Lecture on mass spectrometry Lenka Zajíčková (Faculty of Science MU & CEITEC BUT) i. 1.1 1.2 1.3 1.4 Introduction Applications Principles of Mass Spectrometry Terminology Parts of Mass Spectrometer 2. Ion sources 2.1 Ionization 2.2 Electron ionization 2.3 Chemical ionization 2.4 SIMS and Fast atom/ion bombardment 2.5 Laser desorption ionization and MALDI 2.6 Electrospray more details in the course F7360 Characterization of surfaces and thin films spring semester 2025 3. Mass analyzers 3.1 Quadrupolar Analyzers 3.2 Quadrupole Ion Trap 3.3 Time-of-flight Analyzers 3.4 Magnetic and Electromagnetic Analyzers 4. Detectors 4.1 Photographic Plate & Faraday Cup 4.2 Electron multipliers 4.3 Electro-Optical Ion Detectors 1. Introduction Mass spectrometer (MS) is an analytical tool measuring the mass-to-charge ratio (m/z) of ions created from a sample. Literature • E. de Hoffmann and V. Stroobant, Mass Spectrometry: Principles and Applications, Wiley 1999 • J. H. Gross, Mass Spectrometry, Springer 2011 • J. Benedikt, A. Hecimovic, D. Ellerweg and A. von Keudell, Quadrupole mass spectrometry of reactive plasmas, J. Phys. D: Appl. Phys. 45 (2012) 403001 (23pp) 1. Introduction 1.1 Applications Vacuum systems: □ leak detection in vacuum systems □ determination of gas-specific desorption and adsorption rates of materials for vacuum system components □ partial pressure measurements in high vacuum systems Coating and etching: □ quantitative determination of the composition and purity of process gases □ monitoring the gas composition in vacuum coating processes □ end point detection in vacuum etching □ mass-resolved determination of neutral particles and ions in plasma processes (in this case, it can be coupled with the energy resolved analysis of ions) Other applications: □ mass-selective leak testing of serial production components in the automotive industry □ analyses of complex mixtures or compounds □ analyses of complex reactions on the surface of solid bodies □ investigation of biochemical substance transformations 1. Introduction 1.2 Principles The first step in the mass spectrometric analysis of compounds is the production of gas-phase ions of the compound, for example by electron ionization: M + e" —► M#+ + 2e" This molecular ion normally undergoes fragmentations. Because it is a radical cation with an odd number of electrons, it can fragment to give either a radical and an ion with an even number of electrons, or a molecule and a new radical cation. We stress the important difference between these two types of ions and the need to write them correctly: \ EE* + R* even ion radical OE*+ + N odd ion moleclxe These two types of ions have different chemical properties. Each primary product ion derived from the molecular ion can, in turn, undergo fragmentation, and so on. All these ions are separated in the mass spectrometer according to their mass-to-charge ratio, 1. Introduction 1.3 Terminology Atoms consist of nucleons (protons + neutrons) and electrons □ Z - atomic number (number of protons), N— number of neutrons □ chemical properties determined by the number of electrons (atomic number Z) □ physical properties - mass number (nucleon number) A: A = Z + N A state of ioniz. Isotopes x No. of atoms in molecule □ atom with a determined number of neutrons 4He => 2 protons + 2 neutrons = mass number 4 3He => 2 protons + 1 neutron = mass number 3 □ positively charged => electrons removed from the particle Ions | He+, N2+, C02+,38Ar+, 40Ar+, N2++ □ negatively charged => electrons attached to the particle O", OH" 1.3 Terminology The mass spectrum depends on m/z 15N+ ^ "J 5» 14N2+ * "28" 14|yl5|y+ ^ «2999 14N22+ "14" In mass spectrometry, the ion charge q is indicated as multiples (z) of the elementary charge e (charge of 1 electron) 1 e = 1.602 177xlO"19C q = z e and the mass m is indicated in atomic mass units li/ = 1.660 540xl0"27 kg. For simplicity, a new unit, the Thomson, with symbol Th, has been proposed 1 Th = 1 u/e = 1.036 426 x 108 kg C1 1.3 Terminology Example of mass spectrum - methanol CH3OH analyzed by electron impact ionization: 100-i i 50 31 29 15 i i 18 i '3? 15 mfz m : Relative m/z Relative abundance (%) abundance <%) 12 0.33 28 6.3 13 0.72 29 64 14 2.4 30 3.8 15 13 31 100 16 0.21 32 66 17 1.0 33 0.73 18 0.9 34 -0.1 □ m/z = 32 ... molecular ion □ m/z = 33 ... presence of 13C isotope (1.1% of the molecular ion peak) □ m/z = 15 ... methyl (CH3+) ion □ 32 -15 = 17... accompanying loss of neutral OH □ m/z = 16 ... could correspond to CH4#+, 0+ or CH3OH2+ but doubly charged ion for such a small molecule is not stable enough to be observed and 0+ is unlikely to occur. □ The most intense peak - base peak (normalized to 100%) 1.3 Terminology Mass □ m - mass in atomic mass units (u) or daltons (Da), lu= 1 Da = 1.660 540X10"27 kg □ u / Da used in different contexts: ■ u - masses referring to the particular isotope of each element as used in mass spectrometry ■Da - mean isotopic masses as generally used in stoichiometric calculations □ The mass number A gives rough figure for the atomic mass because of approx. equality of the proton and neutron masses (1.007277u and 1.008665u, respectively) and the relative insignificance of the electron mass (5.48xl0_4u). A state of ioniz. Z No. of atoms in molecule 1.3 Terminology Mass □ For stoichiometric calculations chemists use the average mass calculated using the atomic weights of atoms composing the molecule (weighted averages of the atomic masses for the differently abundant isotopes). Let us consider CH3C1 as an example: Chlorine atoms: mixture of two isotopes, 34.968 852 u and 36.965 903 u with relative abundances 75.77% and 24.23 %. The atomic weight of chlorine atoms is the weighted average mass: (34.968 852x0.7577+36.965 903 x0.2423) = 35.453 Da. The average mass of CH3C1 is 12.01 l+(3x 1.00 794)+35.453 = 50.4878 Da. Carbon and hydrogen are also composed of isotopes, but at much lower abundances. They are neglected for this example. I Mass | In mass spectrometry, the nominal mass or the monoisotopic mass is generally used. □ Nominal mass: calculated using the mass of the predominant isotope of each element rounded to the nearest integer value (element mass number A). □ Monoisotopic mass is calculated by using the exact mass of the most abundant isotope for each constituent element. Let us consider again CH3C1 as an example: The monoisotopic mass is 12.000 000+(3x 1.007 825)+34.968 852 = 49.992 327 u. When the mass of CH3C1 is measured with a mass spectrometer, two main isotopic peaks will appear: 7. w/z=(34.968852+12.000000+3 X 1.007825) = 49.992327 Th, rounded to m/z 50. 2. w/z=(36.96590+12.000000+3x 1.007825) =51.989365 Th, rounded to m/z 52. The abundance at this latter m/z value is (24.23/75.77)=0.3198, or 31.98% of that observed at m/z 50. Carbon and hydrogen isotopes are neglected in this example. The difference between the average mass, the i lass and the monoisotopic mass can amount to several Da for very high molecular weight, depending on the number of atoms and their isotopic composition. The type of mass determined by mass spectrometry depends largely on the resolution and accuracy of the analyzer. 