Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 475 of 707

platinum, potassium, rhodium, ruthenium, silver, sodium, strontium, tin, and zinc. 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 con- ductor 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 (4,000-6,000O K) produce emitted radi- ation higher in energy and in the number of spectral lines than in flame pho- tometry. 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 iden- tify 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 sam- ples of the elements. The density of the wavelengths is proportional to the con- centration of the element being determined. Calibrations are done against stan- dard samples. ES is a useful method for the analysis of trace metallic contaminants in plat- ing baths. The "oxide" method is a common quantitative technique in ES. A sam- ple of the plating bath is evaporated to dryness and then heated in a muffle fur- nace. 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 sam- ple 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 fluores- cent 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 sam- ple. 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 sur- face of the deposit is exposed to an X-ray source and the intensity of the element wave- length 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 bom- barded 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 474

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