Summary: ICP-MS
1 Single-Collector inductively coupled plasma mass spectrometry
1.1 Mass Spectrometry – MS
Mass Spectrometry (MS) = analysis of matter via formation of gaseous ions, separated by their mass-to-charge (m/z) ratio, then
identified and quantified. It is a powerful & versatile technique for identification & quantitative determination of isotopes & molecules.
1.1.1 Components of mass spectrometer
Mass spectrometry (MS) consists of three essential components:
à Ion Source: converts sample into ions
-In ICP-MS: plasma ion source at atmospheric pressure
-In TIMS: ion source under vacuum, sample on heated metal filament
à Mass Analyzer / Mass Spectrometer: separates ions based on mass-to-charge ratio (m/z)
-Quadrupole filter → transmits ions only in a narrow m/z window (~1 amu)
-Magnetic / sector field analyzer → separates ions spatially
-Time-of-flight (TOF) analyzer → separates ions temporally
à Detector: converts incoming ions into a measurable electrical signal
à Vacuum System: High vacuum in mass analyzer and detector region to prevent ion collisions with gas molecules
à Achieved using vacuum pumps (increased instrument cost)
1.1.2 Mass Spectrum
Mass of ions expressed in: u or amu (atomic mass unit) à Defined as 1/12th of the mass of a ¹²C atom
à Approximately equal to the mass of a proton or neutron
(In organic mass spectrometry, the unit Dalton (Da) is used)
A mass spectrum = A graphical representation of signal intensity as a function of m/z (mass-to-charge ratio)
à only the molecular ion (M⁺) is considered
Comparison: similar to electromagnetic spectrum, but instead of plotting energy vs wavelength/
frequency, the mass spectrum plots abundance vs m/z
1.2 Important characteristics of a mass spectrometer
1) Mass Resolution: capability of the instrument to separate ions with very similar m/z values
à ability to distinguish neighboring spectral peaks.
Two equivalent definitions:
a) Peak Width Method: Resolution (R) calculated based on peak width at a certain
height ( 5% of peak height) at mass m.
b) 10% Valley Definition: minimum resolution needed to separate two peaks of equal
height whereby valley between them does not exceed 10% of
peak height. (If peaks have unequal intensity, higher
resolution required to resolve weaker peak.)
Note: Both definitions are equivalent, as a 10% valley =
the sum of two 5% peak-width contributions.
2) Abundance Sensitivity: Quantifies the interference of peak tails from neighboring
masses on the signal of interest.
Factors Affecting It: - Peak symmetry & sharpness → narrow peaks with minimal tails improve
abundance sensitivity
3) Mass Range: The span of m/z values the instrument can measure. (Critical organic MS due to wide MM variation.)
In ICP-MS, mass range of( 0–260 u) is good to cover all naturally occurring elements (heaviest ²³⁸U).
4) Scanning Speed: The time required to scan across the mass spectrum or to switch between monitored m/z values.
Relevance: Affects sample throughput (analysis speed). Crucial for transient signals (e.g., short-lived species or signals
from laser ablation / chromatography coupling).
1.3 Combination of electrostatic and magnetic sector
DOUBLE-FOCUSING SETUP: Purpose: To achieve high mass resolution without excessive loss of ion transmission efficiency.
,Key Principle: In a double-focusing mass spectrometer, the dispersion
(spreading) caused by one sector is exactly compensated by the other. Electrostatic
sector → disperses ions based on kinetic energy. Magnetic sector → disperses ions based
on mass-to-charge ratio (m/z)
à Energy Focusing + Directional Focusing = "Double Focusing"
Ions with the same m/z but different kinetic energies and/or trajectories →
are refocused to a single point at the detector.
à This allows accurate separation without losing too many ions.
