Mass spectrometers negative ions

Figure 22-12 Electrostatic sector of a double-focusing mass spectrometer. Positive ions are attracted toward the negative plate. Trajectories of high-energy ions are changed less than trajectories of low-energy ions. Ions reaching the exit slit have a narrow range of kinetic energies.

Secondary ion sputtering. The surface of a solid sample is sprayed with a primary ion beam (usually negative oxygen ions or positive caesium ions), producing secondary ions (from the sample), which are sputtered into the mass spectrometer. SIMS (ion probe) is particularly useful for obtaining in situ measurements. While SIMS has been widely used to measure element concentrations in biogenic material, there are only a handful of reports on its application to the study of isotope ratios to date (e.g. Aleon et al. 2001). 

The species initially produced in the mass spectrometer are ions derived from normal molecules by loss of an electron. Such an ion will normally contain an odd number of electrons and is consequently described as a radical ion. The same is true of the negative radical ion formed from a neutral molecule by capture of an electron [equation (7.3)]. The relationships between a neutral molecule M, the derived radical ions and M", and the lowest singlet excited state M are shown in Fig. 7.5. 

NMR Spectroscopy Ionization Theory Ion Molecule Reactions in Mass Spectrometry Negative Ion Mass Spectrometry, Methods Photoionization and Photodissociation Methods in Mass Spectrometry Quadru-poles. Use of in Mass Spectrometry Time of Flight Mass Spectrometers. 

Ar, Cs, Ga or other elements with energies between 0.5 and 10 keV), energy is deposited in the surface region of the sample by a collisional cascade. Some of the energy will return to the surface and stimulate the ejection of atoms, ions and multi-atomic clusters (figure Bl.25.8). In SIMS, secondary ions (positive or negative) are detected directly with a mass spectrometer. 

Cheshnovsky O, Yang S H, Pettiette C L, Craycraft M J and Smalley R E 1987 Magnetic time-of-flight photoeieotron spectrometer for mass-selected negative cluster ions Rev. Sci. Instnim. 58 2131... 

A positive ion formed at a positive electrode tip is repelled and travels toward the negative counter electrode, which has a slit in it so that the ion can pass into the mass spectrometer. 

By passing a continuous flow of solvent (admixed with a matrix material) from an LC column to a target area on the end of a probe tip and then bombarding the target with fast atoms or ions, secondary positive or negative ions are ejected from the surface of the liquid. These ions are then extracted into the analyzer of a mass spectrometer for measurement of a mass spectrum. As mixture components emerge from the LC column, their mass spectra are obtained. 

A positive ion formed on such a tip held at a high positive potential is repelled and flies olf the tip almost immediately after formation and into the mass spectrometer, where its m/z value is measured. Similarly, negative ions can be mass measured. 

In field ionization (or field desorption), application of a large electric potential to a surface of high curvature allows a very intense electric field to be generated. Such positive or negative fields lead to electrons being stripped from or added to molecules lying on the surface. The positive or negative molecular ions so produced are mass measured by the mass spectrometer. 

By selecting either a large positive or negative electrical potential on a plate, with a slit in it, held above the target area, the desorbed negative or positive ions are extracted into the analyzer of the mass spectrometer. 

Static SIMS entails the bombardment of a sample surface with an energetic beam of particles, resulting in the emission of surface atoms and clusters. These ejected species subsequendy become either positively or negatively charged and are referred to as secondary ions. The secondary ions are the actual analytical signal in SIMS. A mass spectrometer is used to separate the secondary ions with respect to their charge-to-mass ratios. The atomic ions give an elemental identification (see... 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

The material evaporated by the laser pulse is representative of the composition of the solid, however the ion signals that are actually measured by the mass spectrometer must be interpreted in the light of different ionization efficiencies. A comprehensive model for ion formation from solids under typical LIMS conditions does not exist, but we are able to estimate that under high laser irradiance conditions (>10 W/cm ) the detection limits vary from approximately 1 ppm atomic for easily ionized elements (such as the alkalis, in positive-ion spectroscopy, or the halogens, in negative-ion spectroscopy) to 100—200 ppm atomic for elements with poor ion yields (for example, Zn or As). 

In secondary ion mass spectrometry (SIMS), a beam of energetic primary ions is focused onto the surface of a solid. Some of the ions are reflected but most of the energy of the primary ions is dissipated in the surface by binary collisions that cause neutrals, excited neutrals, and ions (positive and negative) to be ejected or sputtered from the surface. The secondary ions can be analyzed by a mass spectrometer to provide information about the surface composition of the solid. 

Reactions of D with D20 and of 0 with 02, N20, and N02 have been studied with a magnetic sector mass spectrometer. Competition between electron transfer and ion-atom interchange has been observed in the production of 02 by reaction of 0 with 02, an endothermic reaction. The negative ion of the reacting neutral molecule is formed in 02, N2Of and N02 but not in D20. Rate constants have been estimated as a function of repeller potential. 

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