Liquid SIMS

OPTS (Optim i/.ed Potentials for Liquid Simulations) is based on a force field developed by the research group of Bill Jorgensen now at Yale University and previously at Purdue University. Like AMBER, the OPLS force field is designed for calculations on proteins an d nucleic acids. It in troduces non bonded in leraclion parameters that have been carefully developed from extensive Monte Carlo liquid sim u lation s of small molecules. These n on-bonded interactions have been added to the bonding interactions of AMBER to produce a new force field that is expected to be better than AMBER at describing simulations w here the solvent isexplic-... 

Aberth, W. and Burlingame, A.L., "Use of a Cesium Primary Beam for Liquid SIMS Analysis of Bio-organic Compounds", Springer Ser., Chem. Phys., 25, pp. 167-71, 1983. 

Ionization Methods/Processes. The recent development of several new ionization methods in mass spectrometry has significantly improved the capability for the analysis of nonvolatile and thermally labile molecules [18-23]. Several of these methods (e.g., field desorption (FD), Californiun-252 plasma desorption (PD), fast heavy ion induced desorption (FHIID), laser-desorption (LD), SIMS, and fast atom bombardment (FAB) or liquid SIMS) desorb and ionize molecules directly from the solid state, thereby reducing the chance of thermal degradation. Although these methods employ fundamentally different excitation sources, similarities in their mass spectra, such as, the appearance of protonated, deprotonated, and/or cationized molecular ions, suggest a related ionization process. 

The recent use of liquid matrices in SIMS has led to several significant accomplishments, particularly for the analysis of biomolecules by FAB mass spectrometry [101]. In the FAB/liquid SIMS technique, the analyte is dissolved in a liquid matrix, such as glycerol, at some optimum concentration in order to provide a surface which, during particle bombardment, is constantly replenished with sample molecules that diffuse from the bulk to the surface. The mobility of the sample molecules in the liquid matrix is an important property of this type of matrix. 

Selected topics in Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry instrumentation are discussed in depth, and numerous analytical application examples are given. In particular, optimization ofthe single-cell FTMS design and some of its analytical applications, like pulsed-valve Cl and CID, static SIMS, and ion clustering reactions are described. Magnet requirements and the software used in advanced FTICR mass spectrometers are considered. Implementation and advantages of an external differentially-pumped ion source for LD, GC/MS, liquid SIMS, FAB and LC/MS are discussed in detail, and an attempt is made to anticipate future developments in FTMS instrumentation. 

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. 

With the successful implementation of differentially-pumped external ion sources, FTMS is rapidly becoming a routine mass spectrometric technique. Medium-pressure interfaces for the coupling of GC, LC, FAB, and liquid SIMS into the external ionizer are currently under development, and should become available in the near future. 

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. 

The liquid matrix provides continuous surface renewal, so fhaf infense primary beams can be used fo produce intense and long-lasting spectra. Further, the ion source is at ambient temperature, preventing the thermal degradation of labile compoxmds. FAB and liquid SIMS are, however, limited to polar polymers that are miscible with the polar liquid matrices necessary... 

Ionization of condensed-phase analytes occurs by mixing a sample in a suitable matrix and bombarding the matrix-analyte mixture with an energetic beam made of either laser photons as in MALDI, high-energy fission particles as in Cf plasma desorption, or high-energy fast atoms or ions (FAB or liquid SIMS). When an analyte is present in a solution, such as an effluent from a separation device, it can be ionized via thermospray ionization, atmospheric-pressure chemical ionization, atmospheric-pressure photoionization, or electrospray ionization. Desorption electrospray ionization and direct analysis in real time are new modes of ionization that are accomplished in ambient air. 

The high voltages used to generate the primary beam and hence the secondary beam plus the high concentration of sputtered neutral molecules made magnetic sector instruments the most ideal mass analyzer to use with liquid SIMS sources. Linear... 

The use of ion beams precluded the use of the term FAB and the family of techniques became known as liquid SIMS. 

The liquid SIMS system comprises a target stage mounted on the tip of an introduction probe, a source for the primary ionization beam (commonly known as a gun ), and a source comprising beam focusing, extraction, and exit lenses to shape the secondary ion beam and inject it into the mass analyzer (Figure 1). 

The elegance of liquid SIMS is that after such disruptive events the surface is restored by the flow of liquid matrix. Further, the conducting properties... 

From the start liquid SIMS was used for the analysis of important biological molecules that, due to their polarity or their mass, were not amenable to existing mass spectral methods. This was exemplified in Barber s original presentation at the Chemical Society Symposium on Soft Ionization Biological Mass... 

Hgure 5 The fragment ions formed by cleavages involving the peptide bond from protonated peptides following ionization using a liquid SIMS source. 

The revolution in the use of mass spectrometry was extended to other biopolymers, especially sugars. Most enzymes consist of a protein molecule that is modified by the attachment of polysaccharides. The structure of these sugar residues can be vital for the activity of the enzyme and liquid SIMS was used, often in the negative ion mode, to study these. An example showing the fragmentation of two isomeric sugars is shown in Figure 7. 

Although the combination of liquid SIMS and magnetic sector mass spectrometry gave rise to the... 

For this in situ identification method especially FAB (Fast Atom Bombardment), Liquid SIMS (Secondary-Ion MS), or laser desorption are employed as ionization techniques (49,50). The analytes are sputtered from the TLC foil (Fig. 26 (51)), or the TLC plate is placed on a movable table. More details are described in Chapter 9, Thin Layer Chromatography Coupled with Mass Spectrometry. 

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