GC FID

[Kaliane]

Hello guys

I have a problem. While i'm injecting my standard samples, areas of the peaks are proportionals. But, when I inject the same wine sample 2 times, the area and the shape of peaks changes.
The GC settings are the same, so I think the possible reason is the matrix of my sample. Maybe some component could interfere in shape of the peak? Do someone know something about that? Some analyte that changes the shape os some peaks?
My wine samples has SO2 and acetic acid. My sample preparation envolves put 5 mL of CaO 2M, some drops of defoamer and destillation.
Another note: the standards are preparated with ethanol:water 50:50 while the sample has about 12% of ethanol.

Any idea?

[Consumer Products Guy]

I assume that you are injecting aqueous solutions. I'd try a smaller injection volume, start with half as many microliters, water expands a ton.

Also detail whether you are using an internal standard and GC column/conditions.

We assayed for ethanol in hand sanitizer products routinely by GC, validated or method for cGMP.

[Kaliane]

I will try to reduce the injected amount.

I'm using a 4-methyl-2-pentanol as internal standard and CP WAX 57 CB Agilent column. Dimensions: 50mx0,25mmx0,20um. The stationary phase is polyethylene glycol.
Split 1:100
FLow 2 ml/min
Carrier gas: nitrogen
volumn sample: 1uL
Oven temperature: 60°C during 2 minutes; heating rate 15°C/min until 120°C; 3 minutes at 120°C; heating rate 50°C/min until 200°C; 200°C until 1 minute.

[Bigbear]

If you are injecting water, as stated inject no more than 0.5 ul and have your injection pore no higher than 120C. You may need to lower the split ratio.

[Consumer Products Guy]

If you are injecting water, as stated inject no more than 0.5 ul and have your injection pore no higher than 120C. You may need to lower the split ratio.

We also used 0.5ul injection, from a 1ul syringe. We used a 0.53mm i.d. column.

And what's your inlet temperature?

[osp001]

Make sure your liner is compatible with the expansion of your solvent. As others have noted, adjusting to 0.5 uL injection will probably do it.

For future reference, Restek and others have calculators that will determine if your liner volume is compatible with your injection conditions:

https://m.restek.com/images/calcs/calc_backflash.htm