Document 6523142

Transcription

Document 6523142

Performance simulation of a residual gas analyser operating in the first
and third stability zones, T J Hogan, University of Liverpool
Quadrupole Mass Filter (QMF)
Overview
PERFORMANCE SIMULATION OF A
RESIDUAL GAS ANALYSER OPERATING IN
THE FIRST AND THIRD STABILITY ZONES

T.J.HOGAN, J.R.GIBSON, S.TAYLOR
UNIVERSITY OF LIVERPOOL
DEPARTMENT OF ELECTRICAL
ENGINEERING AND
ELECTRONICS

Selective mass to charge ratio
filtering.

Ideal electrodes are hyperbolic.

Circular electrodes more common.

Rectilinear electrodes more
suitable for MEMS fabrication.

End view of micro-engineered
quadrupole lens with 500 µm
electrode radius [1].
2
Performance simulation
Why is it important?

One component of a Quadrupole
Mass Spectrometer/Residual Gas
Analyser.
POTENTIAL FIELD
DISTRIBUTION
HYPERBOLIC ELECTRODES
Quantify effects of electrical and mechanical
tolerance on QMF performance.
Improve product cost performance by targeting
design development to key areas.
Develop new techniques for assessing
performance.
Present understanding may be inadequate when
applied to MEMS electrostatic lenses.
Attempt to apply different technologies to
advance instrument design for novel applications.
Φ ( x, y ) =
2
2
Φ A (x − y )
2r02
CIRCULAR ELECTRODES
∞ A Φ
Φ ( x, y ) = ∑ n n
n
n =0 ro
3
4
Mass Scan Lines
Zones 1 and Zone 3
Stability zones

Large number of stable
operating zones. Only a small
number of practical interest.
Zone 1 most common.
Excitation amplitudes vary with
zone, lowest for zone 1.
Achievable resolution and
sensitivity varies with
operating zone.
U
a-q stability diagram [3]
ZONE
a
q
1
0.23
0.706
3 (upper)
3.16
3.23
3 (lower)
2.52
2.82
Scan line for stability zone 1 a-q plane
4eU
au = a x = − a y =
mω 2ro 2
U
a-q values for stability zones
5
RGA-8, 13 March 2008, Culham
Scan line for stability zone 3 a-q plane
qu = q x = − q y =
2eV
mω 2ro 2
V
6
1
Computer simulation

Simulation Software
Identify optimum r/r0 ratio.

Characterise performance sensitivities.

Increase understanding of novel electrode
topologies.
Public domain and custom software used.
Custom software for hyperbolic electrode mass filter.
QMS2-Hyperbolic for ion trajectory
simulation.
Public domain and custom software for circular
electrode mass filter.
2D Field Solver Poisson/Superfish.
QMS2-Field for ion trajectory simulation.
7
8
Simulation software
Circular rods –

Suite of programs [5,6].
Automesh.
Poisson.
SF7.
WSFPlot.
Third party tools.
DLL allow integration
into custom software.
Simulation Software
QMS2 - Field
GEOM ETRY
D E F IN IT IO N

A U T O M E SH
B IN A R Y
S O L U T IO N
F IL E

P O IS S O N
Solves F = ma by numerical
integration.
Analytically generated field
files and comparison with
previously published results
used validate software.
SF7
W SFPLO T

F IE L D D A T A
F IL E
Uses field data generated
Poisson/Superfish.
QMS2-FIELD user interface
G R A P H IC A L
OUTPUT
9
10
Circular electrodes - Zone 1
Spectral Response

Confirm previously
reported findings
[7,8].
Peak transmission
dependant on r/r0.
Low mass tail varies
with r/r0.
Peak shape and width
varies with r/r0.
Mass peak shifts to
lower m/z value as
r/r0 increases.
Circular rods – Zone 3
Spectral response 2

