Petroleum products and lubricants are routinely analyzed for their elemental content for various reasons. 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 to achieve reliable results in a short time frame.
The concentrations of silicon, aluminum, vanadium, nickel, iron and sodium in crude and residual oils are used to define their quality and value. High amounts of nickel and vanadium in crude oil can deactivate catalysts during processing, but also initiate corrosion in motors and boilers during combustion when present in fuels. Silicon and aluminum present in residual fuel oils cause abrasion within the combustion engine. Products like lubricating oils are tested before use to determine the concentration of additives (which contain metals such as calcium, copper, magnesium, phosphor, sulfur and zinc), which is an important quality control parameter. Prior to measurement utilizing modern measuring techniques such as ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) or ICP-MS (Inductively Coupled Plasma Mass Spectrometry) samples have to be transferred into a measurable (liquid, removed matrix) form.
Current methods for sample preparation of petroleum products
There are several standard methods available which describe the sample preparation of petroleum products. The most common methods currently are:
Method 1: The dilution of the oil is incorporated with an organic solvent, followed by direct introduction into an AAS or ICP-OES. This method is fast but not applicable to samples containing larger particles. Typical drawbacks are clogged nebulizers, unstable plasma conditions and measurement interferences.
Method 2: A 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 analyzed as an aqueous solution. This method allows for processing of large sample quantities (> 10 g) but suffers from significant errors related to loss of volatile elements, resulting in turnaround times in the range of 10 hours.
Microwave-assisted closed-vessel digestion: The new method
A new, increasingly popular method for the sample preparation of petroleum products is microwave-assisted closed-vessel digestion.
Using microwave-assisted closed-vessel digestion, the sample is decomposed with concentrated acids under pressure, resulting in a clear aqueous solution with low residual carbon content. Due to the application of closed pressure vessels, the temperature is not restricted to the boiling point of the acids. According to the Arrhenius Law, a 10 °C increase in temperature means a twofold acceleration of the reaction and halves the reaction time. As a result of the achievable high temperature, the time required for complete digestion is significantly reduced. Microwaves are the perfect heating source as they directly heat the liquid bulk while the surroundings, such as vessels, remain comparably cool. As a consequence, microwave heating is fast and energy-efficient, saving additional process time.
For example, employing Multiwave PRO with Rotor 8NXF100 can reduce the sample preparation time from more than 5 hours to less than 90 minutes.
Microwave Reaction System: Multiwave PRO
Further advantages of this modern sample preparation technique are that no volatiles are lost, the risk of contamination is minimized and a lower amount of reagents is required. This reduces both the consumption of expensive reagents and the costly disposal of hazardous waste. In addition, based on the precise documentation, the sample preparation procedure is highly reproducible and leads to reliable measuring results.