Building a Mass Spectrometer

Transcription

1 Mass Spectrometers

2 Building a Mass Spectrometer Inlet Ionization Chamber Accelerate Mass Analyzer Detector (Ion source) (Mass Filter) M 1, M 2 M 1 + M Separation by Velocity Behavior in E or B field Other methods Ion Detector Can be + or -, or radical ions. Neutral species are left behind.

3 Detectors Electron multiplier tubes Microchannel plates Computer 10 8 multiplication of signal! Ions (already filtered by mass)

4 Mass Analyzers Sector Time-of-Flight Quadrupole Ion Trap All of these select (or filter) ions based on the mass -to-charge ratio m/e (or m/z). M 2+ is detected as 1/2 M +.

5 Magnetic Sector Venerable, simple, capable of very high resolution when multiple sectors are linked.

6 Time of Flight (TOF) M + Flight tube (field free) Detector + - For a given accelerating voltage, heavy ions reach a lower velocity than light ions. Mass is related to arrival time at detector - light ions arrive first, heavy ions later. No mass limit Requires fast electronics, but otherwise simple.

7 Quadrupole Mass Filter Compact, simple, inexpensive. Common in LC-MS and GC-MS systems.

8 Ion Trap Ions can be held, circulating in trap. Ions can be ejected from trap as function of m/e and rf frequency. Detector Very compact High sensitivity Long ion lifetimes Facile MS n experiments

9 Ion Sources Dozens of different ion sources out there, often integrated with inlet systems: GC/MS, LC/MS, ICP/MS EI CI MALDI FAB ESI

10 Direct Probe/Electron Impact (EI) V M + Mass analyzer Only useful for samples with reasonable vapor pressure (low MW only). Heating the probe can decompose the sample. Violent (10 s of ev). M + often breaks up immediately. + e - beam Very simple, inexpensive

11 A Simple EI Spectrum OH C 5 H 12 O m/e 88

12 EI blasts lactose into pieces OH OH OH HO OH O O HO HO O OH C 12 H 22 O 11 m/e 342

13 little tiny pieces.

14 Chemical Ionization V M + Mass analyzer C 2 H H CH 4 CH CH 3 + CH 4 Analyte molecules collide with ionized reagent gas to make MH + ions, as well as (M+15) + and others.

15 Laser Desorption and Matrix-Assisted Laser Desorption (MALDI) Sample in matrix Matrix absorbs most of the energy from laser pulse. Analyte/matrix erupts into gas phase (matrix peaks can interfere). Ions in form of (M+H) +, (M +Na) +, others. Excellent for high masses.

16 Fast Atom Bombardment (FAB) Soft ionization method, suitable for many delicate molecules. Ions are typically (M+H) + Sample in matrix - Mass analyser Beam of fast Xe or Ar atoms

17 Electrospray Ionization Nebulizer gas skimmer cone HPLC Flow Mass analyzer kv Vacuum evaporates solvent rapidly from small droplets Soft ionization method, excellent for delicate, non-volatile analytes Typically ions are (M+H) + or (M+Na )+, not M +, and multiply charged ions are common!

18 Remember Lactose? Electron Impact Electrospray ionization

19 Electrospray Ionization of Myoglobin M/15= M + =16,968 Note - little fragmentation. Lots of different multiply charged ions.

20 Resolution and Sensitivity Analyzer Magnetic Sector Maximum m/e Resolution (roughly) 15,000 +/ (1/10,000) Sensitivity Low TOF No limit +/ High Quadrupole 5,000 +/- 1 High Ion Trap 5,000* +/- 1 High *Much higher with multiply charged ions (to 10 s of k)

21 Parent ion of Benzene M + (M/z = 78) M+1 (M/z = 79, 6.7% of 78) Due to 1.1% 13 C: 6.7/1.1 =~6 carbons!

22 Isotope lines Mass Intensity Due to 35 Cl, 37 Cl Mass Intensity % % % Due to 79 Br, 81 Br

23 What about HIGH resolution? O HN OCH 3 NH Molecular Formula: C 4 H 6 O 2 C 4 H 10 N 2 M Precise Mass (See table 8.4 in PLKV) Differ by mass units (0.055%)

24 Sample Spectra

25 Decane - a linear alkane 43 CH 3 CH 2 CH CH 3 CH 2 CH 2 CH 2 + CH 3 CH CH 3 CH 2 CH 2 CH 2 CH C 6 H C 7 H C 8 H 17 + M + 142

26 Bromobutane 57 M % %

27 Spectrum 1 M M M + is doubled Prominent m/e 91 from loss of 81 and loss of 79. Few other fragments. + Br

28 M/z 31 Loss of 43 M/z 56 Loss of 18 M + =74 M-1 Spectrum 2 Weak parent M-1, M-18 M-43 is prominent OH + + OH M-43 M/z=31 M-18 M/z=56 M/z 31 peak indicates that this is the 1-isomer, not the 2-isomer

29 Benzamide O NH 2 M + 121

30 Dibutylamine N H 86 M N H M + 129

31 1-Pentanol 42 OH M + 88

32 2-Pentanol M + 88

33 M/z 28 + Loss of 46 M/z 45 Loss of 29 M + =74 M/z 59 Loss of 15 M-1 Spectrum 3 Very weak parent M-15, M-29 (indicates alkyls) M-1 M/z 28 indicates CH 2 =CH 2 OH OH M-15 M/z=59 OH M-29 M/z=45

34 M/z 29 M/z 57 M/z 41 Loss of 57 Loss of 29 M/z 85 M/z 72 M + =114 Loss of 42 Spectrum 4 Account for the indicated ions and propose a structure. M Loss of m/e 29 suggests a CH 3 CH 2 - group, leaving m/z 85 (C 7 H 1? C 6 H 13? C 5 H 9 O?). Fragment with even mass (m/z 72) suggests a rearrangement.

35 O CH 3 CH 2 O O m/e 29 m/e 85 m/e 57 m/e 57 McLafferty Rearrangement: O H H O H H m/e 72

36 m/e 120 m/e 138 (loss of 52) m/e 43 M + 180

37