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摘要
The objective of this study was to develop an effective and routine single sample preparation method to accurately and precisely analyze multiple elements (up to 57) in crude oils using both an inductively coupled plasma-optical emission spectrometer (ICP-OES) for high abundance elements and a triple quadrupole-inductively coupled plasma-mass spectrometer (QQQ-ICP-MS) (or ICP-MS) for low abundance trace and ultra-trace elements. A critical aspect of this study was to develop a single sample preparation method that could be effective in analyzing the largest number of quantifiable elements with instruments used in tandem to cover a wider and more practical combined dynamic range. Three sample preparation techniques were investigated and tested, including high pressure combustion with strong-acid digestion, high pressure and high temperature strong-acid digestion within Parr bombs, and high temperature and high pressure digestions in a Single Reaction Chamber (SRC) microwave. For the combustion method, no element showed recovery above 75%, thus this method was deemed unacceptable for accurate and precise analysis of crude oil. Average recoveries ranging from 94% to 106% for 19 analytes were achieved with high pressure and high temperature acid digestion within Parr bombs. A drawback of the method, however, was a rather small sample sizes (100 mg) that could be achieved without venting, which places limitations on detection and quantitation limits of large numbers of low abundance analytes in natural crude oils. The best of the SRC microwave-assisted acid digestion tests, however, was also shown to be acceptable for 19 analytes using a certified organometallic standard, with average recoveries ranging from 93% to 113%. Because of rapid sample throughput and the larger sample sizes (1.2 g sample size per digestion session) that can be accommodated with the single reaction chamber microwave digestion technique, better method detection and quantification limits and high precision on a range of low abundance elements could be achieved. We also then successfully tested the recovery of 53 elements with this SRC microwave method by adding a multi-element spike to Conostan base oil. This optimum SRC digestion method tested was then used to examine a NIST natural crude oil research material (NIST RM 8505) in an attempt to quantify up to 57 elements. The mean vanadium concentration (mass fraction) is reported as 390 +/- 0.4 mu g/g, the mean value of 5 replicate digestions by our method, which agrees well with the recommended value for the only element recommended for NIST 8505 of 390 +/- 10 mu g/g. The number of quantified analytes for the RM 8505 was further extended to a total of 52 elements with RSDs <5% for 38 elements, between 5% and 10% for 10 elements, and between 10% and 15.6% for 4 elements, all of which met our acceptability limits set at <= 20% RSD. Of the 57 elements analyzed in RM 8505, certain elements B, Nb, and W, although above method quantitation limits, were judged unacceptable because RSDs were >= 31.4%, whereas Ag and Tl were below method quantitation limits, and their RSDs were >= 44.2% and judged unacceptable. However, these elements have been quantified in other natural crude oils with higher abundances, which indicates that up to 57 elements can be potentially assessed in crude oils or refined oils with this method. The NIST RM 8505 crude oil reported here and others are being evaluated as potential natural crude oil standards to provide references values for more routine multi-element analysis. Published by Elsevier Ltd.
- 出版日期2017-10-15
