An ion source performs the following two functions:

Transcription

1 Ionization The Ion Source An ion source performs the following two functions: 1) converts sample atoms or molecules to ionized particles (ions) in the gas phase (sometimes the task of introducing the atoms or molecules into the gas phase accompanies the ionization process) and 2) accelerates the ions it produces into the mass analyzer in whatever manner is prescribed by the latter Ionization The Ion Source An ion source performs the following two functions: 1) converts sample atoms or molecules to ionized particles (ions) in the gas phase (sometimes the task of introducing the atoms or molecules into the gas phase accompanies the ionization process) and 2) accelerates the ions it produces into the mass analyzer in whatever manner is prescribed by the latter 1

2 Ionization The Ion Source An ion source performs the following two functions: 1) converts sample atoms or molecules to ionized particles (ions) in the gas phase (sometimes the task of introducing the atoms or molecules into the gas phase accompanies the ionization process) and 2) accelerates the ions it produces into the mass analyzer in whatever manner is prescribed by the latter Desirable for an Ion Source High Sensitivity low sample consumption & high ionization efficiency Stability constant ion production Monochromaticity constant ion energy Low background electronic noise chemical noise: memory or matrix Focusing Minimal mass discrimination The ability to adjust these characteristics to suite specific analytes/experiments. 2

3 Desirable for an Ion Source High Sensitivity low sample consumption & high ionization efficiency Stability constant ion production Monochromaticity constant ion energy Low background electronic noise chemical noise: memory or matrix Focusing Minimal mass discrimination The ability to adjust these characteristics to suite specific analytes/experiments. Desirable for an Ion Source High Sensitivity low sample consumption & high ionization efficiency Stability constant ion production Monochromaticity constant ion energy Low background electronic noise chemical noise: memory or matrix Focusing Minimal mass discrimination The ability to adjust these characteristics to suite specific analytes/experiments. 3

4 Desirable for an Ion Source High Sensitivity low sample consumption & high ionization efficiency Stability constant ion production Monochromaticity constant ion energy Low background electronic noise chemical noise: memory or matrix Focusing Minimal mass discrimination The ability to adjust these characteristics to suite specific analytes/experiments. Desirable for an Ion Source High Sensitivity low sample consumption & high ionization efficiency Stability constant ion production Monochromaticity constant ion energy Low background electronic noise chemical noise: memory or matrix Focusing Minimal mass discrimination The ability to adjust these characteristics to suite specific analytes/experiments. 4

5 Desirable for an Ion Source High Sensitivity low sample consumption & high ionization efficiency Stability constant ion production Monochromaticity constant ion energy Low background electronic noise chemical noise: memory or matrix Focusing Minimal mass discrimination The ability to adjust these characteristics to suite specific analytes/experiments. Desirable for an Ion Source High Sensitivity low sample consumption & high ionization efficiency Stability constant ion production Monochromaticity constant ion energy Low background electronic noise chemical noise: memory or matrix Focusing Minimal mass discrimination The ability to adjust these characteristics to suite specific analytes/experiments. 5

6 Some Ion Source Types (more to come) This list is not comprehensive. Ionization: Internal Energy Changes 1. Any ionization processes involves the transfer of energy. This energy is usually greater than the amount necessary to achieve the ionization itself. 2. Also going from the solid/liquid phase to gas-phase requires energy transfer. 3. Practically, this excess energy has to be dealt with. What does this mean for our newborn ion(s)? Does turning the ionization dial up to 11 make sense? 6

7 Electron Impact (EI) Source Filament e - inlet M M.+ gas-phase molecules Anode (+70 V) to mass analyzer M + e - M e - Ionization Processes in EI Elements: R + e R + + 2e R + e R e R + e R n+ + (n+1)e R + e R (electron capture) Molecules: RXY + e RXY + + 2e RXY + e RXY n+ + (n+1)e RXY + e RX + Y + + 2e RXY + e RX + + Y + e RXY + e RXY The actual process or processes that occur depend on the identity of R (or RXY) and the energy of the electrons. 7

8 Electron Impact Ionization Efficiency Curve Ionization Efficiency ~10 ~70 Electron Energy (ev) Why 70 ev? 70 ev is the Standard electron energy utilized: N.I.S.T. library & others are 70eV Balance ion production and dissociation Essentially all atoms and molecules have ionization energies that are < 70eV. 8

9 Electron Impact Ionization Efficiency Curve Ionization Efficiency 70 ev is the Standard electron energy utilized: NIST library & others are 70eV ~10 ~70 Electron Energy (ev) Characteristics of Electron Impact Ionization Positive molecular ions from gases, vapors, or molecular beams from vaporized liquids of solids Very good stability (overall < 0.1%) Low energy spreads (~ ev) Relatively high ion beam intensities ( A) Notable mass discrimination High background sensitivity High levels of fragmentation (sometimes total!) 9

