Instrumentation MALDI

Unlike many mass spectrometers, which allow the use of several different ionization methods on the same instrument, MALDI-TOF instruments are generally single purpose, being devoted exclusively to MALDI. In practice, El and ESI methods are also used with TOP mass spectrometers. However, the continuous nature of these sources causes a natural mismatch to TOF-MS, which is a pulsed technique. The traditional way to couple a continuous ion source to a TOF-MS is to dynamically deflect the ion beam past a narrow slit, thus generating a pulsed ion beam at the exit side of the siit. However, this results in low sensitivity as more than 99% of the ion beam is discarded. 

Several gas-phase fragmentation techniques have found a niche as a possible means to sequence oligonucleotides. These include ESI in-source CID (nozzle-skimmer voltage to induce fragmentation), IRMPD in an FT-ICR MS instrument, MALDI in-source decay and post-source decay, and CID-MS/MS of the ESI-produced ions. CID generates complementary (a — B )- and in-type ions, which can provide bidirectional sequencing from the 5 3 direction and 3 5 ... 

MIKES of amine molecular ions in the context of H/D isotope effects CID spectrum of toluene molecular ion on a magnetic sector instrument MALDI-PSD spectrum of a peptide... 

Tables 6.27 and 6.31 show the main characteristics of ToF-MS. ToF-MS shows an optimum combination of resolution and sensitivity. ToF-MS instruments provide up to 40000 spectra s-1, a mass range exceeding 100000 (in principle unlimited), a resolution of 5000, and peak widths as short as 200 ms. This is better than quadruples and most ion traps can handle. Unlike the quadrupole-type instrument, the detector is detecting every introduced ion (high duty factor). This leads to a 20- to 100-times increase in sensitivity, compared to QMS used in scan mode. The mass range increases quadratically with the time range that is recorded. Only the ion source and detector impose the limits on the mass range. Mass accuracy in ToF-MS is sufficient to gain access to the elemental composition of a molecule. A single point is sufficient for the mass calibration of the instrument. ToF mass spectra are commonly calibrated using two known species, aluminium (27 Da) and coronene (300 Da). ToF is well established in combination with quite different ion sources like in SIMS, MALDI and ESI.

Many excellent reviews on the development, instrumentation and applications of LC-MS can be found in the literature [560-563]. Niessen [440] has recently reviewed interface technology and application of mass analysers in LC-MS. Column selection and operating conditions for LC-MS have been reviewed [564]. A guide to LC-MS has recently appeared [565]. Voress [535] has described electrospray instrumentation, Niessen [562] reviewed API, and others [566,567] have reviewed LC-PB-MS. For thermospray ionisation in MS, see refs [568,569]. Nielen and Buytenhuys [570] have discussed the potentials of LC-ESI-ToFMS and LC-MALDI-ToFMS. Miniaturisation (reduction of column i.d.) in LC-MS was recently critically evaluated [571]. LC-MS/MS was also reviewed [572]. Various books on LC-MS have appeared [164,433,434,573-575], some dealing specifically with selected ionisation modes, such as CF-FAB-MS [576] or API-MS [577],... 

Lasers have advanced the analytical use of mass spectrometers to characterise additives in polymers, and routine application of MALDI is no longer limited to high molecular masses only. MALDI can now clearly produce isotopically resolved mass spectra of small molecules (<800 Da) in an L-ToF instrument, which can be used successfully for the characterisation of molecules of different chemical classes. High mass resolution with an improvement of mass accuracy to... 

Each of the subsystems can, apart from the others, make a significant diagnostic contribution. For example, the instrumental cell isolation and sample handling component could be used with DNA-based or other non-MS systems for detection and/or identification. As another example, the principles underlying pattern drift compensation can apply to MALDI MS and even non-MS detection systems such as capillary GC of fatty acid methyl esters. 

It should be pointed out that FAB, MALDI, and ESI can be used to provide ions for peptide mass maps or for microsequencing and that any kind of ion analyzer can support searches based only on molecular masses. Fragment or sequence ions are provided by instruments that can both select precursor ions and record their fragmentation. Such mass spectrometers include ion traps, Fourier transform ion cyclotron resonance, tandem quadrupole, tandem magnetic sector, several configurations of time-of-flight (TOF) analyzers, and hybrid systems such as quadrupole-TOF and ion trap-TOF analyzers. 

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