The physics and technology of QMS J H Batey Workshop on
measurement characteristics and use of quadrupole mass
spectrometers for vacuum applications Bled, Slovenia, April 1013,
2012 Slide 2 Paul & Steinwedel 1956 DE 944900 Slide 3 Examples
of quadrupole construction Slide 4 Typical quadrupole RGAs from c.
1982 Anavac SX200 Slide 5 Modern RGAs Slide 6 Typical analytical
quadrupole from c. 1990 Slide 7 A novel geometry: circular axis to
make a compact instrument Slide 8 Liverpool microquadrupole mass
filter 1 2 Rods 0.5 mm diameter r0 0.22 mm Length 20 mm Slide 9
Mass filters come in a wide range of sizes 1: ICP-MS L: 230mm r0:
5.5mm 2: RGA (SX200) L: 125mm r0: 2.7mm 3: RGA (Anavac) L: 50mm r0:
2.7 mm 4: Microquad L: 20mm r0: 0.22 mm 12 3 4 Slide 10 Isotope
separators: quadrupoles on an altogether different scale! Finlan,
Sunderland & Todd, Nucl. Inst & Methods, 195 (1982),
447-456 Von Zahn, Zeitschrift fur Physik, 168 (1962), 129-142 r 0 :
35mm L : 5.86 metres r 0 : 13.5mm L : 3 metres Slide 11 Early mass
spectrometer: Dempster 1918 Recognizable components: Vacuum system
Source Mass analyzer Detector Isotope studies on alkali metals
Slide 12 Main components of a mass spectrometer Main components can
be identified in Dempsters system Slide 13 Quadrupole mass
spectrometer Slide 14 Ion source Electron-impact source is the
commonest. The design can be quite complex for analytical mass
spectrometers. Filament; source electrode; extraction optics Source
voltage; electron energy Repeller; collimating magnets Slide 15 RGA
source For an RGA the source is of relatively simple construction
it resembles an extractor ion gauge. Slide 16 Source General
requirements : Physical size - usually small enough Sensitive -
typically 10 -4 A/mbar Robust Linear - beware of log/log plots
Reproducible Serviceable - easy to dismantle/reassemble Low power;
low voltage Non-invasive; - that is, operating the mass
spectrometer should not alter the vacuum composition Desirable
features Keep electrons confined to source - avoid electron
background signal Variable electron energy - helps separate some
species Low outgassing - minimise materials Avoid trapped volumes -
memory effects Closed or open? - depends on application Choice of
filament material - tungsten, thoria, yttria Slide 17 Linearity:
beware of log-log plots! Which would you rather have? Slide 18
Source General requirements : Physical size - usually small enough
Sensitive - typically 10 -4 A/mbar Robust Linear - beware of
log/log plots Reproducible Serviceable - easy to
dismantle/reassemble Low power; low voltage Non-invasive; - that
is, operating the mass spectrometer should not alter the vacuum
composition Desirable features Keep electrons confined to source -
avoid electron background signal Variable electron energy - helps
separate some species Low outgassing - minimise materials Avoid
trapped volumes - memory effects Closed or open? - depends on
application Choice of filament material - tungsten, thoria, yttria
Slide 19 Electron energy Reduce electron energy to 40eV: eliminates
interferences due to Ar2+ Better detection limit for water in argon
Slide 20 Source General requirements : Physical size - usually
small enough Sensitive - typically 10 -4 A/mbar Robust Linear -
beware of log/log plots Reproducible Serviceable - easy to
dismantle/reassemble Low power; low voltage Non-invasive; - that
is, operating the mass spectrometer should not alter the vacuum
composition Desirable features Keep electrons confined to source -
avoid electron background signal Variable electron energy - helps
separate some species Low outgassing - minimise materials Avoid
trapped volumes - memory effects Closed or open? - depends on
application Choice of filament material - tungsten, thoria, yttria
Slide 21 Filaments Tungsten Simple Mechanically robust Affected by
oxidising/reducing gas Runs hot, so outgassing problems Rapid
burn-out if vacuum leak OK with halogens Thoria-coated iridium
Coating is delicate More stable in oxidizing/reducing gas Cooler,
so less outgassing Resistant to burn-out Not good for halogens Weak
emitter possible health issues? Yttria-coated iridium Generally
similar to thoria, with no radiation worries. Slide 22 Detector
Electron multiplier Higher sensitivity; needs high voltage supply;
more prone to calibration drift; not suitable for coarse vacuum
Discrete dynode multiplier; SCEM; micro-channel plate Faraday
plate/collector Simple and robust. Electron background and/or
secondary electron emission may be a problem (easily prevented).
Slide 23 QUADRUPOLE Hyperbolic electrodes to give 2D hyperbolic
field. Though in practice round rods are often used. (x,y,z) = 0.
