Dear all, yes, as Natalie said, my problem was simply not turning on the backflush. After that the background became ok and I got the peaks. But I still have problems. My peaks come all together in a narrow window between 1650 and 2450s when I set He flow to 1mL min-1, and to 1250 to 2000s when I set it to 3 ml min-1. I have a reference that shows peaks distributing 800 and 2200s, with all peaks very well separated and clearly identifiable. I am following the same method as them: start at 60oC for 2 min, then go up to 120oC at 3oC per min, then go up again to 190oC at 10oC per min. Column length is 50m, ID is 0.32mm. Does anybody have any suggestion to increase peak separation?
Thank you for any help,
Matheus C. Carvalho
Senior Research Associate
Centre for Coastal Biogeochemistry
Southern Cross University
Lismore - Australia

De: Natalie Wallsgrove <[log in to unmask]>
Para: [log in to unmask]
Enviadas: Quinta-feira, 28 de Junho de 2012 10:58
Assunto: Re: [ISOGEOCHEM] amino acid measurement

Hi Matheus,
We regularly analyze AAs in liquid ethyl acetate (no more than 4ul at once), and we leave the backflush on for 550 sec post injection to avoid a solvent peak (we have a 60m column). Hope that's helpful.


On Jun 27, 2012, at 14:26, Geert Van Biesen <[log in to unmask]> wrote:

> Matheus,
> are you injecting pure (liquid) ethyl acetate? In that case you generate a huge
> amount of CO2 and H2O and you are very likely overloading the system (hence the
>> 50 V m/z 44 peak). Why don't you try sampling from the headspace: add about
> 100 microliter of ethyl acetate into a sample vial so that the syringe only
> draws up vapour, and see how that works out. If you would use ethyl acetate as
> solvent for your samples, make sure you have the backflush on long enough to
> divert it from the oxidation reactor.
> Good luck,
> Geert
> Geert Van Biesen, PhD
> Research Laboratory Associate
> CREAIT - Stable Isotope Lab
> Memorial University of Newfoundland
> [log in to unmask]
> 709-864-4037
> Quoting Matheus Carvalho <[log in to unmask]>:
>> Dear all,
>> we are trying to measure amino acids here, and there is a problem that I
>> cannot understand.
>> I installed two 25m columns in the GC oven, one coupled to the other, so that
>> the total length is 50m. I put a new oxidation reactor, and new reduction
>> reactor too. I also immersed the capillary line fully in liquid N2. I checked
>> for leaks, and also injected air in the system (5 to 10uL), and every time I
>> did so I could get nice N2 peaks, of about 12V for mass 28.
>> So far so good, I thought. Then I injected Ethyl Acetate. Almost every time I
>> did that, the background for masses 28 and 29 would increase from nearly
>> 100mV at the beginning to around 2000mV at about 600s, and stay there
>> permanently, even if I change the backflush. The only way that I managed to
>> decrease the background again was to remove the line from the liquid N2 bath.
>> I repeated the procedure several times and it was always the same: good N2
>> peaks for air, strange increase in background for Ethyl Acetate. In addition,
>> when the background increases the way I said, I get no more peaks for air,
>> unless I take out the line from liquid N2 and try again.
>> One more thing that I tried was to inject ethyl acetate without the liquid
>> N2, and check for mass 44. I got a huge peak (>50V) followed by a decrease in
>> the background again, with time.
>> In summary, I think someway CO2 (or H2O?) is blocking the line in the liquid
>> N2 trap, and at the same time air gets inside (so the increase in the
>> background) from somewhere (I have some connections through the capillary
>> line).
>> I appreciate any help.
>> Regards,
>> Matheus C. Carvalho
>> Senior Research Associate
>> Centre for Coastal Biogeochemistry
>> Southern Cross University
>> Lismore - Australia
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