Chromatography Forum

Dear all,

I am trying to change the column of my Varian 3400 GC.

After having stabilised the base line, I tried to inject a solvent and I did not see any peak. I problem with the gas flow, I assumed.

After carefully looking at the manual I realised with horror that only in the 3600 of the series you could enter the new values (Length, ID) of the column in the GC so that the GC automatically changes the flow. No mention to the 3400.

Does this mean that I have to set up the flows manually?

Thanks

It's completely pneumatic, we have several. Just set the flows and you should be good to go. I usually check for flow with the column attached to just the inlet on these pneumatic instruments (dip the detector end into a vial of solvent and look for bubbles). Saves a lot of work and may prevent damaging a column by heating it with no flow.

Thanks God someone is still working with a Varian 3400!!

Thanks for your reply.

At the moment I am conditioning a column with has a 0.32 mm ID compared to the previous 0.25 mm ID column. I see bubbles at the other end in methanol and I have run test and I see peaks.

Does that mean that I have to change the flow manually?
In the manual says that the settings for the gas flows should not be changed as they come already set from the factory.

Regards

In the manual says that the settings for the gas flows should not be changed as they come already set from the factory. ---- Must be for the Air and Hydrogen.

You have to change the carrier gas pressure depending on the column length and diameter. The suggestion linear gas velocity 20 - 40

If possible download the flow calculator from Agilent website.

Yes, you have to set the flow manually. Depending on exactly what plumbing you have you will probably do this by adjusting the head pressure on the column - otherwise known as the inlet pressure - by turning a knob on a mechanical pressure controller, or by entering a setting for an electronic controller. You will probably also have to set the split flow, most likely by changing a parameter called total flow, but maybe by turning a knob on a mass flow controller. You will monitor pressure and flows either by looking at dials (for the pressure) or the front-panel readout on the GC (for flows and pressures). You can measure the split flow with an external flow meter. You should check the flow through the column by injecting some low MW hydrocarbon (e.g. methane or lighter gas) and measuring its retnetion time.

This all makes me feel really old

Peter

No Peter, it's perfect. Real knobs that do real things, isn't it fantastic!!

Tremenda, please note one key thing about Peter's post: you must have a flow meter to do most of this. You can get electronic ones new, you can get electronic ones from e-bay and LabX and you can get glass ones from the same sites. You can also make one from a buret if you have access to one. It is well worth the investment. It is amazing how many times I have been to customer sites and solved problems by measuring flow....

Best regards.

[quote="AICMM"]No Peter, it's perfect. Real knobs that do real things, isn't it fantastic!!
quote]

I love them - the only advantage that I can see to EPC is programmability. Someone told me once that the real reason the intrument manufacturers like electronic controls is that they are about 1/10 the price of the ones with knobs.

Peter

Thank you all!!

I did all what you said with a digital bubble flow meter.

i measured the linear velocity with butane from a lighter (approx 20 cm/s at 6 psi for a 15m 0.25mm ID column)

I also measured the flow at the detector exhaust, with the H2 and the Air turned off (I cannot close the make-up flow) which gave me the make-up flow + the column flow. Then I closed the headpressure knob and I measured again to obtain the make-up flow only.
I got a column flow of 0.72 ml/min. I was surprised at how precise that digital flowmeter is after figuring out how to use it under reproducible conditions.

Then I measured the flow at the Split vent and I set a Split ratio of approximately 30/1 to allow a total flow of Vent + Column to become larger than 20 ml/min.

After doing all this and being incredibly happy after getting a very sharp peak very early of acetonitrile, everything got ruined by horrible peaks right after it.

I think I will post a new thread about this issue.

You are great, thanks!

Your carrier gas flow rate needs to be about double what it is now.

Peter