Mass Spectrometry
A.) Introduction:
Mass Spectrometry (MS) measures the atomic or molecular weight of a ion from the
separation based on its mass to charge ratio (m/z)
- elemental composition of matter
- structures of inorganic, organic and biological molecules
- qualitative and quantitative composition of complex mixtures
- isotopic ratios of atoms in the sample
One of the MOST Routinely used Analytical Techniques
Mass Spectrometry
Nobel Laureates:
Joseph John Thomson
Physics 1906
first mass spectrometer
Wolfgang Paul
Physics 1989
quadrupole and
quadrupole ion trap MS
Francis William Aston
Chemistry 1922
mass spectrometry of isotopes
John B. Fenn
Chemistry 2002
electrospray ionization of
biomolecules
Koichi Tanaka
Chemistry 2002
Matrix-assisted laser
Desoprtion/ionization (MALDI)
A Long and Continuing History of Achievements
Mass Spectrometry
• Qualitative Analysis
- Molecular Weight Determination
- Structure Determination
Quantitative Information
Abundance
Mass Spectrometry Data
• Quantitative Analysis
- Biotechnology
analysis of proteins & peptides
 analysis of oligonucleotides

- Pharmaceutical
drug discovery, combinatorial chemistry
 pharmokinetics, drug metabolism

- Clinical


Qualitative Information
neonatal screening, hemoglobin analysis
drug testing
- Environmental

water, food, air quality (PCDs etcs)
- Geological

oil composition
-Toxicology
- Forensics
Mass Spectrometry
Advantages Over Atomic Optical Spectrometric
•
Detection limits three orders of magnitude
better
•
Remarkably simple spectra that are unique
and easily interpreted
•
Ability to measure isotopic ratios
Disadvantages
•
Instrument costs are two to three times
higher
•
Instrument drift that can be as high as 510% per hour
•
Interference effects
Atomic Weights in MS
•
Discriminates among the masses of isotopes
- differs from other analytical techniques
•
Atomic mass units (amu) or daltons (Da)
12
6
C  exactly 12 amu
- amu or Da equals 1/12 mass of 126 C
- Relative scale:

•
All measured masses relative to 126 C
-
•
1.66054x10-24 g/atom
35
17
35
17
C has a mass 2.91407 times 126 C
C 12.0000 Da x 2.91407 = 34.9688 Da  atomic weight = 34.9688 g/mol
Exact Mass (m)
- Sum of specific set of isotopes within compounds
•

12C1H :
4
m = 12.00000 x 1 + 1.007825 x 4 = 16.031 Da

13C1H :
4
m = 13.00335 x 1 + 1.007825 x 4 = 17.035 Da

12C1H 2H :
3
1

3-4 significant figures to right of decimal
m = 12.00000 x 1 + 1.007825 x 3 + 2.0140 x 1 = 17.037 Da
Nominal Mass (m)
- Whole number precision in mass measurement

12C1H :
4
m = 12 x 1 + 1 x 4 = 16 Da

13C1H :
4
m = 13 x 1 + 1 x 4 = 17 Da

12C1H 2H :
3
1
m = 12 x 1 + 1 x 3 + 2 x 1 = 17 Da
Atomic Weights in MS
• Chemical atomic weight or average atomic weight
n
A  A1 p1  A2 p2    An pn   An pn
i 1

A1, A2, An :atomic masses in Da

p1, p2, pn :fractional abundance of each isotope

n :number of isotopes
•
Weight of interest for most purposes
•
Sum of chemical atomic weights for the atoms in the compound formula
- CH4 (m) =12.01115 + 4 x 1.00797 = 16.0434 Da
- typical atomic masses in periodic table
Boron: 10B 23%
11B 100%
B (m) = (23x10 + 100x11)/123
= 10.81
Zirconium: 90Zr 51.5% 91Zr 11.2%
92Zr 17.1 % 94Zr 17.4% 96Zr 2.8%
Zr (m) = (51.5x90 + 11.2x91 + 17.1x92 + 2.8x96)/100
= 91.22
Molecular Formulas from Exact Molecular Weights
•
Requires identification of molecular ion peak
•
Exact mass needs to be determined

High resolution instruments

detect mass differences of a few thousands of amu
- Purine, C5H4N4 (m = 120.044)
- Benzamidine, C7H8N2 (m = 120.069)
- Ethyltolune, C9H12 (m = 120.096)
- Acetophenone, C8H8O (m = 120.058)

