MALDI magnetic sector

The limited resolution and mass accuracy of the early MALDI-TOF instruments made the combination of MALDI with magnetic sector instruments (Chap. 4.3) desirable, [148,149] but this set-up suffered from low shot-to-shot reproducibility and poor sensitivity getting a full scan spectrum required thousands of laser shots while scanning the magnet. Even though eutectic matrix mixtures were introduced to circumvent such problems, [90,91] the MALDI-magnetic sector combination never became established. 

Electron impact (El), or Electrospray ionization (ESI), or Matrix-assisted laser desorption ionization (MALDI) — Magnetic sector, or Time-of-flight (TOP), or Quadrupole (Q) — Photomultiplier, or Electron multiplier, or Micro-channel plate... 

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. 

The quantification capability is normally limited by the detector and/or the ion source. The MCP that is often utilized in TOF instruments cannot fully handle the ion currents that are produced in MALDI and are often saturated to some extent. With other ion sources, such as SIMS, the detection system is less strained so the detector is less limiting. Instead the ion source will limit the quality in quantification. Magnetic sectors and also qudmpoles are more often utilized when quantification is important. 

V. S. K. Kolli and R. Orlando. A New Strategy for MALDI on Magnetic Sector Mass Spectrometers with Point Detectors. Anal. Chem., 69(1997) 327-332. 

MALDI generated a great demand for a mass analyzer ideally suited to be used in conjunction with a pulsed ion source and capable of transmitting ions of extremely high mass up to several 10 u. [24] Since then, the performance of TOF instruments has tremendously increased. [25,26] TOF analyzers were adapted for use with other ionization methods and are now even strong competitors to the well-established magnetic sector instruments in many applications. [25,27]... 

Strobel, F.H. Solouki, T. White, M.A. Russell, D.H. Detection of Femtomole and Sub-Femtomole Levels of Peptides by Tandem Magnetic Sector/Reflectron TOF Mass Spectrometry and MALDI. J. Am. Soc. Mass Spectrom. 1991,2, 91-94. 

Kumar, N. Kolh, V.S. Orlando, R. A New Matrix for MALDI on Magnetic Sector Instruments With Point Detectors. Rapid Commun. Mass Spectrom. 1996,10, 923-926. 

Harvey, D.J. Hunter, A.P. Use of a Conventional Point Detector to Record MALDI Spectra From a Magnetic Sector Instrum t. Rapid Commun. Mass Spectrom. 1998,72,1721-1726. 

ToF mass spectrometers as dynamic instruments gained popularity with the introduction of matrix assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) as effective pulsed ion sources for the soft ionization of large biomolecules (up to 10s dalton) due to their high ion transmission.38 ToF mass spectrometers, quadrupole analyzers and/or magnetic sector fields can be combined in tandem mass spectrometers (MS/MS) for the analysis of organic compounds. 

Several ionization methods have been applied for CE-MS couphng. Matrix-assisted laser desorption ionization (MALDI), continuous flow fast atom bombardment (FAB), laser vaporization ionization with UV laser, sonic spray ionization and electrospray ionization (ESI) have all been used for coupling CE to MS. However, ESI is now undoubtedly the most widely used ionization technique, employing numerous analyzers including quadrupoles, magnetic sector, Fourier transform ion cyclotron resonance, time-offlight and trapping devices. However, quad-rupole detectors have predominantly been applied in CE-MS [6-8]. 

Preferably, electrospray ionization (ESI) is used in combination with quadrupole mass filters [16,17], whereas MALDI is commonly used in combination with the time-of-flight (TOF) analyzer [18], The relatively simple construction of these two types of analyzers and the resulting price advantage has led to their replacing the traditional magnetic sector instruments as the workhorses of mass spectrometric analysis. A more recent development is the ion-cyclotron-resonance (ICR) analyzer [19] which can be used for both ES-and MALDI-ionization. 

In the past decade, mass spectrometry (MS) has become the method of choice for quality control of synthetic peptides. Historically, plasma desorption (PD) and fast atom bombardment (FAB) were the first ionization methods used for the mass analysis of nonderivatized peptides. More recently, electrospray ionization (ESI) MS and matrix-assisted laser desorption ionization (MALDI) MS have found widespread utility for peptide analysis. Both of the latter methods yield protonated molecules and, thus, provide direct molecular weight information. As will be covered later, ESI can be employed with a variety of mass analyzers, including quadrupole, magnetic sector, ion trap, and time-of-flight (TOF) analyzers. On the other hand,... 

The main advantage of the FAB technique compared to ESI and MALDI is, in fact, not the ionization method itself but the type of mass analyzer it works with. FAB ion sources are usually attached to magnetic sector, double-focusing mass spectrometers, enabling accurate mass measurements and, which is even more important, allowing the study of high-energy collision-induced dissociation (CID) reactions which provide information about the structure of the studied ion (see next section). 

In contrast, the methylated higher ohgosaccharides were overestimated by the other instruments. In the study already mentioned, Naven and Harvey found that there was no discrimination of lower molecular weight analytes when a MALDI ion source and a magnetic-sector field mass analyzer were combined. ... 

Other types of mass spectrometer have been used to record MALDI spectra magnetic sector instruments equipped with an array detector initially offered the advantage of higher resolution than was available with the early linear TOF instruments but these spectrometers have now largely been replaced with reflectron-TOF instruments using delayed extraction ion sources. Mass spectrometers operated with higher than normal ion source pressures to... 

---