Determinação de cloro, enxofre, cobalto, níquel e vanádio em petróleos após combustão iniciada por micro-ondas em frascos descartáveis

Abstract

In this work, sample preparation methods based on microwave-induced combustion in disposable vessel (MIC-DV) for the determination of Cl, S by ICP OES, Co, Ni and V by ICP-MS in petroleum were developed. The main operational parameters were investigated, such as type, concentration and volume of absorbing solution; sample mass; concentration and volume of combustion igniter (NH4NO3); O2 purging time and consecutive combustion cycles. The operational MIC-DV conditions were 10 mg (Cl and S) and 5 mg (Co, Ni and V) of petroleum, H2O (Cl and S) and 7 mol L-1 HNO3 (Co, Ni and V) as absorbing solutions, 40 μL of 10 mol L-1 NH4NO3 as igniter, 60 s of O2 purging time and up to 5 consecutive combustion cycles for Cl and S. For the definition of the best conditions for the MIC-DV method, the results were compared with those after sample digestion by microwave-induced combustion (MIC) for Cl and S determined by inductively coupled plasma optical emission spectrometry (ICP OES), as well as Cl determination by ion chromatography and S determination by elemental analyser, which were considered as reference values. The determination of Co, Ni and V for obtaining the reference values was performed by inductively coupled plasma mass spectrometry (ICP-MS) after microwave-assisted digestion (MAD). The recovery tests presented agreements from 99 to 101% for Cl and from 95 to 97% for S. The accuracy for S was also evaluated by the analysis of a certified reference material (NIST 2721), with agreement of 96%. The accuracy comparing MIC-DV and MAD was 92, 96 and 100% for Co, Ni and V, respectively. After parameters optimization, MIC-DV was applied for the determination of Cl, S, Co, Ni and V in 10 petroleum samples with different °API and H2O content. The limits of quantification after MIC-DV were 73 μg g-1 for Cl and 5.1 μg g-1 for S both determined by ICP OES, and 0.040 μg g-1 for Co, 0.28 μg g-1 for Ni and 0.15 μg g-1 for V determined by ICP-MS. The MIC-DV method was applied for the first time for Cl, S, Co, Ni and V determination in petroleum and proved to be a promising and low-cost alternative comparing with reference methods and with potential application in routine analysis. In addition, the proposed method contributes to the development of methods that are in agreement with the principles of green analytical chemistry.