Ion sources. Ionization and desorption methods

Transcription

1 Ion sources Ionization and desorption methods 1

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3 Processes in ion sources 3

4 Ionization/ desorption Ionization Desorption methods Electron impact ionization Chemical ionization Electro-spray ionisation Ion-spray ionisation Inductively coupled plasma MS Surface ionization Field ionisation Secondary ion mass spectrometrysims Liquid-SIMS, LSIMS Fast atom bombardment, FAB Laser desorption, LD Laser ablation/ laser ionization, LA/LI, Matrix assisted laser desorption, MALDI Plasma desorption, PD 4

5 Introduction: different types of ion sources 5

6 Ionization 6

7 Concept underlying the electron impact ionization The thermoionic electron emission is described by the Richardson equation: (W-tungsten) For T= K, the power of thermoionic emission is described: 7

8 Electron impact ionization The gas phase sample: -N 2, C 6 H 6 etc -Sample diluted in carrier gas Molecular beams, effusion beams Ionization by ev electrons The solid phase sample can be evaporized in an oven T.D. Märk & G.H. Dunn, Electron Impact Ionisation, Springer

9 Characteristic of some suitable materials ev 9

10 Single charged species 10

11 For multiple charged species 11

12 Ionization potentials for selected molecules 12

13 Ionization by collision with electrons 13

14 Excitation scenario diagram 14

15 Appearance potential ionization energy The ionization efficiency curve can be produced by the normal electron impact I ion (Uel). The ionization efficiency curve can be modeled and appearance potential can be determined. One should include into model that electron energy Is not mono-chromatic but it is described by Maxwell-Boltzman thermal distribution. 15

16 Electron Impact In the electron impact ionization method one can generate pool of the excited molecules, which can fragment during the flight to the detector leading to so-called ghosts mass speaks (Geister Linien) : 16

17 Electron impact generation of the negatively charged ions Dissociative ion pair formation Dissociative electron attachment Electron attachment 17

18 Ion formation formulae With the gas density n 0, the cross section σ, the electron pathway s, electron current I e, and probability β that ion will be formed in ionization volume: the ion current can be determined to be: n 0... Density gradient can occur Ie... Ions can be lost on electrodes, Repeller, Wände,... β... depends on formation location and can vary significantly depends on the gas temperature, initial energy of ions and fragments generated in dissociative ionization, The average values are taken for the calculation 18

19 Example 19

20 The ionization cross section, σ 20

21 Partial cross sections for multi-ionization Total cross-section: Ionization is stepwise: 21

22 examples 22

23 The ionization cross section obtained for electrons scattering with 90 ev The absolute cross sections can only be determined for the limmited number of molecules T.D. Märk & G.H. Dunn, Electron Impact Ionisation, Springer

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30 Chemical ionization In the chemical ionization, the sample is ionized in ion-molecule reaction in the environment saturated by ions. One uses Methan, Iso-Buthan, or Ammoniak atmospheres, to generate their cations. 30

31 Chemical ionization Hybrid abstraction Charge exchange The ionization potential of the ionized gas must be larger than reactant. If the IP>5 ev the spectra will be similar to that obtained by electron ionization Ion molecule reactions J. Watson, Introduction to Mass Spectrometry, 2nd Ed., Raven Press, New York, 1985 A.G. Harrison, Chemical Ionisation Mass Spectrometry, CRC Press, Boca Raton,

32 Chemical Ionization 32

33 Chemical Ionization 33

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35 Proton transfer mass spectrometry 35

36 Proton transfer mass spectrometry Chemical ionization for the diagnostic of organic molecules: Hollow cathode discharge generates H 3 O + Drift Tube very small fragmentation Quantitative no GC neccesary 1 part of can be detected Application in the field of environmental research -Medicine, food industry 36

37 Atmospheric pressure ion source Ionization under atmospheric pressure in corona discharge or 63 Ni source API sources ionized in sensitive way trace elements in the air It can be coupled to liquid chromatography (LC): Ion spray ion spray ionization inductively coupled plasma (ICP) High energy electrons interact with Nitrogen contained in the air or through the interface One obtains thermalized electrons: 37

38 Electrospray ionization (ESI) The sample is transported in capillary A high voltage is applied to the end of capillary 38

39 ESI 39

40 Electrospray ionization (ESI) 40

41 Electroionization: determination of the mass from the spectra 41

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43 Inductively coupled plasma mass spectrometry * Highly precise analysis and detection of trace elements: geochemistry, biology * Sample is provided through cappillare and laminar Ar beam is used * High frequency (e.g., 30 MHz) discharge * Atomization and generation of single ionized cations mass spectra are simple to interpret * High sensitivity typicaly 50 ng/l 50 ppt 43

44 Glow discharge ion source/planar cathode 44

45 Gas discharge ion source /pin-shaped cathode 45

46 Grimm-type dc glow discharge ion source 46

47 Surface ionization Ions emited from the hot metal surfaces are partially ionized. The ionization yield: N + is number of evaporated ions per time unit (rate of ion generation), N 0 is number of neutral particles evaporated from the surface, g + statistical weigthing of ions (number of states), g 0 statistical weighting of neutrals Φ the surface exit energy I Ionization potential of evaporated ions Valid for molecular ionization molecules Valid at thermostatic equilibrium between the surface and the sample 47

