Chromatography Forum

I have a Varian GC/Ion Trap MS system, using temp 70C to 310C, ramp 5C / min with flow of 1.0 ml / min on HP-5 MS column.

The column gas flow do not keep stable if split is open. I even reach the GC stabilization for a new injection if split is configured at the beginning or in the end of the run. Someone guest what is the problem?
Looking at how to solve this, I realized that I may need to calibrate the gas flow, using the round black button "flow control". However, I do not know how without specific tools. Someone could explain me how to do this?

Thanks in advance for your attention.
J

Welcome to the forum.

What model of GC is it, and what is the split ratio that you have set ? If you measure the flow out of the split outlet, does it fluctuate, and oes the inlet pressure fluctuate ?

Peter

Also, what is the carrier gas pressure supplied to the inlet? (What does the regulator read compared to what the inlet needs to maintain the set flow?)

Thanks for the replies.

I utilize a Varian 3800 and the split ratio is 20. I can not say if I understand your question Peter, but looking at the display, both the flow and the pressure fluctuates.
The carrier gas suplied is 8.8 psi, ranging from 7-12 psi, while the flow varies between 0.8-1.2 ml/min.
Any thoughts?
Joao

Hi Joao

Probably a partial blockage in the split vent line - does it have a filter installed in the split line, and when was it last changed ?

The EPC module might be malfunctioning, but your mention of a black knob sounds like a mechanical regulator of some sort, it is probably the septum purge regulator. Depending on what software is controlling the instrument you need to adjust the septum purge to 3 ml/min (using a flow meter on the septum purge outlet to measure the flow) and then enter the flow in the software.

A thorough leak check with a leak seeker is also a good idea.

Peter

If I understand you correctly the GC pressure reading is 7 - 12 psi and the regulator is set at something higher than that, probably more than 8.8 psi. If the carrier gas pressure supplied to the GC is too low it will cause variable flow.
If the problem is not obvious you will need a flow meter for diagnosis.

I think Steve is on to something. Most GC's have a minimum pressure setting to enable the injection port to work correctly. At a minimum try setting the tank regulator to 30psi.

Hi,

I tried to look for any obstruction on the split valve by calibrating the split gas flow, but everything was OK. I never saw a filter in the split line, it is something obvious of being found?
Either way, for not having any specific tools to check the gas flow, I decided to call for technical support. I`ll tell after what was the solution. A malfunctioning of the EPC module sounds expensive..

Thanks,
Joao