(x 2 y 2 ) 2r 0 2 Here 0 is 20V Slide 24 QUADRUPOLE Saddle shaped
3D field plot. X field is proportional to the X co-ordinate Y field
is proportional to the Y co-ordinate. Slide 25 QUADRUPOLE The
quadrupole structure can be used as a static device (that is, one
in which the applied voltage 0 is constant) for steering and
shaping an ion beam, with no mass selection. But for a mass filter,
the potential 0 consists of a constant and an alternating
component. Specifically 0 = U V cos (2 f (t-t 0 ) ) where U is the
constant (DC) potential V is the alternating (RF) potential f is
the frequency of the RF supply t is the time t 0 is the initial
phase of the RF component Slide 26 QUADRUPOLE Influenced by this
field, the ions travel on complex trajectories in the X and Y
directions, with a constant drift along the Z axis. Slide 27
QUADRUPOLE Mathieu equation Slide 28 QUADRUPOLE The significance of
the stability region becomes clearer when it is plotted in terms of
V and U for a particular case r 0 = 6 mm f = 2x10 6 Hz (typical
values for a quadrupole ICP-MS) Slide 29 QUADRUPOLE Conceptual mass
spectra, deduced from the stability diagram. Slide 30 QUADRUPOLE
These peak shapes have been calculated using numerical integration
of the Mathieu equation. Field radius (r0): 6 mm Radio frequency: 2
MHz Field length:200 mm Input radius:1 mm Exit radius:6 mm Ion
energy:5 eV Beam divergence5 degrees Ion masses 1, 2, 3, 4 & 5
amu Slide 31 QUADRUPOLE A basic quadrupole model is provided with
the Simion package. The dynamic voltages are programmed using the
Lua language. Slide 32 SIMION QUADRUPOLE -10V -10V +10V +10V -10V
-10V +10V +10V -10V -10V Round rods give a field that is
essentially hyperbolic near the axis, but well away from the axis,
the field is quite different. Potential contours at intervals of 2V
Gradient contours, at intervals of 1V/mm Slide 33 Quadrupole field
in X and Y directions DC constant +20V. No RF applied. r 0 = 2.76
mm Slide 34 Ion motion in RF & DC quadrupole field X component
of ion motion. Vary RF amplitude. r 0 = 2.76 mm F = 2 MHz M = 40
amu DC + 20V DC - 20V Slide 35 SIMION QUADRUPOLE Plot the values of
RF and DC that give stable and unstable X trajectories. Slide 36
SIMION QUADRUPOLE Now add stability for Y trajectories (mirror
image about DC = 0 axis). The ion motion is stable for RF and DC
values within the region bounded by the four coloured lines. Slide
37 A Simion model, using parameters as listed by Taylor &
Gibson. Hyperbolic rods (but note T&G used round rods). S
Taylor & JR Gibson,J Mass Spectrom 2008; 43: 609616 SIMION
QUADRUPOLE S Taylor & JR Gibson,J Mass Spectrom 2008; 43:
609616 Slide 38 Mathieu stability region and scan line Peak
Hyperbolic electrodes. The 50% peak with is 0.117 amu,
corresponding to a resolution of 343. The peak is shifted to lower
mass by 0.015 amu; presumably a smaller grid size would give a
smaller shift. SIMION QUADRUPOLE Slide 39 Now we change to round
rods S Taylor & JR Gibson,J Mass Spectrom 2008; 43: 609616
SIMION QUADRUPOLE Slide 40 Mathieu stability region and scan line
Peak SIMION QUADRUPOLE Slide 41 3D model with fringing field:
transmission is increased and the low- mass tail is reduced SIMION
QUADRUPOLE: 3D 2D 3D Slide 42 The previous slide showed an
unusually narrow peak. Usually a quadrupole is tuned to give a
wider peak. This is data from the same Simion model, but with the
scan line set to give a peak width 1 amu at 50% height. The peak is
much smoother, and there is no low- mass tailing. This would be an
excellent performance for an analytical quadrupole, such as an
ICP-MS, for which abundance sensitivity of 1 ppm or better is
needed. The flat peak top is rarely seen in practice, though
examples have been reported. SIMION QUADRUPOLE: 3D Slide 43 Some
very early quadrupole papers showed flat-topped peaks. Is there
still room for improvement from 21 st century manufacturers?
Flat-topped peaks! Brubaker, Recent developments in Mass
Spectrometry, Proc. Int. Conf. on Mass Spectrosc., Kyoto, Japan,
1969, Pub Univ. of Pank, Baltimore, 1970 R = 1.16 R0 (for round
comparison) L = 25.4 cm; r0 = 6.55 mm Hyperbolic, 1.414 MHz, 1 eV,
aperture 1.27 mm W Paul, HP Reinhard & U von Zahn, Zeitschrift
fur Physik,152 (1958), 143-182 Slide 44 SUMMARY Quadrupole:
versatile wide range of design possibilities The mechanical design
of current RGAs mostly follows long- established design principles
but there is increasing interest in smaller devices Simulation
(e.g. with Simion) allows theoretical performance to be
investigated in considerable detail. AREAS NOT COVERED (in this
talk). Electronics Data systems Calibration
