Mass spectrometers processing

In the mass spectrometer, processes (1) and (2) are comparable for ethyl chloride, but ion formation by process (2) occurs more readily for higher n-alkyl chlorides. Process (1) is of increasing importance for secondary chlorides and most important for tertiary chlorides. It is also of major significance in the case of methyl and aromatic chlorides. 

Siebert P, Petzold G, Hellenbart A, Muller J (1998) Surface microstructure/miniature mass spectrometer processing and applications. Appl Phys A 67 155-160... 

The Data Recorder The data recorder plots the mass spectrum. In the early days this would take the form of a chart recorder. All modern instruments use a specially configured desktop computer system to operate the mass spectrometer, process, record and store the mass spectra for future reference and manipulation. 

Metastable Peaks. If the mass spectrometer has a field-free region between the exit of the ion source and the entrance to the mass analyzer, metastable peaks m may appear as a weak, diffuse (often humped-shape) peak, usually at a nonintegral mass. The one-step decomposition process takes the general form ... 

A laser pulse strikes the surface of a specimen (a), removing material from the first layer, A. The mass spectrometer records the formation of A+ ions (b). As the laser pulses ablate more material, eventually layer B is reached, at which stage A ions begin to decrease in abundance and ions appear instead. The process is repeated when the B/C boundary is reached so that B+ ions disappear from the spectrum and C+ ions appear instead. This method is useful for depth profiling through a specimen, very little of which is needed. In (c), less power is used and the laser beam is directed at different spots across a specimen. Where there is no surface contamination, only B ions appear, but, where there is surface impurity, ions A from the impurity also appear in the spectrum (d). 

It is worth noting that some of these methods are both an inlet system to the mass spectrometer and an ion source at the same time and are not used with conventional ion sources. Thus, with electrospray, the process of removing the liquid phase from the column eluant also produces ions of any emerging mixture components, and these are passed straight to the mass spectrometer analyzer no separate ion source is needed. The particle beam method is different in that the liquid phase is removed, and any residual mixture components are passed into a conventional ion source (often electron ionization). 

This chapter briefly discusses the advantages to be gained from the use of transputers in acquiring and processing data from an instrument like a mass spectrometer, which routinely deals with large-scale input and output at high speed. 

Most mass spectrometers for analytical work have access to a large library of mass spectra of known compounds. These libraries are in a form that can be read immediately by a computer viz., the data corresponding to each spectrum have been compressed into digital form and stored permanently in memory. Each spectrum is stored as a list of m/z values for all peaks that are at least 5% of the height of the largest peak. To speed the search process, a much shorter version of the spectrum is normally examined (e.g., only one peak in every fourteen mass units). 

For the naturally occurring elements, many new artificial isotopes have been made, and these are radioactive. Although these new isotopes can be measured in a mass spectrometer, this process could lead to unacceptable radioactive contamination of the instrument. This practical consideration needs to be considered carefully before using mass spectrometers for radioactive isotope analysis. 

A computer must communicate with a variety of peripheral devices (keyboard, mouse, printer, mass spectrometer). A central processing unit (CPU) controls the flow of information to each, rather like a choreographer directing complicated dance routines. 

Powerful mass spectrometer/computer systems can achieve simultaneous foreground/background operation, especially if transputers are used to provide the advantage of parallel processing. 

A mass spectrometer consists of four basic parts a sample inlet system, an ion source, a means of separating ions according to the mass-to-charge ratios, ie, a mass analyzer, and an ion detection system. AdditionaUy, modem instmments are usuaUy suppUed with a data system for instmment control, data acquisition, and data processing. Only a limited number of combinations of these four parts are compatible and thus available commercially (Table 1). 

The primary process of SiH decomposition is electron impact which produces a large number of different neutral and ionic species as shown in Table 1. The density of S1H2 and SiH neutral species produced has been found to be much larger than the density of the ions. For example, mass spectrometric data for silane discharges indicate that the density of ionic species is lower by 10 compared with the density of neutral species. Further, mass spectrometer signals of ionic species, such as SiH SiH 25 SiH", SiH", and Si2H , increase by more than two orders of magnitude as the r-f power is increased, eg, from 2 to 20 W. A rapid rise in the population of ions, with power, implicitly means an increase in electron density. 

Mass Spectrometer. The mass spectrometer is the principal analytical tool of direct process control for the estimation of tritium. Gas samples are taken from several process points and analy2ed rapidly and continually to ensure proper operation of the system. Mass spectrometry is particularly useful in the detection of diatomic hydrogen species such as HD, HT, and DT. Mass spectrometric detection of helium-3 formed by radioactive decay of tritium is still another way to detect low levels of tritium (65). Accelerator mass spectroscopy (ams) has also been used for the detection of tritium and carbon-14 at extremely low levels. The principal appHcation of ams as of this writing has been in archeology and the geosciences, but this technique is expected to faciUtate the use of tritium in biomedical research, various clinical appHcations, and in environmental investigations (66). 

In the process of SNMS analysis, sputtered atoms are ionized while passii through the ionizer and are accelerated into the mass spectrometer for mass analysis. The ion currents of the analyzed ions are measured and recorded as a function of mass while stepping the mass spectrometer through the desired mass or element sequence. If the purpose of the analysis is to develop a depth profile to characterize the surface and subsurface regions of the sample, the selected sequence is repeated a number of times to record the variation in ion current of a selected elemental isotope as the sample surfiice is sputtered away. 

Figure 1 Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

Demonstration of GDMS feasibility and research into glow-discharge processes has been carried out almost exclusively using the combination of a glow-discharge ion source with a quadrupole mass spectrometer (GDQMS). The combination is inexpensive, readily available and suitable for such purposes. In addition, the quadru-... 

---