1. example is human insulin, a protein having the molecular formula C257H383N65077S6 s of insulin is 5 i using the integer mass of the most abundant isotope of each element: 12 u for carbon, lu for hydrogen, 14 u for nitrogen, 16 u for oxygen and 32 u for sulfur. monoisotopic mass of 5803.6375 u is calculated using the exact masses of the predominant isotope of each element: C=12.0000 u, H=1.0079 u, N=14.0031 u, 0=15.9949 u and S=31.9721 u. average mass of 5807.6559 Da is calculated using the atomic weight for each element: C=12.011 Da, H=1.0078 Da, N=14.0067 Da, 0=15.9994 Da and S= 32.066 Da. 2. example - two alkanes having the molecular formulae C2oH42 and C100H202 Smaller alkane: i monoisotopic mass (20 x 12)+(42x 1.007825)=282.3287u rounded to 282.33 u, average mass (20x 12.01 l)+(42x 1.007 94)=282.5535 Da. Heavier alkane: i monoisotopic mass (100 x 12)+(202x 1.007825)=1403.5807u rounded to 1403.58 u, average mass is (100x 12.011)+(202x 1.007 94)=1404.7039 Da. monoisotopic mass is used when it is possible experimentally to distinguish the isotopes, average mass is used when the isotopes are not distinguishable. is not recommended and should only be used for low-mass compounds. 1. Introduction 1.4 Parts of Mass Spectrometer A mass spectrometer is an apparatus which □ produces a beam of gaseous ions from a sample - sample introduction system + ion source, □ sorts out the resulting mixture of ions according to their mass-to-charge ratio m/z - mass analyzer □ provides output signals which are measures of the relative abundance of each ionic species present - detector + recorder sample gas unresolved accelerated resolved focused ion beam ion beams ion mass analyzer detector source recorder high vacuum Ion sources 2.1 Ionization Although both positive and negative ions can be studied by mass spectrometry, the majority of instruments are used to investigate positive ions because in most ion sources they are produced in larger number (approx. 103 times more) than negative ions. The first ionization potential - a valence e~ from the highest occupied atomic or molecular orbit is removed to form the corresponding atomic or molecular ion (parent ion) in its ground state. To remove 2nd, 3rd etc. electron additional energy is needed (2nd, 3rd,... ionization potentials). □ by electron impact AB + e" -> AB+ + 2e~ □ by photon AB + hv -> AB+ + e" ultraviolet light, lasers (multi-photon absorption), synchrotron radiation □ by impact of high mass particle such as ion - charge exchange AB + C+ ->AB+ + C chemical ionization AB + RH+ -> ABH+ + R such as fast neutral AB + C->AB+ + e+C such as metastable (Penning ionization) AB + C* ->AB+ + e+C 2.1 Ionization - problem of collisions Collisions would produce a deviation of the trajectory and the ion would lose its charge against the walls of the instrument. Ion-molecule collisions could produce unwanted reactions and hence increase the complexity of the spectrum. ^ ^ The mean free path L of projectiles in a stationary target is given by L = -=- on op where k is the Boltzmann constant, o is the collision cross-section (in m2), n is the target density. Approximately o = tucP/4, where d is the sum of the diameters of the colliding particles. Electron - neutral collisions: d = a, i.e. estimated as the diameter of molecule a, o = 7ra2/4 Ion - neutral collisions: d = 2a, i.e. o = Tza1 and the movement of "target" molecules has to be taken into account, thus the mutual speed is V2 g rather than g, i.e. ^ Approximate mean free path of an ion under normal conditions in a mass spectrometer (A=1.38><10"21 JK"1, T ~ 300 K, o ~ 45x 10"20 m2) using the following equations L = 0.66 P where L is in cm and pressure p is in Pa. L = ^2on 1 pascal (Pa) = 1 newton (N) per m2 bar = 106 dyn cm-2 = 105 Pa millibar (mbar) = 10-3 bar= 102 Pa microbar (/xbar) = 10-6 bar = 10_1 Pa nanobar (nbar) = 10-9 bar = 10-4 Pa atmosphere (atm) - 1.013 bar = 101 308 Pa Torr= 1 mmHs= 1.333 mbar = 133.3 Pa psi = 1 pound per square inch = 0.07 atm □ In a mass analyzer working with defined ion trajectories, the mean free path should be at least 1 m and hence the maximum pressure should be 6.6 10~3 Pa. In the instruments using a high-voltage source, the pressure must be further reduced to prevent the occurrence of discharges. In contrast, some trap-based mass analyzers operate at higher pressure. □ Producing efficient ionization collisions requires the mean free path to be reduced to around 0.1 mm, implying at least a 60 Pa pressure in the region of the ion source. □ Introducing the sample to a mass spectrometer often requires the transfer of the sample from atmospheric pressure. ■ ^ These large differences in pressure are controlled with the help of an efficient pumping system using mechanical pumps in conjunction with turbo molecular, diffusion or cryogenic pumps. The mechanical pumps allow a vacuum of about 1-10"1 Pa to be obtained. Once this vacuum is achieved, the operation of the other pumping systems allows a vacuum as high as 10"8 Pa. □ Samples are often introduced without compromising the vacuum using direct infusion or direct insertion methods. For direct infusion, a capillary is employed to introduce the sample as a gas or a solution. For direct insertion, the sample is placed on a probe, a plate or a target that is then inserted into the source through a vacuum interlock. □ For the sources that work at atmospheric pressure and are known as atmospheric pressure ionization (API) sources, introduction of the sample is easy because the complicated procedure for sample introduction into the high vacuum of the mass spectrometer is removed. 2.1 Ionization - overview of different methods Gas-phase ionization (limited to compounds sufficiently volatile and thermally stable) Electron ionization (section 2.2): electron impact causing electron ejection or capture Chemical ionization (section 2.3): collision with other ions - protonation, deprotonation, adduct formation Field ionization: Potential difference 8-12 kV is applied between a filament called the emitter and a counter-electrode (a few mm distant). Gas phase molecules approach the surface of the emitter (positive potential). If the electric field at the surface is sufficiently intense (107 -108 V cm"1), one of the electrons from the sample molecule is transferred to the emitter by quantum tunneling, resulting in the formation of a radical cation M*+. This ion is repelled by the emitter and flies towards the negative counter-electrode. A large number of compounds are thermally labile or do not have sufficient vapor pressure. Molecules of these compounds must be directly extracted from the condensed to the gas phase. Liquid-phase ion sources: electrospray (sec. 2.6), atmospheric pressure chemical ionization and atmospheric pressure photoionization sources Solid-state ion sources: secondary ion mass spectrometry - SIMS (section 2.4), (matrix-assisted) laser desorption ionization - (MA)LDI (section 2.5), plasma desorption and field desorption sources (the analyte is in an non-volatile deposit irradiated by energetic particles or photons that produce ions from the solid surface) 2. Ion sources 2.2 Electron ionization widely used in plasma diagnostics and organic mass spectrometry Works well for many gas-phase molecules but induces extensive fragmentation so that the molecular ions are not always observed. Filament r heater -y-potential L Cathodic filament: electron emitter Electron accelerating potential Electron trajectory Gaseous sample r inlet Ionization space — Anode: electron discharge Extracting lens Focusing lens Accelerating lens TS C 8 * o Hi 10 _ to as? C CL to c o ra iz o ® Z E E I E u E fa >- ZI