Introduction Basic concepts of vacuum Vacuum Hardware (pumps, gauges) Mass Spectrometry OUTLINE...

Introduction Basic concepts of vacuum Vacuum Hardware (pumps, gauges) Mass Spectrometry OUTLINE VACUUM PUMPS AND HARDWARE 1

GETTERS Getters are stripes of material adsorbing the gas NEED OF VACUUM TV TUBES LCD BACKLIGHT GAS LIGHTS (NEON, HIGH POWER LAMPS) DEWAR (FOR DRINKS) Active material: alkali (Cs, Rb), rare earths (Yb, Lu), Hg Support: Al 2 O 3, Zr Interaction of gas (CO 2, O) with getter surface (passivation or oxidation) Role of the surface morphology: surface area/bulk Research applications: impact on everyday life 2

Basic concepts of vacuum UHV Apparatus Gas Kinetics Vacuum concepts Vacuum Pumps Vacuum Gauges Sample Preparation in UHV Cleaving Sputtering & Annealing Fracturing Exposure to gas/vapor Evaporation/Sublimation 3

Ultra High Vacuum Apparatus 4

5 velocity distribution 1D k B = Boltzmann constant Gas kinetics velocity distribution 3D probability of finding a particle with speed in the element dv around v probability density of finding a particle with speed in the element dv around v Maxwell-Boltzmann distribution

Gas kinetics Mean T (C) Molecular speed Quadratic mean Most probable Neon @ 300 K m Ne = 20 1.67 x 10 -27 kg f(v) 6

Gas kinetics 7

for ideal gas n = N/V = number density (mol/cm 3 ) N = total number of molecules 8 Arrival rate R: number of particles landing at a surface per unit area and unit time how many molecules? Consider n molecules with speed v moving towards a surface dS on a surface dS we take the molecules arriving with speed v x in a time dt volume = vdt cos dS total number of molecules with speed v x hitting the unit surface in a time dt dS

9 Gas kinetics

10 p = Pressure (torr) T = Temperature (K) m = M amu O 2 at p = 760 torr, 293 KR = 2.75 10 23 molecules s -1 cm 2 O 2 at p = 1 x 10 -6 torr, 293 KR = 3.61 10 14 molecules s -1 cm 2 k B = Boltzmanns constant (J/K) molecules arrival rate R at a surface (unit area, time) Gas kinetics M Ox =32 g/mol if M=Molar mass m Ne = 20 1.67 x 10 -27 g

11 Gas kinetics: why the UHV 1 Monolayer ~ 10 14 10 15 atoms/cm 2 Residual Gas H2OH2O CO 2 CO CH 4 O2O2 N2N2 Solid Surface Bulk Solid Adsorbed Atoms & Molecules

12 Mean free path Gas kinetics 2r The sphere with 2r is the hard volume The surface of the sphere is the effective section or cross section for impact The number of impacts per unit time is

13 Mean free path Gas kinetics For different molecules A and B rBrB rArA is so large that the collisions with walls are dominant with respect to molecular collisions 2 depends on the fact that we did not consider the presence of other molecules p in torr

15 Sticking probability = 1 1 monolayer of atoms or molecules from the residual gas is adsorbed at the surface in: 1 sec@ p = 1 x 10 -6 torr 10 sec@ p = 1 x 10 -7 torr 100 sec@ p = 1 x 10 -8 torr 1,000 sec@ p = 1 x 10 -9 torr 10,000 sec@ p = 1 x 10 -10 torr 100,000 sec@ p = 1 x 10 -11 torr Utra High Vacuum (UHV):p < 10 -10 -10 -11 torr Why the UHV O 2 at p = 1 x 10 -6 torr, 293 K R = 3.61 10 14

16 Plots of relevant vacuum features vs. pressure

17 Gas flux through a pipe d [Q] = [p][L] 3 [t] -1 pipe p = pressure measured in the plane dV = volume of matter crossing the plane Volumetric flux: variation of number of molecules through an area dV/dt= Volumetric flow rate (portata) Throughput

