split injections on a megabore column

[oncoming storm]

I have a method that asks for the use of a 10:1 split on a 0.53 ID column. It also calls for a >2.0% CV on 5 standard injections. I have to fight the instrument to get this. When I use the exact same method on a 0.32 ID column my CV is down to 0.2-0.5%. Anyone have a good explanation (other than the original method is nuts to begin with)?

[Peter Apps]

Any method that wants a CV bigger than a certain value has to have something odd about it. I cannot thiink why any GC would struggle to give a 10:1 split to a megabore column - column flow is probably between 5 and 10 ml a minute, split flow 50 - 100 ml/min, where exactly is the instrument failing ?

Peter

[larkl]

Could it be with the megabore column you're running at a very low head pressure and the splitter isn't as reproducible as it is with higher pressure (like with the 320 column)? Is it an EPC instrument? I've been told some EPC's are not as reproducible at very low flows.

[oncoming storm]

I typed the wrong ">". :oops: The method wants a CV less than 2.0. This is an EPC instrument; Varian 3800. Column flow is 6 ml/min with head pressure of 3 to 12 psi.

[Peter Apps]

The 3 psi pressure will give a 6 ml/min flow through a 30m megabore at 30-40C, so that looks OK, and presumably the 12 psi is at the top end of the temperature programme, so the constant flow is working more or less right. What exactly is it that is going wrong - inconsistent peak areas, retention times ?? What are you analysing, and what are the other instrument conditions; inlet temp, liner type, injection conditions etc ?

Peter

[Ron]

If you are starting at a 3 psi head pressure, the EPC is probably having a little trouble holding that pressure with a 10:1 split. The total flow is high enough that it is not easy to hold the head pressure that low, and the flow and thus the split will tend to vary a little. With the 0.32 column the head pressure is high enough for the flow controller to control the pressure and flow easily. If you could have a higher starting temperature the precision would probably be better.