often used as e~ energy 2.2 Electron ionization Ionization E * E o C/) c o 15,7 multiply charged ions can be strongly suppressed by lowering the e~ energy, here < 43 eV 200 300 electron energy [ eV ] 2.2 Electron ionization Fragmentation during ionization AB + e~ -> A+ + B + 2e~ appearance potential (AE) - minimum energy required for creation of particular fragment ion. cracking (fractal) pattern - the array of peaks in the complete spectrum of a pure substance. Peak heights in a spectrum are usually normalized by taking the largest peak in the spectrum (base peak) as 100. Every chemical compound has its own distinctive cracking pattern ("fingerprint"). Ionization efficiencies and appearance potentials can be used in many ways to study electron impact phenomena: ■ mechanism of ionization and dissociation ■ calculation of chemical bond strengths ■ energy states of atoms, molecules, free radicals 2.2 Electron ionization Fragmentation during ionization C02 100% 5 Rel. Intensity 10% 5 1% 5 lOOOppm 100 ppm 16n + 12^+ 12C1sO, The fragment distribution of CO: 12c18o2+ ,2C160+ ':C"0' 12C1S01S0+ Hi 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 Mass |amu] 2.2 Electron ionization Mass spectrum of a gas mixture (recorded at 90 eV ionization energy) H* H* »1 0* Ht0* OH* T L ! 25 co+ 16o2+ «CO» J_ Ar' CO, uc«cot* 50 5 10 Wasserstoff Stickstoff 15 20 H Sauerstoff — Wasser 30 35 ■ Kohlenmonoxid 40 45 Argon Kohlendioxid Before carrying out a quantitative gas analysis, the respective calibration factors for each individual component must be determined by feeding suitable calibration gas mixtures with respective non-overlapping components. 2. Ion sources 2.3 Chemical Ionization (CI) production of ions through a collision of the molecule to be analyzed with primary ions (ions of a reagent gas) that are present in the ion source. Ion-molecule collisions are important - the local pressure has to be sufficient to allow for frequent collisions. electron filament be?m ion repeller reagent, gas inlet' . ^ ANALYTE * •* # * \ .V •* REAGENT " \ . . • • • • • • • o • electron trap magnet extraction grid focussing lens To Mass Spectrometer 2.3 Chemical Ionization (CI) □ Both positive and negative ions of the analyzed sample can be formed by chemical reactions with ions in the plasma: proton transfer reactions, hydride abstractions, adduct formations, charge transfers, etc. □ The plasma will also contain low-energy electrons, called thermal electrons. These are either electrons that were used for the first ionization and later slowed, or electrons produced by ionization reactions. These slow electrons may be associated with molecules, thereby yielding negative ions by electron capture. Chemical ionization (CI) is a technique that produces ions with little excess energy. Thus this technique presents the advantage ^ u- -i i-1 -i of yielding a spectrum with less fragmentation in which the Combined El and CI source: i i to . . -i . A nu • 1 • • molecular species is easily recognized. Chemical ionization is a lower energy process than electron ionization. (1) EI/CI switch; in EI mode, the box serves as a pusher; (2) microswitch; (3) entrance for the reagent gas; (4) flexible capillary carrying the reagent gas; (5) diaphragm; (6) filament giving off electrons; (7) path of the ions towards the analyzer inlet; (8) hole for the ionizing electrons in CI mode; (9) sample inlet; (10) box with holes, also named 'ion volume'. The pumping speed is sufficient to maintain a 60 Pa pressure (mean free path is about 0.1 mm) within the box. Outside, the usual pressure in a source, about 10 3 Pa, will be maintained. R rnnHa CA mfiHo 2.3 Chemical Ionization - proton transfer When analyte molecules M are introduced in the plasma of reagent gas, the reagent ions GH+ can often transfer a proton to the molecules M and produce protonated molecular ions MH+. □ It is acid-base reaction: the reagent gas ions GH+ is Bronsted acid (proton donor) and the analyte molecules M is Bronsted base (proton acceptor). □ The proton affinity (PA) is the negative enthalpy change for the protonation reaction. The observation of protonated molecular ions MH+ implies that the analyte molecule M has a proton affinity higher than that of the reagent gas: PA(M) > PA(G) □ If the reagent gas has a proton affinity higher than that of an analyte. PA(M) < PA(G), proton transfer from GH+ to M will be energetically unfavourable. □ The energetics of the proton transfer can be controlled by using different reagent gases. The most common reagent gases: methane (PA=5.7 eV), isobutane C4H10 (PA=8.5 eV) and ammonia (PA=9.0 eV). □ Fragmentation may occur with methane while with isobutane or ammonia the spectrum often presents solely a protonated molecular ion because protonation by these reagent gases is considerably less exothermic. M + GH+ -> MH+ + G ammonia isobutane 2.3 Chemical Ionization - proton transfer CD 03 — -q CO 8000 -J 6000 " 4000 - 2000 - 69 56 1111 n'i 11 p i l'I 11 i* i i'I 111111 p 11 i'i 1111 ■ ■ ^ 11111 p i f\ i'i 1111111111 ii 11111 40 60 80 100 120 140 160 180 mlz 87 87 El 82 i'i 'i 111 100 CD 03 — n cč 4500i 3000 1500- 69 61 methane CI 100 115 143 127 L E * CD 03 C S — _q CO 1200- 900-600 300 H I I I I P I ) | ( i 1 I 111 I ■ l I i i 1 i ■ i i i i I ■ i p ■ ■ i i i i i ? i I i I M I 1 | | i'i I I ) ' I m I I ■ '' ' I I > i 60 80 100 120 140 160 180 m/z isobutane CI 143 73 87 60 81 97 113 11 n 111 i f'i i J n t 111 i i ■ i' i^i'i tíh i'^i i 111 111 111 r 11 i i[ i ii ti n| 60 80 100 120 140 160 180 mlz Mass spectra of butyl methacrylate C8H1402 or CH2C(CH3)COO(CH2)3CH3 • Ionization techniques (EI vs CI) • and the reagent gas (methane vs isobutane) influence: • amount of fragmentation and • prominence of the protonated molecular ions detected at 143 Th. 2.3 Chemical Ionization - adduct formation The sample molecule can be associated with a protonated molecular ion MH+ or a reagent ion F +: MH+ + M —> (2M + H)+ F+ + M —► (F + M)+ Adduct is a product of a direct addition of two or more molecules, resulting in a single reaction product containing all atoms of all components. □ In chemical ionization, all the ions are liable to associate with polar molecules to form adducts. The process is favored by a possible formation of hydrogen bonds. □ For the adduct to be stable, the excess energy must be eliminated, a process which requires a collision with a third partner => reaction rate is of the third order □ A mixture of two species M and N can give rise to associations such as (MH+N)+, (F+N)+, (F+M)+, and so on. □ It is always useful to examine the peaks appearing beyond the ions of the molecular species of a substance thought to be pure. If some peaks cannot be explained by reasonable associations, a mixture must be suspected. 2.3 Chemical Ionization - adduct formation Two examples of chemical ionization (isobutane) spectra. • top spectrum - pure compound 327 Da • bottom spectrum - mixture of two compounds with masses 261 and 270. The substance was initially pure but appears as a mixture in the gas phase because it loses either hydrogen cyanide (HCN) or water. sity 100 c CD ■+—> 80 _c CD 60 > 40 CC CD DC 20 57 MW = 327 dalton o I 0-»-NH-< 176 228 + 272 !n CO LO + 328 co 2841 384 l±L__i_f- C00CH3 CHOH-CH2OS02CH3 CO + Is-CM CO CO CM CM + Is-CM CO 00 CM + CM CO + CM CO X CM 503 555 611 655 -±-!-b-i-1- 100 200 300 400 500 600m/z700 >■ 100 80 a) CD ■*—■ c CD > CD cc 60 40 20 261+1 262 HO N MW 288 dalton ? 