Temperature profile for alcohols

[natalieshen85]

Hey folks,

I'm going to analyze ethanol and butanol in the liquid samples. I was recommended to use Zebron WAXplus capillary GC column[/color] with the guard column (my samples are taken from fermentation broth and they contain a lot of sugars and salts, so a guard column is used to trap those non-volatiles). The GC is Varian 450 with the FID.

I'm a green-hand so I'll appreciate it if anyone could check the method for me as below:

Temperature profile: 35 C for 5 min to 130 C at 10 C/min, then to 210 C at 30 C/min, stay at 210 C for 5 min.
Injector/Oven/Detector temperature: 200 C, 210 C, 220 C
Carrier gas: helium, 5 mL/min
Injection: split 10:1 1 micro-L

Any feedback will be appreciated.
Thank you!

[Don_Hilton]

It looks like a reasonable start at developing conditions. The analytes are the easy part. The question is what does the matrix do to you when you run the chromatogram. The best way to find out if these are good conditions is to make an injection. How does it look?

On column flow. The ideal flow rate expressed in ml/min depends on column diameter. In general somethint between 23 and 45 cm/sec typically works well. Your GC may give you linear velocity as one of front pannel options. Below 23 cm/sec is usually not good. I've seen people run faster than 45 cm/sec.

[chromatographer1]

Instead of using a 30 meter Wax column I would use a 3 meter piece of it as a guard column for a 624 or 1301 column.

If you are using 5mL on a 0.53mm ID column, I would try to increase the flow to at least 8mL. If you are using a 0.32mm ID column then you are fine.

If you insist using a single wax column I would at least cut it into half and use a 15 meter piece for twice the time of use of the column. Why do you need a 30 meter column? Your IPA separation from ethanol on either ID column will be marginal anyway. That is why I would use a 624 column with a wax precolumn.

I have already done this. My HS paper in AC, 1997 June, used a guard and analytical combination column.

The alcohols will elute faster with a 624 column in place of a Wax column and you can use lower temperatures.

best wishes,

Rod

[Bigbear]

I agree with Rod try a 624 column.
If you are concerned with all the extra "junk" in your samples you should consider using headspace. You can do it manualy ,all you need is some headspace vials and caps, and a way to incubate the samples.

[Chenosuke]

Maybe you could also consider distilling the sample and then running the distillate on a 624 column? That for me is the cheapest way if you don't have a headspace sampler.