Nucleic Acid Analysis with UV-vis and NMR - Spectroscopy

18 Spectroscopy 24(11) November 2009
www.spectroscopyonline.com
pressure measurements taken on-site,
with the readings fed into a system
that makes decisions on the fly. These
complex topics deserve a more detailed
exposition, which will appear in this
column eventually. Until then, and to
return to Earth orbit, interested readers
might learn about the Wake Shield
Facility (described at http://www.svec.
uh.edu/wsfp.html), which takes advantage
of the excellent vacuum available
in the wake of the space shuttle.
Earlier columns in “Mass Spectrometry
Forum” covered general topics
of vacuum systems (as well as our
sometimes confusing uses of the terms
vacuum and pressure), and the operation
of high vacuum pumps (1,2). Here,
to keep things simple, we will call any
pressure below 1 atmosphere (760
Torr) a vacuum. There are subsidiary
terms of rough vacuum, low vacuum,
high vacuum, and ultrahigh vacuum,
with such terms corresponding, in
order, to lower and lower pressures.
The pressure in interstellar space, by
the way, is about 10 -16 Torr, and the
pressure within interplanetary space
higher (depending upon where you
are). These are isotropic gas pressures,
and the fact that the interstellar pressure
is so low is one reason why radiation
pressure can be used to exert a
force upon solar sails.
The focus of this column is the
measurement of pressure in a mass
spectrometer, located somewhere on
the surface of planet Earth (+/- 5 km)
(3). The continued growth and diversification
of MS should refocus our
attention on the attainment of vacuum
and the accurate measurement of pressures.
At the heart of MS is the ability
to create ions, to move them around,
to differentiate them by their mass-tocharge
ratios, and to detect them. For
years, and certainly through the dominant
era for electron ionization (EI)
and chemical ionization (CI) sources,
we thought of the MS instrument
as under high vacuum from source
through to the detector. We also came
to know the vacuum pumping system
as a high-cost, high-maintenance part
of the instrument. Certainly, pumping
systems evolved from the crude apparatus
first used by Aston in his
Source
Inlet
Ionization gauge
Analyzer
Diffusion pump
Thermocouple gauge
Rough
pump
Detector
vent
Figure 1: Simple diagram of placement of a thermocouple gauge (bottom) and an ionization
gauge (top) on a generic mass spectrometer.
Table I: Usual operational range for some common pressure gauges used
in mass spectrometers.
Device name
Usual operational range (Torr)
(760 Torr is 1 atmosphere of pressure)
Mechanical gauge (various sorts) 1000 to 100
Piezo or quartz sensor 760 to 1
Capacitance manometer 1000 to 10 -3
Thermocouple gauge 760 to 10 -3
Ionization gauge 10 -4 to 10 -8
parabola mass spectrographs, but the
consistent general model was a highvacuum
diffusion pump (or two) for
the main system to achieve a high vacuum,
with associated backing pumps
(the rough pumps) that also could be
plumbed into source interfaces for separators
or direct insertion probes. Most
pumps (including diffusion pumps
and rotary vane backing pumps) transport
gas molecules against a pressure
gradient, so the ultimate exhaust for
the backing pumps should be a hood
vented to outside. The advent of reliable
turbomolecular pumps did not
shift the basic design of the pumping
systems in our instruments, but added
certain advantages of speed and pump
placement. The basic goal still was to
move the gas molecules from inside the
system to outside of it.
Within this general scheme, much of
the plumbing intricacy of the vacuum
pumping system was designed for the
ionization source of the mass spectrometer,
and the need to transport