Feasibility of medium and extra-heavy crude oil digestion using MA-WD-PDC: alkaline and earth-alkaline elements determination at ppb level by ICP OES

Crude oil samples are usually considered hard matrixes to digest. In this work, a new microwave-assisted wet digestion (MA-WD) method with a pressurized digestion cavity (PDC) was investigated for achieving high digestion efficiency for medium-density to extra-heavy crude oils (American Petroleum Institute (API gravity), 9.40 to 28.9). The following operational parameters of the MA-WD-PDC method were optimized: i) sample mass, ii) volume of HNO3, iii) heating program, and iv) irradiation time. The MA-WD-PDC allowed the digestion of 1,200 mg of crude oil (digestion efficiency greater than 94%) using only 9 mL of HNO3. Alkaline and earth-alkaline elements were determined by using inductively coupled plasma optical emission spectrometry (ICP OES). As the digests presented low carbon content and residual acidity, no interferences were observed in the determination step. Due to the high efficiency for digesting a large crude oil mass, the quantification limits obtained were considerably low for all analytes (0.05 to 1.25 µg g−1) not requiring a more sensitive technique. According to the results, the MA-WD-PDC was adequate for such kinds of crude oil samples and a useful approach for quality control in the petroleum industry.