Green chemistry principles applied to microwave-assisted sample preparation

LUCIMAR L. FIALHO; JOAQUIM A. NOBREGA; MARIANELA SAVIO

Conferencia; 3ª Conferencia sobre tratamiento de muestras de Caparica (www.sampletreatment2018.com).; 2018

One of the main analytical chemistry challenges has been to overcome difficulties to bring samples into solutions, which should be compatible with spectrochemical instrumental methods for inorganic trace analysis. Nowadays it is well-established that organic matrixes can be efficiently microwave-assisted digested in closed vessels using dilute nitric acid solutions [1-3]. In this work sample preparation procedures assisted by microwave radiation and compatible with green chemistry recommendations were developed for grains and cereals used to raise pigs. The digestions were developed using the UltraWAVE TM Single Reaction Chamber (SRC) technology (Milestone) which offers significantly greater digestion capabilities for even the most difficult and dissimilar sample types. Samples were exactly weighed into vials with masses around 0.5 g, and then 4 mL of 7.0 mol L-1 HNO3 plus 1.5 mL H2O2 were added. Vials were placed in a rack, which was automatically lowered into the SRC. The chamber was sealed and pre-pressurized with nitrogen, which physically acts as a cap for the vials, avoiding boiling of the solutions and preventing cross contamination. Microwave-assisted heating program was implemented using: 1st step -5 min to 150 °C, 2nd step - hold at 150 °C for 2 min, 3rd step -5 min to 230 °C, and 4th step - hold at 230 °C for 25 min. All steps were performed at 1,5 kW of applied power. Maximum pressure was set at 110 bar. Efficient digestions were promoted for all samples. The digestion efficiency was evaluated by determining the dissolved carbon content, which was lower than 1%; residual acidity was also evaluated and was lower than 0.65 mol L-1. Aluminum, As, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sb, Se, Si, Sn, Sr, V, Zn were determined either by ICPOES or by ICP-MS depending on respective concentrations. Certified reference materials were evaluated and most determined concentrations were between 80-120%. Finally, after analytical measurements a DuoPur apparatus was used applying low temperature distillation [4]; the purified acid will be reused in further digestions. The combination of these strategies (dilute acid solution followed by purification of the acid) may be seen as an advance step towards green approaches: ?reduce the use, recycle and reuse?, strictly following major green chemistry recommendations.