Chromatography Forum

I am trying to find a good method for GC analysis of an ethanol fermentation reaction. I am using an agilent 7890A FID with an HP-Innowax column. I have not been able to find a good instrument setup for the GC online and was hoping that someone on here had a method that they were currently using or have used in the past. I also have a high amount of salt and simple carbohydrate impurities in my samples.

Thanks in advance

With Innowax, we run ethanol at 35C or 40C isothermal. We use n-propyl alcohol internal standard. We make up samples in water and inject 0.5 microliter, no more, split inlet.

Supelco has an application note T300164.

You may wish to review alternatives.

Rodney George
Supelco

I am also looking at ethanol in fermentation broths. I am using a similar column and am having issues of sugar/salts/solids build up in the injector and that is even with a high split ratio (100:1).

I am looking for some ideas on this also.

The supelco note really is for distilled (already concentrated - 40% ethanol) ethanol products. That is not what fermentation broth is. It has cells, salts, organic acids, and other components along with ethanol (like 0 - 15% ethanol).

FIRST: I no longer work for Supelco.

This application really should be done by static headspace.

Unfortunately, this means for most folks that a special HS analyzer must be purchased, and given the amount of problems that contamination can cause, it could be cost effective if there were a constant need for accurate analysis, as in process control.

I have tried to get instrument makers interested in an inexpensive add-on solution replacing a standard HS analyzer but no one was interested in cannibalizing sales of the profitable HS units for a product that would cost $500 and sell for a few thousand dollars.

So the best I can offer you today is to use SPME headspace analysis which can be done with a few hundred dollars of equipment. At least you won't contaminate injectors and columns with JUNK while still getting the numbers you require.

See Supelco for details.

best wishes,

Rodney George
consultant

Rod is right - static headspace is the best solution. As an interim measure you could try doing it manually with a gas-tight syringe, or if ethanol levels are high you might get away with taking a headpsace sample from a 2 ml vial using an ordinary autosampler with the needle depth adjusted upwards.

As always a lot depends on how accurate and precise you need the results to be.

Are there other volatiles that you need to separate form the ethanol ? - all the broths that I ever analalysed had acetaldehyde, other alcohols and some esters also.

Peter

Headspace may be a cleaner technique and leave some of the nasty stuff in the fermentation broth, but to determine what is going on during the fermentaion you may want to see what is in the mix that hasn't been converted to the volatile oxygenatied species. To do a full analysis of a fermentation mixture LC and GC are usually used. I would suggest looking in the literature to see what methods have been used in the past.

If all you are looking for is the ethanol and other volatile products, use headspece, that will be much simpler.