Ionic emissions

Table 8.62 shows the main characteristics of ICP-MS, which is widely used in routine analytical applications. The ICP ion source has several unique advantages the samples are introduced at atmospheric pressure the degree of ionisation is relatively uniform for all elements and singly charged ions are the principal ion product. Theoretically, 54 elements can be ionised in an ICP with an efficiency of 90 % or more. Even some elements that do not show ionic emission lines should be ionised with reasonable efficiency (namely, As, 52 % and P, 33%) [381]. This is one of the advantages of ICP-MS over ICP-AES. Other features of ICP-MS that make it more attractive than ICP-AES are much lower detection limits ability to provide isotopic ratio information and to offer isotope dilution capabilities for quantitative analysis and clean and simple spectra. The... 

We observe ionic emissions because the temperatures are very high and the plasma is a medium rich in argon ions and in free electrons which provoke ionisation due to collisions with non-ionised atoms. 

Electromagnetic atomic/ionic emission lines complicated... 

The block diagram of a typical ICP emission spectrometer is shown in Figure 28-14. Atomic or ionic emission from the plasma is separated into its constituent wavelengths by the wavelength isolation device. This separation can take place in a... 

At low concentrations, ionization of the analyte can cause nonlinearity in calibration curves. With ICP and DCP sources, the high electron concentrations in the plasma tend to act as a buffer against changes in the extent of ionization of the analyte with concentration. Ionic emission lines are often used with the ICP, and these are not very susceptible to further ionization. Changes in atomizer characteristics (such as flow rate, temperature, and efficiency) with analyte concentration can also be a cause of nonlinearity. 

Inductively coupled plasma (ICP) and direct current plasma (DCP) atomic emission spectrometry have become widely accepted techniques for simultaneous multielemental analysis. These techniques are highly sensitive and have a very wide dynamic range. A wealth of information is contained in the emission signal, including several atomic and ionic emission lines for each element in the sample. In even the simplest sample, there are thousands of observable spectral lines. To make full use of this enormous spectral information the analyst requires an instrument capable of observing a very wide spectral range simultaneously, preferably from 190 nM to 800 nM with a resolution of approximately 0.01 nM. 

The ability to simultaneously examine a wide spectral range opens an entire new realm of possibilities for the analyst. For Instance, if self-absorbtion becomes a problem for an atomic emission line, the possibility now exists to dynamically switch to an ionic emission line. Another possibility is that if a spectral interference becomes a problem at the most sensitive line for a given element, the decision can be made to switch to a less sensitive line, which may have less interference. 

The technique which uses the ionic emission is called photon-stimulated desorption (PSD) The ion current due to PSD is proportional to the number of created core holes., i.e., to the photoabsorption cross section of the absorbate. It is a measurement of the surface absorption with a very high surface contrast in comparison with the previously discussed detection methods. A comparison of different techniques is given by Stohr et al. 

Table 1. Ionic Emissions Identified at Kyushu University...

Qualitatively similar results were obtained by analyzing the fluorescence of a- and /J-naphthol in mixed-solvent water-alcohols with and without added P-CD. The ionic emission of /J-naphthol was quenched the molecular emission of a-naphthol, absent in water-ethanol, grew, owing to the... 

Figure 7.1 Schematic diagram of the excitation and emission process. The energies a and b represent atomic excitation, c represents ionization and d represents ionization and excitation. Four possible emission energies and their respective wavelengths are shown e is ionic emission and /, g, and h are atomic emission. The emission wavelength and energy are related hy AB = hcj. [From Boss and Fredeen, courtesy of PerkinElmer Inc. (www.perkinelmer.com).]...

A related problem is ionization interference. If the analyte atoms are ionized in the flame, they cannot emit atomic emission wavelengths, and a reduction in atomic emission intensity will occur. For example, if potassium is ionized in the flame, it cannot emit at its atomic emission line at 766.5 nm and the sensitivity of the analysis will decrease. If a large amount of a more easily ionized element, such as cesium, is added to the solution, the cesium will ionize preferentially and suppress the ionization of potassium. The potassium ions will capture the electrons released by the cesium, reverting to neutral potassium atoms. The intensity of emission at 766.5 nm will increase for a given amount of potassium in the presence of an excess of cesium. The added cesium is called an ionization suppressant. Ionization interference is a problem with the easily ionized elements of groups 1 and 2. The use of ionization suppressants is recommended for the best sensitivity and accuracy when determining these elements. Of course, as ionization increases, ion emission line intensity increases. It may be possible to use an ionic emission line instead of an atomic emission line for measurements. 

Ablated target constituents can be detected directly in the laser-generated plasma by their atomic and ionic emission... 

Time-gated detectors that allow the optical emission from the laser plasma to be recorded at some time delay after the laser pulse are required to accurately capture the emission spectra. For the first few microseconds after the ignition of the laser spark, the plasma emits a strong white light continuum (also called bremsstrahlung), which decays as the plasma cools the characteristic atomic and ionic emission lines only appear as the plasma cools. A detector delay on the order of several microseconds after the laser pulse is used to eliminate interference from the continuum radiation. The principle is demonstrated in Figure 7.52. 

FIGURE 3.28 Illustration of thermo-ionic-emission (left) and the field-emission (left and right) after (Further Readings on Quantum Solid 1936-1967 Putz, 2006). 

R.L. Kelly, L.J. Palumbo Atomic and ionic emission lines below 2000 Angstroms, hydrogen through krypton. NRL Report 7599 (Naval Research Laboratory, Washington, DC 1973)... 

Detector with thermo-ionic emission Change of emission rate from an oxide-coated cathode... 

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