The physics and technology of QMS

Report
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 10–13, 2012
Paul & Steinwedel 1956 DE 944900
Examples of quadrupole construction
Typical quadrupole RGAs from c. 1982
Anavac
SX200
Modern RGAs
Typical analytical quadrupole from c. 1990
A novel geometry: circular axis to make a compact instrument
Liverpool microquadrupole mass filter
1
2
Rods 0.5 mm diameter
r0 0.22 mm
Length 20 mm
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
1
2
3
4
Isotope separators: quadrupoles on an altogether different scale!
Finlan, Sunderland & Todd,
Nucl. Inst & Methods,
195 (1982), 447-456
r0 : 13.5mm
L : 3 metres
Von Zahn,
Zeitschrift fur Physik,
168 (1962), 129-142
r0 : 35mm
L : 5.86 metres
Early mass spectrometer: Dempster 1918
Recognizable components:
• Vacuum system
• Source
• Mass analyzer
• Detector
Isotope studies on alkali metals
Main components of a mass spectrometer
Main components
can be identified in
Dempster’s system
Quadrupole mass spectrometer
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
RGA source
For an RGA the source is of relatively
simple construction – it resembles an
extractor ion gauge.
Source
General requirements:
Desirable features
• Physical size
• Keep electrons confined to source
- usually “small enough”
• Sensitive
- typically
- avoid electron background signal
• Variable electron energy
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
- 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
Linearity: beware of “log-log” plots!
Which would you rather have?
Source
General requirements:
Desirable features
• Physical size
• Keep electrons confined to source
- usually “small enough”
• Sensitive
- typically
- avoid electron background signal
• Variable electron energy
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
- 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
Electron energy
Reduce electron energy to
40eV: eliminates interferences
due to Ar2+
Better detection limit for water
in argon
Source
General requirements:
Desirable features
• Physical size
• Keep electrons confined to source
- usually “small enough”
• Sensitive
- typically
- avoid electron background signal
• Variable electron energy
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
- 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
Filaments
Tungsten
Thoria-coated iridium
• Simple
• Coating is delicate
• Mechanically robust
• More stable in oxidizing/reducing gas
• Affected by
oxidising/reducing gas
• Cooler, so less outgassing
• Runs hot, so outgassing
problems
• Resistant to burn-out
• Not good for halogens
• Rapid burn-out if vacuum
leak
• Weak a emitter – possible health issues?
• OK with halogens
Yttria-coated iridium
• Generally similar to thoria, with no
radiation worries.
Detector
Faraday plate/collector
Simple and robust.
Electron background and/or secondary
electron emission may be a problem
(easily prevented).
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
QUADRUPOLE
Hyperbolic electrodes to give 2D hyperbolic field. Though in practice round rods are
often used.
F(x,y,z) = F0 . (x2 – y2)
2r02
Here F0 is 20V
QUADRUPOLE
“Saddle” shaped 3D field plot.
X field is proportional to the X co-ordinate
Y field is proportional to the Y co-ordinate.
QUADRUPOLE
The quadrupole structure can be used as a static device (that is, one in which the
applied voltage F0 is constant) for steering and shaping an ion beam, with no
mass selection. But for a mass filter, the potential F0 consists of a constant and
an alternating component. Specifically
F0 = U – V cos (2 p f (t-t0) )
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
t0 is the initial phase of the RF component
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.
QUADRUPOLE
Mathieu equation
QUADRUPOLE
The significance of the stability region becomes clearer when it is plotted in terms of
V and U for a particular case
r0 = 6 mm
f = 2x106 Hz
(typical values for a
quadrupole ICP-MS)
QUADRUPOLE
Conceptual mass spectra,
deduced from the
stability diagram.
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 divergence 5 degrees
Ion masses 1, 2, 3, 4 & 5 amu
QUADRUPOLE
A basic quadrupole model is
provided with the Simion
package.
The dynamic voltages are
programmed using the Lua
language.
SIMION QUADRUPOLE
Round rods give a field that is essentially hyperbolic near the axis, but well away from the
axis, the field is quite different.
-10V
+10V
Potential contours at
intervals of 2V
-10V
+10V
+10V
-10V
+10V
-10V
-10V
-10V
Gradient contours, at
intervals of 1V/mm
+10V
+10V
-10V
+10V
+10V
-10V
Quadrupole field in X and Y directions
r0 = 2.76 mm
DC constant +20V. No RF applied.
RF 0V, DC 20V: potential well in X direction
RF 0V, DC 20V: potential hill in Y direction
20
20
V(x)
V(y)
dV(x)/dx
15
dV(y)/dy
15
r0
10
10
5
5
0
-3
-2
-1
r0
0
0
1
2
3
-3
-2
-1
0
-5
-5
-10
-10
-15
-15
-20
-20
1
2
3
Ion motion in RF & DC quadrupole field
X component of ion motion.
Vary RF amplitude.
RF = 0
RF = 78
RF = 79
RF = 0
RF = 60
RF = 70
RF = 103
RF = 104
RF = 105
RF = 145
RF = 146
RF = 147
DC + 20V
r0 = 2.76 mm
F = 2 MHz
M = 40 amu
DC - 20V
DC - 20V
DC - 20V
SIMION QUADRUPOLE
Plot the values of RF and
DC that give stable and
unstable X trajectories.
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.
SIMION QUADRUPOLE
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: 609–616
S Taylor & JR Gibson,J Mass Spectrom 2008; 43: 609–616
SIMION QUADRUPOLE
12%
10%
Transmission
8%
6%
4%
2%
0%
39.7
39.8
39.9
40.0
40.1
amu
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
Now we change to round rods …
S Taylor & JR Gibson,J Mass Spectrom 2008; 43: 609–616
SIMION QUADRUPOLE
Mathieu stability region and scan line
Peak
SIMION QUADRUPOLE: 3D
3D model with fringing
field: transmission is
increased and the lowmass tail is reduced
2D
3D
SIMION QUADRUPOLE: 3D
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 lowmass 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.
Flat-topped peaks!
Some very early quadrupole papers showed flat-topped peaks. Is
there still room for improvement from 21st century manufacturers?
W Paul, HP Reinhard & U von Zahn,
Zeitschrift fur Physik,152 (1958), 143-182
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
SUMMARY
• Quadrupole: versatile – wide range of design possibilities
• The mechanical design of current RGAs mostly follows longestablished 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

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