Molecular ion peak is 120.070 ± 0.005  only C7H8N2 is possible formula

Precision of a few parts per million is routinely possible
Molecular Formulas from Isotopic Ratios
•
Low-resolution instrument  only differentiate whole number masses
•
Requires sufficiently intense molecular ion peak
•
Requires accurate heights for (M+1)+ and (M+2)+
C6H4N2O4
C12H24
13C
6 x 1.08
2H
4 x 0.0015 = 0.060%
15N
2 x 0.37
= 0.74%
17O
4 x 0.04
= 0.16%
= 6.48
(M+1)+/M+ = 7.44%
13C
12 x 1.08
2H
24 x 0.0015 = 0.36%
= 12.96%
(M+1)+/M+ = 13.32%
Resolution in MS (R)
•
differentiate between masses
R = m/Dm
or
( m1  m 2 )
R
•
Dm
2
Higher the number the better the resolution

500,000 is better than 500

Resolution of 4000 would resolve peaks occurring at m/z values of:

400.0 and 400.1

40.00 and 40.01
Example 22: Calculate the resolution to differentiate (a) C2H4+ (m = 28.0313)
and CH2N+ (m = 28.0187) and (b) N2+ (m = 28.0061) and CO+ (m =
27.9949).
Identification of odd electron ions – The Nitrogen Rule
•
Based on an anomaly in the relationship between the atomic weights and
valences of the common elements
•
An organic molecule containing the elements C, H, O, S, P or halogen
- odd nominal mass if it contains an odd number of nitrogen atoms
- even nominal mass if it contains an even number of nitrogen atoms (including 0)
Element
At Wt
Valence
Compound
MW
H
1
1
CO2
44
C
12
4
CO
28
N
14
3
CH4
16
O
16
2
HCO2H
46
F
19
1
HCl
36/38
S
32
2
HCN
27
Cl
35/37
1
N2
28
NH3
17
Even nominal mass  even valence
Odd nominal mass  odd valence
Nitrogen is the exception
Double Bond Equivalent (D)
• number of rings or double bonds that an ion contains
• Calculated from the elemental formula as follows:
where:




D is unsaturation
imax is the total number of different elements
Ni the number of atoms of element i and
Vi the valence of atom i
• valence of 1 (H, F, Cl), valence of 2 (O, S).
• valence of 3 (N, P), valence of 4 (C, Si).
Hexane: C6H14
Mass of molecular ion: 86
Hexene: C6H12
Mass of molecular ion: 84
D = 1+1/2((6x(4-2) + 14x(1-2)) = 0
D = 1+1/2((6x(4-2) + 12x(1-2)) = 1
Basic MS Instrument Design
• Atomic Mass Spectrometry involves the following steps:
- atomization
- conversion of atoms to ions
 most ions are single charge
- separation of ions based on mass-to-charge ratio (m/z)
- Counting the number of ions of each type or measuring the ion current
•
Principal Components
- Vacuum system  maintain low pressure(10-5 to 10-8 torr)
- Inlet: introduce m-amount of sample into ion source
- Ion source: sample converted into gaseous ion by bombardment with:
 Electrons
 Photons
 Ions
 Molecules
 Thermal/electric energy
- Positive/negative ions accelerated
into mass analyzer
- Mass analyzer: disperse ions (m/z)
- Transducer: convert beam of ions to
electrical signal
Types of Atomic and Molecular MS
• Thermal ionization & Spark source  first MS
• Inductively coupled plasma (ICP)  current common approach
- Differ by types ion sources and mass analyzer
MS Theory:
1.) Mass analyzers use electric and magnetic fields to apply a force on charged particles
F = ma (Newton's second law)
F = e(E+ v x B) (Lorentz force law)
where:
F - force applied to the ion
a - acceleration
v x B - vector cross product
of the ion velocity and the applied
magnetic field
m - mass of the ion
e - ionic charge
E - electric field
2.) Force is therefore dependent on both mass and charge
- spectrometers separate ions according to their mass-to-charge ratio (m/z)
- not by mass alone
Inlet Systems:
•
•
Introduce sample into ion source with minimum loss of vacuum
Spectrometer equipped with multiple inlets for different sample types




