Chromatography Forum

Hello,

I need to connect a CDS 8500 purge and trap to my 8890 GC for isotope analysis of VOCs in water samples. The GC is equipped with a both a s/sless injector (back inlet) and a PTV (front inlet). From previous experience on this type of analyses, I know I will need cryofocusing and I was hoping to be able to use the PTV for this purpose (-100C, I have a LN2 dewar connected to the PTV).

Would that make sense to have the deactivated capillary from the heated transfer line going through the PTV and connect it directly to the GC column in the oven (or something of this order)? Is there any extra parts I need to do that, or tricks I should know?

I know the traditional way of doing it is to connect the P&T to side of the s/sless injector. In this case I will need to buy a cryotrap, which I would like to avoid.

If this is relevant, I do compound-specific isotope analysis, so I need a relatively high mass of compound; hence the P&T, and we use 0.32 mm columns.

Thanks,
Violaine

Whoa! Takes me back.

Long ago and far away, this is how the Tekmar purge and trap worked. That sampler had a metal tee where the LN2 went in through the side port and sprayed out both ends. The column went up through the open inlet of the GC, through the long section of the tee and the transfer line from the P&T connected to the column on the north side of the tee. Thus, the column itself was the cold trap. Prior to injection, the LN2 was turned on to cool section of GC column in the trap. Then, the trap in the Tekmar was heated to liberate the analytes. After desorbing the analytes, the LN2 was turned off and the tee was ballistically heated to liberate the frozen analytes into the flowing stream of carrier gas. It actually worked quite well.

I bet you could you do something like that with your PTV. Run the column up through it from the oven side, connect it to the transfer line outside the oven, use the inlet to cool and heat the column (your cryotrap). Use the column as the cryo trap - not the transfer line from the P&T.

Hi rb6banjo,

Thank you for your answer! I actually used some time ago a heavily customized system with a Tekmar P&T that was very similar to what you described and that was indeed working quite well.

I have a couple more questions:
- With this setup if I understand correctly there won't be flow in the PTV. I am wondering if that won't be an issue for inlet/GC readiness?
- I guess I don't need a liner here?

Thanks,
Violaine

Right. If it needs to have pressure (like what would be for column flow), you could do it. Just set it at a low value. Perhaps you could mimic a column by using a 2-hole ferrule and a piece of fixed restrictor (0.18 mm id, 2-3 m long, outlet in the GC oven) to satisfy the inlet requirements. Run your actual column through the second hole, all the way through the inlet septum so you can connect it to the transfer line from the P&T.

This way, you might be able to trick the system into thinking it's operating as it should yet you're using the P&T to actually feed the flow to the column. You'd just be using the cryo part of the PTV.

Probably don't need an inlet liner but you could just use a straight, glass liner like:

https://www.agilent.com/store/en_US/Pro ... 8740-80220

Great! Thanks for the tips!

Violaine