FT-ICR mass spectrometry

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Linear Iontrap - FT-ICR Mass Spectrometer (magnetic covers are removed)
Superconducting magnet (7 Tesla) for an FT-ICR mass spectrometer.

The FT-ICR mass spectrometry (FT-ICR-MS = Fourier transform ion cyclotron resonance mass spectrometry ) is a method of ion trap mass spectrometry with a very high mass resolution.

history

FT-ICR-MS is based on the Penning trap described in 1936 . In 1974, Alan G. Marshall and Melvin B. Comisarow of the University of British Columbia , inspired by the methods of Fourier transform - nuclear magnetic resonance spectroscopy - (FT-NMR) and ion cyclotron resonance (ICR), developed a Fourier transform mass spectrometer ( FT-ICR mass spectrometry). Marshall developed the technique at Ohio State University and Florida State University . Corresponding commercial devices also became available very quickly. Today the field strengths used in commercial devices are up to 15 Tesla .

Working principle

In the FT-ICR-MS or its ion trap, there is a homogeneous magnetic field that forces the ions on circular paths with a mass-dependent orbital frequency. The ions are first brought into phase with an excitation pulse. A cyclotron resonance can be generated by applying an alternating electrical field perpendicular to the magnetic field . If the frequency of the radiated alternating field and the cyclotron angular frequency of the ion mass coincide, then the case of resonance occurs and the cyclotron radius of the ion in question increases by absorbing energy from the alternating field. These changes in the cyclotron radius lead to measurable signals on the detector plates of the mass spectrometer. In order to detect ions with different masses, the radiated alternating field is varied and the measured signal is Fourier transformed . The time-dependent raw signals are converted into frequency-dependent signals, which can then be correlated to a mass-to-charge ratio .

In contrast to other mass spectrometry systems, the detection does not take place directly with a "discrete" detector . Since frequencies can currently be determined with higher accuracy than any other physical parameter, these systems offer very high resolutions and mass accuracies. The devices achieve mass resolutions that exceed even high-resolution sector field mass spectrometers by up to a hundred times, especially at higher masses. It can be up to R = 2,000,000. The resolution of the FT-ICR-MS increases with the force and also with the homogeneity of the magnetic field. The field strengths used in commercial devices are up to 15 Tesla . This can only be achieved by using superconducting magnets , which greatly increases the expenditure on equipment and the price of the devices.

Commercially available devices today are mostly hybrid mass spectrometers z. B. from an upstream linear trap and the actual FT-ICR-MS. In contrast, smaller FT-ICR-MS with limited mass resolution have not established themselves on the market.

Basic principle

The ion storage principle of the FT-ICR is similar to that of a cyclotron . The ratio of cyclotron frequency and mass-to-charge ratio is somewhat simplified

where f = cyclotron frequency, q = number of charges, B = magnetic field strength and m = ion mass.

This can also be expressed as angular frequency or angular frequency:

where is the angular frequency, relative to .

Since quadrupole fields are present in the trap in practice, the formula only applies as a first approximation.

Areas of application

The most important areas of application of FT-ICR are analyzes in biochemistry and medicine (e.g. proteomics and metabolome research). Applications can also be found in the field of environmental analysis , forensics and the petrochemical industry and the like. a. in the case of complex mixtures in the direct determination of elemental compositions from the exact monoisotopic masses of individual compounds.

Alternatives

The Orbitrap , which has been available since 2005, is also a mass spectrometer with a very high mass resolution . Time-of-flight mass spectrometers can also achieve sufficiently high resolutions for some applications, and this at significantly higher scan rates than with FT-ICR-MS. The use of superconducting magnets is not necessary with either type of device, which keeps the outlay on equipment and the price of the devices significantly lower.

literature

  • Charles L. Wilkins: A History of Ion Cyclotron Resonance (ICR) and Fourier Transform (FTICR) Mass Spectrometry. in: The Encyclopedia of Mass Spectrometry: Volume 9: Historical Perspectives , Elsevier, 2016. ISBN 978-0-08-043848-1 ( Link )
  • Raymond E. March, John FJ Todd: Practical Aspects of Trapped Ion Mass Spectrometry, Volume IV: Theory and Instrumentation. Taylor & Francis, 2010. ISBN 978-1-42-008372-9

Individual evidence

  1. ↑ Frans Michel Penning: The glow discharge at low pressure between coaxial cylinders in an axial magnetic field . In: Physica . tape 3 , 1936, pp. 873 , doi : 10.1016 / S0031-8914 (36) 80313-9 .
  2. ^ Melvin B. Comisarow, Alan G. Marshall: Fourier transform ion cyclotron resonance spectroscopy . In: Chemical Physics Letters . tape 25 , no. 2 , March 15, 1974, p. 282-283 , doi : 10.1016 / 0009-2614 (74) 89137-2 .
  3. ↑ Stefan Bichlmeier, Al Kania: FT-ICR mass spectrometer - a new, compact device for laboratory and process operation . In: Labo . No. 10 , October 2002, p. 24–27 ( siemens.com [PDF]).
  4. ↑ Manoj Ghaste, Robert Mistrik, Vladimir Shulaev: Applications of Fourier Transform Ion Cyclotron Resonance (FT-ICR) and Orbitrap Based High Resolution Mass Spectrometry in Metabolomics and Lipidomics . In: Int J Mol Sci . tape 17 , no. 6 , May 25, 2016, p. 816 ff ., doi : 10.3390 / ijms17060816 .
  5. ↑ Alexander A. Makarov: Electrostatic axially harmonic orbital trapping: A high-performance technique of mass analysis . In: Analytical Chemistry . tape 72 , no. 6 , June 2000, p. 1156-62 , doi : 10.1021 / ac991131p .