Reflectron devices

A special applicability of MALDI is the single bead analysis of peptides (— 1000 Da) with a TOF-analyzer [25] also using different resin linkers [26]. Improved resolution of mass spectra was achieved with the development of reflectron devices and post source decay for a direct structural readout of single compounds [27]. For direct ionization and desorption in the MALDI source, photosensitive linkers were developed [28,29]. 

Figure 2.32. Scheme of a TOF analyzer with the reflectron device, leading to a resolution increase. 

Fig. 15.21 The experimental setup including the laboratory-scale coffee roaster with a sampling unit and a laser mass spectrometer. The homebuilt mobile device consisted of a Reflectron TOFMS analyser, an effusive beam inlet system and a built-in laser operated at 266 nm (Continuum NcL YAG laser SURELIGHT", 266 nm). (Adapted from [203])...

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...

Figure 16.6—Linear time of flight (TOF) and principle of the reflectron. 1) Sample and sample holder 2) MALDI ionisation device 3 and 3 ) extraction and acceleration grid (5 000 V potential drop) 4) control grid 5) multichannel collector plate 6) electron multiplier 7) signal output. The bottom figure shows a reflectron, which is essentially an electrostatic mirror that is used to time-focus ions of the same mass, but which have different initial energies. This device increases resolution, which can attain several thousand.

A TOF mass analyser requires a pulsed ion introduction. In an electrospray-TOF combination, the duty cycle is an important issue. A significant improvement in the duty cycle can be achieved in an ion-trap-TOF hybrid instmment the ions from a continuous ion source are accumulated in the ion trap between two ion introduction events. An ion-trap-TOF hybrid instrument was first described by the group of Lubman [68-69]. The system consists of an atmospheric-pressure ion source with a vacuum interface, a set of Einzel lenses, an ion-trap device, and a reflectron time-of-flight mass analyser. The system was applied for fast analysis in combination with a variety of separation techniques [70]. 

The peptide ions formed in the matrix-assisted source now enter the mass analyzer. The mass analyzer in MALDI-TOF is of time-of-flight (TOE) type. The peptide ions pass down the mass analyzer towards a detector, which is placed at the other end of the mass analyzer. The time of flight of each ion depends upon the miz ratio of that particular ion. The general rule is that greater the m/z ratio, faster the movement of the ion. One of the problems with this process is that peptide ions with the same m/z value are poorly resolved. To overcome this problem, a device called as reflectron is used. The reflectron is placed at the end of the mass analyzer and focuses on ions with the same m/z ratio together and sends them to the detector. The addition of a reflectron has greatly increased the accuracy and sensitivity of the MALDI-TOF analysis. 

Advances in TOF/MS technology, such as reflectron ion optics, delayed ion extraction, and orthogonal ion acceleration, have given rise to bench-top instrumentation with resolution capabilities in excess of that obtained by quadra-pole MS systems. Each of these devices limits or corrects the kinetic energy spread of ions with identical m/ ratios. 

Figure 16.6 A simplified schematic of a time of flight spectrometer and the principle of the ion reflector (reflectron). (1) sample and sample holder (2) MALDI ionization device by pulsed laser bombardment (3 and (3 ) ions are formed between a repeUer plate and an extraction grid (PD 5000V) then accelerated by an other grid (4) control grid (5) microchannel collector plate (6) signal output. Below, a reflectron, which is essentially an electrostatic mirror that is used to time-focus ions of the same mass but which have initially different energies. The widths of the peaks are of the order of 10 and the resolution ranges between 15 to 20 000.

A convenient way to investigate the elemental and isotopic composition of the upper surface layers of small bodies of the Solar system is to use small-size laser mass spectrometers, mounted on a lander. Optimal for the purpose is a reflectron type time-of-flight mass analyzer with laser evaporation and ionization of the target, called LASMA (LASer Mass Analyser). It was created on the basis of a laboratory prototype, initially developed for the LIMA-D laser mass spectrometric system on the PHOBOS experiment, a space mission to the Mars satellite. A spin-off modification of the LASMA device was further developed for the special purposes of environmental research. 

Time-of-flight reflectron mass spectrometry with some form of time-delay extraction has become popular for large-molecule characterizations. Resolutions of up to 15,000 can be routinely achieved over very broad mass ranges, extending up to about 300,000 Da. As noted previously, time-of-flight analysers are particularly compatible with laser-based ionization techniques that produce very short bursts of ions. They are also very fast, with mass spectra often obtainable in about 25 p,s. Finally, time-of-flight analysers have been paired with a quadrupole to produce a hybrid two-dimensional mass spectrometry system that has found widespread use in protein analyses. This device will be discussed in more detail in the section on two-dimensional mass spectrometry. 

---