Chromatography Forum

I want to enlarge the gas chromatography to a preparative scale,but the pressure increases suddenly and can not be back to the initial value when I inject the liquid samples,thus the flow rate of carrier gas changes, and results often are not repeated. For this problem I have read much literature,but they all do not mention it.
Is it really a problem or just a commen phenomenon that can be neglected? Many thanks

Please give in detail how much sample you are injecting and the pressure, flow rate, and column parameters you are presently using.

From your brief description of the problem it appears that you may be trying to inject or process too large a sample at one time.

A more detailed description will be helpful in determining a corrective action.

best wishes,

Rod

Dear Rod

Thanks for your help.

The vaporizer is set at about 140℃,column is 12mm i.d,120cm in length working at 115 ℃.Usually 1 mL mixture of water and ethanol (volume ratio:1/1) is injected in 10 second at a nitrogen flow rate about 400mL/min . the inlet pressure is 0.8atm.


Best wishes.

Your water/ethanol sample when vaporized will produce 'about' 800 to 1000 times the liquid volume.

So you are making enough vapor (1000 cc) to meet your 400cc/min flow rate for 2.5 minutes.

I wonder how much you are back flashing up stream in your carrier gas supply? How much is condensing back to liquid when it flows back beyond the heated zone of your injector? and how long it takes to vaporize at the lower temperature and elute back down through the injector and column?

I think you need to reduce your sample size 10 times and try it again.

best wishes,

Rod

Is your sample just water/ethanol, or is that the solvent?.

If that's the solvent you could try increases the sample concentration whilst reducing the injection volum as suggested by Rod.

Also, if possible reducing the water content would help with the vaporisation. If you're just separating water and ethanol, using the column as a liquid phase separation with thermal purge ( as for Molecular Sieves ) might give higher yields.

Please keep having fun,

Bruce Hamilton

Thanks

I will try to both increase the flow rate and reduce sample size.Many work should be done

Best wishes