Ion trap mass analyzer

-By Rohan Purohit


For beginners to understand ions are charged atoms. They don’t have an equal number of protons and electrons. They may either be positively charged (cations) or negatively charged (anions).

Cations- here proton number is more and electron number are less. Example -Calcium (Ca2+), Potassium (K+), hydrogen Anion- here proton number are less and electron number are more. Example – Iodide (I–), chlorine (Cl–), hydroxide (OH–).

These ions have multiple uses in every aspect of physics and chemistry mostly for industries and research. For which they need to be extracted and this work is done by ion traps.

Ion traps are the machine used to capture ions from an external environment in a system. They have multiple uses like mass spectrometry, basic physics research, and quantum mechanics mostly for controlling it.

There are two types of ion traps

  • Penning trap-they form potential via a combination of electric and magnetic fields, mostly used for precise measurement in spectroscopy.
  • Paul trap-they form potential via a combination of static and electric fields, mostly used for quantum state manipulation this can lead to a trapped ion quantum computer and has been used to create the world’s most accurate atomic clock.

Mass spectrometry and ion trap mass spectrometers

Mass spectrometry is a technique used to measure the mass to charge ratio of ions. The results obtained from the experiment are presented in the form of the mass spectrum which is plot intensity as a function of mass to charge ratio.

An in-charge mass spectrometer may incorporate any of 3 types of traps that are

  1. Panning trap
  2. Paul trap
  3. Kingdom trap or orbitrap
  4. Other types may also use a linear quadrupole ion trap as a selective mass filter.

Panning trap

Panning trap is an instrument invented and made by Frans Micheal panning and Hans George Dehmelt in 1950. The device stores charged particles using a strong homogeneous magnetic field to confine particles radially and a quadrupole electric field to confine particles axially. They are mostly used to measure the properties of ions and their subatomic particles (electrons, protons, and neutrons). Like Dehmet used it to study the magnetic movement of atoms.

Panning traps are used in quantum computation and quantum information processing and are used at the European agency of nuclear research (CERN) for storing animatters. They also form the backbone of fourier transform ion mass spectrometry for determining mass to charge ratio of ions.

Paul ion traps

Paul ion traps use static DC (direct current) and RF (radio frequency) oscillation to trap ions. These types of ion traps are also known as quadrupole ion traps and are used as instruments in a mass spectrometer. Wolfgang Paul got a noble prize in physics in 1989 for the invention of 3D quadrupole ion traps.

This quadrupole consists of hyperbolic metal electrodes with their foci facing each other and a hyperbolic ring electrode halfway between the other two electrodes. Ions are trapped in between these 3 electrodes due to the dynamic (oscillating) and static electric field.

Kingdon trap and orbitrap

A Kingdon trap consists of 3 things that are as follows

  1. Thin wire in the centre
  2. an outer electrode (cylindrical shaped)
  3. And isolated end cap electrodes at both ends.

A static applied voltage results in a radial logarithmic potential between the electrodes. In a Kingdon trap, there is no potential minimum to store the ions; however, they are stored with a finite angular momentum about the central wire and the applied electric field in the device allows for the stability of the ion trajectories.

In 1981, Knight introduced a modified outer electrode that included an axial quadrupole term that confines the ions on the trap axis. The dynamic Kingdon trap has an additional AC voltage that uses strong defocusing to permanently store charged particles. The dynamic Kingdon trap does not require the trapped ions to have angular momentum concerning the filament. An Orbitrap is a modified Kingdon trap that is used for mass spectrometry. Though the idea has been suggested and computer simulations performed neither the Kingdon nor the Knight configurations were reported to produce mass spectra, as the simulations indicated mass resolving power would be problematic.

Uses of ion traps

  1. Ion traps enable scientist all over the world to store even small and tiny particles with high precision and this feature is exploited in mass metrology.
  2. European convention on tapped ion was a meeting series held in the UK to discuss the further prospective and future development of ion traps and its large scale contribution towards quantum physics.
  3. Measurement of g-factors and high precision mass measurement that are the foundation of atomic and molecular physics is done using ion traps.
  4. Used in the direct mass measurement of short-lived isotopes like Rb,   Sr, Cs. This experiment was first performed in ISOLDE, CERN IN Geneva convention. The ion here are first captured and cooled in the first chamber and then transferred to the second chamber, here there mass is determined by measuring their cyclotron frequency.
  5.  Used in time-of-flight (TOF) mass analyzing, here ions are extracted in short bursts (packets) within the ion source and are subjected to an accelerating voltage that causes the ions to “fly” down an evacuated tube of a set length. The flight times are correlated against at least two known masses from an infused tune compound, which allows a simple mass correlation.

Cathode ray tubes

Ion traps were used in television receivers before the introduction of aluminized CRT faces around the year 1958, made to protect the phosphor screen from ions.

Trapped ion quantum computer

Research work based on developing quantum computers uses trapped ions. Units of quantum information also called qubits are stored in stable electronic states of each ion, and quantum information can be processed and transferred through the collective quantized motion of the ions, interacting by the Coulomb force. To induce coupling between the qubit states (for single-qubit operations) or between the internal qubit states and external motional states (for entanglement between qubits) lasers are used.


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Reference for images



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