Newsgroup sci.techniques.mass-spec 1136

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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. 
In my opinion diffusion pumps have there place. How ever if you need a
clean and non-contaminated system stick to turbo's and not diffusion
pumps. 
Diffusion pumps have there place like I said earlier but you would need
liquid nitrogen baffles to stop the back streaming of the DP oil if your
looking for a clean system. 
Jbriggs@sundial.net

The vast majority of VG/Fisons/Micromass sector instruments are pumped
using diff staks - systems must be specially ordered if turbos are
required.  Considering these instruments are used for ultra trace level
analysis, and non of these use liquid nitrogen baffles, I must disagree
with the following statement made by James Briggs.
James J Briggs  wrote in article
<58fb3c$1tn@acmez.gatech.edu>...
} 
} Diffusion pumps have there place like I said earlier but you would need
} liquid nitrogen baffles to stop the back streaming of the DP oil if your
} looking for a clean system. 
} 

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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.
Warning!! The above is a very biased, unobjective opinion. I love TURBOS! 
JJMorelli@aol.com
How do you spell relief?
"G-O-O-D-B-Y-E"

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.
} 
} Warning!! The above is a very biased, unobjective opinion. I love TURBOS!
} 
} JJMorelli@aol.com
} How do you spell relief?
} "G-O-O-D-B-Y-E"
After hearing all of the benefits of turbos, i must stand up for Diff
pumps.  I have never had to replace a diff. pump.  They last "forever". 
Just change oil and a heater once in a while.  I must also say that
backstreaming in modern day diff pumps is negligible.  We have a VG 70SQ
pumped by four diff pumps and have never had any background problems due
to backstreaming oil at all.
-- 
Bill Cotham, Ph.D.
Mass Spectrometry Laboratory
Dept. of Chemistry and Biochemistry
University of South Carolina
Columbia, SC  29208
803-777-2039 (voice)
803-777-9521 (fax)

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.

Hello.
 Recently I'm planning to use HP 5071 MSD with 5890 series II GC to
analyze natural gas derivative samples. I want to separate C-4
hydrocarbons with extreme high accuracy. Especially isomeric
compounds. Please give me advice for column selection. 
Thank you.