Setup Type Mass Resolution Ion Transmission
Advantages Over Single-Sector Systems
Efficiency
Magnetic or High resolution possible, but only Low
Electrostatic Alone with very narrow slit → large ion loss
Double-Focusing High resolution Moderate to high due
(Electrostatic + to wider slit allowed
Magnetic)
1.4 Double focusing set-up: (Reverse) Nier-Johnson Geometry & Mattauch-Herzog geometry
1) Mattauch–Herzog Geometry:
Principle: Provides energy and directional focusing for all ions simultaneously. All ion beams
focus in a single focal plane.
Detection: Old: photographic plates (ion-sensitive
emulsions). Modern: Faraday strip array
detectors or multi-collector arrays.
2) Nier–Johnson Geometry: look at 1.5
Principle: Used in multi-collector ICP-MS (MC-ICP-MS) instruments. Double focusing
is not realized for all ions simultaneously.
3) Reverse Nier–Johnson Geometry à Principle: Order reversed: Magnetic sector
before electrostatic sector. Magnetic sector removes most ions, then ES refines beam.
Advantages: - Beam clean-up → reduced background no
-Improved peak shape and better abundance sensitivity.
1.5 Quadrupole filter
- Advantages: High scanning speed. Operates at relatively high pressure (vs. sector field MS).
Simple design (no energy filter needed). Tolerant of ion kinetic energy spread. Lower cost.
- Disadvantage: Low mass resolution Dm > ½ u (unit resolution only in commercial systems). Cannot
precisely determine ion mass
- Preferred Use: When high mass resolution is not required. When exact mass determination is not needed.
1.5.1 Mass analysis – the quadrupole filter
Structure: Four parallel rods (cylindrical or hyperbolic), conductive or
coated. Diagonally opposed rods electrically connected
à form two electrode pairs with the same (magnitude) but opposite
(sign) potential.
Applied voltages: DC (U) & AC (V) à Acts as a band-pass mass
filter (transmits ions within ~1 u m/z window).
Spectral Scanning: adjustU & V with U/V = cst. Peak hopping or peak
jumping: Discontinuous adjustment in U & V. Higher speed of analysis
1.5.2 Basic operation principle of quadrupole filter
Each electrode pair affects ion trajectories differently. (Seperate evaluation of effect of each pair of quedrupole rods on ion paths)
à Voltages applied: DC (U) + AC (rf, V·sin ωt).
à Opposing electrode pairs → equal magnitude, opposite polarity (phase shift 180°) à XZ vs YZ
1) Electrode Pair 1 (XZ-plane): Applied voltage: +U (DC) + V·sin(ωt) (AC). à Acts as: High-mass filter.
2) Electrode Pair 2 (YZ-plane): Applied voltage: –U (DC) + V·sin(ωt + π) (AC). à Acts as: Low-mass filter.
, Combined Effect:
High-mass filter (XZ) + Low-mass filter (YZ) → narrow band-pass filter. Only ions
within selected m/z range have stable trajectories
→ reach detector.
1.6 Time-of-flight (TOF) analyzer
Principle: Ions accelerated by a potential difference ΔV. Enter a field-free flight tube of length L. All ions (with same charge q) gain
$% !
the same kinetic energy: 𝐸!"# = &
= 𝑞𝑉 = 𝑐𝑠𝑡
'( 𝑳 𝑳√𝒎
Velocity depends on mass-to-charge: 𝑣 = +2 $ Flight time to detector: 𝒕𝒇𝒍𝒊𝒈𝒉𝒕 = 𝒗
=
3𝟐𝒒𝑽
Key point: lighter ions travel faster → reach detector earlier; heavier ions arrive later.
Mass Spectrum Acquisition: Continuous registration of ion arrival times → full mass
spectrum from one ion packet. No scanning required (all m/z detected in parallel). Typically unit
mass resolution (0–260 u).
1.6.1 Use of TOF-analyzer with continuous ion source
Problem: Continuous Ion Source: TOF requires pulse-wise ion
introduction (not continuous).
à If ions entered continuously: A lighter ion (later) could overtake a heavier ion (earlier) → signal overlap.
à ICP (Inductively Coupled Plasma) produces a continuous ion beam → needs beam modulation.
