Optical atomic spectroscopy

Various techniques can be used for quantitative analysis of chemical composition, including (i) optical atomic spectroscopy (atomic absorption, atomic emission, and atomic fluorescence), (ii) X-ray fluorescence spectroscopy, (iii) mass spectrometry, (iv) electrochemistry, and (v) nuclear and radioisotope analysis [41]. Among these, optical atomic spectroscopy, involving atomic absorption (AA) or atomic emission (AE), has been the most widely used for chemical analysis of ceramic powders. It can be used to determine the contents of both major and minor elements, as well as trace elements, because of its high precision and low detection limits. 

X-ray fluorescence spectroscopy (XRF) is another measurement technique for major and minor elemental concentrations, as well as trace elements. However, it has very low sensitivity to elements with atomic number Z < 9. Optical atomic spectroscopy is suitable to analyze solutions, which requires complete dissolution... 

As mentioned earlier, optical atomic spectroscopy is only able to analyze solution sample. As a result, ceramic powders to be tested should be made into solution. The solution is then broken into line droplets and vaporized into individual atoms by heating, which is the step critical to the precision and accuracy of the analysis. Flame is generally used to vaporize the solution, which is therefore also known as flame atomic absorption spectrometry or flame AA. 

Several techniques are currently available for the quantitative analysis of chemical composition, including (1) optical atomic spectroscopy (atomic absorption, atomic emission, and atomic fluorescence), (2) x-ray fluorescence spectros-... 

X-ray fluorescence spectroscopy (XRF) has been used for many years for the measurement of major and minor elemental concentrations. Improvements made in the sensitivity in recent years now allow the technique to be applied to the analysis of trace elements. A problem is that the sensitivity of XRF decreases dramatically for lower atomic number elements so the technique cannot be used for quantification of elements with atomic numbCT Z < 9. When compared to XRF, modern AA or AE instruments are simpla- to use, less expensive, have similar precision of analysis and have bettCT sensitivity. Optical atomic spectroscopy is ideally suited to the analysis of solutions so the method requires complete dissolution of the powder in a liquid. In comparison, XRE is ideally suited to the analysis of solid samples, and this can be a distinct advantage for cCTamic powders that are commonly difficult to dissolve. Table 3.7 includes a summary of the main features of optical atomic spectroscopy and XRF. 

Optical Atomic Spectroscopy Atomic Absorption and Atomic Emission... 

Figure 3.14 Schematic showing the transitions in optical atomic spectroscopy.

For the analysis of ceramic powders by optical atomic specfroscopy, a portion of the powder has to be converted into individual atoms. In practice, this is achieved by dissolving the powder in a liquid to form a solution, which is then broken into fine droplets and vaporized into individual atoms by heating. The precision and accuracy of optical atomic spectroscopy are critically dependent on this step. Vaporization is most commonly achieved by introducing droplets into a flame (referred to as flame atomic absorption spectrometry or flame AA). Key problems with flame AA include incomplete dissociation of the more refractory elements (e.g., B, V, Ta, and W) in the flame and difficulties in determining elements that have resonance lines in the far ultraviolet region (e.g., P, S, and the halogens). While flame AA is rapid, the instruments are rarely automated to permit simultaneous analysis of several elements. 

Atomic x-ray fluorescence spectroscopy (17,18), referred to simply as x-ray fluorescence spectroscopy (XRF), is a technique for qualitatively or quantitatively determining elemental composition by measurement of the wavelength and intensities of the electron transitions from the outer to the inner energy levels of the atom. The energy associated with these transitions (—0.6-60 keV) is significantly greater than that associated with the transitions in optical atomic spectroscopy (approximately a few electron volts). The emitted x-ray spectrum does not depend on the way in which the atomic excitation is produced, but in XRF, a beam of energetic x-rays is used. 

Spectra such as that in Figure Il-I5b led early workers in the field of ICPMS to have hopes of an interference-free method. Unfortunately, this hope was not realized in further studies, and serious interference problems are sometimes encountered in atomic mass spectrometry just as in optical atomic spectroscopy. 

During sputtering, the atoms can then be collisionally excited. These collisions may be with ions, electrons, or other atoms that have been previously excited by collisions with ions, electrons, or atoms. Once excited, atoms lose their energy very quickly. In optical atomic spectroscopy, the wavelength of this photon can be used to determine the identity of the atom, and the number of photons is directly proportional to the concentration of that element in the sample. Some collisions, which are of high energy, result in ionization. By atomic mass spectrometry, these ions are detected. Their mass identifies the type of atoms, and their quantity reveals the amount of that element in the sample. 

Analyses of alloys or ores for hafnium by plasma emission atomic absorption spectroscopy, optical emission spectroscopy (qv), mass spectrometry (qv), x-ray spectroscopy (see X-ray technology), and neutron activation are possible without prior separation of hafnium (19). Alternatively, the combined hafnium and zirconium content can be separated from the sample by fusing the sample with sodium hydroxide, separating silica if present, and precipitating with mandelic acid from a dilute hydrochloric acid solution (20). The precipitate is ignited to oxide which is analy2ed by x-ray or emission spectroscopy to determine the relative proportion of each oxide. 

