Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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the bombarding beam, are characteristic of the molecule; therefore, every chem- ical compound has a distinct mass spectrum. By establishing a mass spectrum of several pure compounds, an observed pattern allows identification and analysis 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 frag- ments 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 450O qualitative and quantitative analysis of liquids, solids, and gases. C. MS permits 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 (7,000O K) 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 sen- sitive 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 compen- sate 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 ener- gy absorbed is proportional to the concentration of the absorbing species in solu- tion. 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 met- als with concentrations of up to 2%. Spectrophotometers consist of a light source (tungsten or hydrogen), a mono- chromator, a sample holder, and a detector. Ultraviolet or visible light of a def- 475

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