Chromatography Forum

I've been using a Tekmar 7000 for a while now. The best sensitivity settings for GC/MS have been 2ml loop, 5:1 split 1.3 ml/min He (~10ml/min) transfer line into the carrier feed.

I am in the process of setting up a MPS-II/CTC system and am curious how to get the best sensitivity for flavor scans with it. I can theoretically do splitless or pulsed splitless since I don't have a transfer-line to sweep. I assume a fast carrier flow and slow plunger to not overload the liner works best?

Thanks for all you work on the 7000. As the application chemist for Tekmar's headspace line, this is always good to hear. I can't offer much advice on how these compare. When you have your system up and running, I would like to hear how things are going. Please contact me at my company email, roger.bardsley@teledyne.com

I've been using a Tekmar 7000 for a while now. The best sensitivity settings for GC/MS have been 2ml loop, 5:1 split 1.3 ml/min He (~10ml/min) transfer line into the carrier feed.

I am in the process of setting up a MPS-II/CTC system and am curious how to get the best sensitivity for flavor scans with it. I can theoretically do splitless or pulsed splitless since I don't have a transfer-line to sweep. I assume a fast carrier flow and slow plunger to not overload the liner works best?

I have used both loop and syringe HS and it seems the syringe give a little better sensitivity.

Splitless will not work. You still need to perform a split injection; it is a gas phase injection and you need to split it.

We use the fastest syringe settings we can get away with, and a low split (1:5) ratio. Depending on how light your light ends are you may need to go to a 1:10 or a 1:20 split.

I love the syringe injections - I much prefer them to the valve.

Splitless will not work. You still need to perform a split injection; it is a gas phase injection and you need to split it.

This is too broad a generalization; if you can use stationary phase focussing and temperature programming to sharpen the peaks, then you can put 1 ml or more of headspace into the column without splitting, with considerable improvements in limits of detection. Also, with a megabore column running at high flows (say 10 ml/min) a 1ml injection is only 12s wide even if it is mixed 1:1 with carrier.

Peter

When I do splitless I always use pulsed splitless that gives me a column flow rate of ~3-4 ml/min during injection. I tried pulsed split with my Tekmar 7000 but found the 6890 could not generate the low split pulses with acceptable reproducibility.

Peter Apps wrote:

Also, with a megabore column running at high flows (say 10 ml/min) a 1ml injection is only 12s wide even if it is mixed 1:1 with carrier.

Peter, he specifically states he is using GC-MS. Hard, really hard, to run 10 mL/minute through your GC-MS.



I agree with the statement about the pulsed splitless. We have tried it in the past and found the variability to be high.

Peter Apps wrote:

Also, with a megabore column running at high flows (say 10 ml/min) a 1ml injection is only 12s wide even if it is mixed 1:1 with carrier.

Peter, he specifically states he is using GC-MS. Hard, really hard, to run 10 mL/minute through your GC-MS.

True - I was broadening the scope. Nevertheless, a smaller volume injection with MS-compatible flows has the same logic.

Peter

This depends on application but I have traditionally had better sensitivity with a headspace plumbed directly into the split splitless inlet.