mass spectrometry in food contaminant mon

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
Francis William Aston
Chemistry 1922
mass spectrometry of isotopes
Wolfgang Paul
Physics 1989
quadrupole and
quadrupole ion trap MS
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
Abundance
Qualitative Information
Quantitative
Information
Mass Spectrometry Data • Qualitative Analysis
- Molecular Weight Determination
- Structure Determination
• Quantitative Analysis
- Biotechnology
 analysis of proteins & peptides
 analysis of oligonucleotides
- Pharmaceutical
 drug discovery, combinatorial chemistry
 pharmokinetics, drug metabolism
- Clinical
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 5-
10% 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)
- Relative scale:  exactly 12 amu
- amu or Da equals 1/12 mass of
 1.66054x10-24 g/atom
• All measured masses relative to
- has a mass 2.91407 times
- 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

12C1H4: m = 12.00000 x 1 + 1.007825 x 4 = 16.031 Da
 13C1H4: m = 13.00335 x 1 + 1.007825 x 4 = 17.035 Da
 12C1H3
2H1: m = 12.00000 x 1 + 1.007825 x 3 + 2.0140 x 1 = 17.037 Da
 3-4 significant figures to right of decimal
• Nominal Mass (m)
- Whole number precision in mass measurement
 12C1H4: m = 12 x 1 + 1 x 4 = 16 Da
 13C1H4: m = 13 x 1 + 1 x 4 = 17 Da
 12C1H3
2H1: m = 12 x 1 + 1 x 3 + 2 x 1 = 17 Da
C
12
6
C
12
6
C
12
6
C
35
17 C
12
6
C
35
17

Atomic Weights in MS
• Chemical atomic weight or average atomic weight
 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







n
1
i
n
n
n
n
2
2
1
1 p
A
p
A
p
A
p
A
A 
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
13C 6 x 1.08 = 6.48
2H 4 x 0.0015 = 0.060%
15N 2 x 0.37 = 0.74%
17O 4 x 0.04 = 0.16%
(M+1)+/M+ = 7.44%
C12H24
13C 12 x 1.08 = 12.96%
2H 24 x 0.0015 = 0.36%
(M+1)+/M+ = 13.32%

Resolution in MS (R)
• differentiate between masses
R = m/Dm
or
• 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
m
2
)
m
m
(
R
2
1
D



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
H 1 1
C 12 4
N 14 3
O 16 2
F 19 1
S 32 2
Cl 35/37 1
Compound MW
CO2 44
CO 28
CH4 16
HCO2H 46
HCl 36/38
HCN 27
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
D = 1+1/2((6x(4-2) + 14x(1-2)) = 0
Hexene: C6H12
Mass of molecular ion: 84
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 m - mass of the ion
a - acceleration e - ionic charge
v x B - vector cross product E - electric field
of the ion velocity and the applied
magnetic 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

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
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
The direct probe is only ¼" in diameter.
Probe moves through various lock system stages permits
for a step-wise increase in the vacuum
Inlet Systems:

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
Desorption 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 252Cf
Fast Atom Bombardment (FAB) Energetic atomic beam
Secondary Ion Mass Spectrometry (SIMS) Energetic beam of ions
Thermospray Ionization (TS)

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●+ + 2e-
 Not 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:
 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:
 Base peak  most intense peak
- Usually a daughter ion or fragment ion
 Peaks at MW greater than molecular 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:
 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
- Usually positive ions are used
 Need to modify electron beam
- Add vacuum pump capacity
- Reduce width of slit for mass analyzer
- 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 proton transfer
C2H5
+ + MH  MH2
+ + C2H4 proton transfer
C2H5
+ + MH  M+ +C2H6 hydride transfer
- Proton transfer  (M + 1)+
- Hydride transfer  (M – 1)+
- C2H5
+ transfer (M + 29)+
EI
Hard
Ionization
CI Soft
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:
 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:
 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:
 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:
 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
- MW > 10,000
- Structural information for MW ~3,000
 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
- 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
Ar+* + Ar0 ----------------> Ar+ + Ar0*
Production of “fast atoms”
Charge
transfer
Accelerated
argon ion
from “ion
gun”
Ground
state
argon
atom
“slow
ion”
“fast
atom”
Focusing
Extraction plate
Analyte ion beam
(secondary ions)
Probe tip
Analyte metrix
Atom beam

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
- Electrostatic analyzer
- Magnetic sector analyzer
 Ions accelerated through slit into curved electrostatic field
- 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:
 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)
where:
V = voltage v = ion velocity e = electronic charge (1.60x10-19C)
z = ion charge
 Path through magnet depends on the balance of two forces:
- Magnetic force (FM) and centripetal force (Fc)
where:
B = magnetic field strength r = radius of curvature of magnetic sector
 Substitute velocity into kinetic energy equation
 Select masses by varying B by changing current in magnet
2
mv
2
1
zeV
KE 

r
mv
Bzev
2

r
mv
F
2
c 
c
M F
F 
Bzev
FM 
m
Bzer
v 
V
2
e
r
B
z
m 2
2


Mass analyzers:

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:
• 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:
- rate of movement through rod
- Mass to charge ratio
- Frequency and magnitude of the ac signal
 dc current effects momentum of ions
- 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:
• 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:
 Electrons, secondary ions, laser-generated photons
 Frequency of pulse 10 to 50 kHz, duration of pulse 0.25 ms
• Ions accelerated by electric field pulse 103 to 104 V
 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
- Lighter particles arrive at detector before heavier particles
- Flight times are 1 to 30 ms
- Requires fast electronics
 Peak broadening due to variability in ion energies and initial position
- 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
- Each dynode held at successfully higher voltage
 Dynodes have Cu/Be surfaces
- Burst of electrons emitted when struck with energetic ions or electrons
 Electron multiplies contain upwards of 20 dynodes
- Typical current gain of 107
 Continuous-Dynode electric multiplier
- 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
- incident ion strikes the dynode surface which emits electrons
- induces a current which is amplified and recorded
 Surrounded by cage
- Prevents the escape of reflected ions and ejected secondary electrons
 Inclined with respect to ion beam
- Particles striking or leaving the electrode are reflected away from entrance
 Connected to ground potential through a large resistor
- 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
+,
H2
16O,16OH+, 14N+
- Serious interference: 14N2
+ with 28Si+,
NOH+ with 31P+,
16O2+ with 32S+,
40ArO+ with 56Fe+,
40Ar2
+ with 80Se+
- 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)
 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
m
zeB
r
v
c 

w

Fourier Transform (FT) MS:
• Detection of ICR
 Ions circulating between plates
induces current between plates
- Image current
- Non-destructive
- Decays over a few seconds
through collision
 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.

mass spectrometry in food contaminant mon

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