Overview of Mass Spectrometry

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Overview of Mass Spectrometry
Instrumentation and Methods Chem 553
Spring 2009
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• Mass Spectrometry
• Unlike other spectroscopies!
• Others
• radiation in, modified radiation out
• infer structure from difference in
• intensity as function of frequency
• MS
• Weigh molecules and their pieces
• Vaporize
• Ionize (can be conjoint with 1.)
• Mass Sort
• Detect
• Interpret

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• Vaporize
• (1) Thermal
• Background vacuum typically 10-6 torr
• need sample at 10-3 torr vapor pressure.
• If you can get it through a GC, it's okay
• T lt250 C typically
• (organics pyrolize at gt250 C)
• Gas/GC/ Solid probe
• Sputter with beam (photon, ion, neutral)
• from solid or liquid matrix
• (3) Spray (LC, Electrospray)

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2. Ionize Electron ionization M
energetic e- ? M. e- ( original e-)
Universal, cheap 1 milliAmp/1 year 0.03
typically 70 V electron acceleration Thermodynam
ics Ionization Energy (IE) ?E0K
Radical cations higher in energy than
isostructural neutral Range of IEs 4-24 eV
1 eV 23.06 kcal 98.5 kJ
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Ionization Energy IE (thermodynamics)
(formerly Ionization Potential IP) Cs
3.89 CH4 12.5 tBu
6.58 H2 15.4 PhCH2 7.6
Ne 21.6 benzene 9.2
He 24.6 MeOH 10.8 Tables NIST
Webbook http//webbook.nist.gov/chemistry -
Recombination energy M. e- ? M - Vertical
vs. adiabatic (next slide) - Appearance Energy
(of fragment) AE - Plateau region
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M.
IEv
IEad
M0
EDEv
M-.
EAad
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Fragmentation ABC ? ABC. ? AB C.
? A BC.
? A B C
? AC B.  (Rearrangements)
Mechanism of Ionization not billiard ball
70 eV 1614 kcal/mol 2 of c gt C, H, CH
- electric field of fast near-miss electron
yields electronic excited state. - If
excited state in continuum, can lose electron or
fragment with excess energy.
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Formation of Anions by EI much rarer (usually
1-3 of amount of cations generated, if
any at all) Reason anion will detach electron
if higher in energy than neutral
(negative electron affinity) if seen, more
stable than neutral EA 0 to 10 eV
(most organic anions are 0 to 4 eV EA) but if
stable anion A e- ?(A-.) ?
A e- electron attachment yields excited
anion, which can detach the electron.
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M.
IEv
IEad
M0
EDEv
M-.
EAad
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• Electron Ionization Practical Aspects
• hot filament (wire or ribbon) emits electrons,
  due
• to low work function (Re, W, ThO2 )
• why is rhenium important to UT Chemistry?
• - filament current few amps (yellow hot)
• - electron beam current nanoamps to microamps
• - Often a small magnet to collimate beam
• - Continuous

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Analyzers Magnetic sector
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Mass Selection F zq(v x B) Lorentz
equation centrifugal
force F mv2/r centripetal force
mv/z q.r.B Momentum spectrometer!
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Thermal energy of ions or neutrals
vave (8kT/pm)1/2 145 (TK/mamu)1/2
m/s 250 m/s for mass 100 at
298K 630 m/s for mass 16
79 m/s for mass 1000
(55 mph 25 m/s) Boltzman
distribution
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Solution Accelerate ions through several
kilovolts! v ( 2qzVaccel/m)1/2 100
amu 124 km/sec at 8 kV (
500 thermal v) m/z qB2r2/(2Vaccel)
thermal ?v now a tiny fraction of total v.
Flight time (1 m) ca. 1 to 10 µsec B 1
Tesla means r 0.5 m For m 100 amu, If
V 1000 volts 8000 volts vaccel
44,000 m/s 124,500 m/s vthermal 0.6
0.2
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r (2Vm/zqB2)1/2 If B 1
Tesla and V 8 kV Then r 12.877 cm for m
100 amu 12.813 cm for m
99 amu or 6.4 mm away.
Can focus mass with either B or V.
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Detection Spatial spread  
Photoplate (original) today photoarray
detector (Fellgett
advantage) Time spread B or V sweep
in time, sequential detect at one point
Faraday cup, to electrometer. measure
current to neutralize impacting ion.
Good down to a picoamp. Picoamp 107
ions/sec
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E
E
Electron multiplier 105 - 106 amplification
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Double focusing high resolution! Single
magnetic sector m/?m ca. 2000 Due to (1)
energy, spatial spread of ions emerging
from source Answer electrostatic and magnetic
fields. High resolution (20,000 to 250,000
m/?m). "Exact mass" (to 1 ppm) ? molecular
formula.  
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Velocity/energy focusing (?v) Electrostatic
sector
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Spatial focusing
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Other analyzers - Quadrupole mass filter
AC and DC fields (100's of volts at 1 MHz) are
high mass/low mass filters.
Cheap, lt2000 Da, lt1000 resolution. - Ion
Trap (J. Am. Soc Mass Spectrom. 2002 13(6).) -
Time of Flight (TOF) Rapid Commun. Mass
Spectrom. 1997 951 - Fourier Transform Ion
Cyclotron Resonance (FT-ICR)
Marshall, A.G. Accounts Chem. Res. 1985 18, 316.
Mass Spectrom. Rev.
1998 17, 1-35. Whole Issue Acc. Chem. Res. 1997
27(11) - Orbitrap J. Mass Spectrom. 2005 40,
430-43.
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Quadrupole analyser ca.. 90 of MSs in the world
Quattro HP GC/MS
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AC high pass filter. DC Low pass filter. AC
ca. 1 MHz and 100-200v
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Quads - Small - High Transmission - Insensitive
to spread in initial ion kinetic
energy Increased rod diameter increase in
transmission
increase in resolution
decrease in mass range Increase
in rod length increase in resolution
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Previous methods (magnetic sector, quadrupole)
are continuous ion beams. Now time-resolved
analysers
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Quadrupole Ion trap Time resolved (msec) He at
milliTorr to help trap ions Ion
accumulation Ion/neutral chemistry Selective ion
ejection Used in Varian GC/MS in BU 665
Used in Varian
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Time-of-flight (TOF) Time resolved
(pulsed) Resolution lt1000
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Reflectron TOF Used in DART, Voyager MALDI

