Process spectrometry

Separation of families by merely increasing the resolution evidently can not be used when the two chemical families have the same molecular formula. This is particularly true for naphthenes and olefins of the formula, C H2 , which also happen to have very similar fragmentation patterns. Resolution of these two molecular types is one of the problems not yet solved by mass spectrometry, despite the efforts of numerous laboratories motivated by the refiner s major interest in being able to make the distinction. Olefins are in fact abundantly present in the products from conversion processes. 

Lover T efa/1997 Electrospray mass spectrometry of thiophenolate-capped clusters of CdS, CdSe and ZnS and cadmium and zinc thiophenolate complexes observation of fragmentation and metal, chalcogenide and ligand exchange processes Inorg. Chem. 36 3711... 

Mass Spectrometry Aside from a peak for the molecular ion which is normally easy to pick out aliphatic carboxylic acids undergo a variety of fragmentation processes The dominant fragmentation m aromatic acids corresponds to loss of OH then loss of CO... 

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. 

Although there has been some controversy concerning the processes involved in field ionization mass spectrometry, the general principles appear to be understood. Firstly, the ionization process itself produces little excess of vibrational and rotational energy in the ions, and, consequently, fragmentation is limited or nonexistent. This ionization process is one of the mild or soft methods available for producing excellent molecular mass information. The initially formed ions are either simple radical cations or radical anions (M ). 

The various stages of this process depend critically on the type of gas, its pressure, and the configuration of the electrodes. (Their distance apart and their shapes control the size and shape of the applied electric field.) By controlling the various parameters, the discharge can be made to operate as a corona, a plasma, or an arc at atmospheric pressure. All three discharges can be used as ion sources in mass spectrometry. 

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

Plasma torches and thermal ionization sources break down the substances into atoms and ionized atoms. Both are used for measurement of accurate isotope ratios. In the breakdown process, all structural information is lost, other than an identification of elements present (e.g., as in inductively coupled mass spectrometry, ICP/MS). 

Confusingly, FAB is sometimes called secondary ion mass spectrometry (SIMS), the secondary referring to the nature of the process (primary bombardment, secondary emission), but see next item. 

All three types of discharge involve the formation of ions as part of the process. For various reasons, most of the ions are positive. The ions can be examined by mass spectrometry. If small amounts of a sample substance are introduced into a corona or plasma or arc, ions are formed by the electrons present in the discharge or by collision with ions of the discharge gas. 

This is the basic process in an inductively coupled plasma discharge (ICP). The excited ions can be examined by observing the emitted light or by mass spectrometry. Since the molecules have been broken down into their constituent atoms (as ions) including isotopes, these can be identified and quantified by mass spectrometry, as happens with isotope ratio measurements. 

In mass spectrometry, the required connectivities (links) can be obtained from observations on metastable ions. For example, finding a metastable ion for a supposed process, M+ F3+, proves the process exists. 

It is recommended that the convention used by Budzikiewicz, Djerassi, and Williams (Mass Spectrometry of Organic Compounds, Holden-Day, 1967, p. 2) be followed in referring to a-cleavage as fission of a bond originating at an atom which is adjacent to the one assumed to bear the charge the definition of p-, y-, then follows automatically. The process ... 

Chromatographic Science European Spectroscopy News European Mass Spectrometry International Journal of Mass Spectrometry International Journal of Mass Spectrometry and Ion Processes (formerly Journal of Mass Spectrometry and Ion Physics)... 

Process monitoring, both in the laboratory and on the production line, is an important appHcation of infrared spectrometry (see Process control). 

The environmental appHcations of infrared spectrometry are many and varied. Many appHcations at industrial sites are analogous to those for on-line process analysis waste streams and recycling processes can be monitored in the same way. Commercial infrared stack-gas monitors are based on either an extractive probe attached to a long-path gas ceU or an open-path (across stack) configuration (69). Stack plume and flare monitoring can be done externally... 

Fig. 4. Examples of emission spectrometry as a diagnostic monitoring tool for plasma processing, (a) The removal of chlorine contamination from copper diode leads using a hydrogen—nitrogen plasma. Emissions are added together from several wavelengths, (b) The etching and eventual removal of a 50-p.m thick polyimide layer from an aluminum substrate, where (x ) and (° ) correspond to wavelengths (519.82 and 561.02 nm, respectively) for molecular CO2...

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