Chromatography Forum

Dear all,

I have a splitless GC method with split ratio 20 and the column flow is 3.00 ml/min. My problem is that the column flow isn't stable during the injection. The value of the total flow, pressure and the split ratio change all the time. What can i do?

Thanks!!!

There are many reasons for that, you can check:
* Leak
* Gas type not correct (in the method)
* Gas type not correct (in the gas supply)
* Column dimension not correct (e.g. too short)
*...

If you are vaporing liquid samples then it is to be expected that your column flow isn't stable, it isn't supposed to be when there is an increase in pressure due to the sample 'explosively' being vaporized.

Do you have any other information and details that you can share?

I do grow tired at times of guessing games, but I do want to help.

best wishes,

Rod

If you have a split ratio of 20 then the method is not splitless, is it ?

Peter

In a splitless method, the split ratio is only for the adjustment of the pressure. Can i have a splitless headspace method?
I am sure that my problem has to do with the method. I used a liquid method in split mode and the column flow was stable. Can someone reply to me?

Thanks!!!

If the split vent is open, there is flow out the split vent. And it soundls like your injection technique runs the pressure up in the inlet - which will result in an electonic flow controller cutting back on gas coming into the inlet - so the only thing that can go out the split vent if no carrier as is being introduced is sample. You do not have a splitless injection. Can you have a splitless headspace injection? Depends on what you are doing. If you are doing a syringe injection of a small enough voume that you can deliver the sample over a short period of time then yes, you can. If you are using an automated headspace sampler, you may not be able to do this.

Tell what kind of equipment and the technique you are using. The devil is always in the details.

In a splitless method, the split ratio is only for the adjustment of the pressure. The split ratio and the inlet pressure have to be independent of one another over wide ranges, otherwise it would not be possible to to set the column flow (which depends on the inlet pressure) and the split ratio, which is simply the raio of the split flow to the column flow, independently of one another

Can i have a splitless headspace method? Yes, but you have to take special precautions against dead volumes in the flow path, and you usually have to modify the headspace sampler in some way if you are using a valve and loop sampler. It would have been really useful if you had methioned in your first post that the method is headspace, and unless you give full details of all your operating conditions and the hardware that you are using this troubleshooting will turn into a long guessing game


I am sure that my problem has to do with the method. It is equally likely that it is due to faulty plumbingI used a liquid method in split mode and the column flow was stable. Can someone reply to me?Many people can reply, and some already have, but the replies cannot be helpful unless you take the trouble to post details of your set-up

Thanks!!!

Peter