TOF, orthogonal acceleration

Up to here, the discussion has focused on MALDI-TOF equipment, because MALDI is a pulsed ionization method. MALDI initiated tremendous improvements of TOF analyzers which makes it attractive to combine such compact but powerful analyzers with any other non-pulsed ionization method desirable. The major breakthrough came from the design of the orthogonal acceleration TOF analyzer (oaTOF), i.e., a TOF analyzer into which pulses of ions are extracted or- 

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Orthogonal TOF is the name commonly given to what should properly be called orthogonally accelerated TOF mass spectrometry. Therefore, it is sometimes referred to by the acronym oa-TOF, especially in official publications, but it is more usual to hear it referred to simply as orthogonal TOF. 

Laser desorption intrinsically is a pulsed ionization process, which is therefore ideally combined with time-of-flight (TOF) analyzers (Chap. 4.2). [16,49] Ever since the first MALDI experiments, MALDI and TOF have been forming a unit, and the majority of MALDI applications are MALDI-TOF measurements. Vice versa, it was the success of MALDI that pushed forth the tremendous delevopment of TOF mass analyzers. More recently, MALDI has also been adapted to orthogonal acceleration TOF analyzers. [147]... 

Schematic diagram of a MALDI orthogonal acceleration TOF instrument.

While the principles of time-of-flight (TOF) mass spectrometry were well established for many years, significant breakthroughs in TOF technology and application were made in the 1990 s [57-58], This can be attributed to the emergence of MALDI as an ionization technique. TOF is considered as the ideal mass analyser for MALDI. This stimulated developments and research in TOF analysers, which in turn led to the rediscovery of the orthogonal-acceleration TOF (oaTOF). 

Time-of-fiight instruments are configured as either a stand-alone TOF mass analyzer (TOF MS) or as a hybrid quadrupole time-of-fiight (QqTOF) mass spectrometer the latter consists of a quadmpole front-end and an orthogonal acceleration TOF back-end for MS/MS experiments (Fig. 6.10). The orthogonal design minimizes the ions initial velocity spread as they are accelerated into the TOF by a pulsed potential. A QqTOF can be operated as a TOF mass analyzer (QqTOF MS, full-scan) or a quadmpole TOF tandem mass spectrometer (QqTOF MS/MS, product ion scan). Compared to a TOF MS, the... 

Figure 1.30a depicts the MALDI-CAD spectrum of [M + Na] of the 9-mer of poly(butyl methacrylate), PBMA A, measured on an EBE-TOF instrument. In this sector-orthogonal acceleration-TOF hybrid, the EBE and TOP sections are used for precursor ion selection (MS-1) and fragment ion analysis (MS-2), respectively, and CAD takes place in an intermediate collision cell at 800 eV collision energy. The CAD spectrum contains several series of fragments separated by 142 Da (repeat unit). Series A and B arise by charge-remote direct cleavages at either end, as shown in Figure 1.30b. Series C/D and E/F...

To overcome the limitations inherent with PSD peptide sequencing using MALDI instrumentation, considerable research efforts are under way to use a MALDI source in conjunction with tandem MS and a CID cell. One approach is the MALDI-TOF/TOF as a linear configuration [105, 106, 107]. Another solution is the use of the more mature orthogonal acceleration TOF technique as the second stage mass analyser with either a quadrupole parent ion selecto [108, 109] or a linear TOF separator upfront to the CID cell [110]. These are rather new developments in the field of tandem MS. The most common and furthest developed ionisation method in conjunction with CID, is elechospray ionisation (ESI). 

Quadrupole-Orthogonal Acceleration TOF Instrument As of today, quadrupole (Q)-orthogonal acceleration (oa) TOF (oa-TOF) instrument is the most popular hybrid instrument [58,59] a simphstic pictorial representation is shown in Figure 4.11. The quadrupole section consists of a normal massresolving quadrupole and an rf-only quadrupole. The latter serves as a collision cell and as an ion-accumulation device. For precursor-ion scan, the ions desired are mass-selected by the main quadrupole, accumulated in the collision cell, and a packet of the CID product ions is pushed into the TOF analyzer. The precursor-ion selection by the quadrupole is at a medium resolution, but the product-ion analysis by the TOF section is at a reasonably high resolution. 

GC/MS system gas chromatograph Agilent 6890N, equipped with automatic injector, Agilent 7683 with a 100-vial tray, and split/splitless injector with electronic pressure control (EPC) up to 150 psi combined with orthogonal acceleration TOF mass spectrometer, model GCT premier (Waters Corp., Milford, MA). 

FIGURE 9.2 A schematic of an orthogonal acceleration TOF analyzer. (From Technical Note 001-0877-00, GBC Scientific, February 1998.)... 

Figure 10.11. Orthogonal acceleration TOF mass spectrometers, (a) Original design of Dawson and Guilhaus using lens focusing, (b) An oaTOF shown with an electrospray source and RF ion guide.

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