Mass Spectrometry Introduction

Transcription

1 Mass Spectrometry Introduction Chem 744 Spring 2013

2 What MS is and is not MS is NOT a spectroscopic method. Molecules are not absorbing EM radiation MS is the generation, separation and characterization of ions in the gas phase according to their mass and charge. Molecules must be ionized Ions must be separated Ions must be detected 2

3 Principles In the late 19th century Thomson accelerated ions in a vacuum tube and deflected them according to their mass with a magnetic field. The ions were recorded by striking a photographic plate. In 1913 MS was used to establish the existence of isotopes by separating Ne 22 from Ne 20 electron impact ioniztion - high energy electron beam MOLECULE vaporize MOLECULE MOLECULE 3 e - fragments

4 Ionization Methods EI - electron impact ionization ev ejects one electron from the molecule leaving a radical cation Only need ~15 ev to ionize ~50 ev excess energy dissipated by breaking covalent bonds lots of fragments often the molecular ion is not observed 4

5 Ionization Methods CI - chemical ionization Soft Ionization A reagent gas is first ionized by an electron beam methane (CH5 + ), isobutane (C4H9 + ), ammonia (NH4 + ) Reagent ions collide with analyte in the gas phase and transfers a proton [M+H] + < 5 ev energy transferred less fragmentation (and less structural info) 5

6 EI and CI Comparison 6

7 Other Methods FAB - fast atom bombardment LDI - laser desorption ionization MALDI - maxtix assisted laser desorption ionization ESI - electrospray ionization APCI - atmospheric pressure chemical ionization 7

8 Principles of Ion Separation An ion accelerated by a voltage (V) will acquire kinetic energy where m is the mass of the ion, ν is its velocity, e is the charge on an electron and z is the number of charges. 1 2 m υ2 = z e V An ion s motion as it passes through a magnetic field of strength B is described by r (radius of curvature). Combining these equations provides the m/z relationship. r = m υ B z e m z = B 2 r 2 e 2 V 8

9 Traditional MS Instrumentation Design 9

10 Quadrupole MS DC voltage alternating at a frequency generates an oscillating electrostatic field. The ratio of this frequency and voltage will affect ions depending on mass. 10

11 Time of Flight MS Ions are accelerated and allowed to drift. Heavier ions will be accelerated to a lesser velocity than lighter ions. They are separated by the velocities over time. Lighter atoms will impact the detector first. z e V = m V 2 /2 v = (2z e V/m) 1/2 t = (L 2 m/2z e V) 1/2 ionization acceleration drift region detector sample inlet heavier ions slower lighter ions faster 11

12 MS - Information Molecular Mass of our analyte Information about molecular formula Fragments provide information about structure Isotopic information The spectrum shows peaks corresponding to the abundance of those ions. The most stable ions will be most abundant (they survive more under the high energy conditions). The Most Abundant Peak in the Mass Spectrum is called the BASE PEAK. 12

13 The Mass Spectrum MS of propane note isotopes and fragments base peak at m/z 29 CH 3 CH 2 CH 3 13

14 MS Fragmentation Ionized molecules will fragment to produce the most stable cations and radicals most abundantly. + CH 3 m/z = 86 m/z = 71 + CH 2 CH 3 m/z = 86 m/z = 57 + CH 2 CH 2 CH 3 m/z = 86 m/z = 43 14

15 MS Spectrum of Hexane MS of n-hexane m/z = 57 m/z = 43 m/z = 71 m/z = 86 15

16 MS of 2,2-dimethylpropane m/z = 72 m/z = 57 + CH 3 16

17 Alkane Fragmentation m/z = 226 m/z =

18 MS to distinguish isomers + CH 3 m/z = 84 m/z = 79 + CH 2 CH 3 m/z = 84 m/z = 56 18

19 Isotopes in the MS Isotopes show up in ratios according to their natural abundance 12 C / 13 C = 100 : 1.1 (Deuterium 0.015%) 19

20 Isotopes Analytes are mixtures of isotopes. To get formulas and structural information you need to calculate using exact isotopic mass, not atomic weights (averages of isotopes) on the periodic table. 20

21 Common Isotope Ratios Relative Abundance of common elements 1 H H C C O O O N N Si Si Si P S S F Cl Cl Br Br I

22 Halogen Isotopes Bromine exists naturally as two main isotopes in about an equal mixture 79 Br / 81 Br = 100:98 Chlorine exists naturally as two main isotopes in ratio of about 3:1 35 Cl / 37 Cl = 100:32 22

23 Halogens in the MS O O Br OH Br m/z = 73 OH m/z = 152/ Br, 81 Br O O Br OH Br OH m/z = 107/109 m/z = 135/137 23

24 Halogens in the MS Cl m/z = 58 OH Cl m/z = 93/95 35 Cl, 37 Cl OH 24

25 Isotope Patterns Halogen Isotope Patterns 25

26 Example of Transition Metal Isotopes 26

27 Molecular Formula from Mass Rule of 13 M / 13 = n+r/13 where CnHn+r for example - take a molecule with a M= / 13 = /13 therefore a MF would be C12H12+8 or C12H20 UN = (2*12+2) = 3 27

28 Mass equivalents for common elements Molecular Formula from Mass 28

29 The Nitrogen Rule A molecule with even numbered MW must have either no nitrogens or an even number of nitrogens A molecule with an odd numbered MW must have an odd number of N Will hold for all compounds with C, N, O, S, X, P, B, Si, As, & alkaline earths For fragmention via 1-bond cleavage: Even # M + will give an odd # fragment Odd # M + will give an even # fragment for fragments containing the N 29