100 137 —t-k— + o CM N CM 8 ^ O CO + + CD 5 C\l CM II s * CO CO 100 200 300 400 + _ o CD £j + |f II co £1 CM 00 500 m/z 600 When interpreting the results, one must always keep in mind that a mixture that is observed may result from the presence of several constituents before the vaporization or from their formation after the vaporization. 2.3 Chemical Ionization - charge-transfer CI Rare gases, nitrogen, carbon monoxide and others with high ionization potential react by charge exchange: Xe + e" —► Xe#+ + 2e~ Xe#+ + M —> M#+ + Xe A radical cation is obtained, as in EI, but with a smaller energy content. Less fragmentation is thus observed. In practice, it is not used very often. Repeat the nomenclature: Radical (more precisely, a free radical) is an atom, molecule, or ion that has unpaired valence electrons. {\/y h-oh -► h. -oh AA h—CH3 -► h • • ch3 Anion is an ion with more electrons than protons, giving it a net negative charge. Cation is an ion with fewer electrons than protons, giving it a positive charge. 2.3 Chemical Ionization - negative ion formation Almost all neutral substances are able to yield positive ions, whereas negative ion creation requires the presence of acidic groups or electronegative elements. This allows some selectivity for their detection in mixtures. Negative ions can be produced by capture of thermal electrons by the analyte molecule AB -I- e —> AB* (associative resonance capture) AB 4- e —> A* + B (dissociative resonance capture) AB 4- e —> A+ 4- B 4- e (ion pair production) □ Associative resonance capture leads to the formation of negative molecular ions, needs electrons in the energy range 0-2 eV. □ Dissociative resonance capture leads to the formation of negative fragment ions, observed with electrons of 0-15 eV. □ Ion pair production is observed with a wide range of electron energies above 15 eV. It is principally this process that leads to negative ion production under conventional EI conditions. Ion pair production forms structurally insignificant very low-mass ions with a sensitivity that is 3-4 orders of magnitude lower than that for positive ion production. Any excess of energy from the negative molecular ion as it is formed must be removed by collision. Thus, in CI conditions, the reagent gas serves not only for producing thermal electrons but also as a source of molecules for collisions to stabilize the formed ions. or by ion-molecule reactions between analyte and ions present in the reagent plasma. These reactions can be an acid-base reaction or an addition reaction through adduct formation. 2.3 Chemical Ionization - reagent gases Methane If methane is introduced into the ion volume through the tube, the primary reaction with the electrons will be a classical El reaction: CRi + e- CrV+ + 2e" This ion will fragment, mainly through the following reactions: CH3+ + H* CH2*+ + H2 However, mostly, it will collide and react with other methane molecules yielding CH/+ +CH4 —HCH5+)+ CHj* Other ion-molecule reactions with methane will occur in the plasma, such as CH3+ + CH4 -AC2H5+\ H2 100 C 50 (5D —LL-j 41 71 —ř*-1-1-ť-+■ 10 20 30 40 50 60 70 m/z Figure 1.7 Spectrum of methane ionization plasma at 20 Pa. The relative intensities depend on the pressure in the source. continues on next slide 2.3 Chemical Ionization - reagent gases Methane A C3Hs+ ion is formed by the following successive reactions: +CH4 —► C2H3+ + H2 + H* C2H3+ +CH4 —* C3H5+ f H2 The relative abundance of all these ions will depend on the pressure. Figure 1.7 shows the spectrum of the plasma obtained at 200 jibar (20 Pa). Taking CHs+, the most abundant ion, as a reference (100%), C2H5+ amounts to 83 % and C3H5+ to 14%. Unless it is a saturated hydrocarbon, the sample will mostly react by acquiring a proton in an acid-base type of reaction with one of the plasma ions, for example M + CH5+ —► MH+ + CH4 proton transfer A systematic study showed that the main ionizing reactions of molecules containing het-eroatoms occurred through acid-base reactions with C2H5 + and C3H5+. If, however, the sample is a saturated hydrocarbon RH, the ionization reaction will be a hydride abstraction: RH + CH5+ —► R+ + CH4 + H2 hydride abstraction Moreover, ion-molecule adduct formation is observed in the case of polar molecules, a type of gas-phase solvation, for example m + ch3+ —► (M + ch3)+ adduct formation The ions (MH)+, R+ and (M + CH3)+ and other adducts of ions with the molecule are termed molecular species or, less often, pseudomolecular ions. They allow the determination of the molecular mass of the molecules in the sample. 100 1 1 1 50 17 29 J 41 I 71 10 20 30 40 50 60 70 mlz Figure 1.7 Spectrum of methane ionization plasma at 20 Pa. The relative intensities depend on the pressure in the source. 2.3 Chemical Ionization - reagent gases Isobutane Isobutane loses an electron upon EI and yields the corresponding radical cation, which will fragment mainly through the loss of a hydrogen radical to yield a /-butyl cation, and to a lesser extent through the loss of a methyl radical: CH3I 100 —- CH3-C-H + 2e" CH3 CH3 CH3-C-H + e" CH3 CH3I CH3-C-H CH3 CH3-C-H + CHi CH3 An ion with mass 39Da is also observed in its spectrum (Figure 1.8) which corresponds to C3H3+. Neither its formation mechanism nor its structure are known, but it is possible that it is the aromatic cyclopropenium ion. Here again, the plasma ions will mainly react through proton transfer to the sample, but polar molecules will also form adducts with the /-butyl ions (M + 57)+ and with C3H3+, yielding (M+ 39)+ among others. This isobutane plasma will be very inefficient in ionizing hydrocarbons because the /-butyl cation is relatively stable. This characteristic allows its use in order to detect specifically various substances in mixtures containing also hydrocarbons. 50 0 V 43 39 j 10 20 30 40 50 60 70 m'z Figure 1.8 Spectrum of the isobutane plasma under chemical ionization conditions at 200 fibar. 2.3 Chemical Ionization - reagent gases Amonia The radical cation generated by EI reacts with an ammonia molecule to yield the ammonium ion and the NH2* radical: NH3*+ + NH3 —► NH4+ I- NH2# An ion with mass 35 Da is observed in the plasma (Figure 1.9) which results from the association of an ammonium ion and an ammonia molecule: nh4+ + NH3 —►((N^TnHj)^ This adduct represents 15 % of the intensity of the ammonium ion at 200 u,bar. I » 100 c - 50 0 1 S. 10 20 30 40 50 60 70 mlz Figure 1.9 Spectrum of an ammonia ionization plasma at 200 ubar. In this gas, the ionization mode will depend on the nature of the sample. The basic molecules, mostly amines, will ionize through a proton transfer RNH2 + NR»+ —► RNH3+ +NH3 Polar molecules and those able to form hydrogen bonds while presenting no or little basic character will form adducts. In intermediate cases, twopseudomolecular ions (M + 1)+ and (M + 18)+ will be observed. Compounds that do not correspond to the criteria listed above, for example saturated hydrocarbons, will not be efficiently ionized. Alkanes, aromatics, ethers and nitrogen compounds other than amines will not be greatly ionized. Comparing spectra measured with various reagent gases will thus be very instructive. For example, the detection, in the presence of a wealth of saturated hydrocarbons, of a few compounds liable to be ionized is possible, as shown in Figure 1.10. 