Plasma background emission intensity

Figure 15. Relationship between the plasma background emission intensity and its associated noise measured with a photomultiplier detection system.

In the direct insertion technique, 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 h ly automated systems have been described. ... 

The application of the Spectroscan DC plasma emission spectrometer confirmed that for the determination of cadmium, chromium, copper, lead, nickel, and zinc in seawater the method was not sufficiently sensitive, as its detection limits just approach the levels found in seawater [731]. High concentrations of calcium and magnesium increased both the background and elemental line emission intensities. 

Nygaard [752] has evaluated the application of the Spectraspan DC plasma emission spectrometer as an analysis tool for the determination of trace heavy metals in seawater. Sodium, calcium, and magnesium in seawater are shown to increase both the background and elemental line emission intensities. Optimum analytical emission lines and detection limits for seven elements are reported in Table 5.8. 

Inductively coupled plasma-atomic emission spectrometry was investigated for simultaneous multielement determinations in human urine. Emission intensities of constant, added amounts of internal reference elements were used to compensate for variations in nebulization efficiency. Spectral background and stray-light contributions were measured, and their effects were eliminated with a minicomputer-con-trolled background correction scheme. Analyte concentrations were determined by the method of additions and by reference to analytical calibration curves. Internal reference and background correction techniques provided significant improvements in accuracy. However, with the simple sample preparation procedure that was used, lack of sufficient detecting power prevented quantitative determination of normal levels of many trace elements in urine. 

A major difficulty encountered with atomic absorption techniques is the presence of incompletely absorbed background emission from the source and scattered light from the optical system. As the background becomes more intense relative to the absorption of the analyte, the precision of the measurement decreases dramatically. For this reason, several background correction techniques have been implemented. A commonly used method is the method of proximity, which was discussed in relation to inductively coupled plasma spectroscopy. 

At complete ionization of the hydrogen (e.g. when added to a plasma with another gas as the main constituent) ne = p/(2 x k x Te) has a maximum at a wavelength of X — (7.2 x 107)/Te or at a fixed wavelength, the maximum intensity is found at a temperature Te = (5.76 x 107)/2. Thus, the electron temperature can be determined from the wavelength dependence of the continuum intensity. As Te is the electron temperature, absolute measurements of the background continuum emission in a plasma, e.g. for the case of hydrogen, allow determination of the electron temperature in a plasma, irrespective of whether it is in local thermal equilibrium or not. Similar methods also make use of the recombination continuum and of the ratio of the Bremsstrahlung and the recombination continuum. 

When the interference is from the plasma emission background, there are background correction options available with most commercial instrumentation. The region adjacent to the line of interest can be monitored and subtracted from the overall intensity of the line. If direct spectral overlap is present, and there are no alternative suitable lines, the interelement equivalent concentration (lEC) correction technique can be employed. This is the intensity observed at an analyte wavelength in the presence of 1000 mg of an interfering species. It is expressed mathematically as ... 

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