18 Mass flux Variation of mass through an area Volumetric flux Magnitude of flow rates Pressure drop at the pipe ends Surface and geometry of pipe Nature of gases Gas flux through a pipe M=molar mass M = total mass Factors affecting the flux

19 Regimes of gas flux through a pipe For < dviscous For dintermediate For > dmolecular d Viscous laminar turbulent pipe The mol-mol collisions are dominant Friction force = viscosity S = layer contact area dv x /dy = mol speed gradient Throughput

20 Volumetric flux Laminar: R e 2200 mass flux For a pipe with diameter d and section d 2 /4 Q mass flux per unit section Reynolds number = viscosity d pipe Regimes of gas flux through a pipe

21 Laminar: Q < 8 10 3 ( T/M)d [Pa m 3 /s] Reynolds number Turbulent: Q > 1.4 10 4 ( T/M)d [Pa m 3 /s] Regimes of gas flux through a pipe

22 For < d For d For > d d viscous intermediate molecular Knudsen number = d/ Only for intermediate and molecular flux intermediate molecular 3 d/ 80 d/ 3 10 -2 p d 0.5 p d 10 -2 For air at RT Regimes of gas flux through a pipe

23 Pipe conductance: In parallel Flux across pipe Pressures at pipe ends [C] = [L] 3 [t] -1 Pipe impedance: SI: m 3 s -1 cgs: lt s -1

24 In series Q 1 = Q 2 = Q T or gas would accumulate

25 Pipe conductance Viscous and intermediate regime (Poiseuille law) Molecular regime Long cylindrical pipe Elbow pipe Laminar Turbulent The molecules must collide with walls at least once before exiting Equivalent to a longer pipe For air at 0 C: 11,6 d 3 /L [lt/s]

26 [S] = [L] 3 [t] -1 Pumping speed S = Q/p 0 Q= flux through aspiration aperture p = Vessel Pressure V = Vessel Volume p0p0 Relevant physical parameters of a pumping system SI: m 3 s -1 cgs: lt s -1 In the presence of a pipe Effective pumping speed in the vessel Q at the pump inlet is the same as Q in pipe C p 0 = pressure at pump inlet Volumetric flow rate

27 [S] = [L] 3 [t] -1 Pumping speed S = Q/p 0 Q= flux through aspiration aperture p = Vessel Pressure V = Vessel Volume p0p0 Relevant physical parameters of a pumping system if C = S Effective pumping speed the S e is halved C

28 Q= flux through aspiration aperture p = Vessel Pressure V = Vessel Volume p0p0 Relevant physical parameters of a pumping system Q 1 = True leak rate (leaks from air, wall permeability) Q 2 = Virtual leak rate (outgas from materials, walls) Outgas rate for stainless steel after 2 hours pumping: 10 -8 mbar Ls -1 cm -2 Sources of flux (molecules)

29 Pump-down equation for a constant volume system True leak rate Only the gas initially present contributes Virtual leak rate Other outgassing sources contribute Short time limit Long time limit Q = Q 0 +Q 1 S = Pumping speed p = Vessel Pressure V = Vessel Volume

30 Pump-down equation for a constant volume system True leak rate Short time limit Q = Q 0 +Q 1 S = Pumping speed p = Vessel Pressure V = Vessel Volume Suppose: Constant S Q = 0 Time needed to reduce p by 50 % V= 1000 L P 0 = 133 Pa S= 20 L/s t = 331,6 s 7.5 L/s = 27 m 3 /h Vol of 1 m 3 = 10 3 L to be pumped down from 1000 mbar to 10 mbar in 10 min = 600 s

31 Pump-down equation for a constant volume system Q = Q 0 +Q 1 S = Pumping speed p = Vessel Pressure V = Vessel Volume Ultimate pressure dp/dt = 0 Virtual leak rate Other outgassing sources contribute Long time limit

32 Pressure versus distance

33 Differential pumping operate adjacent parts of a vacuum system at distinctly different pressures The size of the aperture depends by its function conductance C is determined. A, B to be maintained at pressures P 1 and P 2, P 1 >> P 2 A: gas in with flux Q L gas to B with flux q Q 1 = flux pumped S1 = Q 1 /p 1 Q L /p 1 B: gas in with flux q To keep pressure p 2 S 2 = q/p 2 q = C(p 1 p 2 ) C p 1 S 2 = Cp 1 /p 2 Modern Vacuum Physics, Ch. 5.8

