Liquid-matrix secondary-ion mass

The classicaf FAB ionization technique consists in the bombardment of the sample dissolved in a nonvolatile, viscous, and polar liquid (the matrix) with neutral xenon atoms. However, a much more popular version is Liquid-matrix Secondary-ion Mass Spectrometry (LSIMS) in which the sample solution is bombarded with heavy ions, usually Cs+ or Xe+. Basic information about FAB and LSIMS can be found in mass spectrometry monographs [3, 4]. Comprehensive reviews on the application of FAB for analyzing biomolecules [5-7] and, specifically, saccharides and glycoconjugates [8] are also available. 

Fast Atom Bombardment (FAB) and Liquid-matrix Secondary-ion Mass Spectrometry (LSI MS)... 

Fast atom bombardment, liquid-SIMS (secondary ion mass spectrometry), electrospray (ESI), and matrix assisted laser desorption (MALDI) ionization modes have been applied successfully for the investigations of biomolecules.However, ESI and MALDI are the two most frequently adopted techniques for investigations of biopolymersDetails involving the principles and application of all of these techniques can be found elsewhere. The samples may be introduced either directly or after liquid chromatographic separation. All of the above techniques, with the exception of MALDI, have been adopted for the LC/MS experiments. " Although most of the reported LC/MS investigations involved the electrospray ionization of the molecules, continuous flow-FAB ionization techniques have also been found useful. 

A big step forward came with the discovery that bombardment of a liquid target surface by abeam of fast atoms caused continuous desorption of ions that were characteristic of the liquid. Where this liquid consisted of a sample substance dissolved in a solvent of low volatility (a matrix), both positive and negative molecular or quasi-molecular ions characteristic of the sample were produced. The process quickly became known by the acronym FAB (fast-atom bombardment) and for its then-fabulous results on substances that had hitherto proved intractable. Later, it was found that a primary incident beam of fast ions could be used instead, and a more generally descriptive term, LSIMS (liquid secondary ion mass spectrometry) has come into use. However, note that purists still regard and refer to both FAB and LSIMS as simply facets of the original SIMS. In practice, any of the acronyms can be used, but FAB and LSIMS are more descriptive when referring to the primary atom or ion beam. 

Fast atom bombardment (FAB) Plasma desorption (PD) Liquid secondary-ion mass spectrometry (LSIMS) Thermospray (TSP)/plasmaspray (PSP) Electrohydrodynamic ionisation (EHI) Multiphoton ionisation (MPI) Atmospheric pressure chemical ionisation (APCI) Electrospray ionisation (ESI) Ion spray (ISP) Matrix-assisted laser desorption/ionisation (MALDI) Atmospheric pressure photoionisation (APPI) Triple quadrupole (QQQ) Four sector (EBEB) Hybrid (EBQQ) Hybrid (EB-ToF, Q-ToF) Tandem ToF-ToF Photomultiplier... 

The principle of FAB, less frequently referred to as liquid secondary ionization mass spectrometry (LSIMS), is very similar to secondary-ion mass spectrometry (SIMS). However, FAB utilizes a liquid matrix, such as glycerol, in which a sample is dissolved. The matrix is used to enhance sensitivity and ion current stability. 

In FAB, the sample is usually dispersed in a non-volatile liquid matrix, such as glycerol or diethanolamine, and deposited at the end of a sample probe that can be inserted into the ion source. The sample on the probe is ionised when bombarded by the fast atom beam. However, ionisation of the matrix also occurs, leading to a very large background signal. The technique is thus limited for the analysis of small molecules. Fast-moving ions (Cs+ or Ar+) can be used instead of fast-moving atoms, which is the basis of a technique called liquid secondary ion mass spectrometry (LSIMS). 

SIMS. Secondary Ion Mass Spectrometry is particularly suited for ionization of nonvolatile, polar, and thermally labile molecules. Liquid SIMS, using liquid glycerol matrices, is best done in the differentially-pumped external ion source, because matrix effects and the high vapor pressure of glycerol make liquid SIMS unsuitable for single cell low-pressure FTMS. 