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49 Example of the thermoionic efficiencies (k B T 1 ev for T K). In general, the dependance is more complex, thickness of the surfaces, contamination, lack of thermal equilibrium! 49

50 Elements analyzed by thermal surface ionization 50

51 Field ionization the potential for a external electron in the molecule in the field-free space and in the external field Elektrons originated from A level are in a strong electric field and are not bound; electrons from B level can tunnel through the potential barrier Field ionization conditions requires the electric field of a few V pro atom or molecule radi size, e.g., 1 V/0.1 nm, gives electric field strength of V/m 51

52 Field ionization The stark field can be obtained using a sharpe edges. Approximating the edge by rounded surface one can estimated the field parameters and radius of the edge: For E(R 0 ) = V/m and to get U = 10 kv One need the radius of the edge of: The molecule adsorbed on the edges are desorbed by the field and ionized: field desorption and ionization 52

53 Field ionization Ions generated by field ionization possess relatively narrow energy distribution The fragmentation is minimal Practically no excited ions Ion currents of A A Generation of M +, [M+H] +, [M+Na] +, [M+K] +, H.D. Beckey, Field Ionisation Mass Spectrometry, Pergamon Press, Oxford,

54 Desorption methods 54

55 Particle desorption by energetic ion or neutral beams Atoms and molecules can be sputtered from the surface by fast ion or neutral beams (kev). A part of sputtered material is ionized. Application of neutral beams to sputter the material is known as Fast Atom Bombardment FAB. The efficiency of sputtering depends on kind of primary particles used and sample material, energy and the incident angle charge state, crystal structure and material reorganization. Benninghoven, Rüdenauer, Werner, Secondary Ion Mass Spectrometry, Chemical Analysis, Vol 86 (1987). 55

56 Sputtering 56

57 Sputtering 57

58 α... Dimensionless parameter dependant on M2 / M1 C 0... Constant from the Sputtering theory Sn... Nuclear scattering losses for primary energy Ep US... Binding energie of surface 58

59 Surface oxidation 59

60 Fast atom bombardement FAB 60

61 How to get fast atoms 61

62 FAB Since 1980, FAB is used as a soft ionization technique in organic mass spectrometry. It is convinient method because of a low fragmentation efficiency. The mother molecule (protonated or di-protonated) is the main mass. Sometimes Na is used. Peptides are up to 10 4 amu/q0 observed. A large mass range and a high resolution spectrometers are required. 62

63 Photoionization The ionization energies are larger than 5 ev, in most cases larger than 9 ev. For the one-photon ionization radiation energy of the photons is in VUV region (the sample and the radiation source have to be used in the vacuum environment). High photon energy can be generated using hydrogen, deuterium lamps, synchrotron radiation or in by the laser wave mixing methods. The photoionization cross sections are times smaller than that of electron impact ionization. (hν > IP). For NO (IP = 9.5 ev), Lyman-α radiation with λ =121.6 nm (hν = 10.2 ev) can be used for the ionization. 63

64 Photoionization vs electron impact The photoionization is convinient to measure ionization energy (e.g., synchrotron radiation, laser VUV source). Photoionization spectra are simple with a small fragmentation efficiency. 64

65 Laser photoionization resonant nonresonant IP Resonance ionization mass spectrometry (RIMS) -low laser power resonant steps -selective -high efficiency, low fragmentation Multiphoton ionization (MPI) -high laser power, non-resonant steps -non-selective, but power dependance on laser power -low efficiency, high fragmentation 65

66 Selectivity and high ionization rate are high for atoms 66

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68 Resonance Ionization Mass Spectrometry: example 187 Os / 187 Re measurement 68

69 Resonance Ionisation Mass Spectrometry: example 187 Os / 187 Re measurement 69

70 Photoionisation: non-resonant Laser Ionization 70

71 Non-resonant photoionization:sims-sali SIMS secondary ion mass spectrometry SALI surface analysis mass spectrometry 71

72 SIMS 72

73 Non-resonant photionization 73

74 Non-resonant ionization 74

75 Non-resonant ionization of molecules 75

76 Laser desorption/laser ablation 76

77 Laser desorption 77

78 Laser Ablation / Laser Ionisation 78

79 Laser ablation 79

80 Laser Ablation / Laser Ionization 80

81 Matrix assisted laser desorption and ionization MALDI is soft ionization method for detection of large molecules for which other methods or not sensitive. MALDI can be well applied for the detection of biopolymers (Proteine, Glycoproteine, Oligosaccharide, Oligonucleotide). The samples are cristilized in a matrix. The trix acts as absorber of laser radiation and subsequently enables its evaporation. The sample is mixed with the ratio 1: :5000 with the matrix molecules. Matrix UV Laser (broadband UV absorption): cinnamic acid sinapinic acid... IR Laser: nicotinic acid glycerin Succinsäure, Harnstoff Kaffeesäure,... 81

82 Diagram of MALDI source 82

83 Characteristics: MALDI > Mass range: up to 500 kdalton > Positive Ions Protonated, [M+H] + Protonated, [M+2H] 2+ Negative Ions [M H] > positive und negative cluster Ionen [2M+H] +, [2M H] > No fragmentation > Sensitivity in fmol region > Sample preparation is alchemie > Matrix and sample should have non-overlap absorption bands > Laser pulses should be short 83

84 MALDI matrix 84

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86 MALDI: example 86

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88 Summary of ionization methods 88

89 Summary of desorption methods 89