Reflectron

Electrode stack of a reflectron (right) on the connection pipe to the mass spectrometer (left)

A reflectron is a component of some time-of-flight mass spectrometers that is used to reverse the direction of the ions . This reduces the influence of their kinetic energy distribution on the flight time.

principle

A series of electrodes with different voltages creates an electric field with a gradient that decelerates the ions and then accelerates them again in the opposite direction. If two ions with the same mass but different kinetic energy enter the field, the ion with the higher kinetic energy covers a further path until it is completely decelerated than the one with the lower kinetic energy. Due to this additional distance, both ions then hit the detector at the same time. The electric field used can be static or time-dependent. The reflectron is made up of a pulsed ion source, a field-free area in the connecting tube, an ion mirror and the detector.

application areas

The reflectron is used in some mass spectrometers to detect individual ions, e.g. B. of peptide fragments , after a second fragmentation , to be able to determine their molar mass more precisely and partially only as a result. The equalization of the different kinetic energies of the ions results in an improvement in the measurement accuracy of the mass-to-charge ratio down to a few daltons per electron. In combination with a second fragmentation of the peptide fragments into amino acid fragments, not only a clear identification but also the amino acid sequence of a peptide can be determined in the course of a de novo peptide sequencing .

Reflektron with a field

Scheme of a reflectron with an electric field area

A reflectron with an electric field area ( English single-stage reflectron ) generates a homogeneous electric field in the ion mirror. The distribution of stress along the central axis can be linear or non-linear. The electric field can be constant or time-dependent. With a homogeneous field, the areas without a field ( English zero field ) and the areas with a field in the ion mirror are separated by a permeable metal grid (95% permeable). The reflectron with an electric field allows a comparatively higher resolution for ions whose kinetic energies have relatively small differences (of a few percent) from one another.

The flight time t of the ions with the mass m , the charge q , the acceleration voltage U is in a homogeneous field

${\displaystyle t(U)={\frac {L}{\sqrt {2U}}}{\sqrt {\frac {m}{q}}}\ +{\frac {2L_{m}{\sqrt {2U}}}{U_{m}}}{\sqrt {\frac {m}{q}}}\ }$

with the distance of the ions in the field-free area L , the length of the ion mirror L _m , the voltage across the ion mirror U _m . For a first-order compensation condition for the time of flight t with the spread dU of the kinetic energy U , the following condition should be met:

${\displaystyle {\frac {dt}{dU}}=0}$

Assuming that the kinetic energy of the ions in the field-free area is equal to the potential energy of the ions near the turning point in the ion mirror, and that the turning point is close to the rear electrode in the ion mirror (U _m = U), follows

${\displaystyle L_{m}={\frac {L}{4}}}$

In practice, the length of the ion mirror should be 10–20% longer in order to also measure all ions with scattered kinetic energy.

The electric field E _m in the ion mirror of a reflectron with an electric field should be:

${\displaystyle E_{m}={\frac {4U}{L}}}$

In cases of larger spreading width dU is the relative width of the signal ( English peak ) dt / t in a reflectron with a field by the uncompensated portion of the flight time t (U) determined in proportion to

${\displaystyle {\frac {dt}{t}}=k{\frac {d^{2}t}{dU^{2}}}}$

with k as the constant of the parameters of the reflectron with one field.

Reflectron with two fields

Scheme of a reflectron with two field areas

The ion mirror in this type of reflector has two areas with different electrical fields. This allows both derivatives of t (U) with respect to U to be set to zero. Compared to reflectrons with one field, this allows greater spread of kinetic energy to be compensated. Usually reflectrons with two fields are used in orthogonal acceleration time-of-flight mass spectrometry (oa-TOF-MS, time-of-flight mass spectrometry with orthogonal acceleration). The Mamyrin structure includes two highly permeable conductive grids to separate the two field areas. The resolution in a reflectron with two fields is mainly determined by the scattering of the ions by the grids, the spread of the kinetic energy of the ions after the pulsed ionization source and the accuracy of the setup. To reduce the scatter, the area of ​​the first deceleration should be relatively large. The effect of the scattering of the ions on the resolution can be reduced by a suitable grid structure.

Gridless reflectron

One type of gridless reflectron uses a curved electric field in the ion mirror, in which the potential along the axis is non-linearly dependent on the distance to the entrance of the ion mirror. The compensation of the flight time for ions of different kinetic energies can be achieved by adjusting the voltage. ${\displaystyle x}$

The electrical potential in the ion mirror of a reflectron with a square field is proportional to the square of the distance to the entrance of the ion mirror:

${\displaystyle V(x)=kx^{2}}$

which represents the case of a one-dimensional harmonic field. If both the ionization source and the detector are attached to the entrance of the ion mirror and if the ions are close to the axis of the ion mirror, the time of flight of the ions in the reflectron with a square field is almost independent of the kinetic energy of the ions.

A gridless ion mirror with a nonlinear field and a simplified structure made up of three components has been described.

Post-source decay

With a reflectron with MALDI as the ion source, a further fragmentation of the molecular ions of the first fragmentation after the ionization source ( English post-source decay , PSD, decay after the source) can take place in a vacuum . The post-source decay is used to study complex molecules, including protein sequencing through de novo peptide sequencing .

