Chromatography Forum

I am using a headspace GC with a pressure balanced injection technique. I face the problem of injection needle (SS Needle) getting choked frequently resulting in lower area counts for the solvents. After cleaning the needle a little improvement is seen. Sometimes it is required to replace the needle. I am using PTFE coated butyl rubber septa for my vials. The thermostating temperature is about 90 Deg and the needle temp is about 95 Deg for most of my analysis. How can I get rid of this problem? Is it due to septa problem at high temperature? (Use of silicone rubber septa also tried and of no use) or due to my sample matrix? (The samples are beta lactam antibiotics like penicllins and cephalosporins)

Did you ever try to increase the temperature of needle (and also the temperature of transfer line). As you can read in the literature, e.g. in user manuals, the recommended temperature for needle and transfer line is (minimum!) 10 °C higher than termostatting temperature.
I'm always setting the needle-temp. 20°C-30°C higher than termostatting temperature.

Somewhat obvious but make sure your needle isn't dipping into the solvent.

I would suspect that you are depositing thermal decomposition products from the matrix you are analyzing.

I would try to lower the temperature of your analysis. Do you have to keep the solution at 90°C ?

Can you use a lower temp and do the analysis reproducibly with a different slope to your concentration variation?

You could also try to heat the sample for a shorter time to minimize the decomposition deposits within the needle.
As noted earlier in another response, be certain you are not overfilling the vial and getting liquid solution into your needle.

Hi,

The headspace vial being used is 22 ml capacity and I am taking only 2 ml my analyte. so there is no overfilling of vial. As mentioned by Chromatographer1 I strongly suspect of some decomposition product choking my needle. I will reduce the thermostat temperature and make the trials to find out this. Thanks