Nucleic Acid Analysis with UV-vis and NMR - Spectroscopy

20 Spectroscopy 24(11) November 2009 www.spectroscopyonline.com
mass analyzer may be the cause. The
author of this quote addresses the need
for accurate pressure measurement in
a vacuum chamber used for materials
processing, but the lesson is valuable.
For the mass spectrometer, scattering
collisions reduce ion transmission,
and reduce instrument sensitivity. A
“small” change in pressure can lead
to an amplified degradation in performance.
Pumps “burp” for various
reasons, and the short pressure surge
takes time to dissipate. Erratic instrument
performance may be your only
clue if you are not carefully monitoring
the pressure. The author may be the
only mass spectrometrist in the world
who would become concerned with a
shift in measured base pressure from
2 X 10 -6 to 3 X 10 -6 Torr, but now you
know why.
Performance of the vacuum pumping
system of a mass spectrometer
is monitored through measurement
of the pressure at various points in
the system. These measured pressures
may or may not be logged into
the automated control system of the
mass spectrometer. In process vacuum
chambers, system pressures certainly
are recorded so that the conditions of
materials processing are known precisely.
But in analytical mass spectrometers,
pressure measurement is often
manually monitored. Consider Figure
1, which is a schematic of a diffusion
pump attached to the main vacuum
chamber, supported by the backing
pump (also known as the rough pump),
with the system exhaust routed to a
hood. Pressures within the backing
pump lines are monitored (using the
thermocouple gauge) as an indicator of
the total gas load on the system, and to
ensure that the main diffusion pump is
properly supported. The main chamber
pressure is monitored with the
ionization gauge. The diffusion pump
can operate for a short time at higher
backing pressures, but prolonged operation
leads inevitably to a rise in main
system pressure and a degradation of
the pumping fluid. In a simple EI mass
spectrometer, monitoring the two pressures
(the backing pump line thermocouple
gauge and the main chamber
ionization gauge) usually is sufficient
for reassurance that the instrument
was properly pumped.
Details of how each measurement
device works are explored in specialized
texts (5–8) and commercial manufacturer’s
applications notes, reflecting
the broad application of vacuum
science outside of MS. We concentrate
here on the thermocouple gauge and
the ionization gauge, because these are
the most common devices found on
mass spectrometers, usually configured
approximately as shown in Figure
1. Table I shows the operational pressure
range for the thermocouple gauge
and for the ionization gauge, and a few
other devices included for comparison.
Performance of
the vacuum
pumping
system of a mass
spectrometer is
monitored through
measurement of
the pressure at
various points in
the system.
Note that in addition to monitoring
the pressure in the backing lines, thermocouple
gauges also can be used to
monitor vacuum pressures in sample
introduction interfaces. Note that for
sources that operate at atmospheric
pressure, the ambient pressure usually
is not measured. We emphasize in this
column three basic issues relevant to
the vacuum pumping system in a mass
spectrometer. First, the vacuum gauge
produces an electrical output that can
be related to pressure through a calibration,
and not all residual gas mixtures
follow the same calibration curve.
Second, the pressure measured is the
pressure at the gauge, not necessarily in
the system, and so we have to consider
conductance. Third and finally, proper
vacuum-pumping system care
maintains instrument performance.
Calibration: Like any transducer,
a vacuum gauge must be calibrated.
In MS, that calibration entails some
special issues. The composition of a gas
mixture at Earth atmospheric pressure
contains an expected mix of nitrogen,
oxygen, water, carbon dioxide, argon,
and some trace components. Does this
mixture change in its relative composition
as a pump lowers the pressure in
a vacuum chamber? It most certainly
does, because various pumps may act
to pump one gas component more effectively
than another. Thus the composition
of that starting gas mixture
at 10 -3 Torr will be slightly different
from the composition at atmosphere,
even more different at 10 -6 Torr, and
vastly different at 10 -9 Torr. For the
pressure-measuring devices, a calibration
should be completed so that the
electrical output can be related to the
actual pressure, and the calibration
must factor in the composition of the
gas. Any device reacts to different gases
with a different sensitivity, and thus
exhibits a different calibration curve.
A thermocouple gauge that measures
1 Torr of pressure when the composition
is residual atmospheric gases will
be slightly different in response from
a thermocouple gauge reading 1 Torr
of methane in a CI source. Many commercial
companies offer calibration
services for vacuum gauges. In MS,
traceability to a NIST or other national
standard is not usually necessary. NIST
calibration is described in detail in
publications available on the web (9,10).
Conductance: The connection of
a vacuum gauge to the mass spectrometer
deserves some discussion.
Consider the two situations depicted
in Figure 1 for a thermocouple gauge
and for an ionization gauge. The thermocouple
gauge is connected directly
into the vacuum line, and many gauges
are built into threaded assemblies for
such a purpose. We can be assured
that the pressure measured by the
thermocouple gauge is the pressure in
the line because of this direct connection.
On the other hand, consider the
connection of an ionization gauge to
the main vacuum chamber of a mass
spectrometer. Sometimes the glass-en-