Excitation sources radiofrequency excited

The essential features of an NMR spectrometer shown m Figure 13 5 are not hard to understand They consist of a magnet to align the nuclear spins a radiofrequency (rf) transmitter as a source of energy to excite a nucleus from its lowest energy state to the next higher one a receiver to detect the absorption of rf radiation and a recorder to print out the spectrum... 

Flames and plasmas can be used as atomisation/excitation sources in OES. Electrically generated plasmas produce flame-like atomisers with significantly higher temperatures and less reactive chemical environments compared with flames. The plasmas are energised with high-frequency electromagnetic fields (radiofrequency or microwave energy) or with direct current. By far the most common plasma used in combination with OES for analytical purposes is the inductively coupled plasma (ICP). 

The excitation source comprising the plasma torch, the induction coil connected to a radiofrequency generator and a nebuliser or system for sample introduction... 

An important development in the phase-shift technique is the use of a radiofrequency synthesizer as the driver for the Pockels cell modulator. In this way, the excitation beam can be modulated at any frequency between 1 and 200 MHz [137-139]. This approach allows use of cw lasers such as the He-Cd laser and even mode-locked lasers [139] as the excitation source. If d and M are measured at six to ten suitably spaced frequencies, least-squares curve-fitting techniques can be employed to obtain lifetimes with greatly enhanced precision. Typical data obtained by this multifrequency technique make measurement of decay times as short as 10 ps possible. Gratton and coworkers have developed other curve-fitting procedures to analyze data obtained on a multifrequency phase-shift fluorimeter. These experiments include the construction of time-resolved spectra [140], measurements of ro-... 

The radiofrequency transmitter is the part of the spectrometer which generates the pulses. We start with an RF source which produces a stable frequency which can be set precisely. The reason why we need to be able to set the frequency is that we might want to move the transmitter to different parts of the spectrum, for example if we are doing experiments involving selective excitation (section 3.11). 

The determined frequency is not the exact frequency emitted by the methyl hydrogens. Due to the design of the instrument, the basic frequency of the pulse is not the same as the frequency of the acetone resonance. The observed FID is actually an interference signal between the radiofrequency source (300 MHz in this case) and the frequency emitted by the excited nucleus, where the wavelength is given by... 

A plasma may be defined as a gas containing a relatively large number of ions and free electrons. To produce a plasma, an energy source is required and for analytical atomic spectroscopy three different excitation methods have been used. They are (1) a dc arc, (2) radiofrequency energy coupled through a microwave cavity, and (3) radiofrequency energy inductively coupled to the plasma. 

Plasma sources, dc excited or excited with radiofrequency energy, provide temperatures from 6000 to 10,000°K and are very stable when compared with the dc arc. They are usually used with liquid samples, which are aspirated into the source continually during the excitation period. The problem of fractional distillation thus is avoided and the source is very stable. Dickinson and FasseF devised a system to introduce a solid sample into a plasma by vaporizing the sample from an electrically heated tantalum boat. 

While this technique has found its widest application in conjunction with field ionization detection, it is worth noting that it is equally useful in optical detection. In Eq. (23) we noted that with a cw rf source the optical signal corresponding to radiofrequency transition depends upon the relative radiative decay rates of the two levels, and the sensitivity decreases as the radiative decay rate of the final state. If the final state decays more slowly than the initial state, the signal is nearly proportional to the final state decay rate. In fact, if the pulsed laser excitation is used, and the rf is turned off... 

Inductively coupled plasma-mass spectrometry is a very rapid technique for the determination of long-lived radionuclides. This technique is based on the ionization of elements in the plasma source. Typically, radiofrequency and argon are used to reach plasma excitation temperatures ranging from 4900 to 7000 K [18,19]. The ions produced are introduced through an interface into a vacuum chamber and are analyzed by a quadru-pole mass spectrometer. Other attempts are being made to use faster mass-spectrometer detectors, such as time-of-flight mass spectrometers, but methods are still not available. 

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