Chromatography Forum

Hi,

We want to measure ethylene gas concentration from air samples. We will collect air samples by using gas tight syringe. Our GC is Agilent 7890A. Can you tell me in detail how to do manual injection for this GC? Do we need a certain adaptor for manual injection? Thank you in advance.

Best regards,

Isma

I don't have anything newer than 6890, and my software is older. But isn't the first selection under Instrument labeled "Select injector source" and you would simply select Manual.

Isma,
You should be sampling the air sample by using air sampling equipments like Tedlar bag and personal pump (small pump).
After you get the air sample into your lab, I think you can use the gastight syringe to draw the air sample into syringe and then inject the air sample into the GC inlet as like as liquid injection manually without any adaptors, then press start bottle at GC. For injection precision you can try repeating manual injection several times to get the best precision, I hope it can help you.

Best Wishes
Jetjamnong

Dear Jetjamnong,
Thank you very much for your information...
Since I grow my plants in a closed bottle and put it in a growth chamber, so I think I can directly draw air sample from the bottle with a gas tight syringe, then inject the sample into the GC inlet.


Isma

ismawanto,

Be careful not to shoot too much volume (although the use of FID mitigates this somewhat.) If you do, you will cause a significant pressure pulse in the inlet. (Normally I recommend a sampling valve.) On the other hand, if I understand your other post, you are only shooting 5 - 10 uL so this should not be an issue. I will be surprised if you see plant ethylene at 10 uL with a 30:1 split..... but I have been surprised before.

Best regards,

AICMM

Dear AICMM,

I use 1 ml of air sample and directly inject the sample into GC inlet with 12 : 1 split. When I injected ethylene standard gas I got the peak, but when I injected my sample, there was no peak. Is it because of the ethylene concentration in my sample very low??

Best Regards,

Isma

I use 1 ml of air sample and directly inject the sample into GC inlet with 12 : 1 split. When I injected ethylene standard gas I got the peak, but when I injected my sample, there was no peak. Is it because of the ethylene concentration in my sample very low??

If the standard consistently gives you that peak, and the air sample consistently shoes no peak, then I'd infer that the level in the sample is very low. If I were to pursue, I'd try lower concentration of standard, and zoom in, see if anything in my air sample was 3:1 or 10:1 signal to noise.

Remember, there is no "zero", just "below quantitation level" or below identification level.

If the standard consistently gives you that peak, and the air sample consistently shoes no peak, then I'd infer that the level in the sample is very low. If I were to pursue, I'd try lower concentration of standard, and zoom in, see if anything in my air sample was 3:1 or 10:1 signal to noise.

Remember, there is no "zero", just "below quantitation level" or below identification level.

Actually when I checked & zoom in the chromatogram of my sample, I saw a minor peak exactly at the same position (retention time) as the ethylene standard gas, but the software did not recognize that one as a "peak".

I want to try lower concentration of standard, but my Lab don't have the equipment to make dilution for gas.

You can go 'old school' and dilute the gas yourself. See if you have a gas bulb (has valves on each end with a septum port on the side). Then just purge this nicely with air or nitrogen and manually inject a know amount of you standard into the bulb. Shake to mix and then retrieve the new standard with a syringe (maybe a different one for injection. The difference in injected volume and gas bulb volume is your dilution.