Batch Inlet
Direct Probe Inlet
Gas Chromatography
Liquid Chromatography
Inlet Systems:
External (Batch) Inlet Systems:
• Simplest
• Gas & liquid samples (bp < 500oC)
• Sample heated (<400 °C) in small external oven
• Sample pressure 10-4 to 10-5 torr
• Vapor admitted to ionizer through valve
• Gas stream added to analyte
Inlet Systems:
Direct probe inlet system for solids
Probe moves through various lock system stages permits
for a step-wise increase in the vacuum
The direct probe is only ¼" in diameter.
Direct Probe
• Solids and non-volatile samples
- Less sample is required & wasted (few ng)
- sample held on the surface of glass or aluminum capillary tube, fine wire or small cup
• Sample vial inserted through air-lock into ionizer chamber
- Lock system minimizes amount of air that must be pumped from system
• Vial heated to vaporize sample
• Vial can be reduced to capillary or surface plate for small quantities
Inlet Systems:
•
LC & GC coupled to mass spectrometer
•
Permits separation and determination of components for complex mixtures



Requires specialized inlet systems
Major interface problem – carrier gas dilution
Jet separator (separates analyte from carrier gas)
-
Lighter carrier gas deflected by volume
Heavier sample travels in straight line
Ion Sources:
•
•
•
•
Formation of gaseous analyte ions
Mass spectrometric methods are dictated by ionization techniques
Appearance of spectrum highly dependant on ionization technique
Gas-phase



•
Sample first vaporized then ionized
Thermally stable compounds boiling points < 500oC
MW < 100 amu
Desorption


Solid or liquid directly converted to gaseous ion
MW as large as 105 daltons
Type
Name and Acronym
Ionizing Process
Gas Phase
Electron Impact (EI)
Exposure to electron stream
Chemical Ionization (CI)
Reagent gaseous ions
Field Ionization (FI)
High potential electrode
Field Desorption (FD)
High potential electrode
Electrospray Ionization (ESI)
High electric field
Matrix-assisted desorption ionization (MALDI)
Laser beam
Plasma Desorption (PD)
Fission fragments from
Fast Atom Bombardment (FAB)
Energetic atomic beam
Secondary Ion Mass Spectrometry (SIMS)
Energetic beam of ions
Desorption
Thermospray Ionization (TS)
252Cf
Ion Sources:
•
Hard sources

Sufficient energy so analyte are in highly excited energy state

Relaxation involves rupture of bonds
-
Produces fragment ions with m/z < molecular ion
-
Kinds of functional groups  structural information
Hard
Ionization
Ion Sources:
•
Soft sources

Cause little fragmentation

Mass spectrum consists of molecular ion and only few, if any, other peaks

Accurate mass
Soft
Ionization
Ion Sources:
•
Electron-Impact Source (EI)


Sample heated to produce molecular vapor
Bombard with a beam of electrons
-




Electrons emitted from heated tungsten or rhenium filament
Electrons accelerated by a potential of 70V
Path of electrons and molecular ion at right angles
Form positive ions  electron beam expels electron due to electrostatic
repulsion
M + e-  M●+ + 2eNot very efficient  one molecule in a million ionized
Positive ions attracted to first slit by small potential 5V
High potential applied at accelerator plates 103 to 104 V
-
Generates molecular ion velocity
Ion Sources:
•
Electron-Impact Source (EI)


Hard source 50V higher energy than chemical bond
Highly excited vibrational and rotational state
-

Electron beam does not increase translational energy
Relaxation results in extensive fragmentation
-
Large number of positive ions of various masses
Typically less mass than molecular ion
Lower mass ions called daughter ions
Sometimes molecular ion not present
Ion Sources:
•
Electron-Impact Source (EI)

Base peak  most intense peak
-

Peaks at MW greater than molecular ion
-

Usually a daughter ion or fragment ion
Same chemical formula but different isotope composition
Size of peak depends on relative natural abundance of isotopes
Collision Product peak (M+1)+
-
Collision transfers a hydrogen atom to the ion to generate a protonated
molecule
Second order reaction  depends on concentration
Increases with increase in pressure
Ion Sources:
•
Electron-Impact Source (EI)

Advantages
-

Good sensitivity
Fragmentation  unambiguous identification of analytes
Disadvantages
-
Need to volatize sample  thermal decomposition before ionization
Fragmentation  disappearance of molecular ion peak
-
MW not determined
Ion Sources:
•
Chemical Ionization Source (CI)



Electron Impact and Chemical Ionization are Interchangeable in a
Spectrometer
Chemical Ionization is the second most common procedure for generating
ions
Gaseous atoms from the sample are:
-
Heated from a probe
Collide with ions produced reagent gas bombarded by electrons
-

