The physics and technology of QMS

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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

Von Zahn,

Zeitschrift

fur
Physik
,

168 (1962), 129
-
142

r
0

: 35mm

L : 5.86 metres

r
0

: 13.5mm

L :
3
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
:



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




Linearity: beware of “log
-
log” plots!

Which would you rather have?

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




Electron energy

Reduce electron energy to
40eV: eliminates interferences
due to Ar2+


Better detection limit for water
in argon

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




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
a

emitter


possible health issues?

Yttria
-
coated
iridium



Generally similar to
thoria
, with no
radiation worries.

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).

QUADRUPOLE

Hyperbolic electrodes to give 2D hyperbolic field. Though in practice round rods are
often used.



F
(
x,y,z
) =
F
0

.

(x
2



y
2
)




2r
0
2



Here
F
0
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
F
0

is constant) for steering and shaping an ion beam, with no
mass selection.
But for
a mass filter, the potential
F
0

consists of a constant and
an alternating component.
Specifically

F
0

= U


V cos (2
p

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

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

r
0

= 6
mm

f
= 2x10
6

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


-
10V

-
10V


+10V

+10V

+10V

+10V


-
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

Quadrupole field in X and Y directions

DC constant +20V. No RF applied.

r
0

= 2.76 mm

RF 0V, DC 20V: potential well in X direction
RF 0V, DC 20V: potential hill in Y direction
-20
-15
-10
-5
0
5
10
15
20
-3
-2
-1
0
1
2
3
V(x)
dV(x)/dx
r0
-20
-15
-10
-5
0
5
10
15
20
-3
-2
-1
0
1
2
3
V(y)
dV(y)/dy
r0
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

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

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.

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

SIMION QUADRUPOLE

S Taylor & JR
Gibson,J

Mass
Spectrom

2008;
43
: 609

616

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.

0%
2%
4%
6%
8%
10%
12%
39.7
39.8
39.9
40.0
40.1
amu
Transmission
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 model with fringing
field: transmission is
increased and the low
-
mass tail is reduced

SIMION
QUADRUPOLE: 3D

2D

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 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

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

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