2. Ion sources 2.4a Secondary ion mass spectrometry (SIMS) analyses the secondary ions emitted when a surface is irradiated with an energetic primary ion beam. □ mostly used with solids and is especially useful to study conducting surfaces. High resolution chemical maps are produced by scanning a tightly focused ionizing beam across the surface. □ Static SIMS - low energy beam (less damage), □ Dynamic SIMS - high energy beam (erosion, profiling) More details in the course F7360 Charakterizace povrchů a tenkých vrstev vnvÍT/icr vn lín n v in v 9/19^ 2. Ion sources 2.4b Fast Atom Bombardment and Liquid Secondary Ion Mass Spectrometry Fast atom bombardment (FAB) and Liquid Secondary Ion Mass Spectrometry (LSIMS) □ High primary current beam of neutral atoms/molecules (FAB) or ions (LSIMS) is focused on the sample. □ The sample must be dissolved in a non-volatile liquid matrix. Glycerol is most often used, m-nitrobenzylic alcohol (MNBA) is a good liquid matrix for non-polar compounds, and di-and triethanolamine are efficient, owing to their basicity, in producing negative ions. Thioglycerol and eutectic mixture of dithiothreitol and dithioerythritol (5:1 w/w), referred to as magic bullet, are alternatives to glycerol. □ Energetic particles hit the sample solution, inducing a shock wave which ejects ions and molecules from the solution. Ions are accelerated by a potential difference towards the analyzer. These techniques induce little or no ionization. They generally eject into the gas phase ions that were already present in the solution. Softer than EI and CI. Gives (M+H)+ ions and little fragmentation. Good for more polar compounds. Very efficient for producing ions from polar compounds with high molecular weights. Ions up to 10000 Da and above can be observed, such as peptides and nucleotides. Moreover, it often produces ion beams that can be maintained during long periods of time, sometimes several tens of minutes, which allows several types of analysis to be carried out. Fast atom bombardment □ The neutral atom beam at about 5 keV is obtained by ionizing a compound, most often argon, sometimes xenon. Ions are accelerated, focused and then neutralized: (1) Ionization of argon; the ions are accelerated and focused by the lenses (2). In (3), the argon ions exchange their charge with neutral atoms, thus becoming rapid neutral atoms. As the beam path passes between the electrodes (4), all ionic species are deflected. Only rapid neutral atoms reach the sample dissolved in a drop of glycerol (5). The ions ejected from the drop are accelerated by the pusher (6), and focused by the electrodes (7) towards the analyzer (8). Liquid Secondary Ion Mass Spectrometry (LSIMS) □ Using a 'caesium gun', one produces a beam of Cs+ ions at about 30 keV. It is claimed to give better sensitivity than a neutral atom beam for high molecular weights. However, the advantage of using neutral molecules instead of ions lies in the avoidance of an accumulation of charges in the non-conducting samples. Ion sources 2.5a Laser Desorption Ionization (LDI) □ Laser desorption ionization (LDI) is an efficient method for producing gaseous ions. Generally, laser pulses yielding from 106 to 1010 Wcm"2 are focused on a sample surface of about 10"3-10"4 cm2, most often a solid. □ This technique is used in the study of surfaces and in the analysis of the local composition of samples, such as inclusions in minerals or in cell organelles. It normally allows selective ionization by adjusting the laser wavelength. However, in most conventional infrared LD modes, the laser creates a thermal spike, and thus it is not necessary to match the laser wavelength with the sample. □ Since the signals are very short, simultaneous detection analyzers or time-of-flight analyzers are required. The probability of obtaining a useful mass spectrum depends critically on the specific physical properties of the analyte (e.g. photoabsorption, volatility, etc.). □ Furthermore, the produced ions are almost always fragmentation products of the original molecule if its mass is above approximately 500 Da. This situation changed dramatically with the development of matrix-assisted laser desorption ionization (MALDI)... next slide 2.5b Matrix-Assisted Laser Desorption Ionization (MALDI) □ First, the compound to be analyzed is mixed with a suitable matrix (usually small crystallized organic molecules) using a solvent. The matrix molecules must have a strong absorption at the laser wavelength. This mixture is dried before analysis, i.e. any liquid solvent used in the solution preparation is removed. The result is a 'solid solution' deposit of analyte-doped matrix crystals. The analyte molecules are embedded in the matrix so that they are completely isolated from one another. □ Second, ablation of solid solution by intense laser pulses over a short duration (under vacuum conditions inside the source of the mass spectrometer). Irradiation by the laser induces rapid heating of the crystals by the accumulation of a large amount of energy in the condensed phase through excitation of the matrix molecules. The rapid heating causes localized sublimation of the matrix crystals, ablation of a portion of the crystal surface and expansion of the matrix into the gas phase, entraining intact analyte in the expanding matrix plume. laser sample plate extraction focussing nnrt lone 2. Ion sources 2.6 Electrospray (ESI) Analyte-'EKient flow -M (syringe pump of LC) N2 (80°C) +3 to 6 kV, ions + Metallic capillary Lens r i r i Pump N2(80°C) Spray N^eKJ^^Cepiiary \ Anatyte/Sotvertt droplets (spray) □ □ Summers Transfer canary / To Mass Spectrometef -> ATMOSPHLWC PKLSÜUKL Pumping ESI is produced by applying a strong electric field, under atmospheric pressure, to a liquid passing through a capillary tube with a weak flux (normally 1-10 ul/min). The electric field is obtained by applying a potential difference of 3-6kV between the capillary and the counter-electrode, separated by 0.3-2 cm, producing electric fields of the order of 106 Vm"1 □ The spray starts at an 'onset voltage' that, for a given source, depends on the surface tension of the solvent. In a source which has an onset voltage of 4 kV for water (surface tension 0.073Nm"2), 2.2 kV is estimated for methanol (0.023Nirr2), 2.5 kV for acetonitrile (0.030Nirr2) and 3 kV for dimethylsulfoxide (0.043Nm"2). □ A gas injected coaxially at a low flow rate allows the dispersion of the spray to be limited in space. □ The droplets then pass either through a curtain of heated inert gas, most often nitrogen, or through a heated capillary to remove the last solvent molecules. 2. Ion sources 2.6 Electrospray (ESI) (contin.) If one examines with a microscope the nascent drop forming at the tip of the capillary while increasing the voltage, at low voltages the drop appears spherical, then elongates under the pressure of the accumulated charges at the tip in the stronger electric field; when the surface tension is broken, the shape of the drop changes to a 'Taylor cone' and the spray appears. ooo O O O O Rayleigh: q2 = 8k2 e0yD3 A decomposing droplet in an electrospray source: q is charge; s0 permittivity of the environment; y surface tension and D is diameter of a supposed spherical droplet. Breakdown of the droplets can occur before the limit given by the Rayleigh equation is reached because the droplets are mechanically deformed, thus reducing the repulsion necessary to break down the droplets. The solvent contained in the droplets evaporates, which causes them to shrink and their charge per unit volume to increase. 