Headspace Transferline to front 6890N inlet w/7683 tower

[Peter Apps]

I would have thought that the less resistance there is through the headspacer plumbing the better the GC's EPC will control the gas. The EPC is designed to work with just three lengths of stainless tube between it and the inlet (one for total flow in, one for split out and one for septum purge out). Putting an extra volume and resistance in the total flow input might confuse it.

Peter

[MSCHemist]

I'm not entirely sure what the purpose of the flow element is on the 7000. It is designed to have the option of having it controlled by a manual pneumatics contol knob but the manual does not say to by-pass it with EPC control.

[Peter Apps]

I have vague recollections of there being a resistor in the plumbing of an Agilent G1888, but it was only in the vial pressurization line. Maybe someone messed with the Tekmar's plumbing before you got it ?

Peter

[MSCHemist]

No the plumbing seems to be as per spec. It has the black 40-400 ml/min flow element (highest flow lowest resistance) installed which is what the manual recommends for EPC control. It seems to have trouble abruptly changing flow rates. During gas saver mode it is running a split at 15 ml/min and during a pulse splitless it is running 3 ml/min. When I go to prep run it shoots immediately up to 40 psi then slowly drops back to 30 psi over several minutes.

[Peter Apps]

A slow drop in pressure should not have anything to do with the Tekmar plumbing - the back pressure regulator for the inlet is downstream of the inlet, and the Tekmar is upstream. When the set pressure drops the back pressure regulator should just open up to dump the excess pressure. 3 ml/min total flow is very low for an Agilent EPC though, maybe that is the problem ? What happens if you disable the gas saver (it is not saving gas if your working total flow is lower than the gas saver flow). What is the split flow after the end of the splitless period ?

Doing a pulsed splitless injection needs a transient increase in inlet pressure, with the headspacer in the line this is a big volume that is at higher pressure, and maybe that is the upsetting things. What happens with an ordinary split injection ?

Peter

[MSCHemist]

I think it is just a mater of having all that extra line and volume between the EPC and the inlet. The pressure drops more slowly because there is more volume of pressurized gas between the EPC and the inlet and in splitless it has nowhere to go except into the column. At least that means I probably have very tight connections as if there was a leak it should drop pressure very fast.

I ran that inlet today and it took 3 minutes to go from 15ml/min split to 31psi splitless. So in the future it will take a bit longer to reach ready status but it does get there.

[Bigbear]

Foolish question: what did you do with the "other" end of the inlet gas supply that you cut off? I'm thinking it needs to be attached directly to a port on the 6 way valve of the headspace unit.

[Peter Apps]

in splitless it has nowhere to go except into the column.

I see what you mean - the flow diversion in splitless mode happens inside the EPC (not in the inlet) so in splitless you have only a 3 ml/min flow through the headspacer. This is slow enough to make me worry about band broadening - there is a lot of pipe and connections in there, and with the cut into the inlet supply line some of it is barely warm. If you have a 1 ml loop it will take a minimum of 20 s to empty it, even if it flushes as a clean plug, which if course parabolic flow profiles and diffusion stop it from doing.

Peter

[MSCHemist]

The EPC end of the cut is routed to a 1/16" to 1/8" union and then to the carrier in female fitting on the 7000 with copper tubing. The inlet weldment side of the cut it hooked up to a valco ZDV 1/16" union at the end of the transferline which I was able to run right under the center cover of the GC so it does not interfere with the 7683 tower. I then insulated the heck out of the union and remaining line between the union and weldment with foil tape and cotton.

This is all exactly as stated in the manual. I keep the knob fully opened on the transferline flow controller and there is a resistance element of some sort coded black as 40-400 ml/min. The manual does not state to bypass the flow controller even with EPC control.

[Bigbear]

I'd call Tekmar about how to connect. By connecting to the port on the back of the unit you are introducing flow controllers you do not need.
If it were me I would look at the 6 port plumbing schematic and conect the supply line from the GC directly to the port that "feeds" gas to the sample loop durring desorb with 1/16" tubing. By adding 1/8" tubing into the system you are "confusing" the GC's EPC.

[MSCHemist]

So far it doesn't seem to be a big deal. It takes a few extra minutes to get ready when doing splitless otherwise while doing the run in gas saver mode the EPC seems to have no trouble. It is just the abbrupt reduction in flow in splitless that seems to confuse it.

[MSCHemist]

That is how the manual states it is supposed to be connected for EPC control. They have an entire section devoted to EPC control and they say you should use a 1/8" 1/16" reducing union, 1/8" copper lines and go through the flow controller with the black 40-400 element installed. I suppose I could bypass the manual pneumatics controller and pressure guage but I usually prefer to follow the manual.

[Bigbear]

Yes the manual is usually the way to go. As I recall though the 7000 series were designed to work with 5890's and I had a 7050 connected to a non EPC 5890. I connected the He to the 7000 instead of the 5890 and controlled GC flows with the 7000.
I still think the pressures would stay more consistant when swaping from split to splitless if connected directly to the 6 port. If you're not using the 7000's CG flow control ability, why include all the extra plumbing?

[MSCHemist]

That is acutally what I am considering doing for the vial pressurizer. I have a free auxilary EPC on my 6890 so I am considering using it to supply the vial pressure and sweep gas. However, there are no instructions for doing do.

Never mind I am getting all confused the it goes to the needle flow contoller and then direct to the pressurization controller. I am not quite sure how the vial pressure and needle flow works I can guess that when a vial presses against the needle a valve diverts gas to the vial and when a vial is not the gas flows through the system. I guess in that case it would be tough to regulate both the sweep flow and vial pressure as they'd need different pressures and the GC's aux epc wouldn't know exactly when a vial was being pressurized.

My concern is that I don't want helium running 24/7 though the headspace sampler.

[MSCHemist]

Never mind my last post. I see the needle flow controller provides resistance when there is no back pressure (a vial) and the vial pressurization control sets the maximum line pressure.

Is there any advantage to electronic vs manual control of the vial pressure setting?

I am unable to obtain the inserts for 9/12ml from vials from Tekmar does anyone carry them?