Question -GC-MS

[wqenvat]

Hi. I have a problem with my GC-MS. When I start running a sample the frontline inlet pressure starts increasing over the set point value until the pressure shutdowns.

Any possible causes to this problem?

Thanks

[Don_Hilton]

I assume that you are doing a splitless injection? What solvent? What volume? What liner? What purge off time? And what type of GC?

[wqenvat]

Hi Don

Thanks for replying. The GC that I am using is a Hewlett Packard HP 5973. And yes I am doing splitless injection. The solvent is choloroform, the injection volume is 2 uL. I am performing manual injection. I am not sure of the purge off time but I can double check. The liner is a glass liner, I will check for the part number of the liner or the dimensions. That will be more helpful.

Basically the front inlet pressure start to rise when the instrument is running the oven ramp temperature.

Any ideas of what could be causing the problem will be appreciated.

Thanks

[Don_Hilton]

Even if you are using a 4mm ID liner - I assume the GC is a 5890 or a 5890 with the 5973 MSD - I have a concern about a two microliter injection. If your inlet temperature is sufficiently high, the expansion volume of the solvent can be causing you a problem. If you are using a 2 mm ID liner, it is causing you a problem.

Be sure that you are using a 4 mm ID liner and try a 1 microliter injection. Also look at the backflash calculator on any of a number of web sites. I know that Restek has one.

It seems to me that I used to have this kind of problem - with a 1 microliter injection, different solvent, with a system where the head pressure was barely above atmospheric pressure at injection time. The solutions are to decrease injection size or add flow restriction (narrower or longer column) to increase inlet pressure.

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Without knowing the initial oven temperautre, it crosses my mind that if the column is cool enough, you may have condensation of solvent in the column - which is OK, unless you have enough solvent to condense and restrict the flow of carrier through the column.

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If you have an EPC and have selected the option to maintain constant flow through the column, the inlet pressure will rise - and so will the set point.

[chhubert]

I guess there may be some leak at the injection port. When the pressure builds up during oven ramp, the leak becomes imminent. Possible source may be the septum. Try replacing a new septum and just screw the "green" nut handtight or too much pressure may lead to piercing of the hole after injection.

Hubert

[wqenvat]

Thanks all for the replies. I was thinking on the possibility of leakage. I will try the different options.

My initial oven temperature is 70 and the oven ramp goes up to 275.

[wqenvat]

Thanks all for the replies. I was thinking on the possibility of leakage. I will try the different options.

My initial oven temperature is 70 and the oven ramp goes up to 275.

[wqenvat]

Thanks all for the replies. I was thinking on the possibility of leakage. I will try the different options.

My initial oven temperature is 70 and the oven ramp goes up to 275.

[Peter Apps]

On Agilent GCs you can set two modes of constant flow; constant volume flow and constant linear velocity. In both of these modes the inlet pressure is programmed to increase with oven temperature to compensate for increases of gas viscosity with temperature. So an increase in inlet pressure is expected, but it should not cause an overpressure shutdown. Check what the maximum allowable pressure is set to, and whether you have a sensible pressure at the start of the run. You might also have a blockage between the inlet and the EPC.

Peter

[AICMM]

wqenvat,

It would be very helpful to all of us if you posted your column flow conditions (inlet pressure, posted flow rate, etc...) and your column dimensions.

It would also be very good if you manually measured the flow rate from your split vent port, especially a couple of minutes into the run.

Best regards.