1.6.2 Orthogonal acceleration as a means of beam modulation
Repeller electrode used: No voltage → ions blocked. +V applied → ions in front of
repeller are accelerated perpendicular to beam.
Packets of ions introduced into TOF analyzer( Up to 30,000 ion packets per
second). Well-suited for monitoring transient signals (short duration events).
Energy Spread Problem: Mass resolution decreases if ions of the same m/z have
different kinetic energies. Higher-energy ions travel faster → reach detector earlier.
1.6.3 Use of ion mirror (reflectron) in TOF-ICP-MS to improve mass
resolution
Reflectron (Ion Mirror) = Solution: A series of rings with a retarding voltage applied.
Function: Ions are slowed, stopped, and re-accelerated in reverse direction.
à Higher-energy ions penetrate deeper → travel longer path.
à Lower-energy ions penetrate less → travel shorter path.
à Both reach detector at the same time.
Benefits: Corrects energy spread → improved resolution. Effectively doubles path length (L × 2) without enlarging instrument
footprint. Achieves higher mass resolution (still ~unit mass in 0–260 u range).
1.7 Comparison of characteristics of mass spectrometers
1.8 Ion detection continuous (discrete) dynode electron
multiplier & Faraday collector
Electron Multipliers: -comparison of detector signal with treshold value (background < 0,1 count/s)
-avoid photons reaching detector + limited lifetime (1-2 years)
Types:
1) Continuous dynode electron multiplier (CDEM): Ion strikes inner
surface → eah ion secondary electrons released. Accelerated by potential
difference (front: –2 to –3 kV, back: grounded). Collisions → avalanche of
electrons (multiplication). One ion → 10⁷–10⁸ electrons. Final signal is
measurable current pulse à corresponds to a single ion
2) Discrete dynode electron multiplier (DDEM): Electrons accelerated
stepwise from dynode to dynode (each at higher potential). Same
avalanche principle, but more stable and preferred in modern instruments.
1 Single-Collector inductively coupled plasma mass spectrometry
1.1 Mass Spectrometry – MS
Mass Spectrometry (MS) = analysis of matter via formation of gaseous ions, separated by their mass-to-charge (m/z) ratio, then
identified and quantified. It is a powerful & versatile technique for identification & quantitative determination of isotopes & molecules.
1.1.1 Components of mass spectrometer
Mass spectrometry (MS) consists of three essential components:
à Ion Source: converts sample into ions
-In ICP-MS: plasma ion source at atmospheric pressure
-In TIMS: ion source under vacuum, sample on heated metal filament
à Mass Analyzer / Mass Spectrometer: separates ions based on mass-to-charge ratio (m/z)
-Quadrupole filter → transmits ions only in a narrow m/z window (~1 amu)
-Magnetic / sector field analyzer → separates ions spatially
-Time-of-flight (TOF) analyzer → separates ions temporally
à Detector: converts incoming ions into a measurable electrical signal
à Vacuum System: High vacuum in mass analyzer and detector region to prevent ion collisions with gas molecules
à Achieved using vacuum pumps (increased instrument cost)
1.1.2 Mass Spectrum
Mass of ions expressed in: u or amu (atomic mass unit) à Defined as 1/12th of the mass of a ¹²C atom
à Approximately equal to the mass of a proton or neutron
(In organic mass spectrometry, the unit Dalton (Da) is used)
A mass spectrum = A graphical representation of signal intensity as a function of m/z (mass-to-charge ratio)
à only the molecular ion (M⁺) is considered
Comparison: similar to electromagnetic spectrum, but instead of plotting energy vs wavelength/
frequency, the mass spectrum plots abundance vs m/z
1.2 Important characteristics of a mass spectrometer
1) Mass Resolution: capability of the instrument to separate ions with very similar m/z values
à ability to distinguish neighboring spectral peaks.
