Extração líquido-líquido miniaturizada para determinação de Li, Na, K, Ba, Ca, Mg e Sr em petróleo por ICP-OES

Abstract

In this study, a miniaturized ultrasound combined liquid-liquid extraction (US-LLE) method was developed for the extraction and subsequent Li, Na, K, Ba, Ca, Ca, Mg and Sr determination by ICP-OES in crude oils. For the development of the proposed method, the following parameters, which influence the efficiency of the extraction, were evaluated: type of extraction solution, agitation type and agitation time (manual, vortex and ultrasound), 2 consecutive extractions, addition of toluene to the sample, sample temperature and heating time, volume and concentration of the extraction solution, sample mass, US application time, centrifugation time, use of demulsifier, and sedimentation time. The accuracy of the proposed method was evaluated by comparison with the results obtained after applying the reference method of microwave radiation assisted wet digestion in a single reaction chamber system (MAWD-SRC) and determination by ICP-OES. The digests obtained by the reference method (MAWD-SRC) were also determined by F AAS, which showed no significant difference (t-Student test, 95% confidence level) when compared to the values obtained by ICP-OES. Quantitative results were obtained for all analytes employing the miniaturized US-LLE method with the following conditions: i) sample temperature and heating time: 80 °C and 2 min, ii) sample mass: 2 g, iii) 500 µL of toluene containing 50 mg L-¹ of demulsifier (Dissolvan 961), iv) extraction solution: 500:500 µL of HNO3:HCl 1.0 mol L-¹ , v) manual stirring: 60 s, vi) US application time: 10 min, vii) sedimentation time: 15 min at 80 °C, and viii) centrifugation time: 10 min. In addition, the accuracy was also evaluated from recovery tests with addition of the analytes in the concentration range contained in the sample. Quantitative recoveries of 94 to 101% were obtained for all analytes and RSDs of less than 6%. The limits of quantification for the proposed method were 0.0120, 0.850, 0.0200, 0.130, 2.50, 3.10 and 0.100 µg g-¹ for Li, Na, K, Ba, Ca, Mg and Sr, respectively. The proposed method was suitable for the extraction and subsequent quantification of Li, Na, K, Ba, Ca, Mg and Sr in crude oil samples and showed satisfactory accuracy and precision. In addition, the proposed method presented several advantages such as the possibility of using a diluted extraction solution, which made it possible to reduce the amount of acids about 160 times and consequently the generation of waste was drastically reduced compared to the reference method. In addition, the analytical frequency was increased 7 times and the acid volume was reduced about 160 times. Furthermore, the method proved to be simple, easy to operate, and inexpensive to use for routine analysis in quality control laboratories in the petrochemical industry.