Secondary Ion Generation

Only a small portion of secondary particles ( 1 % of total secondary particles) are ionized and become the secondary ions that are analyzed in SIMS. A sputtered particle faces competition between ionization and neutralization processes when it escapes a sample surface. Ionization probability represents the chance of a sputtered particle being an ion. The ionization probability is strongly affected by the electronic properties of the sample matrix. Ionization directly affects the signal intensity of secondary ions as shown in the basic equation of secondary ion yield. 

The secondary ion current (Im) of chemical species m is collectively determined by the primary ion flux (Ip), the sputter yield (Ym) and the ionization probability. a+ represents the probability for positive ions, 9m is the fractional concentration of species m in the surface layer and rj is the transmission of the detection system. The transmission is defined as the ratio of the ions detected to ions emitted, and it varies from 0 to 1 depending on the analyzer. The sensitivity of SIMS to element m is controlled by the factors Ym, a+ and rj. Ym generally increases with beam energy and with the primary ion mass. It also varies with the types of atomic bonding in target samples. 

The important features of the sputter ionization process can be summarized as follows. First, the point at which the secondary particles escape from the surface is not the point of the initial impact by the primary ion. Instead, there is a surface damage zone that includes the points of primary particle impact and secondary particle escape. Second, the cascade collision results in generation of secondary ions with much lower energy than that of primary ions. Third, there is significant variation in secondary ion yield with chemical elements and chemical states of the surface, which makes quantitative analysis difficult. 

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SIMS is an analytical technique based on the MS analysis of charged species (secondary ions) generated by the interaction between an ion beam (primary ions with an energy between 250 eV and 30 keV) and the surface of a solid sample. The first modern SIMS instruments were developed in 1960s under the NASA supervision and were used to analyze moon rocks [8],... 

In addition to secondary ion generation, SIMS also results in the release of secondary electrons near the sample surface. As previously discussed for SEM analysis, this may result in the buildup of a net electric current for nonconductive surfaces. Analogous to microscopy applications, such surface charging will diffuse the primary beam, making it difficult to perform microanalysis using SIMS. Eur-thermore, charging will deleteriously affect the detection of secondary ions, by... 

Conceptually, SIMS can be considered a straightforward and direct technique. In practice, there are many complexities introduced as a result of the various methodologies that can be applied, whether in the static or in the dynamic mode of SIMS. This exists because there are numerous conditions under which SIMS can be carried out. Each condition is optimized to deal with the analysis of a different elemental or molecular species, from different solid matrices. In addition, relating the output to the compositional variations that may occur on or within the sohd being examined can be problematic. This stems, in part, from the complexities surrounding secondary ion generation, or more precisely, the matrix effect. As the term suggests, the matrix effect describes the effect of the matrix on the population of ions emitted. Matrix effects and their associated transient effects are discussed in Section 3.3.3.1.2. 

Although the dead time is extremely short (typically of the order of 1 ps), its effects will be noted when the frequency of ion impact exceeds this time. For DD-EMs, this frequency is of the order of 1 MHz, or in units of ion impacts per second, of the order of 1 x 10 cps. An example of the effect the dead time can have on a depth profile is illustrated in Figure 4.19. As can be seen, as the count exceeds some value (1 MHz for a DD-EM), the detector no longer can record the secondary ions generated. As a result, the profile is distorted from its actual shape. 

Desorption ionization (DI). General term to encompass the various procedures (e.g., secondary ion mass spectrometry, fast-atom bombardment, californium fission fragment desorption, thermal desorption) in which ions are generated directly from a solid or liquid sample by energy input. Experimental conditions must be clearly stated. 

An important extension of these reactions is the Mannich reaction, in which aminomethyl-ation is achieved by the combination of formaldehyde, a secondary amine and acetic acid (Scheme 24). The intermediate immonium ion generated from formaldehyde, dimethyl-amine and acetic acid is not sufficiently reactive to aminomethylate furan, but it will form substitution products with alkylfurans. The Mannich reaction appears to be still more limited in its application to thiophene chemistry, although 2-aminomethylthiophene has been prepared by reaction of thiophene with formaldehyde and ammonium chloride. The use of A,iV-dimethyf (methylene) ammonium chloride (Me2N=CH2 CF) has been recommended for the iV,iV-dimethylaminomethylation of thiophenes (83S73). 

