Atomic emission spectroscopy sources

The following ionization sources are used mainly in inorganic (atomic) MS, where the elemental composition of the sample is desired. The glow discharge (GD) and spark sources are used for solid samples, while the inductively coupled plasma (ICP) is used for solutions. All three sources are also used as atomic emission spectroscopy sources they are described in more detail with diagrams in Chapter 7. 

The use of a plasma as an atomisation source for emission spectroscopy has been developed largely in the last 20 years. As a result, the scope of atomic emission spectroscopy has been considerably enhanced by the application of plasma techniques. 

Figure 1.2 shows the basic instrumentation necessary for each technique. At this stage, we shall define the component where the atoms are produced and viewed as the atom cell. Much of what follows will explain what we mean by this term. In atomic emission spectroscopy, the atoms are excited in the atom cell also, but for atomic absorption and atomic fluorescence spectroscopy, an external light source is used to excite the ground-state atoms. In atomic absorption spectroscopy, the source is viewed directly and the attenuation of radiation measured. In atomic fluorescence spectroscopy, the source is not viewed directly, but the re-emittance of radiation is measured. 

Atomic emission spectroscopy is applied to the measurement of light emitted by thermal energy caused by the thermal source from the chemical species present. Examples of emission, absorption and fluorescence spectroscopy can be shown schematically, as in Figure 1.5. 

In the application of atomic emission spectroscopy for quantitative analysis, samples must be prepared in liquid form of a suitable solvent unless it is already presented in that form. The exceptions are solids where samples can be analysed as received using rapid heating electro-thermal excitation sources, such as graphite furnace heating or laser ablation methods. Aqueous samples, e.g. domestic water, boiler water, natural spring, wines, beers and urines, can be analysed for toxic and non-toxic metals as received with... 

In the early days of atomic spectroscopy, dc and ac arcs and high-voltage sparks were popular for use in excitation of atomic emission. Such sources have almost entirely been replaced by the ICP. 

In atomic emission spectroscopy, the radiation source is the sample itself. The energy for excitation of analyte atoms is supplied by a plasma, a flame, an oven, or an electric arc or spark. The signal is the measured intensity of the source at the wavelength of interest. In atomic absorption spectroscopy, the radiation source is usually a line source such as a hollow cathode lamp, and the signal is the absorbance. The latter is calculated from the radiant power of the source and the resulting power after the radiation has passed through the atomized sample. 

Atomic spectroscopy can be divided into several broad classes based on the nature of the means of exciting the sample. One of these classes is generally known as atomic emission spectroscopy, in which excitation is thermally induced by exposing the sample to very high electric fields. Another class is known as flame emission spectroscopy or flame photometry, in which excitation is thermally induced by exposing the sample to a high-temperature flame. These methods differ from atomic absorption spectroscopy, in which the absorption of light from a radiation source by the atom is observed rather than the emission from the electronically excited atom. 

The X-ray diffraction (XRD) patterns were obtained by Philips X pert Pro X-ray diffractometer equipped with a Cu-K source at 40 kV and 40 mA. The crystalline sizes of R particles were calculated from Scherrer s equation [15]. Transmission electron microscopy (TEM) images were obtained using the G2 FE-TEM Tecnai microscope at an accelerating voltage of 200 kV. The content of platinum and carbon in the sample was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, RF source Jobin Yvon 2301, 40.68 MHz). 

The routine determination of trace elements in agricultural, geological, and biological samples is of considerable interest. Ideally, many trace elements in each sample should be determined simultaneously using a single group of standards. Atomic-emission spectroscopy using the ICP source provides that capability. 

Classical excitation sources in atomic emission spectroscopy do not meet these requirements. Flame, arc, and spark all suffer from poor stability, low reproducibility, and substantial matrix effects. However, the modem plasma excitation sources, especially ICPs, come very close to the specification of an ideal AES source. 

Because of the unique characteristics of their emitted energies, lasers have been used for sample vaporization and excitation sources in atomic emission spectroscopy. They also have been used as sources for atomic absorption and atomic fluorescence analysis. Their application in these areas will no doubt increase as lasers become cheaper and more readily available. 

Any excitation source for analytical atomic emission spectroscopy must accomplish the following processes (1) the analytical sample must be vaporized (2) it must be dissociated into atoms (3) the electrons in the atoms must be excited to energy levels above the ground state. The three steps... 

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