Simulated spectra for Ar+ ion
at differing r/r0 ratios

11
Peak shape varies
with r/r0, more
marked for lower
resolution setting.
Secondary peaks
more apparent with
lower resolution.
Low mass tail
apparent at lower
resolution and
extremes of r/r0.
Peak height
difference changes
with r/r0.
12
2
Circular electrodes – Zone 3
Performance characteristics

Ion trajectory analysis
Use Discrete Fourier Transform (DFT) to generate power
spatial frequency spectra from time sampled spatial ion
trajectory for x and y axis.
Script written in MatLab to produce ion trajectory spatial
power spectra from time sampled ion trajectories
generated by QMS2-Hyperbolic and QMS2-Field.
Good correlation with analytical calculation.
Spatial power spectra provides an alternative method of
assessing the field quality of circular electrodes QMFs.

Resolution varies with r/r0.
Peak in resolution at r/r0
=1.117.
Resolution sensitivity to r/r0
greater at higher resolution
setting.
Peak width varies with r/r0,
more pronounced at higher
instrument resolution.
Peak width minimum occurs
at r/r0 ≈ 1.117 @ Res. 10%
PH.

13
14
Hyperbolic Electrodes
Ion Trajectories (Zone 1 & 3)
Circular Electrodes
Ion Trajectories-Zone 1
Ion trajectories vary with
stability zone and operating
point.

Ion trajectories vary with
ion entry conditions.

Greater number of spatial
frequencies in Zone3.
Well defined spatial
frequency power peaks.

15
Similar ion trajectories
to hyperbolic rod.
Ion trajectory
dependent on r/r0.
Increased spatial peaks
associated with non
ideal fields.
Increased base power
level.
Circular Electrodes
Ion Trajectories - Zone 3
Alternative Geometries
Square Electrodes 1
Ion trajectories similar
to hyperbolic rod.

Ion trajectory
dependent on operating
point (a,q).

Ion trajectories
dependant on r/r0.

Increased spatial peaks
associated with non
ideal field.
17
16
Approximate circular rod
fields with other geometries.
Ion trajectories similar to
hyperbolic and circular rod
filters.
Requires further investigation
to identify suitable geometry.
18
3
Alternative Geometries
Square Electrodes 2
Misaligned Y axis electrode
Zone 1
EFFECTS OF MANUFACTURING TOLERANCE FOR OPERATION IN STABILITY ZONE 1 ION = 40 amu

10
10
9
9
8
8
7
7
6
-0.005r0
-0.0025r0
5
-0.001r0
4
0.000r0
T ra n s m iss io n (% )

Geometry results in noisy
spatial frequency spectrum.
Multipole coefficients far
from ideal. Quadrupole
term not close to unity.
Correlation between spatial
frequency plot and
multipole coefficients.
Incorrect peak shape.
T ra n s m is s io n (% )

6
4
3
3
2
2
1
1
0
0
39.60
+0.005r0
+0.0025r0
+0.001r0
0.000r0
5
39.65
39.70
39.75
39.80
39.85
39.90
39.95
40.00
40.05
40.10
39.60
39.70
39.80
39.90
40.00
40.10
m/z
m/z
Mass peak shifts with increasing mass positional error [13].
Shift direction different for inward and outward displacements.
Pre cursor evident for both inward and outward displacement
19
Misaligned X and Y electrodes
Zone 1
20
COMPENSATION METHOD - 1
Y electrode displacement results
in slightly smaller shift
accompanied by significant changes
to peak shape and structure.
10
9
8
-(U-VOcosωt)/2
7
T r a n s m is s io n ( % )
X electrode displacement results
in shift to lower mass position with
minor change to peak shape and
amplitude.
+
DISPLACEMENT
-
6
-0.000r0
5
-0.005r0 X Electrode
-0.005r0 Y Electrode
4
(U-VOcosωt)/2
Y
r0
X
3
2
1
0
39.60
39.70
39.80
39.90
40.00
40.10
m/z
21
COMPENSATION METHOD - 2
Conclusions
YGAIN = 1 + 2α
-(U-VOcosωt)/2
XGAIN = 1
Combination of custom and standard software packages
produce powerful and flexible simulation toolset.
Effects of r/r0 different for Zone 1 and Zone 3 operation.
Multipole coefficients give a measure of field quality.
Not obvious how to select optimum.
Power spatial frequencies provide a fast method of
identifying an area of interest.
Methods exist to compensate for mechanical tolerance.
+
DISPLACEMENT
-
(U-VOcosωt)/2
Y
r0
22
X
Where α = displacement of
electrode as a fraction of r0
This technique is the subject of a
number of patents [14].
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4
References and Bibliography