10 Typical Uses of Electron Impact Ionization Isotope abundance measurements Atomic mass measurements Chemical analysis of organic molecules Good for some molecules < 800 g/mol Leak detection (takes advantage of high background sensitivity) Ionization and dissociation studies Electron Ionization Mass Spectrum How What What are fragmentation What is the the is molecular ions the base that produce process peak s ion signal s m/z? the leads m/z? base peak to ions formed with from m/z the 15? 29? molecular ions? 10

11 Examples of Electron Impact spectra M + Caffeine m/z Examples of Electron Impact spectra Nicotine m/z M + 11

12 Examples of Electron Impact spectra O O N M + 3-nitrobenzyl alcohol m/z H O m/z Electron Impact Ionization & Organic MS EI-MS of acetone, C 3 H 6 O m/z M + 58 (M-15) + 43 CH M + What are these peaks things likely to be? 12

13 Identifying what you began with: the [M] + peak 1) [M] + will usually be the highest m/z peak with significant abundance. May still be only 0-20% of largest peak though! 2) Look at intensities of M+1 and M+2 peaks; must be consistent with proposed formula. 3) Losses from [M] + peak should be explainable; not 3-14 or Da. i.e., a significant M - 5 peak is impossible. 4) If a fragment clearly contains 3 atoms of an element then the precursor (i.e., [M] + ) must contain at least this many. 5) Just because you are in the gas-phase does NOT mean normal bonding and reactivity chemistry disappears! Identifying what you began with: the [M] + peak 6) Charge and mass and thus, number of electrons need to be conserved in any fragmentation mechanism. i.e., if we begin with 1 unpaired electron and 1 positive charge with a total nominal mass of 100 u, we need to end each fragmentation mechanism with a charged and neutral which add up to this amount too! So for this analyte we should be able to draw reasonable mechanisms for the formation of each major peak. If you can t, then perhaps your Molecular assignment is wrong. 13

14 Fragmentation from [M] + ions Identifying what you began with: the [M] + peak Beware: Structural isomers can unfortunately produce very similar (sometimes identical) electron ionization mass spectra. 14

15 Nominal mass determination C c H h O o containing compound The nominal mass of a substance is 140. What is its molecular formula? The rule of 13 (From JC s notes): 1. Divide the nominal mass by 13: 140/13 = ; A hydrocarbon with this molecular weight would have 10 C atoms 2. Multiply the remainder by 13: 0.769*13 = or 10; A hydrocarbon with this molecular weight would have (10+10) or 20 H 3. For every O subtract 16 (1C + 4 H or 16 H) = C 9 H 16 O; C 8 H 12 O 2 : C 7 H 8 O 3 ; C 6 H 4 O 4 etc. The nominal mass of a substance is 140. What is its molecular formula? The rule of 13 (From JC s notes): number of C = MW/13 (the digits before the decimal point) number of H = (the number of C atoms and 13*digits after the decimal) Once determining the number of carbons and hydrogens, subtract for each oxygen: 16 (1C+4H; 16 H) for each sulfur : 32 (2C+8H; 32 H) for each nitrogen: 14 (C+2H, 14 H).. 15

16 Using Exact Mass Measurements Suppose you determined the exact mass of an ion by mass spectrometry to be Nominal mass 56 How can you figure out all the possible formulas that add to 56? First use the Rule of 13 Divide the nominal mass by thirteen; the number in front of the decimal is the number of carbons; multiply the number following the decimal by 13 and add it to the number of carbons; this equals the number of hydrogens. a. To add an oxygen: remove a carbon and 4 hydrogens b. To add a nitrogen: remove a carbon and 2 hydrogens c. To add a sulfur: remove two carbons, 6 hydrogens; or 2 oxygens Nominal Mass of 56 56/13 = ; The number of carbons is 4 13* = 4; therefore the number of hydrogens is Therefore the hydrocarbon formula is C 4 H 8 Other possible molecular formulas are: C 4 H 8 -CH 4 = C 3 H 4 O C 4 H 8 -CH 2 = C 3 H 6 N X C 4 H 8-2CH 4 = C 2 O 2 ; C 4 H 8-2CH 4 = C 2 S; C 4 H 8-2CH 2 = C 2 H 4 N 2 C 4 H 8 -CH 4, CH 2 = C 2 H 2 NO X C 4 H 8-3CH 2 = CH 2 N 3 X C 4 H 8 - C = C 3 H 20 X C 4 H 8 -CH 4, 2CH 2 = CN 2 O C 4 H 8-4CH 2 = N 4 X = fails Nitrogen rule; => structurally impossible NITROGEN RULE: All neutral compounds (or [M] + ions) with an odd number of nitrogens must have an odd molecular weight. 16

17 The exact mass of an ion by mass spectrometry was determined to be u Nominal mass 56 exact mass N4 4* CN2 O * CH2 N C2 O C2 H2 NO C2 H4 N C3 H4 O C3 H6 N C4 H Identifying what you began with: the [M±nH] n± peak If you used a different ionization method which instead generated [M±nH] n± ions you need to adjust to the neutral before applying the prior rules! 17

18 For you to do at home: Why does this obviously contain Bromine? Can you explain the fragment M-29? 18