Ultra-trace Hg determination in crude oils by CV-ICP-MS: overcoming the limitations of sample preparation to determine sub-ppb levels

Abstract

A method was developed to determine Hg in crude oil at sub-ppb levels by cold vapor generation inductively coupled plasma mass spectrometry (CV-ICP-MS) after wet digestion in a microwave-assisted system with a pressurized cavity. Operational parameters for crude oil digestion were optimized, such as sample mass, nitric acid volume, digestion temperature, and the heating program, overcoming the current limitations of closed vessel digestion for this type of sample. The accuracy of the proposed method was evaluated by spike recovery experiments and using standard reference materials (SRMs) of crude oil (NIST SRM 2722) and residual fuel oil (NIST SRM 1634c). The optimized conditions (8 mL of concentrated HNO₃, 270 °C, and a heating program of 85 min plus 20 min for cooling) led to complete digestion of up to 1.2 g of crude oil (API gravity ranging from 9.5 to 28.8). The carbon concentration and residual acidity in digests were about 3500 mg L⁻¹ and 10%, respectively, which were considered suitable for CV-ICP-MS analyses. In addition, Hg recoveries ranged from 92 to 105% for all samples and both SRMs. By overcoming the limitations for digestion of a relatively higher sample mass of crude oil (1.2 g), the proposed method enabled us to obtain a very low limit of detection (0.16 ng g⁻¹). It is important to mention that this study reports the highest sample mass of crude oil feasible to be digested using closed vessels, thus contributing to determine sub-ppb Hg levels, as required by the industry.