Turbopump preselection of analytes
based on molecular weight
Scientific Basis
• The compression ratio of a turbomolecular pump
increases rapidly with the mass of the molecules being
pumped.
William B. Whitten
•
Oak Ridge National Laboratory
Peter T. A. Reilly
It should be possible to harness this effect to
preconcentrate analyte molecules based on their mass
for improved measurement sensitivity.
Washington State University
This research was sponsored by the ORNL Laboratory-Directed Exploratory Research and
Development program, by the DOE Office of Proliferation Detection, and by the DOE Office
of Nuclear Nonproliferation.
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for the Department of Energy
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Applications
Compression ratio for different
molecules for a V-70D turbopump from
the Varian catalog
Molecules lighter than air
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• He, H2, HT, T2
Molecules heavier than air
•
•
•
•
•
•
•
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CW agents
Explosives
Drugs
Unexploded ordinance
Chlorinated VOCs
Toxic gases, trace VOCs
Tracer molecules
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1
Inverted compression ratio vs mass,
extrapolated to 267 amu (VX nerve
agent)
Model of a single turbopump stage
compression ratio
K max 

M12
3 1/ 2 
 exp(
)
M 21
8 sin 
vb
m
 vb
vg
2kT
where vb is the speed of the blades
and vg is the speed of the gas molecules
T. N. Schneider et al., J. Vac. Sci. Technol.
A 16, 175-180 (1998).
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6
Experimental results for 9-stage
turbopump compression ratio vs blade
speed
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for the Department of Energy
Cross section of a typical turbopump
A practical pump has many
stages connected to a single
rotating shaft.
• gas was dry air
• vg was constant
• vb was varied,  = vb/vg
• solid curves are from theory
M. H. Hablanian, High-Vacuum Technology, Marcel Dekker, 1990
J. G. Chu and Z. Y. Hua, J. Vac. Sci. Technol. 20, 1101 (1982)
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8
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2
Steady-state concentrator requires 3
streams – these units can be ganged
Scheme for molecules lighter than air
to rough pump
concentration
element
125 l/s N2
concentrated stream for light molecules
Vacuum
Chamber
concentrated stream
sample inlet
TV 141
Navigator
Turbo
RGA
V 81-M
Turbo
bleeder
sample inlet
50 l/s N2
leak valve
waste stream
KNF Diaphragm
Pump
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Photo of bleeder input for lower mass
ions
waste stream
Background mass spectrum for light
molecules
V-81M Turbopump
bleeder valve
leak valve
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3
RGA with 1% SF6 in He into vacuum
chamber
RGA with 1% SF6 in He into bleeder
valve
The heavier molecules (SF6) are
substantially attenuated
relative to helium!
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for the Department of Energy
Scheme for molecules heavier than air
RGA with input of 1% SF6 in He into
vacuum chamber
waste stream
to rough pump
TV 141
Navigator
Turbo
125 l/s N2
Vacuum
Chamber
RGA
V 81-M
Turbo
50 l/s N2
sample inlet
KNF Diaphragm
Pump
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concentrated stream
for heavier molecules
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4
RGA of exhaust from diaphragm pump
transferred to chamber with some extra
air
Preliminary Results
Light Molecules
(He/SF5)bleeder = 5.5 E+3
(He/SF5)chamber = 1.1 E+2
Change in He relative to SF5 = 50
Heavy Molecules
(SF5/He)exhaust = 1.8 E-1 (neglecting additional air)
(SF5/He)chamber = 6.1 E-3
Change in SF5 relative to He = 30
These results are only approximate because there was no background
correction and SF5 peak in the light molecule measurement was below
the background.
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for the Department of Energy
18 Managed by UT-Battelle
for the Department of Energy
Small Turbo Pumps
Conclusions
• We have shown that the mass dependence of compression
ratio for a turbomolecular pump can be used to
preconcentrate either molecules lighter than air or heavier
than air depending on the experimental arrangement.
•
Further enhancement should be possible with multiple
stages.
• Very small turbopumps are now commercially available
making targeted applications in portable instrumentation
attractive.
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20 Managed by UT-Battelle
for the Department of Energy
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