Computers will be integrated more and more into commercial SEMs and there is an enormous potential for the growth of computer supported applications. At the same time, related instruments will be developed and extended, such as the scanning ion microscope, which uses liquid-metal ion sources to produce finely focused ion beams that can produce SEs and secondary ions for image generation. The contrast mechanisms that are exhibited in these instruments can provide new insights into materials analysis. 

The instrumentation for SSIMS can be divided into two parts (a) the primary ion source in which the primary ions are generated, transported, and focused towards the sample and (b) the mass analyzer in which sputtered secondary ions are extracted, mass separated, and detected. 

What concerns us here are three topics addressing the fates of bromonium ions in solution and details of the mechanism for the addition reaction. In what follows, we will discuss the x-ray structure of the world s only known stable bromonium ion, that of adamantylideneadamantane, (Ad-Ad, 1) and show that it is capable of an extremely rapid degenerate transfer of Br+ in solution to an acceptor olefin. Second, we will discuss the use of secondary a-deuterium kinetic isotope effects (DKie) in mechanistic studies of the addition of Br2 to various deuterated cyclohexenes 2,2. Finally, we will explore the possibility of whether a bromonium ion, generated in solution from the solvolysis of traAU -2-bromo-l-[(trifluoromethanesulfonyl)oxy]cyclohexane 4, can be captured by Br on the Br+ of the bromonium ion, thereby generating olefin and Br2. This process would be... 

Mass Spectrometry. Mass spectrometry holds great promise for low-level toxin detection. Previous studies employed electron impact (El), desorption chemical ionization (DCI), fast atom bombardment (FAB), and cesium ion liquid secondary ion mass spectrometry (LSIMS) to generate positive or negative ion mass spectra (15-17, 21-23). Firm detection limits have yet to be reported for the brevetoxins. Preliminary results from our laboratory demonstrated that levels as low as 500 ng PbTx-2 or PbTx-3 were detected by using ammonia DCI and scans of 500-1000 amu (unpublished data). We expect significant improvement by manipulation of the DCI conditions and selected monitoring of the molecular ion or the ammonia adduction. 

Iminium ions, generated in aqueous solution from secondary amines and formaldehyde, undergo a Barbier-type allylation mediated by tin, aluminum, and zinc. The reaction is catalyzed by copper and produces tertiary homoallylamines in up to 85% yield.67 The imines generated in situ from 2-pyridinecarboxaldehyde/2-quinolinecarboxaldehyde and aryl amines undergo indium-mediated Barbier allylation in aqueous media to provide homoallylic amines.68 Crotyl and cinnamyl bromides... 

Secondary Ion Yields. The most successful calculations of secondary in yields are based on the local thermal equilibrium (LTE) model of Andersen and Hinthorne (1973), which assumes that a plasma in thermodynamic equilibrium is generated locally in the solid by ion bombardment. Assuming equilibrium, the law of mass action can be applied to find the ratio of ions, neutrals and electrons, and the Saha-Eggert equation is derived ... 

At a finite gas pressure and in the condensed phase, secondary ions are generated by reactions of primary ions with neutral gas molecules. 

The Mg isotopic measurements were performed with a modified AEI IM-20 ion microprobe [13,14]. Secondary ions were generated by bombarding the sample with a focussed ion beam to excavate a small volume of the sample. A fraction of the sputtered material is ionized during the sputtering process and is drawn off into the mass spectrometer. A duoplasmatron ion source produces a... 

The generation of large numbers of complex, poly-atomic and multiply charged ions in the sputtering process [15,16,17] creates potentially severe problems for low resolution secondary ion isotopic analysis. To minimize the formation of hydride and... 

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