Chromatography Forum

The GC I'm using has a current flow rate setting of 1.0cc/min Helium. I have recently been trying to do an experiment which requires a split ratio of 5:1, but have not been able to achieve this split ratio. The machine will not fucntion properly at split ratios of 20:1 or lower.

The helium flow and subsiquent experiment is being affected.

Could someone explain the science behind this, Im having trouble soaking it into my brain!

is there a Helium pressure problem?

Im using a GC/MS

Thanks

if your GC has a EPC or some other computer controlled means of adjusting flow and split then the problem lies in that you are trying to operate outside the parameters of which the hardware can operate.

If you have strictly a manually adjusted system then it would be a matter of the needle valves which you use to control the splitter flow not being able to adjust to such a low flow rate.

Hardware design limitation: septum purge flow and flow needed for backpressure regulator.
See manual on minimum injector vent flow.
Agilent service recommends greater than 50 ml/min -- have never been able to get below 10 for various commercial instruments.

There are limitations while using EPC. The limitations vary for the different gases. Can you try using nitrogen instead of helium? If you can get a lower split ratio with it and if it suits your analysis, then why not use nitrogen?

Electronic regulation systems will not be able to cope with 5:1 split ratios. Whilst old GCs with head pressure regulation can running just 5:1 split ratios will most likely lead to poor reproducability as the needle valves do not control well at low flows. Why do you need to run split ? Surely running splitless will solve your concentration problems and then the split flow can be set to a value your Electronic flow control system can cope with ?