The post-source decay fragment the precursor ions with a kinetic energy of a few kilos electro volt by laser or high-energy collisions ( English high-energy collision-induced dissociation , HCD). The time interval suitable for a measurement begins with the exit of the precursor ions from the ion source and ends with the arrival of the ions at the ion mirror. The kinetic energy of the resulting fragments (PSD) ions with the mass m differs significantly from that of the precursor ions of mass M and is proportional to m / M . As a result, the distribution of the kinetic energies is comparatively large, but E / m and thus the speed distribution is small. Therefore, the ions are then accelerated to comparatively high kinetic energies (factor 4 to the precursor ions) in order to achieve sufficient resolution for the PSD. The use of gridless reflectrons with ion mirrors with a curved field or with a time-dependent field can also improve the resolution.

history

The idea for a reflectron was first developed in 1956 by SG Alichanow. In 1973 it was implemented in the laboratory of Boris Alexandrovich Mamyrin .

literature

• TJ Cornish, RJ Cotter: A curved-field reflectron for improved energy focusing of product ions in time-of-flight mass spectrometry. In: Rapid communications in mass spectrometry: RCM. Volume 7, Number 11, November 1993, pp. 1037-1040, ISSN  0951-4198 . doi: 10.1002 / rcm.1290071114 . PMID 8280914 .
• Robert J. Cotter, Serguei Iltchenko, Dongxia Wang: The curved-field reflectron: PSD and CID without scanning, stepping or lifting. In: International Journal of Mass Spectrometry. 240, 2005, pp. 169-182, doi: 10.1016 / j.ijms.2004.09.022 .

Individual evidence

1. ↑ A. Staub, J. Schappler, S. Rudaz, JL Veuthey: CE-TOF / MS: fundamental concepts, instrumental considerations and applications. In: Electrophoresis. Volume 30, Number 10, May 2009, pp. 1610-1623, ISSN  1522-2683 . doi: 10.1002 / elps.200800782 . PMID 19441039 .
2. ^ RJ Cotter, W. Griffith, C. Jelinek: Tandem time-of-flight (TOF / TOF) mass spectrometry and the curved-field reflectron. In: Journal of chromatography. B, Analytical Technologies in the Biomedical and Life Sciences. Volume 855, Number 1, August 2007, pp. 2-13, ISSN  1570-0232 . doi: 10.1016 / j.jchromb.2007.01.009 . PMID 17258517 .
3. ↑ E. Pittnauer, G. Allmaier: High-energy collision induced dissociation of biomolecules: MALDI-TOF / RTOF mass spectrometry in comparison to tandem mass spectrometry sector. In: Combinatorial Chemistry & High Throughput Screening . Volume 12, Number 2, February 2009, pp. 137-155, ISSN  1875-5402 . PMID 19199883 .
4. ↑ T. Bergmann, TP Martin, H. Schaber: High-resolution time-of-flight mass spectrometers: Part I. Effects of field distortions in the vicinity of wire meshes . In: Rev. Sci. Instrum . tape 60 , 1989, pp. 347 , doi : 10.1063 / 1.1140436 .
5. ↑ DS Selby, V. Mlynski, M. Guilhaus: Demonstrating the effect of the 'polarized grid geometry' for orthogonal acceleration time-of-flight mass spectrometers. In: Rapid Communications in Mass Spectrometry. Volume 14, Issue 7, 2000, p. 616.
6. ↑ J. Flensburg, D. Haid, J. Blomberg, J. Bielawski, D. Ivansson: Applications and performance of a MALDI-ToF mass spectrometer with quadratic field reflectron technology. In: Journal of biochemical and biophysical methods. Volume 60, Number 3, September 2004, pp. 319-334, ISSN  0165-022X . doi: 10.1016 / j.jbbm.2004.01.010 . PMID 15345299 .
7. ^ J. Zhang, CG Enke: Simple cylindrical ion mirror with three elements. In: Journal of the American Society for Mass Spectrometry . Volume 11, Number 9, September 2000, pp. 759-764, ISSN  1044-0305 . doi: 10.1016 / S1044-0305 (00) 00145-8 . PMID 10976882 .
8. ↑ T. Bergmann, TP Martin, H. Schaber: High resolution time ‐ of ‐ flight mass spectrometers. Part III: Reflector design. In: Rev. Sci. Instrum. 61, 1990, p. 2592.
9. ↑ R. Kaufmann, D. Kirsch, B. Spengler: Sequenching of peptides in a time-of-flight mass spectrometer: evaluation of postsource decay following matrix-assisted laser desorption ionization (MALDI). In: International Journal of Mass Spectrometry and Ion Processes. 131, 1994, pp. 355-385, doi: 10.1016 / 0168-1176 (93) 03876-N .
10. ↑ S. Kurnosenko, E. Moskovets: On the high-resolution mass analysis of the product ions in tandem time-of-flight (TOF / TOF) mass spectrometers using a time-dependent re-acceleration technique. In: Rapid Commun Mass Spectrom. 24 (1), 2010, pp. 63-74.
11. ↑ SG Alikhanov: A new impulse technique for ion mass measurements. In: Soviet J Exptl Theoret Phys. Volume 31, 1956, pp. 517f.
12. ↑ SG Alikhanov: A new impulse technique for ion mass measurement. In: Sov. Phys. JETP. Volume 4, 1957, pp. 452f.
13. ↑ BA Mamyrin, VI Karataev, DV Shmikk, VA Zagulin: The mass-reflectron, a new nonmagnetic time-of-flight mass spectrometer with high resolution. In: Sov. Phys. JETP. Volume 37, 1973, p. 45.
14. ^ BA Mamyrin: Time-of-flight mass spectrometry (concepts, achievements, and prospects). In: International Journal of Mass Spectrometry. 206, 2001, pp. 251-266. doi: 10.1016 / S1387-3806 (00) 00392-4 .