Chromatography Forum

We have a new GC that splits an injected sample to two detectors. One dector is connected to the A/D board on the GC, I have a needle valve to control flow to this detector but even when the needle valve is fully closed, we show 5ml/min flow out of the detector. I need to slow this flow to approximately 2-3ml/min in order to send a majority of the injected stream to the second detector. Any ideas on what to use or where to purchase it (flow restrictor or better needle valve)?

Thanks much!!! Kevin (Gasman)

It is quite possible that the needle valve is not fully closed. On many needle valves, the knob that you use to turn the needle valve is adjustable on the stem of the valve. Manufacturers set this so that the knob hits the body of the valve before the needle is fully closed. This is to prevent users jamming the needle into the valve seat. You may be able to adjust the knob on the stem so that you can close the needle valve a little more.

Gasman

Sorry, you just call me Kevin. I'll assume you had Gasman first. That is my name at work as well.

Do you know of any inline restrictors that would fit a 1/8" packed column?

Also, on our new Agilent, we have the option to adjust the inlet pressure (and thus column flow). Higher pressure (at least in my range) equals higher flow. My injection is split to FID and Licor. How can I tighten up tailing in the Licor measurement (plenty large peak) and improve my FID resolution?

A simple 1/8" to 1/16" adapter from Swagelok and a piece of 1/16" OD tubing of 0.004" ID or 0.010" ID will give you a restrictor. Depending upon the amount of restriction you require 1 to 10 meters of tubing may be required.

These parts and other fittings are available from many vendors, Supelco for an example, my former employer.

best wishes,

Rodney George
consultant

Hi Rodney, Thanks for the response. I have done exactly what you said, my length of tubing is only 12 inches though. Maybe I shoud try to crimp it?

Crimping is risky due to the problem of the restriction changing with time, or even becoming blocked. It is better to use a longer piece of tubing or a piece with a smaller Id with a clear passage. Even better use uncoated FSOT
tubing.

You can use a piece of FSOT tubing of 0.25m ID or less to get more restriction with minimal dead volume/mass. Remember 0.25mm = 0.010"

You could use a 1/16" ID tubing with an inert packing such as Supelcoport fitted with frits or screens, both available from Supelco to act as a restrictor.

Indicate the flow drop required and I am sure they are able to deliver you the required restriction (if they remember everything I taught them )

Best wishes,

Rodney George
consultant

Rod,

So, if I wanted 1/2 cc/min at 10 psi, Supelco could make a custom restrictor to give that kind of flow rate? Something I have been interested in for a while now.

Best regards.

The limits are PRICE and Parameters: Temperature, GAS, and flow rate variation are important parameters that have to be fixed.

If I had a lab in which to make such a restrictor I would make them myself. I would also set one up for someone else if they offered to pay me for my trouble. It would only cost a few thousand dollars of test equipment and much less than that for my time.

The problem is doing one or two, or making them with a variety of restrictions, for a reasonable price, given the equipment it requires. I don't think it can be done for a customer without a capital investment.

However, if you offer the right price, many companies would make restrictors in bulk, given a single fixed set of parameters.

Email me privately if you want to discuss options that aren't of public interest.

Rodney George

I'd try one of these first, they've worked for me in the past in a similar application. http://www.vici.com/cv/vmicmetr.php

Those VICI valves are good products and as long as you are not using micropacked or capillary applications (ie as long as you ARE using packed columns with corresponding higher flows) they work very well. One has to be careful not to over tighten or error in making adjustments to the needle valve.

But if you are capable of adjusting them correctly, they are excellent. Trying to get one to give you less than 0.5cc/min flow at 10 psi might be a bit daunting for many users.

best wishes,

Rodney George
consultant