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