Inlet pressure too high

[Nance]

We have been conducting glucosinolate analysis with the same method for a year on a 6890. Following routine replacement of merlin microseal, inlet liner, gold seal, etc. the inlet pressure is higher than the set point at the initiation of a run. We cleaned the split vent trap and line. Our derivitization method produces a lot of contaminants that are caught in the trap. The actual inlet pressure is equal to the set point before a run is started and achieves whatever set point is entered when either the column flow or pressure set point is changed. Once a run is started the inlet pressure rises above the set point and so no injection can occur. Our method uses a 50:1 split but we found that we had to decrease the split to 35:1 so that the inlet pressure could match the set point. This worked for a while and then we had to reduce the split again. Can the problem be anything other than a plugged split vent trap and line? Thanks.

[gcguy]

The inlet weldment on the 6890 is prone to blocking and contamination at the connection where the split line connects to the inlet. We take our entire weldment out and clean them once a year, we usually find bit of wool and brown muck in them. Not sure if this is what is causing your problems. Have you checked the epc units to make sure they are working correctly?

GCguy

[Peter Apps]

Yes, the problem could be something other than a plugged line or trap, but that is by far the most likely cause of the problem, and so should be the first thing that you try to fix.

Peter

[Nance]

Are there any special instructions for removing the weldment assembly itself and cleaning it? We can't find any in the literature. We did notice some material in the inlet connection to the assembly and tried to remove it with K wipes.

Thanks.

[Nance]

Continuing on, we have cleaned the split vent trap, split vent line, inlet weldment assembly. We replaced the column with one of known performance and now we find that even in the splitless mode, the actual inlet pressure is higher than the set point.

What can be the problem? As far as I can see, in the splitless mode, the only way to have a restriction in flow even before any injection occurs, is to have a blockage in the septum purge line. Is this possible? If so, how can I clean that?

Thanks.

[gcguy]

Nance,
I am thinking that there could be a problem with the EPC units. Have you checked the actual flow using a flow meter? It might answer a few questions as to whether the split line or the septum purge is the problem. I would also check that the column itself is not blocked with a bit of ferrule.

GCguy

[Nance]

The problem exists with several columns. I have checked the septum flow and the flow varies from +2 to negative values. I finally found some old lab notes relating to the instrument which describes a diagnostic test. I could not find reference to this test anywhere else. By going to Options-Diagnostics-front inlet-pneumatics control-split valve control-front inlet split valve-manual control off or on, and comparing the responses of each of our gcs, I found that switching the valve stopped the split vent inlet flow on the second gc but NOT on the one with which I have had problems. This was fixed before by replacing the complete EPC manifold. I guess this must be the problem

I am not sure if this would also cause the septum purge flow to be so low.

Thanks.

[gcguy]

Nance,

Sounds like this may be the answer. We would normally just check the epc units by setting a flow and ensuring that the measured flow using an external flow meter matched the set point. We had some issues in the past with epc units "going" until we started filtering our gas supplies. It was in the air supply to the FIDs. The supply quality changed and the compressors inline filters had been removed without us being told!!

GCguy