30 The Nitrogen Rule Examples CH 4 16 CH 3 CH 3 + CH CH 3 CH 3 CH 2 OH 46 CH 3 CH 2 NH 2 45 NH 2 CH 2 CH 2 NH CH 3 NHCH + 3 CH 3 NH + + CH CH 3 NH + CH

31 M + or a Fragment? How do we know if we are looking at the M + or a fragment? Must obey the nitrogen rule Must generate a lower mass fragment by loss of a reasonable neutral fragment 31

32 M + or a Fragment? How do we know if we are looking at the M + or a fragment? Must obey the nitrogen rule Must generate a lower mass fragment by loss of a reasonable neutral fragment 32

33 M + or a Fragment? Stability of the M + ion is key Decreasing ability to give a prominent M + peak aromatics > conjugated alkenes > cyclic compounds > organic sulfides > alkanes > mercaptans Decreasing ability to give recognizable M + peak ketones > amines > esters > ethers > carboxylic acids, aldehydes, amides, halides M + usually not detectable aliphatic alcohols, nitrites, nitrates, nitro compounds, nitriles, highly branched compounds 33

34 Low and High Resolution MS For low resolution instruments we can obtain mass to the nearest AMU 16 O = C = For high resolution we can obtain mass to four decimal places 1 H = C = HRMS allows us to differentiate molecules of the same nominal mass + C O N N H 2 C CH Exact Mass within 10 ppm #.XXXX difference of 2 = 1 ppm 34

35 HRMS 35

36 HRMS 36

37 Mass Spectrometry Fragmentation Chem 744 Spring 2013

38 Unimolecular Reactions Organic Cations Fragmentation Chemistry Usually a neutral species is lost (not detected) Most common fragmentation is cleavage of one bond generating a cation and a neutral radical When a molecule is ionized, where is the charge? 38

39 EI Ionization Generally controlled by HOMO Loss of electrons from heteroatoms and pi-bonds nonbonded electrons R O R - e - R O R pi electrons arenes - e - R O R - e - R O R hydrocarbons - e - R R R R pi electrons - e - R + + R conjugated systems 39

40 Fragmentation General Considerations Even electron ions cannot cleave to a pair of odd-electron fragments (thermodynamics) Mass loss of 14 is very rare (can occur in 2 sequential fragmentations) Fragmentation occurs to form the most stable cations/radicals (ion stability takes precedence) Vinyl radicals difficult but possible Several fragmentations can occur especially if it leads to a highly stabilized cation or radical Loss of nuetral stable molecules is possible and often favored (dehydration, dehydrohalogenation, alkenes, CO, etc) Probability of cleavage a function of bond strength and stability of fragments Not possilbe to identify every peak in a spectrum. 40

41 One bond Cleavage Most common fragmentation is cleavage of one bond generating a cation and a neutral radical C C C + C C C Z + C C Z C C Z C + C Z 41

42 Two bond Cleavage Usually generates a stable neutral molecule 1,2-elimination H C X C C C + HX retro Diels-Alder + McLafferty Rearrangement R H R O Z ketones, esters, carboxylic acids + H O Z 42

43 Alkanes Dominated by C-C bond breaking Straight chain - relatively large M + (only 1 carbocations/radicals can be formed) Branching reduces M + Cycloalkanes show large M + - need to break 2 bonds to break off fragments. Usually loses ethylene (M-28) 43

44 Alkane Fragmentation m/z = 226 m/z =

45 Alkane Fragmentation

46 Alkane Fragmentation CH

47 Alkene Fragmentation EI usually removes an electron from the pi-bond leaving a relatively stable molecular ion Alkenes migrate easily - difficult to determine regioisomers Will fragment to afford allylic cations preferentially R Cyclic alkenes similar 41 + R Cyclohexenes - retro Diels-Alder 47

48 Alkene Fragmentation 48

49 Alkene Fragmentation 49

50 Alkene Fragmentation 50

51 Arene Fragmentation 51

52 Arene Fragmentation tropylium ion 91 52

53 Arene Fragmentation 53

54 Arene Fragmentation 54

55 Halides Fragmentation -X and -HX most common M+ greatest for I > Br > Cl > F Br and Cl isotopes R Br + R Br R Br + HBr R 55

56 Halide Fragmentation Br 164/166 Br 135/137 + CH 3 CH 2 56

57 Halide Fragmentation 57

58 Alcohol Fragmentation M + very small or absent Alpha cleavage is common - the more stable R' R C O H R' C O H + R R" R" Dehydration - often happens in the injector R H OH + H 2 O R 58

59 Alcohol Fragmentation 1,4 eliminations and multiple bond fragmentations possible R OH H R + H 2 O R OH H R + H 2 O + Cyclic alcohols cleave H - what else can it do? H OH OH + H 59

60 Alcohol Fragmentation 60

61 Ether Fragmentation 61

62 Ether Fragmentation 62

63 Ether Fragmentation 63

64 Amine Fragmentation 64

65 Amine Fragmentation 65

66 Amine Fragmentation 66

67 Aldehyde Fragmentation 67

68 Aldehyde Fragmentation 68

69 Ketone Fragmentation 69

70 Ketone Fragmentation O O O O O + CO

71 Ester Fragmentation 71

72 Ester Fragmentation 72

73 Carboxylic Acid Fragmentation O O OH OH O C OH 45 73

74 Other Functional Groups Fragmentation Amides O R O C NH 2 + R R NH 2 44 Nitriles (M + usuallly not observed) H O NH 2 R OH NH 2 59 R C N R C N + H Nitro Compounds ( NO2 + 46, NO + 30) 74