Need to modify electron beam
-
Add vacuum pump capacity
Reduce width of slit for mass analyzer
-

Usually positive ions are used
Allow a reagent pressure of 1 torr in ionization area
Keep pressure below 10-5 torr in analyzer
Concentration ratio of reagent to sample is 103 to 104
-
Electron beam preferentially interacts with reagent instead of sample
Ion Sources:
•
Chemical Ionization Source (CI)


Soft Source
Methane is common reagent
-
Also use propane, isobutane and ammonia
Reacts with high-energy electron beam to generate several ions
CH4+, CH3+ (~90% of product) and CH2+
React with other methane molecules
CH4+ + CH4  CH5+ + CH3
CH3+ + CH4  C2H5+ + H2

Collisions Between sample molecule and CH5+ & C2H5+ are highly reactive
-
Involve proton or hydride transfer
CH5+ + MH  MH2+ + CH4
C2H5+ + MH  MH2+ + C2H4
C2H5+ + MH  M+ +C2H6
-
proton transfer
proton transfer
hydride transfer
Proton transfer  (M + 1)+
Hydride transfer  (M – 1)+
C2H5+ transfer (M + 29)+
CI Soft
Ionization
EI
Hard
Ionization
Ion Sources:
•
Matrix-Assisted Laser Desorption/Ionization (MALDI)

Accurate MW for polar biopolymers
-
DNA, RNA, Proteins
Few thousands to several hundred thousand Da

Sample is mixed with large excess of radiation-absorbing matrix material

Solution is evaporated onto solid surface
Sample exposed to pulsed laser beam

-
Sublimation of analyte ions
MS spectra recorded between laser beam pulses
Ion Sources:
•
Matrix-Assisted Laser Desorption/Ionization (MALDI)

Low background noise
-




At high MW, matrix causes significant background at low MW
Absence of fragmentation
Multiple charged ions (+2, +3)
Observe dimers trimers
Mechanism is not completely understood
Matrix compound must absorb the laser
radiation
Soluble enough in sample solvent to be
present in large excess
Analyte should not absorb laser radiation

Fragmentation will occur
Simulation of MALDI
Ion Sources:
•
Matrix-Assisted Laser Desorption/Ionization (MALDI)

MALDI spectra are greatly influenced by type of matrix, solvent and additive
-
At high MW, matrix causes significant background at low MW
Dariusz Janecki et al. (2002) ASMS
Ion Sources:
•
Electrospray Ionization (ESI)

One of the most important techniques for analyzing biomolecules
-



Polypeptides, proteins and oligonucleotides
Inorganic species synthetic polymers
MW >100,000 Da
Uses atmospheric pressure and temperature
Sample pumped through a stainless steel capillary
-
Rate of a few mls per minute
-
Needle at several KVs potential
Creates charged spray of fine droplets
Ion Sources:
•
Electrospray Ionization (ESI)

Passes through desolvating capillary
-
Evaporation of solvent
Attachment of charge to analyte molecule
Molecules become smaller  charge density becomes greater  desorption of
ions into ambient gas
Ion Sources:
•
Electrospray Ionization (ESI)



Little fragmentation of thermally fragile biomolecules
Ions are multiply charged
-
m/z values are small
-
Detectable with quadrupole with mass range of 1500 or less
Average charge state increases ~linearly with MW
-
MW determined from peak distribution
Ion Sources:
•
Fast Atom Bombardment Sources (FAB)


Major role for MS studies of polar high molecular-weight species
Soft Ionization technique
-

Samples are in a condensed state
-

Glycerol solution matrix
Liquid matrix helps reduce lattice energy
Ionized by bombardment with energetic (several keV) xenon or argon
atoms
-

MW > 10,000
Structural information for MW ~3,000
Very rapid sample heating
Reduces sample fragmentation
Positive & negative analyte ions are sputtered from the surface
-
Desorption process
Must overcome lattice energy to desorb an ion and condense a phase
“healing” the damage induced by bombardment
Ion Sources:
•
Fast Atom Bombardment Sources (FAB)