Two equivalent definitions:
a) Peak Width Method: Resolution (R) calculated based on peak width at a certain
height ( 5% of peak height) at mass m.
b) 10% Valley Definition: minimum resolution needed to separate two peaks of equal
height whereby valley between them does not exceed 10% of
peak height. (If peaks have unequal intensity, higher
resolution required to resolve weaker peak.)
Note: Both definitions are equivalent, as a 10% valley =
the sum of two 5% peak-width contributions.
2) Abundance Sensitivity: Quantifies the interference of peak tails from neighboring
masses on the signal of interest.
Factors Affecting It: - Peak symmetry & sharpness → narrow peaks with minimal tails improve
abundance sensitivity
3) Mass Range: The span of m/z values the instrument can measure. (Critical organic MS due to wide MM variation.)
In ICP-MS, mass range of( 0–260 u) is good to cover all naturally occurring elements (heaviest ²³⁸U).
4) Scanning Speed: The time required to scan across the mass spectrum or to switch between monitored m/z values.
Relevance: Affects sample throughput (analysis speed). Crucial for transient signals (e.g., short-lived species or signals
from laser ablation / chromatography coupling).
1.3 Combination of electrostatic and magnetic sector
DOUBLE-FOCUSING SETUP: Purpose: To achieve high mass resolution without excessive loss of ion transmission efficiency.
,Key Principle: In a double-focusing mass spectrometer, the dispersion
(spreading) caused by one sector is exactly compensated by the other. Electrostatic
sector → disperses ions based on kinetic energy. Magnetic sector → disperses ions based
on mass-to-charge ratio (m/z)
à Energy Focusing + Directional Focusing = "Double Focusing"
Ions with the same m/z but different kinetic energies and/or trajectories →
are refocused to a single point at the detector.
à This allows accurate separation without losing too many ions.
Setup Type Mass Resolution Ion Transmission
Advantages Over Single-Sector Systems
Efficiency
Magnetic or High resolution possible, but only Low
Electrostatic Alone with very narrow slit → large ion loss
Double-Focusing High resolution Moderate to high due
(Electrostatic + to wider slit allowed
Magnetic)
1.4 Double focusing set-up: (Reverse) Nier-Johnson Geometry & Mattauch-Herzog geometry
1) Mattauch–Herzog Geometry:
Principle: Provides energy and directional focusing for all ions simultaneously. All ion beams
focus in a single focal plane.
Detection: Old: photographic plates (ion-sensitive
emulsions). Modern: Faraday strip array
detectors or multi-collector arrays.
2) Nier–Johnson Geometry: look at 1.5
Principle: Used in multi-collector ICP-MS (MC-ICP-MS) instruments. Double focusing
is not realized for all ions simultaneously.
3) Reverse Nier–Johnson Geometry à Principle: Order reversed: Magnetic sector
before electrostatic sector. Magnetic sector removes most ions, then ES refines beam.
Advantages: - Beam clean-up → reduced background no
-Improved peak shape and better abundance sensitivity.
1.5 Quadrupole filter
- Advantages: High scanning speed. Operates at relatively high pressure (vs. sector field MS).
Simple design (no energy filter needed). Tolerant of ion kinetic energy spread. Lower cost.
- Disadvantage: Low mass resolution Dm > ½ u (unit resolution only in commercial systems). Cannot
precisely determine ion mass
- Preferred Use: When high mass resolution is not required. When exact mass determination is not needed.
1.5.1 Mass analysis – the quadrupole filter
Structure: Four parallel rods (cylindrical or hyperbolic), conductive or
coated. Diagonally opposed rods electrically connected
à form two electrode pairs with the same (magnitude) but opposite
(sign) potential.
Applied voltages: DC (U) & AC (V) à Acts as a band-pass mass
filter (transmits ions within ~1 u m/z window).
Spectral Scanning: adjustU & V with U/V = cst. Peak hopping or peak
jumping: Discontinuous adjustment in U & V. Higher speed of analysis
1.5.2 Basic operation principle of quadrupole filter
Each electrode pair affects ion trajectories differently. (Seperate evaluation of effect of each pair of quedrupole rods on ion paths)
à Voltages applied: DC (U) + AC (rf, V·sin ωt).