3. Mass Analyzers Type of analyser Symbol Principle of separation Electric sector E or ESA Kinetic energy Magnetic sector B Momentum Quadrupole Q m/z (trajectory stability) Ion trap IT m/z (resonance frequency) Time-of-flisht TOF Velocity (flight time) Fourier transform ion cyclotron resonance FTICR m/z (resonance frequency) Fourier transform orbitrap FT-OT m/z (resonance frequency) Measuring the performance of a mass analyzer Mass resolution or resolving power Mass range limit - The mass range determines the limit of m/z over which the mass analyzer can measure ions. It is expressed in Th, or in u for an ion carrying an elementary charge, that is z = 1. Analysis speed - The analysis speed, also called the scan speed, is the rate at which the analyzer measures over a particular mass range. It is expressed in mass units per second (u s"1) or in mass units per millisecond (u ms"1). Transmission - The transmission is the ratio of the number of ions reaching the detector and the number of ions entering the mass analyzer. Sensitivity of spectrometer is related to trasmission. Mass accuracy - Mass accuracy indicates the accuracy of the m/z provided by the mass analyzer. It is the difference that is observed between the theoretical m/z (^theoretical) and the measured m/z (mmeasmed). It can be expressed in millimass units (mmu) but is often expressed in parts per million (ppm). Resolution or resolving power resolving power R = M / AM where AM is the resolution □ A measure of ability to separate and identify ions of slightly different masses Usually defined in terms of the largest mass at which a given criterion is met. □ The most popular "valley" definition: highest mass at which two adjacent peaks of equal height, differing in mass by AM, exhibit a valley between the peaks not greater than a certain percentage such as 2 or 10 %, of the peak height. □ In practice, resolution must often be determined using an isolated peak. Then AM is often taken as the width of the peak at 50 % peak height level (FWHM). For example resolving power of 2500 is required to separate the N2+ peak (mass = 28.006148) from CO peak (mass = 27.994915), even though the nominal mass is only 28. Sensitivity Sensitivity and resolving power are inversely proportional! □ A measure of the instrument's response to ions of a particular component at an arbitrary m/z value. It is expressed for a particular peak and a particular sample □ various materials exhibit different efficiencies for ionization in the source, □ there might be differences in the efficiencies of the transmission of ions through the mass analyzer □ the detector may exhibit a higher or lower efficiency for a particular mass or type of ion. instrument noise level, i.e. the spurious instrument response not due to ions striking the ion collector - signal-to-noise at least 2:1 for good measurement instrument background, instrument response, at a given mass, without the sample 3. Mass Analysers 3.1 Quadrupole Analyzers The quadrupole analyzer is a device which uses the stability of the trajectories in oscillating electric fields to separate ions according to their m/z ratios. The 2D or 3D ion traps are based on the same principle. (A.y) = ^()(-V2 - y2)/rl = (.V2 - J2)(C/ - V COS örf)/>Q Differentiating and rearranging the terms leads to the following equations of the movement (Paul equation): d2x 2ze — H--— (U - V cos cot)x = 0 dt- mrfi dry 2ze —----=■ (U - V cos cot) v = 0 3. Mass Analyzers 3.1 Quadrupole Analyzers (contin.) The trajectory of an ion will be stable (ion will be able to pass the quadrupole ) if the values of x and y never reach r0, thus if it never hits the rods. Solution: based on the solution of equation established in 1866 by the physicist Mathieu to describe the propagation of waves in membranes: ^2 M —7 + (au - 2qu cos 2£) u = 0 u stands for either x or y. Comparing the preceding equations with this one, and taking into account that the potential along y has opposite sign to the one along x, the following change of variables gives to the equations of the movement the form of the Mathieu equation: 2*2 COt 9 CO t First, £ is defined as £ = — and thus t~ = - 2 4 In the first term of the Paul equation, replacing t2 by ^2 introduces a factor co2/4. To compensate for this factor, the whole equation must be multiplied by the reverse, 4/co2. In the cosine term, 2^ is equal to cot, as needed in the Paul equations. Incorporating these changes and rearranging the terms yields the following expressions: SzeU 4zeV au = ax = —ay =-— and qu = c/x = -c/v =-— 3. Mass Analyzers 3.1 Quadrupole Analyzers (contin.) Stability areas along x or y: a, xz plane yz plane Stable along both x and y -v^|| xz plane yz plane 1/ X Stable along x ^ ■<7u Stable along y u represents either x or y. The four stability areas are labelled A to D and are circled. The area A is the one used commonly in mass spectrometers. In practice, the highest detectable m/z ratio is about 4000 Th, and the resolution hovers around 3000. Thus, beyond 3000 u the isotope clusters are no longer clearly resolved. Usually, quadrupole mass spectrometers are operated at unit resolution, that is a resolution that is sufficient to separate two peaks one mass unit apart. Stable along y, unstable along x Quadrupoles are low-resolution instruments. 3. Mass Analyzers 3.1 Quadrupole Analyzers (contin.) 2 2 2 2 ---± and V= the initially less energetic ions receive more kinetic energy and join the initially more energetic ions at the detector. Energy focusing can be accomplished by adjusting the amplitude of the pulse and the time delay between ion formation and extraction. For optimal focusing, both the pulse and the delay is adjusted separately, and it is mass dependent. Lower pulse voltages or shorter delays are required to focus ions of lower m/z ratio. 3.3 Time-of-Flight Analyzers - Reflectrons Another way to improve mass resolution is to use an electrostatic reflector also called a reflectron: create a retarding field that acts as an ion mirror by deflecting the ions and sending them back through the flight tube. Reflectron consists usually of a series equally spaced grid electrodes or more preferably ring electrodes connected through a resistive network of equal-value resistors. REFLECTRON 3000 V SOURCE P DETECTOR \ c E = VR/D □ The reflectron increases the mass resolution at the expenses of sensitivity and introduces a mass range limitation. The performance of the reflectron may be improved by using a two-stage reflectron, to reduce the size and to improve the homogeneity of the electric field. In this reflectron, two successive homogeneous electric fields of different potential gradient are used. The first stage is characterized by an intense electric field responsible for the strong deceleration of the ions while the second stage is characterized by a weaker field. These two-stage reflectrons have the advantage of being more compact devices because of the strong deceleration of the ions at the first stage, but they suffer from a lower transmission. 