ReTOF

Instrumental developments concern micro ion traps (sub-mm i.d.) [193], extension of the mass range, mass resolution and capture efficiency for ions generated externally. Fast separations at very low detection levels are possible by means of hybrid QIT/reToF mass spectrometry [194]. 

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. 

Example The curved-field reflectron of a coaxial ReTOF instrument occupies a large portion D) of the total flight path s = li + D) and uses 86 lens elements... 

Example Regardless of the manufacturer of the hardware, the effect of a time lag on resolution is quite dramatic. The resolving power of linear instruments is improved by a factor of 3-4 and reflector instruments become better by a factor of about 2-3. [36] The advantages are obvious by comparison of the molecular ion signal of Ceo as obtained from a ReTOF instrument with continuous extraction (Fig. 4.7) and from the same instrument after upgrading with PIE (Fig. 4.12), or by examination of MALDI-TOF spectra of substance P, a low mass peptide, as obtained in continuous extraction mode and after PIE upgrade of the same instrument (Eig. 4.11). 

Fig. 4.11. MALDI-TOF spectra of substance P, a low mass peptide, as obtained from a Bruker Biflex ReTOF in continuous extraction mode (left) and after PIE upgrade (right).

Fig. 4.13. Bruker Reflex IV MALDI-ReTOF instrument. With kind permission of Bruker Daltonik, Bremen.

Fig. 4.15. Tandem-in-space setups for different beam instruments magnetic four-sector instrument (a), magnetic sector-quadmpole hybrid (b), triple quadmpole (c), and ReTOF (d).

Methods for the detection of metastable ion dissociations in ReTOF-MS in combination with secondary ion mass spectrometry (SIMS) and Cf plasma desorption ( Cf-PD, Chap. 9.5.2) mass spectrometry were known before the advent of MALDI. [56-59]... 

Despite the comparatively poor precursor ion resolution and being a time- and sample-consuming procedure, MS/MS on the ReTOF works so well that this method has become established as one of the major tools of biomolecule sequencing for ions in the m/z 500-3000 range (examples are shown in Chap. 10). 

Note In particular the MALDI-TOF community has coined some sort of an own terminology, e.g., in-source decay (ISD) for all fragmentations occurring within the ion source, post-source decay (PSD) instead of metastable ion dissociation and fragment analysis and structural TOF (FAST) for the specific mode of operation of a ReTOF to detect metastable ions. 

Figure 5.6 MALDI-RETOF mass spectrum of Qo re with matrix...

The RETOF technique has applications far beyond mass analysis and determination of metastable dissociation fraction. In particular, it provides a very valuable approach to determining kinetic energy release (KER) during evaporative dissociation. As shown in this section, these data also find application in determining absolute values of cluster bond energies for systems without a barrier to... 

The major deficiency of a simple linear TOF instrument is its insufficient mass resolution, resulting from flight time variations of ions of the same m/e ratio. The ionization process adds a certain amount of initial kinetic energy to the molecules before acceleration. In addition, the different spatial positions in the source from where the ions are formed lead to varying values of l, and thus to flight time variations. As a result, modern TOF instruments commonly employ a technique called reflection to enhance resolution. A reflectron TOF (RETOF) [4] (Fig. 3) is used to focus ions of the same... 

FIGURE 9. Schematic drawing of the central part of a ZEKE spectrometer. 1,2,3 and 4 are skimmers RETOF denotes retardation of time-of flight and MCP is a multichannel plate. Reproduced by permission of the American Institute of Physics from Reference 27... 

ABSTRACT. Details of the novel method of laser evaporation of intact neutral molecules (LEIM) with a low powered IR-laser and the multiphoton ionization (MUPI) combined with a high-resolution Reflectron-Time-of-Flight (RETOF) mass spectrometer are explained. Some features of the method are discussed. Mass Spectra of biomolecules obtained with this method are displayed and their differences to other mass spectrometric techniques are discussed. It is shown, that Multiphoton Ionization is a general activation method for forming ions in a mass spectrometer, with additional features for an easy deducing of structures and intrinsic properties of biomolecules in contrast to other mass spectrometric ionization methods. 

In the last years several techniques have been developed to improve the mass resolution in TOF-mass spectrometers such as the use of a reflecting field/2/ in the ion beam path or the method of velocity compac-tion/3/. Especially by using a Reflectron-Time-of-Flight (RETOF) ins-trument/4/ some shortcomings of the Time-of-Flight technique can be overcome like the limited mass resolution. The necessity of pulsed ion sources in Time-of-Flight mass spectrometry makes this technique excellent for pulsed ionization methods like laser ionization or secondary ion emission. 

A common problem to all Time-of-Flight mass spectrometers is the so called "turn around time" that is due to different initial velocities of the neutrals in the ion source. Even in a RETOF these kinetic energies can destroy the achievable mass resolution of the instrument. The low translational cooling, resulting from the supersonic beam expansion, yields small kinetic energy distributions of the neutral molecules /17/. 

In conclusion we will sununarize the advantages of MUPI-RETOF-Mass Spectrometry using a laser desorption source for free neutral molecules. 

ReTOF, reflectron time-of-flight Mass analyzer separating ions according to their flight time a reflectron serves for refocusing an ion beam. 

The ability of the ReTOF to compensate for the initial energy spread of ions largely increases the resolving power of TOF instruments. While a typical continuous extraction TOF instrument in linear mode cannot resolve isotopic patterns of analytes above about m/z 500, it will do when operated in reflector mode (Fig. 4.7). At substantially higher m/z, the ReTOF still fails to resolve isotopic patterns, even though its resolution is still better than that of a linear TOF analyzer. 

There are two basic instrumental concepts for MS/MS. The first is tandem mass spectrometry in space (or tandem-in-space MS). In order to perform two consecutive mass-analyzing steps, two mass analyzers may be mounted in tandem. Thus, tandem-in-space refers to MS/MS instrumentation where product ion spectra are recorded using spatially separated m/z analyzers. Speedfic m/z separation is performed so that in one section of the instrument ions are selected, then dissociated in an intermediate region, and the products thereof are finally transmitted to a second analyzer for mass analysis (Fig. 9.1). All beam transmitting devices, e.g., multiple sector, ReTOF, TOF/TOF, QqQ, and QqTOF instruments follow this route to tandem MS (Fig. 9.2) [4]. The second approach, tandem mass spectrometry in time (or tandem-in-time MS), enploys a single m/z analyzer (QIT, LIT, FT-ICR) that may be operated by executing the discrete steps of ion selection, activation, and product ion analysis in the very same place but sequentially in time [4]. 

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