Chromatography Forum

Hello everyone,

I use a gastight syringe to take a gas sample from a 5LTedlarbag which is filled 2L sample. After drawing the sample, is the pressure in the syringe is atmosphere pressure (760mmHg)?

Thanks,

Grace

If you expose the tip of the sample needle to the atmosphere then if there is any pressure difference between the atmospheric pressure and the pressure inside the syringe it will either intake air or expel sample until the pressure is equal.

best wishes,

Rod

Whilst Rod's answer is correct and the best option, you risk the possibility of contaminating your sample. If you want to try that technique, one option is to slightly squeeze the tedlar bag to create an above-atmosphric pressure in the syringe, and quickly seal the needle after gas has vented from the needle on removal from the sample bag.

In general, sampling bags often are slightly above-ambient pressure and shrink when the sample is taken. If your gas tight sryinge has a freely-moving plunger, the plunger should move to bring the sample to atmospheric pressure.

Unfortunatly most syringes have sticky plungers. I tend to sample, close the valve, and push the punger gently in - just to ensure the sample is at positive pressure.

Bruce Hamilton
Bruce Hamilton

Thank both of you!

Could you please recommend a good syringe (10ul-500ul ) for this purpose?


My purpose is that using cylinder gas standard , test unknown gas samples in Tedlarbags. therefore I need to know the pressures in syringes or ensure the presuures are same.

Thanks,

Grace

Thank both of you!

Could you please recommend a good syringe (10ul-500ul ) for this purpose?


My purpose is that using cylinder gas standard , test unknown gas samples in Tedlarbags. therefore I need to know the pressures in syringes or ensure the presuures are same.

Thanks,

Grace

I would recommend Hamilton (series 1700) sample lock or VICI Series A-2 syringes, both available from Alltech among others.

If possible, I would recommend using side-port needles.

Thank JGK very much!

Grace