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An Optimum Sample Preparation Method for Multi-element Analysis in Crude or Fuel Oils with ICP-OES and QQQ-ICP-MS by Mineralization Utilizing a Single-Reaction-Chamber Microwave System


A sample preparation method was developed to accurately and precisely analyze up to 57 elements in crude oils and fuel oils by ICP-OES, used in tandem with QQQ-ICP-MS. To eliminate spectral and polyatomic interferences caused by the complex organic matrix of crude oils, we decomposed oil samples to form aqueous solutions using strong-acid digestions. Our test of many strong acid preparation techniques included combustion followed by digestion, Parr Bomb acid digestions, and single reaction chamber (SRC) microwave assisted acid digestions. A synthetic multi-element metallo-organic standard from Conostan Oil Analysis Standards (USA) was tested thoroughly for recovery of analytes by each digestion method. No element showed recovery above 75% in our best combustion test, whereas based on 5 replicate digestions, Parr Bomb average recoveries ranged from 94% to 106% for 19 elements in the standard and SRC microwave test average recoveries ranged from 93% to 113% for 19 elements in the standard as well. A drawback of the Parr method was a rather small sample sizes (0.1g), which placed limits on method detection limit (MDL) and method quantitation limit (MQL). Because of rapid sample throughput and the larger sample sizes (1.2g), SRC microwave digestion method was chosen to achieve better MDL and MQL on a range of elements. We then successfully tested SRC recovery (90-110%) for 57 elements in standards added to a Conostan base oil. The SRC technique was used to digest a NIST RM 8505 natural crude oil in an attempt to quantify up to 57 elements, based on 5 replicate digestions and analyses. We were able to precisely and accurately match the only recommended value referenced by NIST RM 8505 for vanadium, which is reported as 390±10 µg/g. Our result is 390±0.4 µg/g. We were able to precisely quantify 52 elements, with %RSDs of < 5% for 38 elements, 5-10% for 10 elements, and 10-15.6% for 4 elements, all meeting our acceptability limits of < 20% RSD. Successful quantification was achieved by ICP-OES for higher abundance elements and by QQQ-ICP-MS for low abundance elements using the SRC microwave-assisted preparation. B, Nb, and W, although above MQL, were judged unacceptable because their RSDs were > 31.4%, whereas Ag and Tl were below MQL, and their RSDs were > 44.2% and judged unacceptable as well; although these elements have been quantified in other natural crude oils.