Calibration in Plasma Emission Spectrometry

Objective lens Upper illuminator Sample cell Carrier gas (Ar, He) 

Most samples for analysis by plasma emission spectrometry are dissolved in acids, because metal elements are more soluble in acids. The acidic nature of the solution prevents elements from adsorbing onto the surface of the glassware. Some bases, such as tetramethyl ammonium hydroxide (TMAH), are used to prepare solutions but care needs to be taken to avoid the formation of insoluble metal hydroxides. Organic solvents may be used to dissolve organometallic compounds directly or to extract chelated metals from aqueous solution into the solvent as part of separation and preconcentration steps in sample preparation. 

For most analyses, standard solutions are aqueous solutions containing several percent of a mineral acid such as HCl or nitric acid. When mixing different elements in acids, it is important to remember basic chemistry and solubility rules for inorganic compounds. The elements must be compatible with each other and soluble in the acid used so that no precipitation reactions occur. Such reactions would change the solution concentration of the elements involved in the reaction and make the standard useless. Combinations to be avoided are silver and HCl, barium and sulfuric acid, and similar 

The alkali metals Li, Na, and K are easily ionized in flames and plasmas. Concentrations of an easUy ionized element (EE) greater than 1000 ppm in solution can result in suppression or enhancement of the signals from other analytes. The EE effect is wavelength dependent ion lines are affected differently than atom lines. The EE effect can be minimized by optimizing the RF power and plasma conditions, by matrix-matching, by choosing a different wavelength that is not subject to the EE effect, or by application of mathematical correction factors. 

Spectral interference is much more common in plasmas than in flames due to the great efficiency of excitation in plasmas. Elements such as Fe, Mn, Ta, Mo, W, and U emit thousands of lines in a plasma source. Ideally, the analyte wavelength chosen should have no interference from other emission lines, but this is often not possible. 

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