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Ethanol analysis irreproducible results
Posted: Tue Sep 20, 2011 3:03 pm
by YDY
Hello,
I am using 6890N Agilent GC to measure ethanol and the column is Econocap-EC 1000 capillary column. The oven temperature is 120 C and run time is 3 min. I am trying to develop a calibration curve for ethanol concentrations between 500mg/L to 10 mg/L ethanol and most of the time the peaks are very good and areas of three injections from the same sample is very close. But at every 10-15 injections I got distorted peaks and the area is quite low. I was thinking that it might be because of the auto injector syringe and replaced it with a new one and the problem is still there. Could you please advise? What can be the reason?
Thanks
Dilsad
Re: Ethanol analysis irreproducible results
Posted: Tue Sep 20, 2011 4:24 pm
by Consumer Products Guy
Are you using water as the solvent? If so, how many microliters are you injecting? Water has huge expansion volume in the inlet.
We assay for alcohol routinely, and we inject 0.5ul, do not inject larger volumes of water. I assume (probably wrong) that you are using an internal standard, such as n-propyl alcohol or ACN as used in <USP 611>.
Re: Ethanol analysis irreproducible results
Posted: Tue Sep 20, 2011 5:38 pm
by YDY
Solvent is water and the injection volume is 1 uL (splitless). Now I am testing a ramp of 10 C/min and will start from 80 C and goes up to 120 C. What else can I test? Thanks
Re: Ethanol analysis irreproducible results
Posted: Tue Sep 20, 2011 10:03 pm
by Consumer Products Guy
Inject smaller volume, 0.5 ul. Use Split injection technique.
Re: Ethanol analysis irreproducible results
Posted: Fri Sep 23, 2011 2:28 pm
by Bigbear
120! How short is your run? You could get ETOH and N prop out within 7 min or so @ 40.
In shooting 0.5 ul water I wouldn't set the injection port any higher than 125.
Re: Ethanol analysis irreproducible results
Posted: Fri Sep 23, 2011 2:36 pm
by YDY
Thank you guys, I got it. I tried a different program with initial temp of 80C and then up to 120 C with a ramp of 10/min and this helped. still the injection volume is 1 ul. I got a nice calibration curve and peaks are ok. Thanks a lot for your help.
Re: Ethanol analysis irreproducible results
Posted: Fri Sep 23, 2011 5:16 pm
by Consumer Products Guy
In shooting 0.5 ul water I wouldn't set the injection port any higher than 125.
<USP 611> uses 210 °C inlet.
Re: Ethanol analysis irreproducible results
Posted: Fri Sep 23, 2011 7:18 pm
by aldehyde
http://www.chem.agilent.com/en-US/Suppo ... fault.aspx
Here is the Agilent/HP vapor volume calculator. You can use this to identify how much vapor you are creating. I suggest you put in your parameters and see if you are exceeding for liner capacity, even if you have things working correctly. You may be able to get better results with smaller injection volume as others have said.