Chromatography Forum

Does anyone know anything about how the split and split vent actually works? Is there a valve in there? If so, do you have a schematic of it someplace I could look at? My boss suggested that the split wasn't working well because I wasn't getting reproducible results and I cleaned and replaced the tubing today –it was quite clean surprisingly. The split vent filters are backordered about two weeks, so I’m waiting on that. I’m wondering since the tubing was so clean with methanol rinses that perhaps the split material is just not reliably going out. I replaced the tubing anyway just in case. Any thoughts would be helpful. thanks.

Can you specify what type of GC you have?

All of the major GC manufacturers have diagrams of their split systems on their websites, some of them have animations of where the gas goes and what valves are open and closed.

Peter

My GC is an Agilent 6890.

If you have a split/splitless inlet, there is a split valve. The PTV inlet has a similar valve - and off the top of my head, I don't recall if it gets a different name in this inlet.

The information available on the front panel should tell you what kind of inlet you have. And, if you can set a split ratio, you have a split valve.

okay.
thank you.

I looked into this and found that the inlet liners i have are for splitless injection because this is what Agilent sells as UltraInert and I was using them and doing both splitless and split injection work with it depending on the sample. Apparently the splitless liner doesn't work well or repeatedly if you are doing split injection method.

I had wanted to use single taper inlet liners for split injection but Agilent's engineers tell me there is no such thing as using single taper inlet liners for split injections.. is this true? I had thought that previously I had seen this used.

So I think the answer is that I have to change the liner if I change from split to splitless method. Is this true?

Yes it is true. You can use a tapered liner in split mode for single analysis diagnostic type run but it won't be very reproducible as the split flow has to flow out the channels of the gold seal at the bottom of the inlet. The taper doesn't allow for this to flow very well. At least this is my experience. I just wouldn't do it. Split liners are usually pretty cheap. If your compounds aren't highly reactive then you probably don't need the most expensive type of deactivated liners either. This can really run up the cost. Simple 4 mm ID split liner should work fine.

okay.
thank you.

I looked into this and found that the inlet liners i have are for splitless injection because this is what Agilent sells as UltraInert and I was using them and doing both splitless and split injection work with it depending on the sample. Apparently the splitless liner doesn't work well or repeatedly if you are doing split injection method.

I had wanted to use single taper inlet liners for split injection but Agilent's engineers tell me there is no such thing as using single taper inlet liners for split injections.. is this true? I had thought that previously I had seen this used.

So I think the answer is that I have to change the liner if I change from split to splitless method. Is this true?

Having done split and splitless injections on ordinary taper-at-the-bottom Agilent liners my experience has been that you can do split injections to a splitless liner, but sometimes not vice versa since the narrower split liner cannot accommodate all the vapour from a splitless injection.

The critical test of whether you need to be buying specific liners is whether you can get the performance you need from the liners that you have - if you can then save your money. The presence or absence of glass wool in the liner will have a much greater impact.

Peter

FWIW the split vent filters are just some charcoal I think, so that you don't get gassed by the traces of whatever you're analysing.

I replaced the split vent filters. That wasn't the problem though.

I still can't get reproducibility within runs. What troubles me the most is that in a mixture of flavor components I get varying RSDs for each component. t he percentages are anywhere from 2 to 48% for an individual component within the overall flavor say for example something like orange. If I run the same orange sample liquid injection seven times, and then look at each peak, the separate components that I know make up the flavor since we made it in house don't have the same peak areas. Any thoughts as to why this might be?

What is the history of your current GC method? Has it worked in the past or are you starting from scratch? Did it just start "acting up"? Let us know some more details (i.e., injection volume, injection temp, solvent oven start temp, injector pressure, flow rate etc.)

Yes it is true. You can use a tapered liner in split mode for single analysis diagnostic type run but it won't be very reproducible as the split flow has to flow out the channels of the gold seal at the bottom of the inlet. The taper doesn't allow for this to flow very well. At least this is my experience. I just wouldn't do it. Split liners are usually pretty cheap. If your compounds aren't highly reactive then you probably don't need the most expensive type of deactivated liners either. This can really run up the cost. Simple 4 mm ID split liner should work fine.

thank you for the information! i really appreciate it a lot!