My work is on CO2 photoreduction in gas-solid phase, and i'm doing it in a batch mode. i analyze my gas samples every 1 hour using GC TCD/FID by manually withdrawing the gas samples (0.5ml) using a gas tight syringe and injecting the samples into GC. The products of my reaction are CO, H2, C2H6 and C2H4. Throughout the reaction period, i obtain a linear plot for the formation of H2, C2H6 and C2H4 however, CO peak area always fluctuating, sometimes its very high (up to 20mol%) and sometimes near to 0. I tried to perform few runs to see if the CO formation trend matches each other, but the deviation is too high.
Is anyone have experience in handling a similar situation? I would appreciate your help.
Dear Tharani Kulandaivalu
Very interesting! I would suggest revising the chemical equations and the number of mole for reactants and products, if everything is fine, then please to consider the adsorption capability of CO. Carbone monoxide can be adsorbed to susceptible surfaces unlike the others (i.e. H2, C2H6, and C2H4).
Another important issue to look at; does the amount of CO laying within LLoD for the GC detector? otherwise, it is drifted and a kind of noise.
All the best though
Zaid
As correctly stated by Prof Zaid Al-Obaidi , if the concentration is close to the limit of detection (LoD) you would have fluctuation in peak areas: however, I do not think it is the cause of peak fluctuation in the experiment of Dr Tharani Kulandaivalu , since concentration is up to 20%mol, and I expect that LoD is far lower.
So, I recommend that you carry out an accurate validation of your GC method: you should know exactly what is the accuracy and repeatibility of your analytical method, so that you will be able to calculate the uncertainty of your measurement. Only after doing this, you can discuss if the area variation is due to analytical noise or other causes.
After having assessed that your area fluctuations are statistically significant, then you can discuss if the reason is due to the photoreduction experiment, or if there are other causes.
Best regards
Gas injection through syringes hardly achieves good reproducibility. If you have the means, you should try gas-sampling valves as injection technique. They are extremely reliable and attain the best reproducibility you can have in gas chromatography (GC). It is an old-fashion technique and not so many people are acquainted with it since gas sampling is not as ubiquitous as liquid sampling in GC. However, they can reach the same reproducibility you can have in HPLC valve-sampling.
If your gas-tight syringes are really good, this should work. I did not see use of a liquid plug but you have a choice of "nearly gas" How about butane/pentane mix.....blocking the loss of your gases from the syringe: set up the mixture with a bit of pentane then butane last before your sampling and injection. If the butane is lost or diminished, the syringe leaks. Otherwise gas-sampling precision valves are good!
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