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Digestion of Petroleum Products and Lubricants

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.

Introduction

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. Silicon and aluminum serve as indicators for the presence of abrasive silicate particles, causing harm to the combustion engine. Being present as particles adds the necessity for very thorough high-speed mixing to the sampling and the sample preparation procedures. Otherwise lower recoveries for particle bound elements like silicon and partially aluminum can be experienced.

In unused products like lubricating oils the concentration of additives (which contain metals such as calcium, copper, magnesium, phosphorus, sulfur and zinc) is an important quality control parameter.

There are several standard methods available, which describe the sample preparation prior to elemental analysis in petroleum products. Basically these methods apply mainly two different approaches:

  • Dilution of the oil with an organic solvent such as xylene and kerosene and direct introduction into an AAS or ICP-OES. This method is fast but not applicable for samples containing metal particles. Typical drawbacks are clogged nebulizers, instable plasma conditions and measurement interferences. Common methods applying dilution are ASTM D4951, D5708 (Test Method A), D5863 (Test Method B), or D5185.
  • Dry ashing (with subsequent acid digestion) of the sample burns off the organic matrix. The inorganic residues (ash) are dissolved with (diluted) 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 and ashing times of several hours. Common methods for dry ashing are IP 501, ASTM D5708 (Test Method B), or D5863 (Test Method A).

Due to the lower limits of determination, modern measuring techniques such as ICP-OES or ICP-MS 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.

In order to prove the excellent suitability of microwave-assisted closed-vessel digestion, four different reference materials were digested and the results obtained thereof were compared with conventional methods ASTM D5708, D4951, and D5185.

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