Metal Finishing Guide Book


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Emission Spectrometry In emission spectrometry (ES), a sample composed of a solid, cast metal or solution is excited by an electric discharge such as an AC arc, a DC arc, or a spark. The sample is usually placed in the cavity of a lower graphite electrode, which is made positive. The upper counterelectrode is another graphite electrode ground to a point. Graphite is the preferred electrode material because of its ability to withstand the high electric discharge temperatures. It is also a good electrical conductor and does not generate its own spectral lines. The arc is started by touching the two graphite electrodes and then separating them. The extremely high temperatures (4,000-6,000OK) produce emitted radiation higher in energy and in the number of spectral lines than in flame photometry. Characteristic wavelengths from atoms of several elements are separated by a monochromator and are detected by spectrographs or spectrophotometers. Qualitative identification is performed by using available charts and tables to identify the spectral lines that the emission spectrometer sorts out according to their wavelength. The elements present in a sample can also be qualitatively determined by comparing the spectrum of an unknown with that of pure samples of the elements. The density of the wavelengths is proportional to the concentration of the element being determined. Calibrations are done against standard samples. ES is a useful method for the analysis of trace metallic contaminants in plating baths. The "oxide" method is a common quantitative technique in ES. A sample of the plating bath is evaporated to dryness and then heated in a muffle furnace. The resultant oxides are mixed with graphite and placed in a graphite electrode. Standards are similarly prepared and a DC arc is used to excite the sample and standards. X-ray Fluorescence X-ray fluorescence (XRF) spectroscopy is based on the excitation of samples by an X-ray source of sufficiently high energy, resulting in the emission of fluorescent radiation. The concentration of the element being determined is proportional to the intensity of its characteristic wavelength. A typical XRF spectrometer consists of an X-ray source, a detector, and a data analyzer. Advantages of XRF include the nondestructive nature of the X-rays on the sample. XRF is useful in measuring the major constituents of plating baths such as cadmium, chromium, cobalt, gold, nickel, silver, tin, and zinc. Disadvantages of XRF include its lack of sensitivity as compared with ES. X-ray spectroscopy is also used to measure the thickness of a plated deposit. The X-ray detector is placed on the wavelength of the element being measured. The surface of the deposit is exposed to an X-ray source and the intensity of the element wavelength is measured. A calibration curve is constructed for intensity against thickness for a particular deposit. Coating compositions can also be determined by XRF. Mass Spectrometry In mass spectrometry (MS), gases or vapors derived from liquids or solids are bombarded by a beam of electrons in an ionization chamber, causing ionization and a rupture of chemical bonds. Charged particles are formed, which may be composed of elements, molecules, or fragments. Electric and magnetic fields then separate the ions according to their mass to charge ratios (m/e). The amount and type of fragments produced in an ionization chamber, for a particular energy of the bombarding beam, are characteristic of the molecule; therefore, every chemical compound has a distinct mass spectrum. By establishing a mass spectrum of several pure compounds, an observed pattern allows identification and analysis 484

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