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Ion Cyclotron Resonance (ICR) Mass
Spectrometry Fourier Transform Mass Spectrometry
(FTMS) FT-ICR MS Pulsed/time resolved Inherently
highest mass resolution known Trapped ions (msec
to sec to minutes) bimolecular ion-molecule
chemistry Parallels to nmr!
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FT-ICR Cell, trapping and detection
Ion made in magnetic field Ions not accelerated
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Frequencies in an ICR cell   Cyclotron  
fcyc qB/2?m   1535.7 BT
/mamu   Axial (due to trapping plates)   fax
( 2qVtrap/md2)1/2   d distance between
trapping plates (1 term)   Net f
(qB/2?m)2 - qVtrap/?md21/2
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FT-ICR Signal FID
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FT-ICR resolution as a function of transient
length
Record resolution 32,000,000,000 at Mass 3
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e- mass 0.5 mmu
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• FT-ICR Performance
• Resolution (mass x resolution const record
  30x109)
• Mass Accuracy
• Mass range
• Dynamic Range more limited than others
• Sensitivity 1 ion (remeasurement!)
• positive/negative switching near-instant
• Magnetic field higher the better
• - Background Pressure lower the better
• Acquisition time msec
• Signal averaging remeasurement of same ions

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Orbitrap MS Makarov, A. J. Mass Spectrom. 40
(2005) 430
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Other Sources and Hyphenated combinations!
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GC/MS open-split interface
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LC/MS Micro LC into 1 torr vacuum Differential
pumping EI etc. after desolvation
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• Chemical Ionization soft ionization
• 1 torr CH4 in source (high pressure)
• plus analyte at 10-3 torr or so
• CH4 e- ? CH4.
• CH4 . CH4 ? CH5 CH3. self-ionization
• CH5 B ? BH CH4
• less exothermic ionization (0-4 eV) means little
  fragmentation,
• often just quasi-molecular ion BH
• Other reagent gases, specific for functional
  groups
• Negative ions (electron attachment or
  deprotonation)

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Negative Ions Electron attachment e- ArX
? ArX-. selective for Ar-EWG Dissociative
Attachment XY e- ? X. Y- NF3 ?
F- N2O ? O-. ( RH ? HO-) NH3 ?
NH2- Ac2O ? AcO-
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Tandem MS, or MS/MS the structure of ions
triple quadrupole MS
Also multiple magnetic sector instruments,
FT-ICR, Ion Trap
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iBuC(O)Et MW 114
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Sputtering Methods
Energetic Beam onto surface SIMS Secondary Ion
Mass Spec Ion beams easy to create, but
charges the sample, with degradation of
signal
FAB Fast Atom Bombardment 8 kV neutral Xe or
Ar beam, onto glycerol (liquid in vacuum!)
drop with dissolved analyte
MALDI matrix assisted Laser Desoprtion and
Ionization photon beam (neutral and easy!)
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MALDI Matrix Assisted Laser Desorption/Ionization
Typical matrix substituted cinnamic acid,
ArCHCHCOOH Absorbs at laser wavelength
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External Sources Atmospheric Pressure
Ionization Differential pumping Ion guides
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Electrospray Ionization (ESI) from LC, but
ionization is in liquid phase
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One molecule, variously charged peptides 2
of 20 AA side chains are basic and
positively charged ca. 10 charges/10,000 amu
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MW 5727 nominal 5730.6 (3.6) exact 5733.6
(6.6) ave C254H377N65O75S6
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Tetramer Mass 147,550 daltons
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Calibration Perfluorokerosene (CF2 50 C2F4
100) PFTBA N(C4F9)3 671 PEG H(OCH2CH2)nOH
steps of 44 Ultramark (1621) perfluoroalkylphosph
azene
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DART Direct Analysis in Real Time
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DART Chemistry
He discharge ? He. (filtered out) e-
He
metastable ca. 19 eV Cations He H2O ?
H2O. e- H2O. H2O ? H3O HO. H3O
B ? BH H2O Anions e- O2 ?
O2-. O2-. B ? (B-H)- HOO.
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APCI Atmospheric Pressure Chemical Ionization
LC source, gas phase ionization
APPI Atmospheric Pressure Photoionization
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DESI Desorption Electrospray Ionization Atmosphe
ric pressure Electrospray beam onto open
surface from wand Desorbed material sucked
into vacuum orifice
Handheld Startrek Tricorder! (for chemicals,
not biologicals)