Continuous-Flow Fast Atom Bombardment Interface

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]. 

The Continuous-Flow/Frit (Dynamic) Fast-Atom-Bombardment Interface... 

Several other interface designs were introduced over this period, including continuous flow fast atom bombardment (CFFAB)" and the particle beam interface (PBI)," but it was not until the introduction of the API source that LC/MS applications really came to the forefront for quantitative analysis. Early work by Muck and Henion proved the utility of an atmospheric pressure interface using a tandem quadrupole mass spectrometer. 

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. 

M. A. Moseley, L. J. Deterding, K. B. Tomer, and J. W. Jorgenson. Nanoscale Packed-Capillary Liquid Chromatography Coupled with Mass Spectrometry Using a Coaxial Continuous-Flow Fast Atom Bombardment Interface. Anal. Chem., 63(1991) 1467-1473. 

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. ... 

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... 

Moseley, M. A. Deterding, L. J. Tomer, K. B. Jorgenson, J. W. 1991. Nanoscale packed-capillary liquid chromatography coupled with mass spectrometry using a coaxial continuous-flow fast atom bombardment interface. Anal. Chem., 63,1467-1473. 

Mass spectrometry is a valuble tool with which an abundancy of structural information may be obtained from a minute amount of material. Capillary electrophoresis may be interfaced with mass spectrometry by electrospray ionization [124-126] or continuous-flow, fast-atom bombardment methods [127,128]. Several reviews discuss applications of the interfacing techniques, and address the attributes and disadvantages associated with these methods [129,130]. Critical parameters involved in the optimization of CE-electrospray ionization mass spectrometry analysis have been reviewed as well [131],... 

The instrumentation and interfaces that had been used up to 1998 in CWC-related LC/MS analysis were summarized previously (4). At that time, sources that operate at atmospheric pressure, using electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI), were displacing their predecessors that used thermospray ionization or continuous flow fast atom bombardment. Atmospheric pressure ionization (API), either ESI or APCI, is now the method of choice in CWC-related analysis and will be the focus of this current review. A small number of recent applications involving alternative types of ionization are also included. For earlier applications of LC/MS to chemical weapons (CW) analysis, using thermospray and other ionization methods, the reader is referred to our previous review (4). The other major trend has been the increasing availability and ease of use of less-expensive bench-top quadrupole and... 

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. 

Over 30 years of liquid chromatography-mass spectrometry (LC-MS) research has resulted in a considerable number of different interfaces (Ch. 3.2). A variety of LC-MS interfaces have been proposed and built in the various research laboratories, and some of them have been adapted by instmment manufacturers and became commercially available. With the advent in the early 1990 s of interfaces based on atmospheric-pressure ionization (API), most of these interfaces have become obsolete. However, in order to appreciate LC-MS, one carmot simply ignore these earlier developments. This chapter is devoted to the older LC-MS interfaces, which is certainly important in understanding the histoiy and development of LC-MS. Attention is paid to principles, instrumentation, and application of the capillary inlet, pneumatic vacuum nebulizers, the moving-belt interface, direct liquid introduction, continuous-flow fast-atom bombardment interfaces, thermospray, and the particle-beam interface. More elaborate discussions on these interfaces can be found in previous editions of this book. 

Nanoscale LC for LC-MS was pioneered by Deterding et al. [52] using a coaxial continuous-flow fast-atom bombardment interface. Nano-LC was commercialized by LC Packings, both with respect to column technology and instrumentation [53]. Nowadays, nano-LC is available from several instrument and column manufacturers. 

Various types of HSCCC-MS have been developed using frit fast-atom bombardment (FAB) including continuous flow (CF) FAB, frit electron ionization (El), frit chemical ionization (Cl), TSP, atmospheric pressure chemical ionization (APCI), and electrospray ionization (ESI). Each interface has its specific features. Among those, frit MS and ESI are particularly suitable for directly interfacing to HSCCC, because they generate low back-pressures of approximately 2 kg/cm, which is only one-tenth of that produced by TSP. 

Reinhoud, N. J., Niessen, W. M. A., Tjaden, U. R., Gramberg, L. G., Verheij, E. R. and van der Greef, J. Performance of a liquid-junction interface for capillary electrophoresis mass spectrometry using continuous-flow fast-atom bombardment Rapid Commun. Mass Spectrom. 3 348-351, 1989. 

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. 

The most recent significant advance in plant hormone analysis has been the use of combined HPLC-MS for the analysis of GA conjugates, lAA conjugates and cytokinins. A number of interfaces have been developed for HPLC-MS, including thermospray, atmospheric pressure chemical ionisation, electrospray, particle beam, continuous flow fast atom bombardment (FAB) and frit-FAB (see reference [94]). GA standards have been analysed by HPLC-MS with a thermospray interface [95], an atmospheric pressure chemical ionisation interface has been used with GA conjugates [96] and cytokinins [97] while ion spray and plasma spray have been used to analyse ABA and its metabolites [98]. There are, however, many more reports on the use of frit-FAB HPLC-MS for the analysis of not only standards, but also endogenous hormones and their isotopically-labelled metabolites [18-23,99-101]. 

Continuous-Flow Fast Atom Bombardment Interface Although currently not a popular approach, the coupling of CE mass spectrometry was once achieved via a CF-FAB probe. Makeup flow is required in this coupling because of the mismatch of the low flow rates of the CE solution with the liquid flow rates of stable CF-FAB operation. The sheath-flow and liquid-junction designs discussed above have been used successfully for this purpose [69-71], The sheath-flow design has the advantages that the composition and the flow rates of the CE effluents and of the FAB matrix solution can be optimized independently, and that the separation efficiency is higher. 

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