Aluminum is best detected quaUtatively by optical emission spectroscopy. SoHds can be vaporized direcdy in a d-c arc and solutions can be dried on a carbon electrode. Alternatively, aluminum can be detected by plasma emission spectroscopy using an inductively coupled argon plasma or a d-c plasma. Atomic absorption using an aluminum hoUow cathode lamp is also an unambiguous and sensitive quaUtative method for determining alurninum. 

Tungsten is usually identified by atomic spectroscopy. Using optical emission spectroscopy, tungsten in ores can be detected at concentrations of 0.05—0.1%, whereas x-ray spectroscopy detects 0.5—1.0%. ScheeHte in rock formations can be identified by its luminescence under ultraviolet excitation. In a wet-chemical identification method, the ore is fired with sodium carbonate and then treated with hydrochloric acid addition of 2inc, aluminum, or tin produces a beautiful blue color if tungsten is present. 

In Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), a gaseous, solid (as fine particles), or liquid (as an aerosol) sample is directed into the center of a gaseous plasma. The sample is vaporized, atomized, and partially ionized in the plasma. Atoms and ions are excited and emit light at characteristic wavelengths in the ultraviolet or visible region of the spectrum. The emission line intensities are proportional to the concentration of each element in the sample. A grating spectrometer is used for either simultaneous or sequential multielement analysis. The concentration of each element is determined from measured intensities via calibration with standards. 

Knowledge on the plasma species can be obtained by the use of plasma diagnostics techniques, such as optical emission spectroscopy (OES) and mass spectroscopy (MS). Both techniques are able to probe atomic and molecular, neutral or ionized species present in plasmas. OES is based on measuring the light emission spectrum that arises from the relaxation of plasma species in excited energy states. MS, on the other hand, is generally based on the measurement of mass spectra of ground state species. 

Plasma sources were developed for emission spectrometric analysis in the late-1960s. Commercial inductively coupled and d.c. plasma spectrometers were introduced in the mid-1970s. By comparison with AAS, atomic plasma emission spectroscopy (APES) can achieve simultaneous multi-element measurement, while maintaining a wide dynamic measurement range and high sensitivities and selectivities over background elements. As a result of the wide variety of radiation sources, optical atomic emission spectrometry is very suitable for multi-element trace determinations. With several techniques, absolute detection limits are below the ng level. 

Fabre, C., Boiron, M.-C., Dubessy, J., Moissette, a. 1999. Determination of ions in individual fluid inclusions by laser ablation optical emission spectroscopy development and applications to natural fluid inclusions. Journal of Analytical Atomic Spectrometry, 14(6), 913-922. 

W.C. Martin and W.L. Wiese From Atomic Spectroscopy, inG.W.F. Drake (Ed.), Atomic, Molecular and Optical Physics Handbook, American Institute of Physics, Press, Woodbury, N.Y., 135 (1996). 

The most frequently applied analytical methods used for characterizing bulk and layered systems (wafers and layers for microelectronics see the example in the schematic on the right-hand side) are summarized in Figure 9.4. Besides mass spectrometric techniques there are a multitude of alternative powerful analytical techniques for characterizing such multi-layered systems. The analytical methods used for determining trace and ultratrace elements in, for example, high purity materials for microelectronic applications include AAS (atomic absorption spectrometry), XRF (X-ray fluorescence analysis), ICP-OES (optical emission spectroscopy with inductively coupled plasma), NAA (neutron activation analysis) and others. For the characterization of layered systems or for the determination of surface contamination, XPS (X-ray photon electron spectroscopy), SEM-EDX (secondary electron microscopy combined with energy disperse X-ray analysis) and... 

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Analysis by atomic (or optical) emission spectroscopy is based on the study of radiation emitted by atoms in their excited state, ionised by the effect of high temperature. All elements can be measured by this technique, in contrast to conventional flames that only allow the analysis of a limited number of elements. Emission spectra, which are obtained in an electron rich environment, are more complex than in flame emission. Therefore, the optical part of the spectrometer has to be of very high quality to resolve interferences and matrix effects.-... 

The besl isolation of radiant energy can he achieved with flame spectrometers that incorporate either a prism sir grating monochromator, those with prisms having variable gauged entrance and exii slits. Both these spectrometers provide a continuous selection of wavelengths with resolving power sufficient lo separate completely most of the easily excited emission lines, and afford freedom from scattered radiation sufficient lo minimize interferences. Fused silica or quartz optical components are necessary to permit measurements in Ihe ultraviolet portion of the spectrum below 350 nanometers Sec also Analysis (Chemical) Atomic Spectroscopy Photometers and Spectra Instruments. 

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