34 Example CVD coatings on panels Antireflective coatings, p-n junction growth for solar panels P0P0 P1P1 P2P2 P1P1 P0P0 S1S1 S2S2 S3S3 S 1 = Cp 0 /p 1 CCC S 2 = Cp 1 /p 2 S 3 = Cp 2 /p 1

35 Gas-solid interaction H2OH2O CO 2 CO CH 4 O2O2 N2N2 He H2H2 elastic inelastictrapped physical adsorption (shortened to Physisorption): bonding with structure of the molecule unchanged Chemisorption: bonding involves electron transfer or sharing between the molecule and atoms of the surface Can be thought of as a chemical reaction

36 Gas-solid interaction H2OH2O CO 2 CO CH 4 O2O2 N2N2 He H2H2 Origin: Van der Waals forces The well depth is the energy of adsorption E to be supplied to desorb the molecule Typical q: 6 - 40 kJ/mol = 0,062 - 0,52 eV /molecule Physisorption

37 Gas-solid interaction H2OH2O CO 2 CO CH 4 O2O2 N2N2 He H2H2 Origin: Electron sharing or transfer between molecules and surface atoms The well depth is the energy of adsorption Typical q: 40 - 1000 kJ/mol = 0,52 - 10 eV /molecule Chemisorption P is a precursor state the molecules have to overcome

38 Gas-solid interaction How does this affect vacuum? probability per second that a molecule will desorb O2O2 Molecule trapped in the adsorbed state at temp. T potential well of depth q Dilute layer (no interactions with other mol.) How long does it stays? Surface atoms have E vib = h = K B T = K B T/h At RT = 0.025/(6.63 10 34 1.6 10 19 ) = 6 10 12 s 1 10 13 s 1 = number of attempts per second to overcome the potential barrier and break free of the surface. Boltzmann factor probability that fluctuations in the energy will result in an energy q

39 Gas-solid interaction probability per second that a molecule will desorb O2O2 p(t) = probability that it is still adsorbed after elapsed t p(t+dt) = p(t) x (1- dt) probability of not being desorbed after dt dp = p(t+dt) - p(t) = - dt p(t) average time of stay

40 Gas-solid interaction O2O2 average time of stay At RT 10 13 s1 97 kJ / mol = 1 eV / molecule Temperature dependance Molecular dependance Note: Simple model Neglects all other interactions, surface diffusion, adsorption sites so a can change

41 Desorption P = 1000 mbarP = 10 -7 mbar Equilibrium pumping Far from equilibrium till. Experimental relation Gas flux /area = 1 for metals = 0.5 for elastomers = 0.5 = 1 q 1 5x10 8 mbar L s 1 cm -2 1 mbar L N at 2.6x10 19 Outgassing rate 10 12 molec s 1 cm -2

42 Desorption How important is the molecule/surface interaction energy? H2OH2O N2N2 Rate of desorption Simple model calculation idealized UHV system RT, V= 1 L, A = 100 cm 2 S = 1 L/s only gas source: initially complete ML of specified binding energy adsorbed at the wall fall of pressure at RT q integrate

43 Outgassing Origin of fluxes: Permeation Adsorption Solubility Desorption Gas is continuously released, (at relatively small rates) from walls Principally water vapor Limit to attainable vacuum achievable in reasonable times (hours) 10 6 mbar

44 Gas-solid permeation p 1 = 1000 mbar Residual Gas H2OH2O CO 2 CO CH 4 O2O2 N2N2 p 2 = 1x10 -8 mbar He H2H2

45 Gas-solid permeation p 1 = 1000 mbar Residual Gas p 2 = 1x10 -9 mbar Permeation is a complex process Adsorption Dissociation Solution into the solid Diffusion Recombination Desorption