Beam of fast energetic atoms are generated by:
-
Passing accelerated argon or xenon ions from an ion source through a chamber
Chamber contains argon or xenon atoms at 10-5 torr
High-velocity ions undergo a resonant electron-exchange reaction without
substantial loss of translational energy
Focusing
Atom beam
Extraction plate
Analyte ion beam
(secondary ions)
Probe tip
Analyte metrix
Production of “fast atoms”
Ar+*
+
Accelerated
argon ion
from “ion
gun”
Charge
Ar0 ---------------->
Ar+ +
transfer
Ground
state
argon
atom
“slow
ion”
Ar0*
“fast
atom”
Example 23: Identify the ions responsible for the peaks in the following
mass spectrum for 1,1,1,2-tetrachloroethane (C2H2Cl4):
MW = 167.85
Mass analyzers:
•
Double-Focusing analyzer

Two devices for focusing an ion beam
-

Ions accelerated through slit into curved electrostatic field
-

Electrostatic analyzer
Magnetic sector analyzer
Focus beam of ions with narrow band of kinetic energies into slit
Ions enter curved magnetic field
-
Lighter ions deflected more than heavier ions
Mass analyzers:
•
Double-Focusing analyzer

Curved magnetic field of 180, 90 or 60 degrees
-

Vary magnetic field strength or accelerating potential to select for ions of different mass
Kinetic energy of accelerated ion (before magnetic field)
KE  zeV 
where:
V = voltage v = ion velocity
z = ion charge

e = electronic charge (1.60x10-19C)
Path through magnet depends on the balance of two forces:
-
Magnetic force (FM) and centripetal force (Fc)
FM  Bzev
where:
B = magnetic field strength
FM  Fc

1
mv 2
2
mv 2
Fc 
r
r = radius of curvature of magnetic sector
mv 2
Bzev 
r
Bzer
v
m
Substitute velocity into kinetic energy equation
m B 2 r 2e

z
2V

Select masses by varying B by changing current in magnet
Mass analyzers:
•
Double-Focusing analyzer
Mass analyzers:
•
Quadrupole mass analyzer


•
More compact, less expensive, rugged
High scan rate  spectrum in < 100ms
Four parallel cylindrical rods serve as electrodes

Opposite rods are connected electrically
-

•
One pair attached to positive side of variable dc source
One pair attached to negative side of variable dc source
Variable radio-frequency ac potential (180o out of phase) applied to each
pair of rods
Ions accelerated through space between rods



Potential of 5 to 10 V
ac and dc voltages increased simultaneously with ratio being constant
All ions without specific m/z strike rods and become neutral
-
only ions having a limited range of m/z reach transducer (detector)
Mass analyzers:
•
•
Quadrupole mass analyzer
Positive rods


Alternating ac causes ions to converge during positive arc and diverge
during negative arc
If ions strike rod during negative arc  neutralized and removed by
vacuum
Striking a rod depends on:
-

dc current effects momentum of ions
-

rate of movement through rod
Mass to charge ratio
Frequency and magnitude of the ac signal
Momentum directly related to square-root of mass
More difficult to deflect heavy ions than lighter ions
Prevents heavier atoms from striking rods
High-pass mass filter
Mass analyzers:
•
•
Quadrupole mass analyzer
Negative rods



•
In the absence of ac, all positive ions drawn to rods  annihilated
Offset for lighter ions by ac
Low-pass mass filter
For ions to pass through the rods (band of ions):


Significantly heavy to pass positive rods
Significantly light to pass negative rods
-
•
One pair attached to positive side of variable dc source
One pair attached to negative side of variable dc source
Adjusting ac & dc moves the center of band of ions which pass the rods
Mass analyzers:
•
•
Time of Flight (TOF) Mass Analyzers
Ions generated by bombardment of the sample with a brief pulse of:


•
Ions accelerated by electric field pulse 103 to 104 V

•
Electrons, secondary ions, laser-generated photons
Frequency of pulse 10 to 50 kHz, duration of pulse 0.25 ms
Same frequency of ionization pulse, but lags behind
Accelerated particle enter field-free drift tube


Ions enter tube with same kinetic energy
Ion velocity vary inversely with mass
-

Peak broadening due to variability in ion energies and initial position
-

Lighter particles arrive at detector before heavier particles
Flight times are 1 to 30 ms
Requires fast electronics
Limits resolution compared to magnets and quadrupole
Less widely used than quadrupole
Advantages: unlimited mass range, rapid data acquisition, simplicity, ruggedness,
ease of access to ion source
Transducers (detectors) for Mass Spectrometry:
•
Electron Multipliers


Detects positive ions
Similar to photomultiplier used in UV/vis
-

Dynodes have Cu/Be surfaces
-


Typical current gain of 107
Continuous-Dynode electric multiplier
-

Burst of electrons emitted when struck with energetic ions or electrons
Electron multiplies contain upwards of 20 dynodes
-

