GC/MS column issue (megabore VB-624 to mass spec)

[djpegram]

I am trying to run EPA method 524.2 (http://www.epa.gov/waterscience/methods ... /524_2.pdf) - see section 6.3.

Our Varian CP3380 GC (Saturn 2000 mass spec) needed to have some repairs to the oven and EFC, so we had a varain tech come in. I had just installed a brand new VB-624 (75m, 0.53mm ID, 3micrometer film) two days before, and he is saying that this megabore column CANNOT be used with the mass spec. I trust his judgement but I figure there has to be some type of adapter that could work for it, otherwise 3 of the 4 columns listed in the EPA method cannot be used and column 3 (a DB-5) says that it needs a cryo interface.

If anyone has any advice or experience with this please resond here or email me (djpegram@memphis.edu).

Thanks

Derek

[Peter Apps]

This is a flow rate issue - the MS vacuum pumps cannot pump fast enough to keep up with the input from the column. You can put a short length of narrow bore silica at either end of the column to throttle the flow. Varian has applications notes on using restrictors upstream of megabore columns, which is the preferred option. Jack Cochran from Restek published something early this year ? on how to do the plumbing.

There is also a recent thread on here, have a look in the archives.

Peter

[CE Instruments]

He will have been advised that the pumping system on the MS cannot cope with the high flow on the Megabore column. You however do have a 75m column so it should cope with the volume of gas provided you are running at about 2mls per minute. It will reduce the life of the turbo pump as it will have to work hard. To help you could fit a narrow bore restrictor as a transfer line from your column into the MS. The vacuum will currently be evacuating the last few meters of your column so will be affecting the chromatography. Why have you chosen a Megabore column when the use of Megabore with MS is not advised by MS suppliers ? You could have chosen an alternate column that was 0.32 mm i.d.

[Yuva]

0.32 or .53 id column can not be used directly with MS. To use the 0.32 or 0.53 id column use the restrictor column. ie connect 1 meter of 0.10 is open tubular column at the output. Connect this at the end of the analytical column by Vacuum Union ( Restek). Connect other end to the MS. You may have to buy 0.10 mm uncoated capillary, corresponding ferrule and Vacuum Union, The same is available with Restek. I have used 0.53 column by this way. It worked fine.

[djpegram]

Thanks for the advise Peter, CEI, and Yuva.

@CEI, My advisor wanted to stick as close to the actual method as possible, in the 524.2 method it lists 4 columns (section 6.3 in the link above) that have been approved for use and 3/4 are megabore columns. The third is a 30m 0.32 ID 1micron DB-5 (we didn't choose this because he wants a longer column and doesn't want to use cryo as suggested with this column).

On the EPA method link above, in section 11.3 they talk about use of a jet separator or an open split interface.

The boss says now the new plan is to exchange the column for a 60m 0.32 ID 1micron DB-5 (apparantly he spoke with the previous varian instrument tech and he said that one should work without me having to make some FS restrictor). I've got 3 people who have told me this column should be able to operate at that flow; Yuva, are you sure 0.32 ID is still to large to run unrestricted into the ion trap?

[Yuva]

0.32 column, 30 meter length may work. 60 meter should be better. The flow through the column is depends on the difference pressure between the ends. Back end of the column is at vacuum. With head pressure 2 psi, the flow through the column is around 1.2 ml/min and 4 psi will give 1.5 ml/min. You can try with pressure 2- 4 psi

[Peter Apps]

A 60 m 0.32 m column will not cause any problems for the MS.

Peter

[carl.nott]

Well, the varian tech was incorrect in saying the megabore column cannot be used. I've used megabores with jet separators in the past. They have some analytical advantages (and disadvantages). In your case I would advise swapping/buying a 624 60m 0.25id column and skipping the jet separator tho (plugged/active site jets SUCK).