46 Gas-solid permeation p 1 = 1000 mbarp 2 = 1x10 -9 mbar Permeation process can be quantified trough phenomenological quantities permeability =Q/(p 1 -p 2 )A Q=flux trough wall A= unit area [Q] = [p][L] 3 [t] -1 = [L] 3 [t] -1 [L] -2 m 3 s -1 m -2 ls -1 cm -2 Residual Gas

47 Gas-solid permeation K p = Permeability coefficient For a given gas A = wall area d = wall thickness m 3 s -1 m -1 Pa -1 He cm 3 s -1 cm -2 Pa -1 p = 13 mbar d = 1 mm depending on diffusion mechanisms

48 Gas-solid permeation Metal gas K p GlassMetalsPolymers He, H 2, Ne, Ar, O 2 No rare gasAll gases p p p p p p Table of gas permeability

49 Solubility Is the quantity of substance A that can be dissolved in B at given T and p For a gas Gas quantity dissolved in solid volume unit at standard conditions For undissociated molecular gas (interstitial) c = gas concentration Henrys law Valid for low concentrations and for glass and plastic materials No formation of alloys

50 Solubility For dissociated gas Sieverts law Valid for low concentrations and for metals Interstitial or substitutional H 2 on metals Note the high solubility of H 2 in Ti,Zr

51 Vacuum Pumps Capture pumps Pistons Gears Turbines Jet stream Cold traps Ionization Getters Throughput pumps Differences: pressure range, speed, gas selectivity

52 Pressure Ranges Spanned by Different Vacuum Pumps More than one pump to HV and UHV

53 What pump to use? S = [L] 3 [t] -1 Pumping speed S = Q/p p = inlet pressure For a pressure range where S does not depend on p, i.e. the pumping speed is constant Compression ratio: This can be used to measure S or to estimate the time to reach p u Depends on the gas type S varies with p Q=Q 0 cost p u S = Q 0

54 Ultimate pressure Time to reach the u.p. Residual gas composition Other (absence of magnetic fields) What pump to use?

55 Rotary Roughing Pump P u : 10 -2 mbar Rotor blade Eccentric rotor inlet Exhaust valve Spring Cylindric body Oil S: 2,5 10 2 m 3 /h 0.7 28 l/s 1 m 3 /h = 0.28 l/s CR: 10 5 Starting operating pressure: 10 3 mbar

56 Dual stage Rotary Roughing Pump P u : 10 -3 10 -4 mbar Advantages No saturation Heavy duty Low cost (2500 ) Disadvantages Oil backstreaming Need traps for oil vapor Noisy Rotor blade Eccentric rotor inlet Exhaust valve Spring

57 Rotary Roughing Pump: gas ballast CR=10 5 Op. temp T 70 C Pump water vapor at 70 C when P reaches 3.3 10 4 Pa The vapor liquefies and does not reach P > 1 10 5 Pa So the exhaust valve does not open The vapor remains inside the pump and is mixed with oil Decrease pump speed, and can damage the rotor by increasing the friction The gas can liquefy inside the rotation chamber Vapor pressure

58 Rotary Roughing Pump: gas ballast Ballast valve Solution: gas ballast NO gas ballast Gas ballast liquid The valve is set to decrease the CR to 10 The vapor does not liquefy

59 Diaphragm Pump Housing Valves Head cover Diaph. clamping disc Diaphragm Diaphragm supp. disc Connecting rod Eccentric bushing P u : ~ 1 mbar CR: 10 2 10 3 Starting operating pressure: 10 3 mbar

60 Diaphragm Pump Advantages Oil-free No saturation Low cost Disadvantages High ult.pressure (4 mbar) Low pump speed Noisy

61 Root Pump Advantages Oil-free No saturation High throughput Disadvantages Need prevacuum Medium cost delicate Eight-shaped rotor turning in opposite direction Clearance between rotors ~ 0.3 mm No lubricants CR depends on clearance

62 Root Pump S and CR of a root pump depend on the preliminary pump installed ahead The gas flux is the same for both pumps rootpalette prpr p p atm SpSp SrSr Palette: 60 m 3 /h = 16,8 l/sS r = 16,8 x 40 = 672 l/s

