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Sample preparation of petroleum products

Petroleum products and lubricants are routinely analyzed for their elemental content[1]. Before use, products like lubricating oils are tested to determine the concentration of additives (which contain metals such as calcium, copper, magnesium, phosphor, sulfur, and zinc) as this is an important quality control parameter. 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.

Trace metal analysis is also often performed to protect the environment: Waste products from refineries, such as waste waters and sludge, have to be investigated to determine how to dispose of them correctly. Prior to analysis via modern techniques[2] 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 form. This article summarizes the options for the sample preparation of petroleum products prior to trace metal analysis.

Conventional 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 the loss of volatile elements, resulting in turnaround times in the range of ten hours.

Microwave-assisted closed-vessel digestion

Microwave Reaction Platform: Multiwave 5000

Using microwave-assisted closed-vessel digestion[3], 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. 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 a microwave digestion system with the appropriate high-performance rotor can reduce the sample preparation time from more than five hours to less than 90 minutes.

Digesting petroleum-based samples often means dealing with heightened reactivity and samples that are difficult to handle. Safety is therefore a major concern and additional active and passive safety features (self-checks, resealing safety door, status lights, control mechanisms) as well as comprehensive reaction control (temperature- and pressure control) are indispensable.

Besides increased safety and reduced processing time, the main advantages of this modern sample preparation technique are:

  • no volatiles are lost
  • the risk of contamination is minimized
  • 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.

Conclusion

Conventional methods for the sample preparation of petroleum products and lubricants, like dilution or ashing, suffer from severe drawbacks, as they are unsafe, unreliable, time-consuming and/or cost-intensive. Microwave-assisted closed-vessel digestion is a modern alternative which significantly reduces the time required for digestion and does not lose volatile components.

References

  1. Bulska, E., Matusiewicz, H. (2017). Inorganic Trace Analytics: Trace Element Analysis and Speciation. Walter de Gruyter GmbH & Co KG.
  2. Crouch, S. R., Holler, F. J., Skoog, D. A. (2017). Principles of Instrumental Analysis. Cengage learning, pp 231–273.
  3. https://www.astm.org/Standards/D7876.htm 

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