Each dynode held at successfully higher voltage
Glass that is heavily doped with lead
Potential of 1.8 to 2 V across length of transducer
Trumpet shaped
Ions strike surface near entrance that ejects electrons
Electrons then skip along surface ejecting more electrons with each impact
Typical current gain of 105 to 108
Rugged, reliable with high current gains
Positioned directly at the exit slit of magnet
-
Ions have enough energy to eject electrons
Requires accelerator with quadrupole MS
Transducers (detectors) for Mass Spectrometry:
•
Faraday Cup

hollow collector, open at one end and closed at the other, used to collect
beams of ions
-

Surrounded by cage
-



Particles striking or leaving the electrode are reflected away from entrance
Connected to ground potential through a large resistor
-

Prevents the escape of reflected ions and ejected secondary electrons
Inclined with respect to ion beam
-

incident ion strikes the dynode surface which emits electrons
induces a current which is amplified and recorded
Ions striking plate are neutralized by flow of electrons from resistor
Causes a potential drop across resistor
Independent of the energy, mass or chemical nature of ion
Inexpensive and simple mechanical and electronic device
Disadvantages:
-
Need for a high-impedance amplifier

Limits speed at which spectrum can be scanned
Less sensitive than electron multipliers
Atomic Mass Spectra and Interferences:
•
Spectroscopic interference


An ionic species in the plasma has the same m/z values as an analyte
Isobaric interference
-
-

Two elements that have isotopes with nearly the same mass
Differ by less than 1 amu
113In+ overlaps with 113Cd+ and 115In+ overlaps with 115Sn+

Isobaric interference occurs with the most abundant and most sensitive isotope
40Ar+ overlaps with 40Ca+ (97%) need to use 44Ca+ (2.1%)

58Ni+ overlaps with 56Fe+ need to use 56Fe+

Exactly predictable from abundance tables
Polyatomic ion interference
-
-
More serious than isobaric
Polyatomic species form from interactions with species in plasma, matrix or
atmosphere
Several molecular ions can form and interfere
Typically observed for m/z < 82 amu
Potential interference: 40Ar2+, 40ArH+, 16O2+,
H216O,16OH+, 14N+
Serious interference: 14N2+ with 28Si+,
NOH+ with 31P+,
16O + with 32S+,
2
40ArO+ with 56Fe+,
40Ar + with 80Se+
2
Correct with blank
Atomic Mass Spectra and Interferences:
•
Spectroscopic interference

Oxide and Hydroxide species interference
-
-

Most serious
Oxides an hydroxides of analyte, matrix solvent
plasma gases
MO+ and MOH+ ions
Formation depends on injector flow rate, RF power,
sampler skimmer spacing, sample orifice size,
plasma gas composition, oxygen elimination
solvent removal efficiencies
Matrix Effects
-
-
Occur at concentrations > 500 to 1000 mg/ml
Reduction in analyte signal, sometimes signal enhancement
General effect
Minimized by using more dilute solutions, altering sample introduction
procedure or by separating out offending species
Use internal standard to correct effect
Affect of pesticide
response to matrix effects
Add protectant to remove
matrix effect
Analytical Sciences 2005, 21, 1291
Fourier Transform (FT) MS:
•
Improves signal-to-noise ratios
•
Greater Speed
•
Higher sensitivity and resolution
•
Requires an Ion Trap


Ions are circulated in well-defined orbits
for extended periods
Ion cyclotron resonance (ICR)
-
-
Ions in a magnetic field circulate in a
plane perpendicular to the direction of the
field
Angular frequency of this motion 
cyclotron frequency (wc)
v zeB
wc  
r
m

Ion trapped in a magnetic field can
absorb energy from an ac electric field
-
Frequency of field must match cyclotron
frequency
Absorbed energy increases velocity of ion
and radius without affecting wc
Fourier Transform (FT) MS:
•
Detection of ICR

Ions circulating between plates
induces current between plates
-

Image current
Non-destructive
Decays over a
through collision
few
seconds
Decay over time of the image
current after applying an RF pulse
is transformed from the time
domain into a frequency domain
Ions of two different m/q ratios excited on resonance for the same
amount of time with the same excitation voltage. Ion [A] has the
lower m/q ratio and thus has a higher cyclotron frequency. Ion [B]
has the higher m/q ratio and thus a lower cyclotron frequency.