à Opposing electrode pairs → equal magnitude, opposite polarity (phase shift 180°) à XZ vs YZ
1) Electrode Pair 1 (XZ-plane): Applied voltage: +U (DC) + V·sin(ωt) (AC). à Acts as: High-mass filter.
2) Electrode Pair 2 (YZ-plane): Applied voltage: –U (DC) + V·sin(ωt + π) (AC). à Acts as: Low-mass filter.
, Combined Effect:
High-mass filter (XZ) + Low-mass filter (YZ) → narrow band-pass filter. Only ions
within selected m/z range have stable trajectories
→ reach detector.
1.6 Time-of-flight (TOF) analyzer
Principle: Ions accelerated by a potential difference ΔV. Enter a field-free flight tube of length L. All ions (with same charge q) gain
$% !
the same kinetic energy: 𝐸!"# = &
= 𝑞𝑉 = 𝑐𝑠𝑡
'( 𝑳 𝑳√𝒎
Velocity depends on mass-to-charge: 𝑣 = +2 $ Flight time to detector: 𝒕𝒇𝒍𝒊𝒈𝒉𝒕 = 𝒗
=
3𝟐𝒒𝑽
Key point: lighter ions travel faster → reach detector earlier; heavier ions arrive later.
Mass Spectrum Acquisition: Continuous registration of ion arrival times → full mass
spectrum from one ion packet. No scanning required (all m/z detected in parallel). Typically unit
mass resolution (0–260 u).
1.6.1 Use of TOF-analyzer with continuous ion source
Problem: Continuous Ion Source: TOF requires pulse-wise ion
introduction (not continuous).
à If ions entered continuously: A lighter ion (later) could overtake a heavier ion (earlier) → signal overlap.
à ICP (Inductively Coupled Plasma) produces a continuous ion beam → needs beam modulation.
1.6.2 Orthogonal acceleration as a means of beam modulation
Repeller electrode used: No voltage → ions blocked. +V applied → ions in front of
repeller are accelerated perpendicular to beam.
Packets of ions introduced into TOF analyzer( Up to 30,000 ion packets per
second). Well-suited for monitoring transient signals (short duration events).
Energy Spread Problem: Mass resolution decreases if ions of the same m/z have
different kinetic energies. Higher-energy ions travel faster → reach detector earlier.
1.6.3 Use of ion mirror (reflectron) in TOF-ICP-MS to improve mass
resolution
Reflectron (Ion Mirror) = Solution: A series of rings with a retarding voltage applied.
Function: Ions are slowed, stopped, and re-accelerated in reverse direction.
à Higher-energy ions penetrate deeper → travel longer path.
à Lower-energy ions penetrate less → travel shorter path.
à Both reach detector at the same time.
Benefits: Corrects energy spread → improved resolution. Effectively doubles path length (L × 2) without enlarging instrument
footprint. Achieves higher mass resolution (still ~unit mass in 0–260 u range).
1.7 Comparison of characteristics of mass spectrometers
1.8 Ion detection continuous (discrete) dynode electron
multiplier & Faraday collector
Electron Multipliers: -comparison of detector signal with treshold value (background < 0,1 count/s)
-avoid photons reaching detector + limited lifetime (1-2 years)
Types:
1) Continuous dynode electron multiplier (CDEM): Ion strikes inner
surface → eah ion secondary electrons released. Accelerated by potential
difference (front: –2 to –3 kV, back: grounded). Collisions → avalanche of
electrons (multiplication). One ion → 10⁷–10⁸ electrons. Final signal is
measurable current pulse à corresponds to a single ion
2) Discrete dynode electron multiplier (DDEM): Electrons accelerated
stepwise from dynode to dynode (each at higher potential). Same
avalanche principle, but more stable and preferred in modern instruments.
