an industrial ethanol sample. And maximum impurities are present in ppm levels (50-500), except some which might be around 0.1% (1000ppm).

The detector i am using is FID.

[Willy]

Heya!

First carboxylic acid are difficult to be detected directly by GC-FID. One way will be to do esterification of the CA into their methylester correspondant. Usually we use headspace to do that type of analysis rather than direct injection. If ethanol is use as solvent you may have few peaks eluting at the same time.

As i know: acetic acid is feasible by HPLC;
all other Carboxylic acids can be derivatized and analysed by GC-Headspace; for the other solvent you post they are analysed by GC-headspace too. Im never in favor of doing direct injection with solvent because ive experienced poor reproducibility.

I wish you good luck if you want to do eveything with only one method. 2 or 3 method should be needed to perform all the analysis.

If you need more details, post them and will be glad to answer back.

Hope it will help a bit.

Willy the GC

[m.banerjee]

Thanks!!! The information is very helpful.

I have tried other methods too and will be using them.

As far as Headspace GC is concerned, the problem is that maximum of the components in the sample are not volatile... most of them are heavy alcohols, acids and esters.

And can anyone tell me what column can i use for analysing acids? Rod has clearly said that acids do not chromatograph well on methyl silicone columns.

I am reading about the option of headspace GC also. Will get back if i find anything.

[krickos]

Hi

Been a while since I did carboacids analysis but I know that a DB-wax column (at least up to heptanoic acid) alternatively a DB-FFAP column (at least up to decanoic acid) usually works fine.

Personally I would proberbly try a 30m*0,32mm*0,5um DB-wax column. Keeping the start temp in the column low (40°'C) to start with and then slowly increase temperature in the begning in order to separate the ethanol (bp 78°) from the diacyl (bp 88°) and other alcohols.

Cheers Chris

[chromatographer1]

T195890 Packed Column Application Guide

This technical guide at Supelco web site will be informative.

http://www.sigmaaldrich.com/Brands/Supe ... raphy.html

is the web page that has the link for the free download.

best wishes,

Rod

[m.banerjee]

Hey There!!

Thanks a lot for the link.

It's very useful.

I am sure it will help a lot.