Metal Finishing Guide Book


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of complex mixtures. The mass spectrum of a compound contains the masses of the ion fragments and the relative abundances of these ions plus the parent ion. Dissociation fragments will always occur in the same relative abundance for a particular compound. MS is applicable to all substances that have a sufficiently high vapor pressure. This usually includes substances whose boiling point is below 450OC. MS permits qualitative and quantitative analysis of liquids, solids, and gases. Inductively Coupled Plasma Inductively coupled plasma (ICP) involves the aspiration of a sample in a stream of argon gas, and then its ionization by an applied radio frequency field. The field is inductively coupled to the ionized gas by a coil surrounding a quartz torch that supports and encloses the plasma. The sample aerosol is heated in the plasma, the molecules become almost completely dissociated and then the atoms present in the sample emit light at their characteristic frequencies. The light passes through a monochromator and onto a detector. The high temperature (7,000OK) of the argon plasma gas produces efficient atomic emission and permits low detection limits for many elements. As with atomic absorption (AA), ICP does not distinguish between oxidation states (e.g., Cr3+ and Cr6+) of the same element—the total element present is determined. Advantages of ICP include complete ionization and no matrix interferences as in AA. ICP allows simultaneous analysis of many elements in a short time. It is sensitive to part-per-billion levels. Disadvantages of ICP include its high cost and its intolerance to samples with greater than 3% dissolved solids. Background corrections usually compensate for interferences due to background radiation from other elements and the plasma gases. Physical interferences, due to viscosity or surface tension, can cause significant errors. These errors are reduced by diluting the sample. Although chemical interferences are insignificant in the ICP method, they can be greatly minimized by careful selection of the instrument's operating conditions, by matrix matching, or by buffering the sample. ICP is applicable to the analysis of major components and trace contaminants in plating solutions. It is also useful for waste-treatment analysis. PHOTOMETRIC METHODS Photometric methods are based on the absorption of ultraviolet (200-400 nm) or visible (400-1,000 nm) radiant energy by a species in solution. The amount of energy absorbed is proportional to the concentration of the absorbing species in solution. Absorption is determined spectrophotometrically or colorimetrically. The sensitivity and accuracy of photometric methods must be frequently checked by testing standard solutions in order to detect electrical, optical, or mechanical malfunctions in the analytical instrument. Spectrophotometry and Colorimetry Spectrophotometry involves analysis by the measurement of the light absorbed by a solution. The absorbance is proportional to the concentration of the analyte in solution. Spectrophotometric methods are most often used for the analysis of metals with concentrations of up to 2%. Spectrophotometers consist of a light source (tungsten or hydrogen), a monochromator, a sample holder, and a detector. Ultraviolet or visible light of a definite wavelength is used as the light source. Detectors are photoelectric cells that measure the transmitted (unabsorbed) light. Spectrophotometers differ from photometers in that they utilize monochromators, whereas photometers use fil485

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