63 Turbomolecular Pump P u : 10 -10 mbar S: 50 5000 l/s CR: 10 5 10 9 Starting operating pressure: 10 -2 mbar

64 Turbomolecular Pump Molecular speed distribution without blades (only v) Molecular speed distribution plus blade speed

65 Turbomolecular Pump Principle of operation High pressure side Low pressure side The pumping action is provided by the collisions between blades and molecules Molecular regime The speed distribution (ellipse) depends on the angle between V and blade

66 Turbomolecular Pump Pumping speed: depends on gas type Residual gas: H 2 After bake out

67 Compression Ratio of a Turbomolecular Pump

68 Turbomolecular Pump Advantages No saturation Clean (magnetic) UHV Any orientation Disadvantages Cost Delicate Quite noisy 70 l/s ~ 4000 250 l/s ~ 9000 2000 l/s ~ 20000 Rotor suspension Ball bearings (lubricant required) Magnetic (lubricant absent)

69 Molecular drag pump Turbo disk Threaded stator Cylindrical Rotor Forevacuum flange (outlet) Threaded stator Safety ball bearing Gas entry Magnetic bearing Lubricant reservoir Electrical socket Operating principle: Same as turbo but different geometry No blades but threads

70 Molecular drag pump P u : 10 -7 mbar S: 40 100 l/s CR: H 2 : 10 2 10 9 He: 10 3 10 4 N 2 : 10 7 10 9 Starting operating pressure: 1-20 mbar They are use in combination with turbo in a single mounting so Use a low CR backing pump (i.e. membrane for clean operation) Higher backing vacuum pressure

71 baffle vapor diffusion pump Fluid is heated and ejected from nozzles at high speed due to the nozzle shape and pressure difference between inside and pump cylinder. Fluid speed up to Mach 3-5 The gas molecules are compressed to the pump base through collisions with oil vapor

72 vapor diffusion pump Advantages No saturation Heavy duty Low cost Disadvantages gas reaction Liquid vapor tension Contamination Needs water cooling P u : 10 -9 mbarS: 20 600 l/s Starting operating pressure: 10 -2 mbar The pumping speed and the pressure strongly depends on oil type

73 Getter pumps The active material is sublimated by thermal heating Sublimation getters - Gas-surface chemical interaction - Chemisorption - Solution of gas inside material Pumping mechanism Non evaporable getters The active material is constituted by porous medium

74 Sublimation getter pumps Sublimation getters - Gas-surface chemical interaction - Chemisorption - Solution of gas inside material Ti or Ti Mo filaments Pumping mechanism The material form a thin film on the pump walls that becomes the active layer The molecules are chemisorbed on the film

75 Non evaporable getter pumps Cartridge of porous material (Zr-16%Al) Pumping operation Problem: saturation of getter material requires cartridge change Activated by heating (750 C) and kept at operating T 300 C to increase molecule diffusion

76 sublimation S strongly depends on gas > 10 3 l/s Zr-Al Getter pumps Pumping speed (l/s) A= sublimation, A=non evaporable Non evaporable 800- 2x10 3 l/s S depends on active surface saturation Molecular weight (g) area Adsorption probability

77 Gas-surface weak interaction Physisorption and diffusion into the bulk Plus: Wall cooling Pressure limit: 10 -10 10 -12 mbar Advantages Pump H 2 Heavy duty Low cost No contamination Disadvantages Saturation Metal vapours No rare gas pumping Stripes of active material Getter pumps With a number of panels one can obtain S > 1x10 4 l/s But if warmed it releases the gas

78 Ion-getter pump 7 KV ~1 Tesla Ti - Gas-surface chemical interaction - Chemisorption - Solution of gas inside material Pumping mechanism - Ionization of gas molecules - Burying inside the active material Ion-getter with cathodic grinding

79 Basic processes occurring within a single cell e - ionize molecules Secondary e - ionize molecules Ions are accelerated to cathodes produce secondary e - grind up cathode material make craters Ions buried into cathode material Produce cathode vapors Depositing also on anodes to work as getters H 2 : accumulates into the cathodes Need regeneration by annealing