3.3 Time-of-Flight Analyzers - Reflectrons Gridless Reflection Electrodes-""*— M2 M3 Detector = _ Ion reflection ~ section ~ Ion deceleration ~ section 1 Ion Packet 3 Pulsed Ion Source □ Ions with more kinetic energy and hence with more velocity will penetrate the reflectron more deeply than ions with lower kinetic energy, i S Consequently, the faster ions will spend more time in the reflectron and will reach the detector at the same time than slower ions with the same m/z - correction of the initial kinetic energy dispersion view down the reflectron http://www.auburn.edu/cosam/departments/chemistry/facilities/massspecl/education/malditof/reflectro 3. Mass Analyzers 3.4 Magnetic and Electromagnetic Analyzers Action of the magnetic field FM = qvB qvB = mv or ion beam source lighter atom or molecule' ion detectors heavier atom or molecule Faradays Detector If the radius r is imposed by the presence of a flight tube with a fixed radius r, for a given B only ions with corresponding m/q go through. Changing B as a function of time: successive observations of ions with various m/q, provided that they all have the same kinetic energy. The magnetic analyzer is fundamentally a momentum analyzer and can be used as a mass analyzer provided that the kinetic energy of the ions or at least their velocity is known. The kinetic energy can be controlled with an electrostatic analyzer {next slide). Instead of positioning a guide tube and detecting the ions successively while scanning the magnetic d, it is also possible to use the characteristic that ions with the same kinetic energy but different m/q ratios have trajectories with different rvalues. Such ions emerge at different positions (these instruments are said to be < e). 3.4 Electromagnetic Analyzers - Electrostatic field Suppose a radial electrostatic field E is produced by a cylindrical condenser. The trajectory is then circular and the velocity is constantly perpendicular to the field. 2Ek cjE = mv Introducing the entrance kinetic energy Ek S0^ce r = Electric sector r=2v/E Magnetic sector r = mv/Bz Centrifugal force = Centripetal force Kinetic energy = Potential energy nVz= B2r2/2V m/z= Er/v2 Since the trajectory is independent of the mass, the ; not a mass analyzer, but rather a , just as the magnetic field is a momentum analyzer. The electric sector separates the ions according to their kinetic energy. 3.4 Electromagnetic Analyzers - Dispersion and resolution Resolution depends inversely on the dispersion at the analyzer outlet. Three factors favor the dispersion, and thus the loss of resolution: 1. If the ions entering the field do not have the same kinetic energy, they follow different trajectories through the field. This is called energy dispersion. 2. If the ions entering the field follow different trajectories, this divergence may increase during the trip through the field. This is called angular dispersion. 3. The incoming ions do not originate from one point, but from a slit. The magnetic or electric field can only yield, at best, a picture of that slit. The picture width depends on the width of the slit and on the magnifying effect of the analyzer. energy dispersion angular dispersion I 3.4 Electromagnetic Analyzers - Direction focusing Direction focusing in a magnetic sector Direction focusing in an electric sector □ An ion entering the magnetic field along a trajectory perpendicular to the field edge follows a circular trajectory. An ion entering at an angle a with respect to the previous perpendicular trajectory follows a circular trajectory with an identical radius and thus converges with the previous ion when emerging from the sector. Choosing correctly the geometry of the magnetic field (sector field) allows focusing of the incoming beam. □ An ion entering the electric sector perpendicular to the field edge follows a curved trajectory. However, if the ion trajectory the inlet is perpendicular to the edge, its trajectory is longer if it enters the sector closer to the outside and shorter if it enters the sector closer to the inside. A suitably chosen geometry results in a direction focusing 3.4 Electromagnetic Analyzers - Energy focusing qV ^& Aston (1919) B Source Dempster (1918) Photographic plate Source Multiplier Photographic plate Mattauch-Herzog (1935) Nier-Johnson (1953) Source Multiplier q Photographic plate Bainbridge _ (1933) Quadrupolar lenses I Multiplier Source Matsuda(1981) When a beam of ions with different kinetic energies issues from the source, the electric and magnetic sectors produce an energy dispersion and a direction focusing. If two sectors with the same energy dispersion are oriented as shown below, the first sector energy dispersion is corrected by the second sector convergence. Double-focusing instruments use this principle (B - magnetic sector, E - electric sector) Instruments 4, 5 and 6 are mostly used. 3.4 Electromagnetic Analyzers - Practical considerations □ The magnetic instrument's sources must function with potentials Vs of about 10 kV. The vacuum in the source must thus be very high to avoid arcing. □ Classical magnets were not well suited to fast scanning because of the hysteresis phenomenon and the magnet heating up by the Foucault currents induced by rapidly changing magnetic fields. Lamellar magnets avoid such inconveniences; they have been well developed and are now widely used. □ Magnetic instruments function at constant resolution R = m/Sm. As a result, dm is proportional to m. Consider R = 1000; ion m] = 100 (Sml = 0.1) is observed while scanning from mass 99.95 to 100.05, whereas for ion m2 = 1000 (Sml = 1) masses range from 999.5 to 1000.5. If the scanning is carried out to cover a mass unit within a time t, the ion mx is observed during 0.1/, whereas m2 during It. Since detection time interval differ, the number of detected ions does not correspond to the number of ions produced scanning time decreases exponentially to correct for this error. □ Method to increase the mass precision: peak matching technique consists of comparing the masses of two compounds that are simultaneously ionized in the spectrometer source: one is unknown and its exact mass is sought; the other is a reference and its mass is known with accuracy. This comparison is achieved by a very rapid alternative modification of the acceleration voltage to focus the two ions, the intensities of the magnetic and electric fields being kept constant. The match is perfect when the two mass profiles exactly overlap. If the acceleration voltages necessary for the focusing of the two ions are known with accuracy, the mass of the unknown compound can be determined with accuracy. 4. Detectors Detectors are able to generate from the incident ions an electric current that is proportional to their abundance. A variety of approaches are used to detect ions: □ The measurement of direct charge current that is produced when an ion hits a surface and is neutralized (Farraday cup). □ The kinetic energy transfer of incident ions by collision with a surface that in turn generates secondary electrons, which are further amplified to give an electronic current (electron multipliers or electro-optical ion detectors). Because the number of ions leaving the mass analyzer at a particular instant is generally quite small, significant amplification is often necessary to obtain a usable signal. Indeed, 10 incident ions per second at the detector corresponds to an electric current of 1.6 x 10"18 A. □ the detector consists of a pair of metal plates within the mass analyzer region close to the ion trajectories. Ions are detected by the image current that they produce in a circuit connecting the plates (image current detectors at FTICR or orbitrap). Ion detectors can be divided into two classes: 1. point ion collectors: made to count ions of a single mass at a time and therefore they detect the arrival of all ions sequentially at one point. 