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 Performance simulation of a residual gas analyser operating in the first and third stability zones, T J Hogan, University of Liverpool 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 RGA-8, 13 March 2008, Culham 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 Performance simulation of a residual gas analyser operating in the first and third stability zones, T J Hogan, University of Liverpool 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 RGA-8, 13 March 2008, Culham 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 Performance simulation of a residual gas analyser operating in the first and third stability zones, T J Hogan, University of Liverpool Alternative Geometries Square Electrodes 2 Misaligned Y axis electrode Zone 1 EFFECTS OF MANUFACTURING TOLERANCE FOR OPERATION IN STABILITY ZONE 1 ION = 40 amu 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 EFFECTS OF MANUFACTURING TOLERANCE FOR OPERATION IN STABILITY ZONE 1 ION = 40 amu 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]. 23 RGA-8, 13 March 2008, Culham 24 4 Performance simulation of a residual gas analyser operating in the first and third stability zones, T J Hogan, University of Liverpool 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. 25 RGA-8, 13 March 2008, Culham THANK YOU THE END 26 5