Evaluation of DS-GF AAS for Ca determination in crude oil

Crude oil is a complex matrix containing hydrocarbons as main compounds and variable amounts of heteroatoms such as N, O, S and metals. Metals are common contaminants present in crude oil and can occur as soluble salts associated to emulsified water.1 Hydrolysis reactions of Ca, Mg and Na with chloride can cause problems of midstream petrochemical industry. Calcium chloride may form hydrochloridric acid and hydroxychloride salt causing corrosion and clogging of pieces.2 Therefore, the development of high sensitive and simple analytical strategies for Ca determination in crude oil is an important task. Generally, sample preparation is necessary previously to the determination of most elements. However, sample digestion has some drawbacks such as sample dilution, contamination and require the use of corrosive acids. Direct sampling graphite furnace atomic absorption spectrometry (DS-GF AAS) present some advantages such as low consumption of reagents, fast analysis, low risk of contamination and relatively low cost.3 In this sense, the use of DS-GF AAS was evaluated for Ca determination in crude oil. The wavelength was set at 422.7 nm and three low-sensitivity conditions using variable magnetic field strength were used: i) 3-field dynamic mode (0.4-0.8 T) with gas stop flow during atomization, ii) 3-field dynamic mode (0.6-0.8 T) with gas stop flow during atomization and iii) 3-field dynamic mode (0.6-0.8 T) with gas flow during atomization. The heating program for Ca was established based on pyrolysis and atomization curves for sample and aqueous reference solution. The temperatures for pyrolysis and atomization were 1400 and 2600 °C, respectively. The feasibility of calibration curves using aqueous reference solutions was evaluated and the limit of quantification (LOQ) for Ca was 0.1 µg g-1 (using a high sensitivity condition). Ca concentration in 12 samples of crude oil ranged from 0.153 to 39.5 µg g-1. Accuracy of the DS-GF AAS method was evaluated by results comparison with those obtained by GF AAS after microwave-assisted digestion and the agreement ranged from 99 to 113%. Recovery tests were also performed and recoveries ranged from 92 to 107%. The DS-GF AAS method is a good tool for Ca determination in crude oil and is feasible for routine analysis. In addition, several advantages should be highlighted such as the low LOQs, no use of hazardous reagents and low residues generation.