Ion trap mass spectrometer
The ion trap mass spectrometer (also ion trap mass spectrometer , IT mass spectrometer) is a special type of mass spectrometer that is used in connection with HPLC , gas chromatography or also with a high-resolution sector field mass spectrometer. In contrast to a conventional mass spectrometer, in which ionization and mass analysis take place continuously, the ion trap works discontinuously. Therefore, with "in-trap fragmentation" there is the possibility of MS-MS experiments or MS n .
Device types
The following types of ion trap mass spectrometers exist:
- Quadrupole ion trap ( Paul trap )
- Linear trap
- Fourier transform ion cyclotron resonance (FT-ICR, also Penning trap )
- Orbitrap
Quadrupole ion trap
Technical development
The principle of the ion trap (Engl. Ion trap , IT) was at the beginning of the 1950s by the German Nobel laureate Wolfgang Paul develops and corresponded to the underlying theory in about a quadrupole mass filter. In contrast to a conventional mass spectrometer (e.g. quadrupole or sector field mass spectrometer), in which the ionization and mass analysis are carried out continuously but separately, namely in the ion source and the quadrupole field or magnetic field, the ion trap works discontinuously. However, the ion trap concept has only been used in practice in analysis since George Stafford , an employee of the then MS manufacturer Finnigan MAT , made some improvements around 1983. With them it was possible to store mass ranges of ions in the ion trap at the same time and to specifically release them from the " trap ". In addition, the Stafford group found that letting in helium at around 10 −5 Pa dramatically improved the mass resolution of an IT mass spectrometer.
Originally, the same space was used for the generation and separation of the ions. Devices with an external ion source are now also used. If electron impact ionization is used in the trap, the sample molecules interact with energetic electrons after entering the ion trap, thereby forming positive ions. The three processes that take place in the trap - ionization, mass analysis and detection - are usually controlled by a process known as Automatic Gain Control (AGC) . The AGC scan function consists of a short prescan and the actual measurement. The AGC software automatically selects an ionization time. For low concentrations such as B. the baseline or small GC peaks, the maximum ionization time for the actual ionization is selected. If the concentration of the substance to be analyzed increases, the ionization time is automatically reduced to prevent the ion trap from being overloaded with ions. However, when different compounds are co-eluted, this leads to a deterioration in the detection limits for the less concentrated compounds and makes quantification in the trace range difficult for complex mixtures of substances .
Advantages and disadvantages of the analyzer system
The advantages of the quadrupole ion trap are the high detection sensitivity in scan mode, the compact design of the device and the comparatively high masses that can be achieved. In ion trap mass spectrometers, multiple repetition of excitation and mass selection is possible without the need for an additional component. A disadvantage is the relatively poor linearity of detector response (even when using the AGC) and related problems in quantifying and occurring space charge effects that the mass spectra can be different from those of classic mass spectrometer.
Linear trap
Instead of being held in a 3D quadrupole field in the linear trap, the ions are held in a 2D quadrupole field . An additional fringe field is applied to keep the ions in the trap. Compared to a 3D quadrupole field, the 2D quadrupole field increases the storage capacity of the ion trap and thus also the detection sensitivity of the mass spectrometer. This increased detection sensitivity goes hand in hand with an increased data rate and chromatographic resolution.
Newer chromatographic methods such as Rapid Resolution HPLC or Nano-HPLC, which require a high chromatographic resolution, make use of 2D ion traps (such as Thermo Fisher Scientific LXQ, LTQ) if ion traps are used . B. in the detection of proteins. Like the Pauli trap, the linear trap has the option of MS-MS experiments (also MS n ) with “in-trap fragmentation”.
Fourier transform ion cyclotron resonance mass spectrometry
The Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) based on the above-1936 Penning trap . In the 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 .
FT-ICR-MS devices achieve mass resolutions that are up to a hundred times higher than high-resolution sector field mass spectrometers, especially at higher masses. 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. The resolution is very high and can be up to R = 2,000,000.
Orbitrap
The latest development in ion trap mass spectrometers is the Orbitrap technology published by Alexander Alexejewitsch Makarov in 2000 . A central, spindle-shaped electrode is located in the ion trap of the Orbitrap . The ions are shot into the orbitrap radially to this electrode and, due to the electrostatic attraction, move in circular paths ( orbits ) around the central electrode. Since the ions are not injected in the center of the chamber, but in a decentralized manner, they vibrate simultaneously along the axis of the central electrode. The frequency of this oscillation generates signals in detector plates, which are converted into the corresponding m / q ratios by Fourier transformation . The principle is therefore similar to the FT-ICR-MS (see above), but works with an electrostatic field instead of a magnetic field. Orbitraps therefore manage without the complex cooling with liquid helium .
In practice, the mass resolution R of Orbitraps is only slightly worse than that of an FT-ICR device with a 7-Tesla magnet.
literature
- Hans-Joachim Hübschmann: Handbook of GC-MS: fundamentals and applications. 2., completely rev. and updated ed., Wiley-VCH, Weinheim 2008, ISBN 978-3-527-31427-0 .
- Raymond E. March, Richard J. Hughes: Quadrupole Storage Mass Spectrometry. Wiley-Interscience, New York 1989, ISBN 0-471-85794-7 .
Individual evidence
- ↑ Patent DE944900 : Process for the separation or separate detection of ions with different specific charges. Registered on December 24, 1953 , inventor: W. Paul, H. Steinwedel (German priority December 23, 1953).
- ↑ George Stafford Jr .: Ion trap mass spectrometry: a personal perspective . In: Journal of the American Society for Mass Spectrometry . 13, No. 6, 2002, pp. 589-596. doi : 10.1016 / S1044-0305 (02) 00385-9 .
- ↑ A. Makarov: Electrostatic Axially harmonic orbital trapping: A high-performance technique of mass analysis . In: Analytical Chemistry . 72, No. 6, 2000, pp. 1156-1162. doi : 10.1021 / ac991131p .