80 Ion-getter pump Advantages Heavy duty No traps No contamination Any mounting position Silent Disadvantages High magnetic fields Low pump S for H 2 Medium - high cost Pressure limit: 10 -11 10 -12 mbar S: 4 1000 l/s Starting operating pressure: 10 -3 10 -4 mbar

81 Adsorbing pumps Liquid N 2 cooled Adsorbing material - Gas cold surface interaction - Physisorption Pumping mechanism Adsorbing porous material High surface/volume ratio Zeolites Al 2 O 3, SiO 2 H 2 O and N 2 pumping Liquid He cooled Cold walls - Gas cold surface interaction - Physisorption, condensation Pumping mechanism Cryogenic pumps

82 Cryopump - Gas cold surface interaction - Physisorption and condensation Pumping mechanism Metal wall

83 Cryopump Pressure limit: 10 -10 10 -11 mbar Advantages Heavy duty No contamination Low cost Disadvantages Saturation Noisy Needs other UHV pumps The gas condensation if gas pressure > vapor pressure at wall T S: 4 100 l/s Starting operating pressure: 10 -9 mbar vapor pressure

84 Ionization in gases Type of collisions: - neutral Molecule electron - neutral Molecule ions - neutral molecule neutral molecule (Penning) - radiation absorption - neutral Molecule hot metal surface - + - Ionization of a molecule (atom) from collisions with e - Ion - Ion + - -

85 Ionization in gases Ionization energy eV Ion +Electron affinity Ion - - + - - - Less probable More probable

86 Collision type: - elastic - atom excitation - molecule dissociation - Ionising ( e) Considering the relative speed and energy conservation Atom or neutral molecule electron collision Very small Elastic collision In the collision the kinetic energy of electrons (and of the molecules) remains almost unchanged relative energy loss for electrons

87 Total energy loss e - suffers very small energy loss for each elastic collision e - mean free path e = average space between two elastic collisions e - collision rate e = collisions number per unit time number of collisions Elastic collision

88 Apply external electric field E Maximum kinetic energy of an e - moving in a gas Depends on electric field and pressure Elastic collision If e - has v in ~ 0

89 Ionization Ionization energy e - can ionize an atom if But it can also - Increase the atom kinetic energy - Excite an e - to unoccupied bound states Ionization probability i = ionizing collisions/total collisions - - +

90 Ionization Long path to produce more ions But it can also - e - trapped inside atom with formation of negative ions - - e - with E k unit pressure unit lenght Specific ionization coefficient Due to practical measurements e - can ionize an atom

91 Vacuum measurement Different types of vacuometers depending on pressure range Mechanical, thermal, ionization

92 Vacuum measurement Mechanical Bourdon To vacuum Membrane Pin wheel tube index 10 5 10 2 Pa (10 3 1 mbar) The tube curvature changes with pressure Needs calibration Precision: 1-2% fsr 10 5 10 2 Pa (10 3 1 mbar) The membrane or bellow bends with pressure Needs calibration Precision: 1-2% fsr

93 Thermal conductivity vacuometers Pirani heated filament The filament temperature, and hence the resistance depends on heat dissipation in the gas, i.e. on the gas pressure Pressure variation means T variation i.e. resistance variation. This is measured through the W. bridge V variation

94 Thermal conductivity vacuometers unbalanced Hence Thermal dissipation radiative dissipation contact dissipation For small p, the reference bridge is The pressure is obtained by measuring the Wheatstone voltage In general it depends on the gas type = cost Stephan-Boltzmann =wire emissivity K gas = gas thermal conductivity K f = wire thermal conductivity =coefficient

95 Ionization vacuum gauges Hot cathodeCold cathode Based on gas ionization and current measurements

96 Ionization vacuum gauge I + = I - i e p Sensitivity K = i e Directly proportional to pressure Sensitivity K = i e I + = ion current i = specific ionization coefficient I - = electron current from filament e = electron mean free path The gauge measure the total pressure Range: 10 -4 10 -12 mbar K depends on gas, gauge geometry, gauge potential Usually one increases by designing the gometry

97 Ionization vacuum gauge electrons from gas or field emission similar to the behavior inside the ion getter pumps Less precise due to problem of discharge current at low pressure 1 tesla Range: 10 -4 5 x10 -10 mbar Cold cathode No filament so less subject to Filament faults Note: discharge starts only by mag field to avoid high E field - induced currents

