Instrument radiofrequency

In two other implementations of electron impact SNMS, a plasma is generated in the ionizer volume to provide an electron gas sufFiciendy dense and energetic for efficient postionization (Figure 2c). In one instrument, the electrons are a component of a low-pressure radiofrequency (RF) plasma in Ar, and in the second, the plasma is an electron beam excited plasma, also in Ar. The latter type of electron-gas SNMS is still in the developmental stages, while the former has been incorporated into commercial instmmentation. 

The basic components of the solid state spectrometer are the same as the solution-phase instrument data system, pulse programmer, observe and decoupler transmitters, magnetic system, and probes. In addition, high-power amplifiers are required for the two transmitters and a pneumatic spinning unit to achieve the necessary spin rates for MAS. Normally, the observe transmitter for 13C work requires broadband amplification of approximately 400 W of power for a 5.87-T, 250-MHz instrument. The amplifier should have triggering capabilities so that only the radiofrequency (rf) pulse is amplified. This will minimize noise contributions to the measured spectrum. So that the Hartmann-Hahn condition may be achieved, the decoupler amplifier must produce an rf signal at one-fourth the power level of the observe channel for carbon work. 

A schematic diagram of an ICP-MS instrument is shown in Fig. 5.1. The TCP part bears an almost exact resemblance to the ICP used for atomic emission spectrometry, with the obvious exception that it is turned on one side. Indeed, sample introduction systems, radiofrequency generators and the nature of ICP itself are often the same for ICP-MS and ICP-AES systems, with the usual variations between individual manufacturers. 

The newer instruments (Figure 2.4c) utilize a radiofrequency pulse in place of the scan. The pulse brings all of the cycloidal frequencies into resonance simultaneously to yield a signal as an interferogram (a time-domain spectrum). This is converted by Fourier Transform to a frequency-domain spectrum, which then yields the conventional m/z spectrum. Pulsed Fourier transform spectrometry applied to nuclear magnetic resonance spectrometry is explained in Chapters 4 and 5. 

Figure 5.29 Instrumental setup of a radiofrequency (rf) glow discharge mass spectrometer by combining an ion source developed in house with a commercial double-focusing sector field mass spectrometer with reverse Nier-Johnson geometry (Element, Thermo Fisher Scientific, Bremen, Germany), (f S. Becker et al., IntJ. Mass Spectrom., 164, 81 (1997). Produced by permission of Elsevier.)...

For a given ICP-OES instrument, the intensity of an analyte line is a complex function of several factors. Some adjustable parameters that affect the ICP source are the radiofrequency power coupled into the plasma (usually about 1 kW), the gas flow rates, the observation height in the lateral-viewing mode and the solution uptake rate of the nebuliser. Many of these factors interact in a complex fashion and their combined effects are different for dissimilar spectral lines. The selection of an appropriate combination of these factors is of critical importance in ICP-OES. This issue will be addressed in Chapter 2, where experimental designs and optimisation procedures will be discussed. Many examples related to ICP and other atomic spectrometric techniques will be presented. 

Figure 16.23—Triple quadrupote MS MS instrument. In the triple qnadrupole arrangement, the middle quadrupole is used as a collision chamber. It is operated in the radiofrequency voltage mode only, where it will transmit all masses. A gas pressure introduced in the second quadrupole is responsible for collision activation. Triple quadrupole instruments can conduct all three types of MS — MS analysis described above.

NMR instruments use either constant magnetic fields and variable radiofrequencies or constant frequencies and variable magnetic fields, but the abscissa of NMR spectra is always measured in units of frequency (cps) in relation to an added reference compound such as tetramethyl-silane (TMS) and divided by the radiofrequency of the instrument to yield dimensionless numbers on either the 8-scale, which sets the reference signal equal to zero, or on the T-scale, which sets it equal to 10 thus 8 -f- t = 10. The spectra in this report use the 8 scale and show the reference signal of TMS at 8 = 0. 

Identification of Minerals in Coal. Once the low-temperature mineral matter residue has been obtained by radiofrequency ashing, the minerals can be identified, and their concentrations can be determined by a variety of instrumental techniques. The best developed, most inclusive, and probably most reliable method used thus far in distinguishing minerals in coal is x-ray diffraction analysis. It has been used extensively by Gluskoter (15), Wolfe (17), O Gorman and Walker (2), and Rao and Gluskoter (1) and has been somewhat successful in quantifying mineral analyses. 

MICROWAVE SPECTROSCOPY. A type of adsorption spectroscopy used in instrumental chemical analysis that involves use of that portion of the electromagnetic spectrum hav ing wavelengths in the range between the far infrared and the radiofrequencies, i.e.. between 1 nun and. 111 cm. Substances to be analyzed are usually in the gaseous state. Klystron tubes are used as microwave source. 

An intermediate level of sophistication was achieved by semiautomated sorting of radiofrequency tagged containers (e.g., MicroKans,8 Lanterns11). A simple instrument that integrates the process of one-dimensional necklace encoding by creating a linear sequences of solid-phase particles is the Encore synthesizer. 

High-power Radiofrequency Electronics. - The system must produce and transmit r.f. power levels of several hundreds of watts, and the probe circuit must not arc during the high-power pulses. Modern instrumentation is designed to conform to the necessary standards. 

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