Signal change in FID measuring methane

[kahmark]

I have an Agilent 7890 with a Gerstel sampler. When I inject CH4 stds from 0.6 to 3.3 ppm I see now linear trend. The peak hts and areas are very similar, any suggestions?

Kevin

[chromatographer1]

I suspect your 'blank' levels are close to 3 ppm so adding small amounts at 1-3 ppm are not showing up in a linear fashion.

and your blanks may not contain consistent amounts of methane as well.

Does that make sense to you?

ROdney George

[kahmark]

Excellent comment. The difference from my lower end std to the blk is quite close, an area count of 16 to 13.5 for the blk (vertical scale is in pA).

As it turns out one of my Gerstel syringes may have a leak or is of a short lifespan before the seal leaks....still haven't tested this well yet.

So point well taken, I suspect my MDL for this techniques is not much lower than 2ppm, which is a problem.

Methane in the blk....how can I decrease the concentration in the blank this besides adding a sample loop injection.

Kevin

[chromatographer1]

Catalytic oxidation of the methane to CO2, much like the converters on our automobiles can bring the levels of methane down to near zero levels.

Good luck,

Rodney George

[AICMM]

Try helium or hydrogen in your blank vials as a proof of problem...

Best regards.