[1] S. Taylor, B. Srigengan, J. R. Gibson, D. Tindall, R. Syms, T. Tate and M. Ahmed, “ A miniature mass spectrometer for chemical and biological
sensing,” In Proc. SPIE-Int. Soc. Opt. Eng. 4036, pp. 187–193, 2000.
[2] P. H. Dawson, “Principles of operation,” in Quadrupole Mass Spectrometry and its Applications, Amsterdam: Holland: Elsevier, 1976, pp. 9-64.
[3] D. J. Douglas and N. V. Konenko, “Influence of the 6th and 10th spatial harmonics on the peak shape of a quadrupole mass filter with round rods,”
Rapid Commun. in Mass Spectrom., vol. 16, pp. 1425-1431, 2002.
[4] R. E. Peddar, “Practical Quadrupole Theory: Graphical Theory,” Extrel Application Note RA_2010A, Poster, 49th. ASMS Conference on Mass
Spectrometry and Allied Topics May/June 2001.
[5] F. L. Krawczyk, J. H. Billen, R. D. Ryne, H. Takeda and L. M. Young, The Los Alamos Accelerator Code Group, Proc. IEEE Particle Accelerator
Conference, Vol. 4, pp. 2306-2308, 1995.
[6] Poisson/Superfish User Manual Available from : http://laacg.lanl.gov/laacg/services/download_sf.phtml.
[7] J. R. Gibson and S. Taylor, “Prediction of quadrupole mass filter performance for hyperbolic and circular cross section electrodes,” Rapid Commun.
Mass Specrom., vol. 14, pp. 1669-1673, 2000.
[8] J. R. Gibson and S. Taylor, “Numerical investigation of the effect of electrode size on the behaviour of quadrupole mass filters,”Rapid Commun.
Mass Spectrom., vol. 15, pp. 1960-1964, 2001.
[9] T.J.Hogan and S.Taylor, “Performance simulation of a quadrupole mass filter operating in the first and third stability zones,” IEEE Trans. Instr.
Meas. Accepted for publication, June, 2007.
[10] Z. Ouyang, G. Wu, Y. Song, H. Li, W. Plass and R. G. Cooks, “Rectilinear Ion Trap: Concepts, Calculations, and Analytical Performance of a New
Mass Analyser,” Anal. Chem., vol. 76, No. 16, 2004.
[11] G. Wu, R. G. Cooks and Z. Ouyang, “Geometry optimization for the cylindrical ion trap: field calculations, simulations and experiments,” Int. J.
Mass Spectrom. No. 241, pp. 119-132, 2005.
[12] T.J.Hogan, S.Taylor, “Performance simulation of a quadrupole mass filter operating in the first and third stability zones,” IEEE Trans. Instr. Meas.,
vol. 57, no.3, pp. 498-508, Mar. 2008.
[13] S.Taylor, J. Gibson, “Prediction of the effects of imperfect construction of a QMS filter,” J. Mass Spectrom. Awaiting publication.
[14] D. M. Burns, S. Taylor, J.R.Gibson, Quadrupole Mass Filter, UK Patent GB2390222, 2004; US Patent 6940068 2005; European Patent EP1649488
2006.
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THANK YOU
THE END
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