A two-jet arc plasma: matrix effects and ways to their suppression

A two-jet plasma is used for direct atomic emission analysis of powdered samples. It is characterized by relatively weak matrix effects, which allows using unified calibration samples on the basis of graphite powder for analysis of the samples with inorganic, organic, and organomineral matrix. In the present paper the effects limiting the usage of the unified approach due to different thermal stability and evaporation efficiency of the samples are discussed. The understated concentrations of a set of elements (Al, Ba, Ca, La, Mg, Mn, Sr, Ti, and Y) were obtained in analysis of certified reference materials of geological samples. It was shown that determination of rare earth elements should be carried out in the region behind the jet confluence providing their complete evaporation. For other elements, registration of the spectra in this region improves the results to some extent but they do not achieve the certified values. To speed up evaporation of these elements, the experimental conditions were chosen for plasma chemical reactions which provide conversion of the matrix elements into more volatile compounds. Addition of ammonium hydrofluoride to powdered sample considerably increased the line intensities of Al and Ca strongly associated with the silicon matrix. Incomplete evaporation was observed in analysis of biological samples with particle size more than 100 μm. A decrease in consumption of carrier argon is quite enough for effective decomposition of the organic matrix in plasma; the value of gas consumption depends on thermal stability and particle size of the sample. Preliminary sample carbonization is another way to improve evaporation of biological samples.

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