Setting the split ratio on a modified 5890 Series II GC

[bedlamVR]

Hi,
This may be a simple quetion but we have a modified injection port ( to take a CDS 5000 pyroliser)on a 5890 series II GC linked to a Trio 2000 Mass spec running a 0.25mm cappilary colulp (CP SIl 5 i think).

The question I have is there a way to check the split ratio on the system without having to inject an unretained species onto the column...because thats no as easiy as it would sound . I am getting some stange chromatography and I was wondering if something had changed in the settings .

Any help would be appreciated. Sorry if this is the wrong forum.

[AICMM]

BedlamVR,

I am assuming you are using a standard splitless/split 5890 injection port, probably one you have cut into so you can tie in a transfer line? If not, can you provide more of a description?

Is this EPC controlled or manual flow control and can you describe the symptoms a bit more...?

Best regards.

[bedlamVR]

Hi,

Yes it is a standard split feed which I tap into to run through the pyroliser . The 5890 is manually controllled. and Helium is getting to the column the question I really have is how to calculate the flow rate. I symptoms we are having are reduced sensitivity with longer than expected retention times though the tune on the linked trio seems fine and there is a gap at the begining of acquisition followed by a large hump solvent peak . The baseline was fine and I am running the same standards I was previously but getting totally different results .

Thanks

[AICMM]

bedlamVR,

First, use air as your un-retained peak. I am pretty sure there will be a small amount of air left in your pyrolyzer before you turn the heat or valve and this should be the first thing off, right? In fact, I would start with a low EM setting the first time I did this since I would be uncertain about the response. If at first I did not succeed, I would try again with a fresh sample and a higher initial EM. Or... simply inject into the septum on top of the cut into injector nut and measure the air peak retention time. Then you calculate the column flow. Second, the 5890 is back pressure regulated so if the head pressure gauge has not changed you should be getting the same flow (in theory.) Third, reduced sensitivity speaks to an increase in the split flow rather than a change in the column flow and that you should be able to measure off the split vent port. I am assuming in all of this that you are running split the whole time?

While I am at it I would trim the column (~20 cm) and replace the liner and septum. I would also question the condition of the transfer line. Again, an injection directly in the injection port should enlighten you on this, perhaps an alkanes mix C10-C14 or C10-C28, available from most supply houses. Plus, we are dividing the system in 1/2 which is always a good way to troubleshoot.

Best regards.