98 Mass Spectrometry Need to distinguish the intensity of specific gas molecules Collect molecules Molecule ionization Separation of different molecules Current measurement Specific mass = ion mass (a.u.)/ion charge = n = ion ionization multiplicity Specific mass of Ar + = 40 Specific mass of Ar ++ = 20 For a single molecule there are many peaks, depending on n

99 Mass Spectrometry Specific mass table

100 Mass Spectrometry detector Faraday cup All ions measured No filaments Low sensitivity sturdy Channeltron - electron multiplyer High sensitivity Delicate Fast response To remove secondary electrons Amplifier time constant large

101 Quadrupole Mass Spectrometry (QMS) Vacuum Chamber Ion source (filament) Analyser (Quadrupole field) Detector (Channeltron) Storing system Quadrupole field between the rods Ions of varying mass are shot axially into the rod The applied quadrupole field deflects the ions in the X and Y directions, causing them to describe helical trajectories through the mass filter.

102 Quadrupole Mass Spectrometry (QMS) r 0 = rod separation (~3mm) U+Vcos( t) -(U+Vcos( t)) Superimpose an oscillating field Vcos( t) The forces are uncoupled along x,y,z axis Quadrupole potential

103 Quadrupole Mass Spectrometry (QMS) ion equation of motion Constant speed along z Stability parameters

104 Quadrupole Mass Spectrometry (QMS) Solved numerically for different a and q All solutions outside are imaginary and give increasing oscillation amplitudes Neutralization of the ions on the rods Ions oscillate in the xy plane Only some e/m values reach detector Solutions inside are real (stable trajectory)

105 Quadrupole Mass Spectrometry (QMS) Zoom to region I The line shrink to one point Only one ion with m/e ratio can reach detector Stable solutions fixed U, V and the overall ion motion can (depending on the values of a and q) result in a stable trajectory causing ions of a certain m/z value to pass the quadrupole for

106 Quadrupole Mass Spectrometry (QMS) Zoom to region I The line enter the stable solutions region Work line All the ions with a/q on the line will reach detector V=V 0 cos( t) for Reducing U relative toV, an increasingly wider m/z range can be transmitted simultaneously. qq the width q of the stable region determines the resolution. By varying the magnitude of U and V at constant U/V ratio an U/V = constant scan is obtained ions of increasingly higher m/e values to travel through the quadrupole

107 Quadrupole Mass Spectrometry (QMS) How to select different m/e ratios Change U, V keeping the ratio constant This results in different m/e values allowed to pass the quadrupole In the U,V space a changing in m/e ratio means moving along the straight line

108 Quadrupole Mass Spectrometry (QMS) V AC > U DC the two pairs act as a band pass filter, low and high mass ions outside the band being rejected, with limits determined by the values of U, V, and . U+Vcos( t) (x) -(U+Vcos( t)) (y) Ion traveling in the z direction X Motion will tend to be stable for (t) >0 and unstable (t)

109 Quadrupole Mass Spectroscopy (QMS) profiles of the residual gas p 3x10 -7 mbar Before bake-out p 5x10 -11 mbar After bake-out H2OH2O CO+N 2 CO 2 H2OH2O H2H2

110 VACUUM SEALING Clamps Low Vacuum No bake at high temperatures Reusable Viton rings

111 UHV VACUUM SEALING HV Bake at high temperatures Reusable (maybe once) Plastic deformation and shear

112 VALVES Diaphragm Butterfly

113 VALVES Dynamometric sealing Stem All metal

114 VALVES Gate Leak High conductance UHV to air compatible Large clearance for instruments Bakeable

115 FEEDTHROUGH Multi-pin for signal or Low currents Multi-pin for high currents

116 MANIPULATION Rotation

Introduction Basic concepts of vacuum Vacuum Hardware (pumps, gauges) Mass Spectrometry OUTLINE...

Documents

Transcript of Introduction Basic concepts of vacuum Vacuum Hardware (pumps, gauges) Mass Spectrometry OUTLINE...