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A decomposição de amostras que contêm elevados teores de Si, utilizando mistura com os ácidos nítrico e fluorídrico, pode provocar a precipitação de fluoretos de Al(III), Ca(II), Fe(III) e Mg(II) durante a digestão, impedindo a recuperação total desses elementos. Visando minimizar esse problema, foram testados quatro procedimentos aplicados a diferentes amostras de solos. O procedimento mais eficiente envolveu a decomposição assistida por radiação microondas, seguida pela separação do material sólido residual com elevado teor de Si do sobrenadante ácido. A fração residual foi solubilizada usando HF concentrado e, após complexação com ácido bórico, essa solução foi misturada à fase líquida do digerido. Os teores de metais nos digeridos foram determinados por espectrometria de emissão óptica em plasma induzido (ICP OES) e a exatidão foi demonstrada aplicando o procedimento proposto em amostras provenientes do programa colaborativo International Soil-Analytical Exchange (ISE, Wegening Agricultural University, Holanda) e em amostra certificada de solo proveniente do National Institute of Standard and Technology (NIST, SRM 2709), que apresentou respectivamente, os seguintes percentuais de recuperação: 100,0 ± 2,6; 99,5 ± 1,9; 108,5 ± 2,7; 94,6 ± 8,9; 92,2 ± 9,7; 95,7 ± 1,8; 96,7 ± 2,7; 95,2 ± 0,6; 103,6 ± 2,6 e 96,1 ± 1,6 para, Al, Ba, Ca, Cr, Cu, Fe, Mg, Mn, V e Zn. The use of pre-mixed nitric and hydrofluoric acids for the decomposition of samples containing high amounts of Si can cause the precipitation of Al(III), Ca(II), Fe(III), and Mg(II) fluorides during digestion, reducing the total recovery of these elements. Four procedures were applied to certified soil samples in an attempt to circumvent this problem. The most efficient procedure was based on microwave-assisted acid digestion decomposition of the samples, followed by separation of the residual silicate compounds. This fraction was solubilized using concentrated HF and the remaining fluoride ions were complexed by adding boric acid. This solution was mixed with the liquid phase of the acid digest. Elemental analysis of the acid digests was performed by inductively coupled plasma optical emission spectrometry (ICP-OES). The accuracy of the procedure was confirmed by applying it to samples provided by the International Soil-Analytical Exchange (ISE- Wageningen Agricultural University, Netherlands) and to certified soil reference material from the National Institute of Standards & Technology (NIST, SRM 2709). The following elemental recovery (%) was obtained: 100.0 ± 2.6; 99.5 ± 1.9; 108.5 ± 2.7; 94.6 ± 8.9; 92.2 ± 9.7; 95.7 ± 1.8; 96.7 ± 2.7; 95.2 ± 0.6; 103.6 ± 2.6 and 96.1 ± 1.6 for Al, Ba, Ca, Cr, Cu, Fe, Mg, Mn, V, and Zn, respectively

Decomposição de amostras de solos assistida por radiação microondas: estratégia para evitar a formação de fluoretos insolúveis