The ionization methods reported for IMS included MALDI [41,76-80], Secondary Ion Mass Spectrometry (SIMS) [19, 81-86], Matrix-enhanced (ME)-SIMS [87, 88], Desorption Electrospray Ionization (DESI) [89-99], Nanostructure Initiator Mass Spectrometry (NIMS) [100-102], Atmospheric Pressure Infrared MALDI Mass Spectrometry (AP-IR-MALDI-MS) [103], Laser Ablation-inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) [104-106], Laser Desorption Postionization (LDPI) [107], Laser Ablation Electrospray Ionization Mass Spectrometry (LAESI) [108, 109], and Surface-assisted Laser Desorption/ioniza-tion Mass Spectrometry (SALDI) [110-112], Another method was called probe electrospray ionization (PESI) that was used for both liquid solution and the direct sampling on wet samples. 

These direct ion sources exist under two types liquid-phase ion sources and solid-state ion sources. In liquid-phase ion sources the analyte is in solution. This solution is introduced, by nebulization, as droplets into the source where ions are produced at atmospheric pressure and focused into the mass spectrometer through some vacuum pumping stages. Electrospray, atmospheric pressure chemical ionization and atmospheric pressure photoionization sources correspond to this type. In solid-state ion sources, the analyte is in an involatile deposit. It is obtained by various preparation methods which frequently involve the introduction of a matrix that can be either a solid or a viscous fluid. This deposit is then irradiated by energetic particles or photons that desorb ions near the surface of the deposit. These ions can be extracted by an electric field and focused towards the analyser. Matrix-assisted laser desorption, secondary ion mass spectrometry, plasma desorption and field desorption sources all use this strategy to produce ions. Fast atom bombardment uses an involatile liquid matrix. 

Liquid Secondary Ion Mass Spectrometry Linear Time of Flight Matrix-Assisted Laser Desorption Matrix-Assisted Laser Desorption/Ionization Molecular Beam... 

A wide variety of desorption ionization methods is available [7] desorption chemical ionization (DCI), secondary-ion mass spectrometry (SIMS), fast-atom bombardment (FAB), liquid-SIMS, plasma desorption (PD), matrix-assisted laser desorption ionization (MALDI), and field desorption (FD). Two processes are important in the ionization mechanism, i.e., the formation of ions in the sample matrix prior to desorption, and rapid evaporation prior to ionization, which can be affected by very rapid heating or by sputtering by high-energy photons or particles. In addition, it is assumed that the energy deposited on the sample surface can cause (gas-phase) ionization reactions to occur near the interface of the solid or liquid and the vacuum (the so-called selvedge) or provide preformed ions in the condensed phase with sufficient kinetic energy to leave their environment. 

Mass spectra were obtained with the following conditions ES (Electrospray) on anLCTOF MICROIVIASS El (Electronic Impact) and D/CI (Desorption/Chemical Ionization ammonia) on a SSQ7000 FINNIGAN LSIMS (Liquid Secondary Ion Mass Spectrometry) with a SCIEX PERKIN ELMER (Cs 35 KeV 3-nitrobenzyl alcohol matrix). 

In recent years, several techniques have been developed for mass spectrometry, whereby samples are ionized and analysed from a condensed phase, without prior volatilization. These desorption techniques have permitted the extension of mass spectrometric analyses to sulfate and glutathione conjugates, as well as to underivatized and labile glucuronic acid conjugates. Primary among these techniques are field desorption 6, plasma desorption (7), laser desorption (8), fast atom bombardment (or secondary ion mass spectrometry with a liquid sample matrix) ( ) and thermospray ionization ( O). The latter can also serve to couple high pressure liquid chromatography and mass spectrometry for analysis of involatile and thermally labile samples. 

Aberfh, W., Straub, K. M., and Burlingame, A. L., Secondary ion mass spectrometry with cesium primary beam and liquid target matrix for analysis of bio-organic compoimds. Anal. Chem., 54, 2029, 1982. 

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