Chromatography Forum

Hi
I am trying to develop a calibration curve for cis-dichloroethylene using GC/FID. But I haven't been able to see any peak so far.

I have a stock solution of cis-DCE (0.85mg/ml) in methanol and inject 0.005 of the stock solution in a 10ml vial containing 5ml of water. This gives me a concentration of 0.85 mg/L in water and approximately 0.1 mg/L in gas. After one hour I take a 25microliter headspace sample and inject it in the GC.
I am running the samples in a GC with a Rt-QPLOT column (from restek). The oven temperature that I am using is 60 degree C hold 2 min, 10 degreeC/min to 150 C hold 5 min. The detector temperature is at 250 C. The injector at 200 C.

My idea is that I am injecting a small volume to the GC. The lab manager recommended me to inject at most 25uL. Do you have an idea on what could be the reason for such a small volume of injection? Do you have any idea of what is wrong with my method? Do you have some suggestions to improve the method?

THANKS
ITZA

Calculating from the gas phase concentration that you give, a 25 microliter injection contains only 2.5 ng of dichloroethylene. Since the chlorines weaken the FID response, this is just too little for the detector to see.

Presuming that the PLOT column is 530 micron i.d you could try to inject a larger volume.

Peter

Hi Peter

thanks for your help. I have another question:

Is there a way to know what would be the maximum volume that I can inject or the volume that would give me better results?

THANKS
ITZA

Hi Itza

In principal it is possible to calculate this, but experimantation is more straightforward and more robust. Keep increasing the injection volume - you will first see a peak that gets bigger as volume increases, peak height will increase in proportion to volume injected, and the top of the peak will be sharp. When the volume gets too big for the conditions that you are runing under the peak will get wider, not taller, and its top will flatten. When you see this you have a condition known as volume overloading.

Beware that the maximum volume depends on gas flows into the column and the temperature programme.

Peter