3.4 Interferences
ICP systems greatly reduce the number of interferences over those created in flame-based systems. Nebulizer, chemical, ionization, and spectral interferences are all present in ICP systems, but spectral interferences are most prominent. Nebulizer interferences (also known as matrix effects) can arise from physical and chemical differences between reference standards and samples, or between samples, such as the inconsistence presence of matrix salts and organic compounds or different viscosities and surface tension of the liquid. Each of these can be overcome by the use of standard addition calibration techniques discussed in Chapter 6, but at a significant increase in the cost of analysis (primarily due to labor costs). For low ionic strength samples, nebulizer interferences are less prominent. Chemical interferences are common in FAAS and FAES but are less common or practically nonexistent in ICP-AES due to the relatively high temperature of the plasma, long residence time in the plasma, and inert atmosphere of the Ar plasma. Ionization interferences, in direct arc- and microwave-produced plasmas, usually only occur for easily ionized elements such as alkali and alkaline earth elements. The net result of ionization interferences is an increase or decrease in the intensity of emission lines for these elements. Few ionization interferences occur for these elements in ICP-AES.
Spectral interferences can be common in high temperature plasmas as opposed to flame-based systems given the complete excitation and subsequent emission of all compounds in the sample (including the argon). Spectral interferences can be divided into three different classes. The first type is spectral line coincidence when resolution of the monochromator of the system is too poor to separate the analyte line from a matrix line. The use of Echelle monochromators with higher resolution eliminates spectral interferences. The second cause of interferences occurs when a wavelength of interest overlaps completely with a nearby “broadened line wing”. This can be solved by monitoring a secondary emission line for that particular element. Most instruments use a background correction technique to overcome this type of interference. The final type of spectra interference referred to as spectral continuum occurs when stray light results from the recombination of electrons with Ar ions in the plasma that emits multiple intense lines. This can be avoided in some instruments by adjusting the temperature of the plasma or monitoring alternate lines. Stray light from matrix emissions can be avoided by use of high-end optical components such as “solar-blind PMTs”.
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