Mass Spectrometers

Building a Mass Spectrometer

Inlet

M1, M2

Mass Analyzer

Accelerate

(Mass Filter)

Separation by •  Velocity •  Behavior in E or

B field •  Other methods

Detector

Ion Detector

Ionization Chamber

M1•+

M2•+

+ -

(Ion source)

•  Can be + or - , or radical ions.

•  Neutral species are left behind.

Detectors

Electron multiplier tubes Microchannel plates Computer

Ions (already filtered by mass)

108 “multiplication” of signal!

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). M2+ is detected as 1/2M+.

Magnetic Sector

http://www.wooster.edu/chemistry/analytical/ms/default.html

•  Venerable, simple, capable of very high resolution when multiple sectors are linked.

Time of Flight (TOF)

M•+

- +

Flight tube (field free)

•  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.

Detector

http://www.chem.vt.edu/chem-ed/ms/quadrupo.html

Quadrupole Mass Filter

Compact, simple, inexpensive. Common in LC-MS and GC-MS systems.

Ion Trap

Detector

•  Very compact •  High sensitivity •  Long ion lifetimes •  Facile MSn experiments

Ions can be held, circulating in trap. Ions can be ejected from trap as function of m/e and rf frequency.

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

Direct Probe/Electron Impact (EI)

•  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.

V M•+ Mass analyzer

e- beam

- +

Very simple, inexpensive

- -

A Simple EI Spectrum

OH C5H12O m/e 88

EI blasts lactose into pieces…

O O O

OHOH

HO

OH HOHO

OH

OH

C12H22O11 m/e 342

…little tiny pieces….

Chemical Ionization

V M•+ Mass analyzer

- - -

CH4 CH4•+ + CH3

+

C2H5+ + H2

+……..

CH4 Analyte molecules collide with ionized reagent gas to make MH+ ions, as well as (M+15)+ and others.

Laser Desorption and Matrix-Assisted Laser Desorption (MALDI)

•  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.

Sample in matrix

Fast Atom Bombardment (FAB) Sample in matrix

Beam of fast Xe or Ar atoms

-

Mass analyser

•  Soft ionization method, suitable for many delicate molecules.

•  Ions are typically (M+H)+

Electrospray Ionization

•  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!

HPLC Flow

“skimmer cone”

Mass analyzer

kV

Nebulizer gas

Remember Lactose?

Electron Impact

Electrospray ionization

Electrospray Ionization of Myoglobin

•  Note - little fragmentation. Lots of different multiply charged ions.

http://www.chm.bris.ac.uk/ms/theory/esi-ionisation.html

M/15=1131.2 M+=16,968

Resolution and Sensitivity

Analyzer Maximum m/e

Resolution (roughly)

Sensitivity

Magnetic Sector

15,000 +/- 0.0001 (1/10,000)

Low

TOF No limit +/- 0.0001 High

Quadru-pole

5,000 +/- 1 High

Ion Trap 5,000* +/- 1 High

*Much higher with multiply charged ions (to 10’s of k)

“Parent ion” of Benzene

“M+” (M/z = 78)

“M+1” (M/z = 79, 6.7% of 78)

Due to 1.1% 13C: 6.7/1.1 =~6 carbons!

Isotope lines

Mass Intensity 156.0 61.9% 157.0 4.4% 158.0 59.9%

Mass Intensity 112.0 100.0 113.0 6.8 114.0 31.8

Due to 35Cl, 37Cl Due to 79Br, 81Br

What about HIGH resolution?

Molecular Formula: C4H6O2 C4H10N2

M+ 86.0 86.0 “Precise Mass” 86.0845 86.0368 (See table 8.4 in PLKV)

Differ by 0.0477 mass units (0.055%)

OCH3

O

NH

HN

Sample Spectra

M+ 142 29

43

57

71

85

113 99

CH3CH2+

CH3CH2CH2+

CH3CH2CH2CH2+

CH3CH2CH2CH2CH2+

C6H13+ C7H15

+ C8H17

+

Decane - a linear alkane

M+ 136 0.6% 138 0.6%

57

Bromobutane

Spectrum 1 M•+-81

M•+-79 •  M+ is “doubled’ •  Prominent m/e

91 from loss of 81 and loss of 79.

•  Few other fragments.

Br

• +

OH•+

+M-18M/z=56

OHM-43M/z=31

M+=74

M-1

M/z 56

Loss of 18 M/z 31

Loss of 43

Spectrum 2 •  Weak parent •  M-1, M-18 •  M-43 is

prominent

M/z 31 peak indicates that this is the 1-isomer, not the 2-isomer

M+ 121

105

77

Benzamide

NH2

O

M+ 129

86

N

H

N

H

Dibutylamine

M+- 43

OH

M+ 88

1-Pentanol

70

55

42

31 29

2-Pentanol

M+ 88 73

45

55

Spectrum 3 •  Very weak

parent •  M-15, M-29

(indicates alkyls)

•  M-1 •  M/z 28 indicates

CH2=CH2

M/z 59 Loss of 15

M+=74

M-1

M/z 45

Loss of 29

M/z 28

Loss of 46

• +M-15M/z=59

OHM-29M/z=45

OH OH

Spectrum 4 Account for the indicated ions and propose a structure.

M+=114

M/z 57

Loss of 29

M/z 41

Loss of 57

M/z 29

M/z 85

M/z 72

Loss of 42

M+ 114. Loss of m/e 29 suggests a CH3CH2- group, leaving m/z 85 (C7H1? C6H13? C5H9O?). Fragment with even mass (m/z 72) suggests a rearrangement.

OH H

OH

H

m/e 72

O

CH3CH2O

m/e 29 m/e 85

O

m/e 57 m/e 57

McLafferty Rearrangement:

M+ 180

m/e 138 (loss of 52)

m/e 120

m/e 43