Less than 90 Minutes Turnaround Time for Trace Metal Analysis on Petroleum Products

Conventional methods for the sample preparation of petroleum products and lubricants like dilution or ashing suffer from severe drawbacks. Microwave-assisted acid digestion according to ASTM D7876 is a beneficial approach which achieves reliable results in a short time frame.

Petroleum products and lubricants are routinely analyzed for their elemental content for various reasons. In crude and residual oils the concentrations of silicon, aluminum, vanadium, nickel, iron and sodium are used to define their quality and value. Nickel and vanadium in crude oil can deactivate catalysts during processing, but also initiate corrosion in motors and boilers during the combustion when present in fuels. The presence of silicon and aluminum in residual fuel oils causes abrasion within the combustion engine. Before use, products like lubricating oils are tested to determine the concentration of additives (which contain metals such as calcium, copper, magnesium, phosphorus, sulphur and zinc) as this is an important quality control parameter.

Standard Methods
There are several standard methods available which describe the sample preparation prior to elemental analysis in petroleum products. These methods mainly apply two different approaches. The first method incorporates the dilution of the oil with an organic solvent and direct introduction into an AAS or ICP-OES (ASTM D4951, D5708 – A, D5863 – B, D5185). This method is fast but not applicable for samples containing larger particles. Typical drawbacks are clogged nebulisers, instable plasma conditions and measurement interferences. The second frequently used method is dry ashing (with subsequent acid digestion); dry ashing burns off the organic matrix of the sample. The inorganic residues, the ashes, are dissolved with acids in an open digestion system and subsequently analysed as an aqueous solution (IP 501, ASTM D5708 – B, or D5863 – A). This method allows for processing of large sample quantities (> 10 g) but suffers from significant errors related to loss of volatile elements and results in turnaround times in the range of 10 hours.

Current Change of Technology
More and more laboratories are changing their analytical measuring equipment from FAAS to modern measuring techniques such as ICP-OES or ICP-MS. There are several advantages provided by these techniques: they are rapid, provide multi-elemental capability and, especially for ICP-MS, have a wide dynamic range. Additionally, they have much lower limits of determination and therefore do not require several grams of sample to obtain accurate analytical data. This fact opens the door for modern sample preparation techniques like microwave-assisted closed-vessel digestion. The release of the new standard practice ASTM D7876 (which covers sample decomposition by using microwave heating) confirms the increased importance of this sample preparation method in the petrochemical industry.

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