Newsgroup sci.techniques.mass-spec 1147

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R. E. Haufler wrote:
} 
} On 8 Dec 1996 16:14:20 -0500, "James J Briggs" 
} wrote:
} 
} }It has been my experience in working with vacuum that turbo molecular
} }pumps do not need larger roughing pumps, except when your pumping large
} }volumes. As a rule a turbo pump only becomes efficient when the pressure
} }is ~ 15u's(microns) because of it's compression ratio.  The effect of
} }increasing the capacity of a rough pump would only increase pump down time
} }and not the over all vacuum. A rough pump only reaches ~ 1x10-4 as to most
} }turbo's ultimate pressure reaching down to 1x10-8 1x10-9 range.
} 
} The choice of mechanical pump displacement also depends strongly on
} the load.  For example, molecular beam work, API, or GC/MS, require
} greater pumping speed than and MALDI-TOF spectrometer.
In reference to a number of issues raised in the previous comments I
would like to add the following:  
The main reason that turbo molecular and for that matter diffusion pump
efficiency improves so dramatically as you've observed, is that gaseous
flow mechanism changes from laminar to  diffusion at about that
pressure. i.e. the gas molecules are getting further apart from each
other so that the average distance traveled in the time between impacts
with other molecules becomes significant. 
I agree with the comment on diffusion pumps being very robust.  However,
I would have to qualify that statement saying that maintenence of a good
clean vacuum system requires meticulous operation of the pumping
system.  Often the new persons working with vacuum science
instrumentation insist on learning this lesson the hard way. i.e. not
letting the diffusion pump cool sufficiently prior to bringing the
system up to atmosphere, venting the system with air or worse a high
humidity air rather than nitrogen or argon.  With such lack of respect
for the integrity of the diffusion pump such a system will have chronic
problems with backstreaming due to degradation of the pumping oil.  This
lack of understanding the operating needs of the system is an added
frustration factor for such people.
Finally, in relatively clean systems having low pumping load I would
definitely prefer a diffusion pumped system. Again though, it must be
properly designed to remove or minimize the possibility of operator
foulups to compromise the integrity of the pumping system.  This would
best be done by an interlocked pnuematically controlled gate valve
system for insuring that the proper sequence is followed for bringing
the system up to atmospheric pressure when needed.  Of course if at all
possible any time a vacuum system is brought up to atmosphere all means
should be taken to isolate as much of the system as possible by means of
gate valves i.e. only bringing that part of the system up to atmosphere
which is absolutely necessary. 
For systems with high pumping loads or particularly dirty sources which
will ultimately contaminate the diffusion pump oil a turbomolecular pump
is the obvious choice for pumping systems.  
The major concern then for either system (which is commonly overlooked)
is to maintain the roughing pump on the system with a rigidly followed
PM every three months with a high quality pump oil (19, 19+, 20 grade). 
Duoseal pump oil is great for low level vacuums but should never be used
in UHV applications.

Hi,
We found some polymeric contamination of a protein in a spectrum obtained
by electrospray MS. The difference between the monomers is about 157 Da,
and we observed peaks at 752.3, 909.4, 1066.5, 1221.6 and 1383.7, with
decreasing ion intensities. Has anybody an idea about the nature of this
polymer ? 
Thanx,
Bart 
*********************************************************************
Bart Devreese
Lab. of protein biochemistry and protein engineering
University of Gent
KL Ledeganckstraat 35
9000 Gent 
BELGIUM
http://allserv.rug.ac.be/~jvbeeume
************************************************************

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In article <58jpa6$boh@acmey.gatech.edu>, jjmorelli@aol.com wrote:
} One added advantage of Turbo pumps irrespective of their pumping speed and
} performance is that air cooled models are available.  Often water cooling
} can be difficult or impossible. Anyone who has had to repair a water leak
} from an aging water cooled system can attest to this problem.  There is
} also the issue of the quality of water supplied.  In a closed system,
} chillers are required.  These can develop salting problems and be an
} additional maitanence item.
  I agree that water cooled diff pumps are more trouble prone than air
cooled ones. The Leybold pumps in our VG SIRA mass spec developed leaks
from the copper cooling tubes, presumably due to electrolytic corrosion.
OTOH, we've never had problems with our Edwards water cooled diff
pumps, which use stainless steel coils.
  The Edwards air cooled diff pumps in one of our machines have operated
without downtime for 20 years. This includes the fan and heater.
  Our limited experience with turbo pumps has not been good. The machine
with Varian turbo pumps has required pump replacement on a less-than-yearly
basis.
Douglas McIntosh
Institute of Marine Science
University of Alaska
Fairbanks,AK