Microwave digestion of 'residual fuel oil' (NIST SRM 1634b) for the determination of trace elements by inductively coupled plasma-mass spectrometry

摘要

A microwave procedure for the digestion of the NIST l634b reference material "residual fue.l oil" in closed pressurized vessels was developed in an attempt to facilitate routine analysis and obtain reproducible condi- tions or comparable results. The influence of sample size, reagent composition and volume, microwave power, and duration of heating on the digestion procedure was stud- ied. Pressure and temperature inside the reaction vessels were monitored to determine the progression of the reac- tion and to develop optimal conditions. A nine-step heat- ing program requiring 36.5 min with microwave power not exceeding 450 W in the pulsed mode was found suit- able for the digestion of- 250 mgfuel oil with a mixture of nitric acid (5.0 mL) and hydrogen peroxide (2.0 mL). The reproducibility of microwave power was determined in terms of the relative standard deviations (n = 3) for temperature (2.7%) and pressure (4.9%) data. The vapor pressures obtained with 5.0 mL Milli-Q water (heated) in an 80-mL digestion vessel showed good agreement with literature data. The excess acid in the resulting digests was removed by evaporation and the concentrations of 24 elements (Ag, AI, As, Ba, Bi, Ca, Cd; Co,Cr, Cu, Fe, Hg, Mg, Mo, Ni, Pb, Sb, Sn, Sr, Ti, TI, V, U, and Zn) were determined in the diluted digests by inductively coupled plasma mass spectrometry (ICP-MS). Th~ experimental results were in good agreement with the certified and rec- ommended concentrations for eight elements (AI, As,Co, Cr, Ni, Pb, V, Zn) in solutions obtained after one digestion step. An additional digestion step, consisting of interme- diate cooling and venting stages, was required for the accurate determination of Fe.No agreement was reached for Ca and Ba even after two-step digestion.The proposed method of digestion provided precise results with relative standard deviations generally less than 5% for most of the elements determined.