GC-FID Question
Posted: Wed Oct 09, 2013 9:19 pm
Hi,
I work at an alcoholic beverage company and one of my tasks is to quantify ethyl acetate (among other compounds) in our products. The method we use is pretty straightforward in that there is minimal sample prep - just add an aliquot of internal standard to the sample and go.
Now, the issue I have here is this: I noticed recently that there is a fairly uniform decrease in peak response for all compounds depending on the alcoholic strength of the sample. The trend is: the higher the alcoholic strength (%alc/vol), the higher the peak response. We have products and ingredients that span a range of strengths from 40-96%alc/vol, but I ran a quick experiment recently which has me questioning the method a bit. I ran a product at 80% strength, then diluted it with distilled water to 40%. The peak areas for all compounds dropped by 35-40%. This, fortunately was also true for the internal standard, so it compensated for the loss when calculating the final concentrations. Also, I should point out that all retention times remained repeatable to within 0.002 minutes.
The question I have is, what exactly does water do to my system to reduce the response of every single compound in my run? I am now preparing more samples to run to see if the amount of reduction is repeatable and also I will test at different strengths to see if it is a linear reduction as the water content of my sample increases. The reduction was observed on both GC systems with identical methods loaded.
I am running 2 Agilent 6890 GCs with J&W DB-624 (60m x 0.25mm ID) columns installed, identical auto-injectors, identical FIDs.
Carrier gas is H2 @ 50cm/sec
Inlet temp = 250 deg
Injection volume = 0.5 microlitre (to reduce chance of backflash considering water's expansion volume)
Split Ratio = 10:1
FID temp = 250 deg
I think I've posted all the relevant information about my setup, but if you need any more, feel free to ask.
Any ideas about this would be greatly appreciated, thanks for your time.
Chen.
*edit*In my rambling, I forgot to specify one thing in my question. The reduction in peak response that I have observed so far does not appear proportional to the dilution. Meaning, the reduction I saw in response based on a dilution from 80% to 40%, was different from what was expected from a dilution from 80% to 60% and from 80% to 70%, which suggests that there is another factor here at play other than simply the dilution effect.
I work at an alcoholic beverage company and one of my tasks is to quantify ethyl acetate (among other compounds) in our products. The method we use is pretty straightforward in that there is minimal sample prep - just add an aliquot of internal standard to the sample and go.
Now, the issue I have here is this: I noticed recently that there is a fairly uniform decrease in peak response for all compounds depending on the alcoholic strength of the sample. The trend is: the higher the alcoholic strength (%alc/vol), the higher the peak response. We have products and ingredients that span a range of strengths from 40-96%alc/vol, but I ran a quick experiment recently which has me questioning the method a bit. I ran a product at 80% strength, then diluted it with distilled water to 40%. The peak areas for all compounds dropped by 35-40%. This, fortunately was also true for the internal standard, so it compensated for the loss when calculating the final concentrations. Also, I should point out that all retention times remained repeatable to within 0.002 minutes.
The question I have is, what exactly does water do to my system to reduce the response of every single compound in my run? I am now preparing more samples to run to see if the amount of reduction is repeatable and also I will test at different strengths to see if it is a linear reduction as the water content of my sample increases. The reduction was observed on both GC systems with identical methods loaded.
I am running 2 Agilent 6890 GCs with J&W DB-624 (60m x 0.25mm ID) columns installed, identical auto-injectors, identical FIDs.
Carrier gas is H2 @ 50cm/sec
Inlet temp = 250 deg
Injection volume = 0.5 microlitre (to reduce chance of backflash considering water's expansion volume)
Split Ratio = 10:1
FID temp = 250 deg
I think I've posted all the relevant information about my setup, but if you need any more, feel free to ask.
Any ideas about this would be greatly appreciated, thanks for your time.
Chen.
*edit*In my rambling, I forgot to specify one thing in my question. The reduction in peak response that I have observed so far does not appear proportional to the dilution. Meaning, the reduction I saw in response based on a dilution from 80% to 40%, was different from what was expected from a dilution from 80% to 60% and from 80% to 70%, which suggests that there is another factor here at play other than simply the dilution effect.