Varian 3800 GC

[WK]

Can anyone help?
I have an interesting little observation here:
I am using a 1079 injector in what I believe should be splitless mode.
I have the split programmed off at 0.00mins and then to 100mls/min at 0.70mins.
But the total flow is about 103ml/min at the start and not about 3mls/min. Even bubble flow meter tells me 100mls/min. Why does the split not shut off? Even if I program it off at 0.01mins it still stays on!?
I have tried setting split to 1-2mls/min but it doesn't like to stabilise at this.
Best Regards
WK

[Peter Apps]

Hi WK

The 3800, and nearly all other modern GCs has back pressure regulation of the inlet pressure, which means that when it is in splitless mode the split flow is diverted away from the inlet rather than being shut off completely as it was on the good old front pressure systems. This means that the total flow, and the flow out through the split vent stays the same no matter whether the inlet is in split or splitless mode.

The 3800 users manual has some flow diagrams of inlet operation.

Back pressure regulation supposedly gives pressure control that is more robust to leaks, but at the cost of being more difficult to troubleshoot.

As a general rule it is a good idea to close the split at the end of the preceding run (you just programme this in as a timed event) so that there are no valves opening or closing just as the sample is injected.

Peter