2. array collectors: ability to count multiple masses and detect the arrival of all ions simultaneously along a plane (photographic plates, image current detectors or array detectors). Efficiency of detectors generally decreases exponentially when the mass of the ion increases. 4. Detectors 4.1 Photographic Plate & Faraday Cup Photographic Plate Ions sharing the same m/z ratio all reach the plate at the same place and the position of the spots allows the determination of their m/z values after calibration. The darkness of the spots gives an approximate value of their relative abundance. This detector, which allows simultaneous detection over a large m/z range, has been used for many years but is obsolete today. Faraday Cup ion hits a surface and is neutralized. Because the charge associated with an electron leaving the wall of the detector is identical to the arrival of a positive ion at this detector, secondary electrons emitted when an ion strikes the wall of the detector are an important source of errors. The accuracy of this detector can be improved by suppression of secondary electrons and preventing the escape of reflected ions and secondary electrons. The cup is coated with carbon because it produces few secondary ions. The shape of the cup and the use of a weak magnetic field prevent also any secondary electrons produced inside to exit. The disadvantages of this simple and robust detector are its 1 i and its s e time. Ion beam Amplifier A/V Ions Electrons 4. Detectors 4.2 Electron multipliers (EMs) - discrete dynode Electron multiplier - The most widely used ion detector in mass spectrometry. The discrete dynode electron multiplier is made up -3000 v -1000 v of a series of 12 to 20 dynodes that have good secondary emission properties. Ions from the analyzer are accelerated to a high velocity towards an electrode (conversion dynode) held at a high potential from ±3 to ±30 kV (polarity depends on the charge of the detected ions). -5000 V -2000 V -600 V A positive or negative ion striking the conversion dynode causes the emission of several secondary particles. When positive ions strike the negative high-voltage conversion dynode, the secondary particles of interest are negative ions and electrons. When negative ions strike the positive high-voltage conversion dynode, the secondary particles of interest are positive ions. The secondary particles are converted to electrons at the first dynode. A cascade of electrons is created and the final flow of electrons provides an electric current at the end of the electron multiplier that is then increased by conventional electronic amplification. 4.2 Electron multipliers - Channeltron A type of continuous-dynode electron multipliers (CDEM), which is called a channeltron, is made from a lead-doped glass with a curved tube shape that has good secondary emission properties. amplified current toward electrometer □ The amplifying power is the product of the conversion factor (number of secondary particles emitted by the conversion dynode for one incoming ion) and the multiplying factor of the continuous dynode electron multiplier. □ The lifetime is limited to 1 or 2 years because of surface contamination from the ions or from a relatively poor vacuum. □ The conversion factor is highly dependent on the impact velocity of the detected ions and on their nature (mass, charge and structure), so these detectors are not as precise as Faraday cups. □ Because of their slower velocity, large ions produce fewer secondary electrons and thus the efficiency decreases when the mass of the ion increases. The conversion dynodes kept at high potential reduce the mass discrimination effect, especially with mass analyzers delivering ions at low kinetic energy, such as quadrupoles or ion traps. 4.2 Electron multipliers - MicroChannel plate (MCP) MCP is a plate in which parallel cylindrical channels have been drilled. The channel diameter ranges from 4 to 25 jim with a centre-to-centre distance ranging from 6 to 32 urn and a few millimetres in length. The plate input side is kept at a negative potential of about 1 kV compared with the output side. □ Electron multiplication is ensured by a semiconductor substance covering each channel and giving off secondary electrons. □ The snowball effect within a channel can multiply the number of electrons by 105. A plate allows an amplification of 102-104, whereas by using several plates the amplification can reach 108. □ This detector is characterized by a very fast response time because the secondary electron path inside the channel is very short. In consequence, it is well suited to TOF analyzers, which need precise arrival times and narrow pulse widths. □ The large detection area of the microchannel plate allows the detection of large ion beams from the analyzer without additional focalization. □ They are fragile, sensitive to air and large microchannel plates are expensive. 4.2 Electron multipliers - Microsphere plate (MSP) Towards electrometer MSP electron multiplier consists of glass beads with diameters from 20 to 100 um that are sintered to form a thin plate with a thickness of 0.7 mm. □ This plate is porous with irregularly shaped channels between the planar faces. The surfaces of the beads are covered with an electron emissive material and the two sides of the plate are coated to make them conductive. □ A potential difference 1.5-3.5 kV is applied across the plate, with the output side of the plate at the more positive potential. □ The microsphere plate offers some advantages over the microchannel plate: • less expensive, gain is higher, 106-107, because nearly the entire surface of the input side is active and therefore emits secondary electrons that will be accelerated onto and through the plate to give the final signal. In comparison, the surface of the microchannel plate between the microchannels, which corresponds to about 50% of the entire surface, is inactive. 4. Detectors 4.3 Electro-Optical Ion Detectors Conversion of ions to electrons and then to photons. The most common - the Daly detector: two conversion dynodes, scintillation or phosphorescent screen and photomultiplier. Detection of positive and negative ions. In the positive mode, ions are accelerated towards the negative dynode, whereas in the negative mode towards the positive dynode. I ; that are given off are then j s. The photons are detected by the j Conversion dynode < -20 kV Ion beam (detector off) Conversion dynode <+10kV Phosphorescent screen < +20 kV Photomultiplier Ions Electrons Photons □ The phosphorescent screen surface is covered with a thin layer of aluminum conductor to avoid the formation of a charge that would prevent new electrons from reaching it. □ Its lifetime is longer than the lifetime of electron multipliers because the photomultiplier is sealed in glass and held under vacuum. □ It has a fast response time and a similar sensitivity to electron multipliers with an amplification 104 - 105. Another electro-optical ion detector is the electro-optical array detector: allows the simultaneous measurement of ions spatially separated along the focal plane of the mass spectrometer. It combines the microchannel plate and Daly detector.