Chromatography Forum

Hello.

Our GC has two detectors - a MS and an FID. We are trying to use a "Dean's Switch" to split the gas flow from a single column onto both detectors. This would allow us to simultaneously generate MS and FID data from the same injection.

However, two of the 5 Dean's switch inlets are blocked with what looks like short lengths (2 cm) of capillary column. I've gently pulled on them, but they are firmly fixed in place. I don't want to yank them, because I'm starting to think they may be functional.

Can anyone advise me on how to proceed?

Thanks

Please post a photo of your Dean’s switch. Who manufactured it?

You do not want a Deans switch if you need to generate simultaneous chromatograms on the two detectors; a Deans switch sends the column effluent to one detector or the other, not to both at the same time - that's why it's called a switch. What you need is a splitter.

We need to know what hardware you are working with.

How comfortable are you with DIY gas plumbing ?

Peter

Peter is correct but you still might be able to use the plumbing of your switch as the splitter. Just don't apply any pressure/flow to the switch. Use your fixed restrictors to dictate how much of the flow from the outlet of the column goes to the MS and FID. This is dictated by ID and length of the restrictors.

What is your application? If you have a system with 2 detectors and a dean's switch, you'll have a much more powerful system if you use the switch in the manner for which it was designed.

A while back, someone was having trouble separating menthol from camphor in a rub/pharmaceutical-type product. I showed that you can easily separate them using a 2-dimensional system with heart cutting (with the dean's switch). Here's the link:

https://onedrive.live.com/?authkey=%21A ... ot&o=OneUp

The problem with this split is that one outlet is at atmospheric pressure and the other under vacuum

Just to pick up on previous valid comments by rb6banjo and Peter.

How comfortable are you with DIY gas plumbing ?

Use your fixed restrictors to dictate how much of the flow from the outlet of the column goes to the MS and FID. This is dictated by ID and length of the restrictors. What is your application?

I have found it also possible to do it by

a) using a 2 hole ferrule in a coupling for the split or

b) If your analysis could be satisfied by the use of a wide bore 530u column then a simpler low dead volume tee piece will work very well. I can give details and calculations later of this open split interface technique later if required

Thank you for all of the replies.

Firstly - I have solved my original problem - the small pieces of capillary tubing were attached to lengths of column that had broken. I've removed the ferrules.

Here is a picture of the Dean's switch. It is from Thermo Fisher.



I'm confident with DIY gas plumbing.

It is my understanding that a Dean's Switch with two auxiliary gas flows can be used to split the from from a single column onto two detectors. The extra flow to the MS vacuum is counteracted by attaching a length of narrow bore capillary column from the Dean's switch to the MS. The fine control of the flow split is then provided by the auxiliary gas flows.

The application is straightforward: FID quantitation with simultaneous MS peak identification. This allows us to adhere to certain industrial standard methods, while also generating MS data to ID unusual peaks that pop up from time to time. It also allows us to use direct MS quant for other applications.

@ GOM: That low dead volume, two hole ferrule sounds like a much simpler solution, but I think the Dean's Switch allows more flexibility i.e. if the MS vacuum is higher or lower on a given day, the gas split can be controlled by adjusting the aux gas flows to the Dean's Switch.

It looks like that Thermo switch works on the same premise as the Agilent switch that I use.

Are you back up and running now?