Continuous flow-fast atom bombardment CF-FAB

The mass spectra of mixtures are often too complex to be interpreted unambiguously, thus favouring the separation of the components of mixtures before examination by mass spectrometry. Nevertheless, direct polymer/additive mixture analysis has been reported [22,23], which is greatly aided by tandem MS. Coupling of mass spectrometry and a flowing liquid stream involves vaporisation and solvent stripping before introduction of the solute into an ion source for gas-phase ionisation (Section 1.33.2). Widespread LC-MS interfaces are thermospray (TSP), continuous-flow fast atom bombardment (CF-FAB), electrospray (ESP), etc. Also, supercritical fluids have been linked to mass spectrometry (SFE-MS, SFC-MS). A mass spectrometer may have more than one inlet (total inlet systems). 

The combination of CE with continuous-flow fast atom bombardment (CF-FAB-MS) requires the use of an interface, because of the incompatibility of the CF-FAB process and CE for liquid flow [888], The CF-FAB source requires a solvent, usually water/glycerol (95-5 v/v), which is maintained at a steady flow-rate of 2-15mLmin 1. Flow-rate in CE does not exceed 1 nLmin-1. 

Several years later, the next step in the application of MS-MS for mixture analysis was developed by Hunt et al. [3-5] who described a master scheme for the direct analysis of organic compounds in environmental samples using soft chemical ionisation (Cl) to perform product, parent and neutral loss MS-MS experiments for identification [6,7]. The breakthrough in LC-MS was the development of soft ionisation techniques, e.g. desorption ionisation (continuous flow-fast atom bombardment (CF-FAB), secondary ion mass spectrometry (SIMS) or laser desorption (LD)), and nebulisation ionisation techniques such as thermospray ionisation (TSI), and atmospheric pressure ionisation (API) techniques such as atmospheric pressure chemical ionisation (APCI), and electrospray ionisation (ESI). 

In standard FAB, the surface of the matrix solution is depleted of analyte and suffers from radiational damage during elongated measurements. Refreshment of the surface proceeds by diffusion (limited by the viscosity of the matrix) or evaporation. Continuous-flow fast atom bombardment (CF-FAB) continuously refreshes the surface exposed to the atom beam. [107,108] The same effect is obtained in slightly different way by the frit-fast atom bombardment (frit-FAB) technique. [109,110] In addition, both CF-FAB and frit-FAB can be used for online-coupling of liquid chromatography (LC, Chap. 12) [111] or capillary electrophoresis (CE) to a FAB ion source. [112]... 

The most commonly used FAB interface in LC/MS is known as continuous-flow fast-atom bombardment (CF-FAB) ionization, in which the fast atoms or ions are directed at a target along which the LC eluent flowsd In a CF-FAB, the LC eluent, mixed with a FAB matrix such as 5% aqueous glycerol, is continuously introduced and deposited on the tip of a FAB probe. The maximum flow rate is in the range of 5 to 15 pL/min. A comprehensive review of the principles and application of CF-FAB for LC/MS has been written by Caprioli. ... 

The first approaches to the coupling of liquid-phase separation techniques with mass spectrometry were designed for HPLC needs, starting in the 1970s with since-forgotten techniques such as direct liquid introduction (DLI) and moving belt. In the 1980s, techniques such as thermospray, continuous-flow-fast atom bombardment (CF-FAB), and particle beam arose. 

During the last decade, research efforts in the field of LC-MS have changed considerably. Technological problems in interfacing appear to be solved, and a number of interfaces have been found suitable for the analysis of flavonoids. These include TSP, continuous-flow fast-atom bombardment (CF-FAB), ESI, and APCI. LC-MS is frequently used to determine the occurrence of previously identified compounds or to target the isolation of new compounds (Table 2.11). LC MS is rarely used for complete structural characterization, but it provides the molecular mass of the different constituents in a sample. Then, further structural characterization can be performed by LC-MS-MS and MS-MS analysis. In recent years, the combination of HPLC coupled simultaneously to a diode-array (UV-Vis) detector and to a mass spectrometer equipped with an ESI or APCI source has been the method of choice for the determination of flavonoid masses. Applications of LC-MS (and LC-MS-MS) in flavonoid... 

Earlier methods of ionization applied to carotenoids, including electron impact (El), chemical ionization (Cl), a particle beam interface with El or Cl, and continuous-flow fast atom bombardment (CF-FAB), have been comprehensively reviewed elsewhere (van Breemen, 1996, 1997 Pajkovic and van Breemen, 2005). These techniques have generally been replaced by softer ionization techniques like electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), and more recently atmospheric pressure photoionization (APPI). It should be noted that ESI, APCI, and APPI can be used as ionization methods with a direct infusion of an analyte in solution (i.e. not interfaced with an HPLC system), or as the interface between the HPEC and the MS. In contrast, matrix-assisted laser desorption ionization (MALDI) cannot be used directly with HPEC. 

Other commercially available systems which come equipped with the direct liquid injection (DLI) or the continuous flow-fast atom bombardment (CF-FAB) interfaces are used less frequently (21). Because of their limited use, little information has been published about their application. 

In a continuous-flow fast-atom bombardment (Cf-FAB) interface, typically a 5-15 pl/min liquid stream, mixed with 5% glycerol as FAB matrix, flows through a narrow-bore fused-sihca capillary towards either a stainless-steel frit or a (gold-plated) FAB target. At the target or frit, a uniform liquid film is formed due to a subtle balance between solvent evaporation and solvent dehvery. Ions are generated by bombardment of the liquid film by fast atoms or ions, common to FAB. The Cf-FAB interface for LC-MS have been reviewed [37-38]. 

Combined liquid chromatography/mass spectrometry (LC/MS)can play an important role in both qualitative and quantitative bioanalysis. LC/MS can be performed with a number of interfaces. Three interfaces are presently available in our laboratories i.e., the thermospray interface (TSP), the moving-belt interface (MBI), and continuous-flow fast atom bombardment (CF-FAB). These interfaces are supplementary with respect to their applicability and the type of information that can be obtained. 

A variety of interfaces was developed for on-line LC-MS coupling. A number of these found wide application when they became commercially available, e.g. the moving-belt interface, direct-liquid introduction, thermospray, particle-beam and continuous-flow fast-atom bombardment (Cf-FAB), but are hardly used any longer due to the introduction of interfaces based on atmospheric-pressure ionization (API), i.e. electrospray and atmospheric-pressure chemical ionization (APCI). Before discussing the API-based interfaces in detail, brief attention is given to three of the older LC-MS interfaces. 

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