Spectrometer power

A computer attached to a mass spectrometer is used both to acquire data and to control the operation of the spectrometer. Powerful transputer systems can be used to ensure that both modes of operation can be carried out almost simultaneously. 

The i-LAB Visible Hand Held Analyzing Spectrometer from Microspectral Analysis, LLC (www.microspectralanalysis.com) is a portable miniaturized spectrometer powered by 3 AA batteries that covers the 400-700 nm region and weighs only 200 g (Figure 5.37). The instrument has a bandwidth of 4-7 nm, spectral resolution of 1.4 nm, and no moving parts. 

For the highest resolution and sensitivity, laser-based spectrometers must be used. These have tire advantage that the resolution depends on the linewidth of the laser, rather than the monocln-omator. Furthennore, at any given moment, all of the power is at the frequency of mterest, rather than being spread out over the whole IR... 

Probably the simplest mass spectrometer is the time-of-fiight (TOP) instrument [36]. Aside from magnetic deflection instruments, these were among the first mass spectrometers developed. The mass range is theoretically infinite, though in practice there are upper limits that are governed by electronics and ion source considerations. In chemical physics and physical chemistry, TOP instniments often are operated at lower resolving power than analytical instniments. Because of their simplicity, they have been used in many spectroscopic apparatus as detectors for electrons and ions. Many of these teclmiques are included as chapters unto themselves in this book, and they will only be briefly described here. 

The low MW power levels conuuonly employed in TREPR spectroscopy do not require any precautions to avoid detector overload and, therefore, the fiill time development of the transient magnetization is obtained undiminished by any MW detection deadtime. (3) Standard CW EPR equipment can be used for TREPR requiring only moderate efforts to adapt the MW detection part of the spectrometer for the observation of the transient response to a pulsed light excitation with high time resolution. (4) TREPR spectroscopy proved to be a suitable teclmique for observing a variety of spin coherence phenomena, such as transient nutations [16], quantum beats [17] and nuclear modulations [18], that have been usefi.il to interpret EPR data on light-mduced spm-correlated radical pairs. 

The design of a pulsed EPR spectrometer depends heavily on tlie required pulse lengdi and pulse power which in turn are mainly dictated by the relaxation times of tlie paramagnetic species to be studied, but also by the type of experiment perfomied. When pulses of the order of a few nanoseconds are required (either to compete... 

Pulsed, or time-domain, EPR spectrometers have also been developed at higher frequencies up to 140 GHz [55. 56]. They are generally low-power units with characteristically long pulse lengths (typically 50 ns for a n/2-pulse) due to tire limited MW powers available at millimetre wavelengths and the lack of fast-switching... 

Analytical chemistry has in recent years been equipped with a number of powerful means of investigation. Their application, especially that of gas-phase chromatography coupled with a mass spectrometer, has demonstrated the presence of a certain number of thiazoles in natural products such as fruits or cereals (287. 288, 297). The many results are shown in Table III-59. 

Although GGMS is the most widely used ana lytical method that combines a chromatographic sep aration with the identification power of mass spectrometry it is not the only one Chemists have coupled mass spectrometers to most of the mstru ments that are used to separate mixtures Perhaps the ultimate is mass spectrometry/mass spectrome try (MS/MS) m which one mass spectrometer gener ates and separates the molecular ions of the components of a mixture and a second mass spec trometer examines their fragmentation patterns ... 

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). 

If a sample solution is introduced into the center of the plasma, the constituent molecules are bombarded by the energetic atoms, ions, electrons, and even photons from the plasma itself. Under these vigorous conditions, sample molecules are both ionized and fragmented repeatedly until only their constituent elemental atoms or ions survive. The ions are drawn off into a mass analyzer for measurement of abundances and mJz values. Plasma torches provide a powerful method for introducing and ionizing a wide range of sample types into a mass spectrometer (inductively coupled plasma mass spectrometry, ICP/MS). 

By being able to obtain an unequivocal relative molecular mass, or even a molecular formula derived from that mass, the hybrid mass spectrometer becomes a powerful tool for investigating single substances or mixtures of substances. With an APCI inlet, fragmentation can be induced to obtain structural information (see Chapter 9). 

Mixtures passed through special columns (chromatography) in the gas phase (GC) or liquid phase (LC) can be separated into their individual components and analyzed qualitatively and/or quantitatively. Both GC and LC analyzers can be directly coupled to mass spectrometers, a powerful combination that can simultaneously separate and identify components of mixtures. 

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

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