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Encycopedia of Materials Characterization (surfaces_ interfaces_ thin films) - C. Brundle_ et al._ (BH_ 1992) Episode 12

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Tham khảo tài liệu 'encycopedia of materials characterization (surfaces_ interfaces_ thin films) - c. brundle_ et al._ (bh_ 1992) episode 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Direct Introduction of Samples from Solids Surfaces or Thin Films There are advantages to direct solid sampling. Sample preparation is less time consuming and less prone to contamination and the analysis of microsamples is more straightforward. However calibration may be more difficult than with solution samples requiring standards that are matched more closely to the sample. Precision is typically 5 to 10 because of sample inhomogeneity and variations in the sample vaporization step. In the direct insertion technique 1 214 the sample liquid or powder is inserted into the plasma in a graphite tantalum or tungsten probe. If the sample is a liquid the probe is raised to a location just below the bottom of the plasma until it is dry. Then the probe is moved upward into the plasma. Emission intensities must be measured with time resolution because the signal is transient and its time dependence is element dependent due to selective volatilization of the sample. The intensity-time behavior depends on the sample probe material and the shape and location of the probe. The main limitations of this technique are a time-dependent background and sample heterogeneity-limited precision. Currently no commercial instruments using direct sample insertion are available although both manual and highly automated systems have been described.4 Arc and spark discharges have been used to ablate material from a solid conducting sample surface.1 2 The dry aerosol is then transported to the plasma through a tube. Detection limits are typically in the low ppm range. The precision attainable with spark discharges that sample over a relatively large surface area 0.2-1 cm2 is typically 0.5 to 5.0 . Calibration curves are linear over at least 3 orders of magnitude and an accuracy of 5 or better is realized. Commercial instruments are available. In some cases it is possible to use pure aqueous standards to produce the calibration curves used for spark ablation ICP-OES. In general calibration .