Reflectron performance

Although the first generation of TOF analyzers were formed by just a drift tube and had low resolution (Figure 2.10a), many improvements have been made over the years. In particular, the introduction of a reflectron , located at the end of the flight tube, improved theTOF performances a lot. In fact, once accelerated in the source, ions with the same m/z value do not have exactly the same kinetic energy. Thus they arrive at the detector at... 

Instruments are calibrated with substances of known relative molecular mass and very accurate mass measurements can be made with this type of analyser. Sensitivity is very high and there is virtually no upper limit to the working mass range. The performance of TOF instruments can be improved further by the incorporation of an electrostatic reflector (reflectron) which ensures that all ions of identical mass reach the detector simultaneously by correcting for any differences in their kinetic energies. 

J. Flensburg, D. Haid, J. Blomberg, J. Bielawski, and D. Ivansson. Applications and Performance of a MALDI-ToF Mass Spectrometer with Quadratic Field Reflectron Technology. J. Biochem. Biophys. Meth., 60(2004) 319-334. 

The reflector or reflectron has been developed by Mamyrin. [35] In the reflector TOF analyzer - often abbreviated ReTOF - the reflector acts as an ion mirror that focuses ions of different kinetic energies in time. Its performance is improved by using two-stage or even multistage reflector designs. 

TOF High resolution and mass accuracy when operated in reflectron mode or in Q-TOF systems, elemental composition can be obtained Tandem MS available for generation of sub-structure information or quantitative analysis High sensitivity Very fast scan speed Unlimited mass range Initial energy and spatial distribution must be corrected for ions High-performance electronics needed... 

Fig. 8.1.1 Simple illustrations of a various mass spectrometers, a The triple-quadrupole tandem mass spectrometer (top panel). The middle set of quadrupoles are part of the collision cell (CC) and do not perform mass separation. MSI and MS2 indicate the first and second quadrupole mass separation devices, respectively. The bold arrow shows the path of ions, b Ion-trap mass spectrometer (middle left). The charged sections of the ion trap are not elliptical as drawn, but rather hyperbolic. The diagram is also two-dimensional, whereas the ion trap is three-dimensional. The ion path is such that ions enter the device and are trapped until a specific voltage ejects these ions, c Time of Flight mass spectrometer with a Reflectron (middle left). Ions are separated by the time it takes to pass through the instrument. The Reflectron improves/focuses the ions, d Hybrid Tandem mass spectrometer (bottom). The diagram shows that a quadrupole instrument can be combined with a different type of mass spectrometer, forming a tandem hybrid instrument...

TOFSIMS analyses were performed on a Kratos PRISM instrument. It was equipped with a reflectron-type time-of-flight mass analyzer and a pulsed 25 kV liquid metal ion source of monoisotopic 69Ga ions with a minimum beam size of 500 A. Positive and negative spectra were obtained at a primary energy of 25 keV, a pulse width of 10-50 ns, and a total integrated ion dose of about 10" ions/cm2. This is well below the generally accepted upper limit of 5 x 1012 ions/cm2 for static SIMS conditions in the analysis of organic materials [12], The mass resolution at mass 50 amu varied from M/AM= 1000 at 50 ns pulse width to about 2500 at 10 ns pulse width. 

The mass spectrometry analysis was performed by the matrix assisted laser desorption/ionisation time-of-flight (S8-MALDI) technique using a Voyager-DE PRO Biospectrometry Workstation (Applied Biosystems, USA). Radiation pulses of 0.5 ns and 3 Hz frequency from N2 laser operating at 337 nm were used to desorb the species and negative/positive ions formed were detected in reflectron mode. Sulfur used as a matrix material was also dissolved in toluene and mixed with the samples solution prior to deposition onto a target. 

Common space instruments have two mass analysers, allowing MS/MS experiments to be performed. A frequently used instrument of this type uses quadrupoles as analysers. The QqQ configuration indicates an instrument with three quadrupoles where the second one, indicated by a lower case q, is the reaction region. It operates in RF-only mode and thus acts like a lens for all the ions. Other instruments combine electric and magnetic sectors (E and B) or E, B and qQ, thus electric and magnetic sectors and quadrupoles. TOF instruments with a reflectron, or a combination of a quadrupole with a TOF instrument, are also used. 

Due to the pulsed nature of most of these experiments, much of the work to date has been performed using time-of-flight mass spectrometers. Quadrupole mass spectrometers are also well suited, especially with higher duty cycle systems. Design of the instruments has followed conventional approaches, for which the resolution limits the size and complexity of the cluster and cluster-adduct species that can studied. One serious problem is the isotopic abundance of many of the metals, which serves to complicate mass spectra. Isotopi-cally pure materials, such as used in measurements of hydrogen uptake on Fe clusters, " simplify the mass spectra. Use of the reflectron time-of-flight mass spectrometer allows the study of metastable clusters and cluster adducts. Details of different instrument designs are described in the references. 

ToF measurements can be performed using a linear or reflectron mass analyzer. The former comprises a basic ToF layout, whereas the latter uses an ion mirror to time-focus the ions onto the detector. The linear detector is used for intact protein analysis owing to its ability to detect a very wide mass range, up to 100,000 Da (26). The reflectron geometry offers higher mass resolution capabilities, but has a more limited mass range, typically below 5000 Da. 

Figure 6.31. Positive-ion MALDI-TOF reflectron mode mass spectrum of grape seed extract (matrix 2,5-dihydroxybenzoic acid). (Reprinted from Journal of Agricultural and Food Chemistry, 48, Yang and Chien, Characterization of grape procyanidins using high-performance liquid chromatography/mass spectrometry and matrix-assisted laser desorption time-of-flight mass spectrometry, p. 3993, Copyright 2000, with permission from American Chemical Society.)...

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