Chromatography Forum

I would like to hear from anyone who has experience with manifolding a single high capacity rough pump with individual isolation valves on two or more mass specs with small diffusion pumps, specifically, HP MSD's. I can guess what practical problems to expect with such an arrangement and they may be manageable but I would like others thoughts about the idea.

Why?

I forsee some isues with the isolation of multipule instruments - when you pump down an instrument you would have to cut off any other instruments to maintain vacuum. However carrier gas flowing into these instruments would cause a small but real pressure rise within the closed off instrument - so you could not run with them cut off from the rough pump. Yes, the turbo or diff pump woluld keep running - but you are beginning to pressurize the closed off tubing. Which may not be much of a rise when compared to atmospheric - but depending of tubing volumes, it can become significant as backpressure on the turbo or diff pump.

I have my doubts that you could run any of the instruments if you have any of the others in a pump-down process.

Last concern with such a manifold - if you get the valve opening/closing sequence out of order you can back stream a mass spec. Back, years ago, I was involved in stripping a set of quads with solvent to get oil off of them. As I recall, we did get the instrument working again. (It also took a new electron multiplier.) Evaluate the cost and risk of accident as well as potential savings.

I have considered all of these problems as you mentioned and they are real. I think I could manage them alone but I doubt others with less experience could. Of course someone could or probably would come along and screw it up.

My reasons for considering this have to do with funding of course. I must recommend purchase of a replacement rough pump and rebuilt is the only option. Because a higher capacity pump doesn't cost much more I wondered could it work for both.

Thanks for the comments.

I don't think this would work. You are risking stopping the runs on the other instruments. The sudden change in the vacuum as you are starting a pump down sequence as the others are running is likely. Frankly, this is false economy.

If you think you can get this to work reliably then outline the procedure. Step by step. How many instruments on one rough pump? Pumping capacity of that rough pump etc....back streaming will happen and then cost the downtime as well because it will not likely happen but "will" happen. Stop thinking this way. Timothy

formalhaut,

Questions of economy aside, I would say this would be an issue of ballast. With a big enough ballast I would think you could do this since this would slowly pull low vacuum on one instrument while maintaining on others. The danger I see is the problem of putting all you eggs in one basket (the one pump) which means if it dies (for what ever reason at all) all of you MSD's are dead in the water. I generally push people towards redundancy, not the direction you are proposing.

Best regards,

AICMM

Thank you for the response. Of course you are correct. This is a situation with two identical MSD's which are not frequently used but the desire is to keep them both up and running at all times. I think one pump would suffice but the cost of valve hardware to isolate each instrument would be prohibitive. The issue is moot now since we have acquired an additional pump.