Nonlinear response in GC-FID

[skooma]

Hi!

I have a situation where I want to use single point calibration to determine compounds concentration. The results I get using single point calibration show the following trend (the numbers are illustratory):

Concentration - Recovery calculated from 100ppm Response Factor
200ppm - 150%
150ppm - 120%
100ppm - 100%
50 ppm - 80%
25 ppm - 50%
10 ppm - 25%

The peak area/concentration scatter is curved(Upward concave) making these recoveries reasonable, but at the same time other compounds during the same run give perfectly linear responses and recoveries.

What could be reason for this response?

[rb6banjo]

I plotted your data and it looks pretty good from 50-200 (just eyeballing it). It's those 2 low standards that are the big problem.

How are you introducing the sample to the instrument? What is the identity of the analyte? What are your analysis conditions (column, oven, inlet, etc.)?

[skooma]

I should have been more clear with the explanation.
The percentage is not peak area% compared to the calibration level, but recovery calculated from (concentration found in sample)/(concentration added).

So it's not ok if it's 100% +- the error limit.

I use autosampler, 1ul injection, 1/10 split. It's MeOH solution.
The system is Agilent 7890B, DB-ZB-1 Column, 80-280 gradient, injector temp is 280(I have tested different temperatures, the trend is the same)

[rb6banjo]

A plot of your concentrations vs. the % you give is the same thing (proportionally), correct?

It's actually linear from 10-50 and 50-200. The slopes of the lines are different, however.

10:1 split should be ok. Are you using an inlet liner that contains glass wool? What is the material you're trying to determine? If your analyte is something that's prone to problems (e.g., carboxylic acids, amines, etc.) then your problem could be adsorption of your analyte to active sites in the inlet/on-the-column.

If it were me, I'd try a model system - decane in methanol for instance - to see what I get with that. Takes the adsorption phenomenon out of the system. If that works, then you know your system is ok.

[skooma]

Thanks for the info!
I will try inert liner, because I allready added some DMSO to the samples which gave OK results during the same run. The compound is an amine so maybe it is absorption problem.

[rb6banjo]

I've never had success with amines on any other stationary phase except a wax. You may have to switch columns. Amines are very difficult.