IP 346 Determination of Polycyclic Aromatics - Dimethyl Sulfoxide Extraction Refractive Index Method

The standard “IP 346 Determination of polycyclic aromatics in unused lubricating base oils and asphaltene-free petroleum fractions - Dimethyl sulfoxide extraction refractive index method,” published by the Energy Institute as part of IP test methods, describes a test method for determining polycyclic aromatics (PCA) in 5% recovered samples of unused, additive-free lubricating base oils with an atmospheric boiling point of at least 300 degrees Celsius, at concentrations ranging from 1-15% (m/m) by mass.

This method can also be applied to PCA concentrations outside this range and to other asphaltene-free petroleum fractions, but the sensitivity has not been determined.

The analytical test method defined in the IP 346 standard measures the content of polycyclic aromatics (PCA) or polycyclic aromatic compounds (PAC) in lubricating base oils and similar petroleum fractions. These compounds, particularly certain 3-7 ring structures, are associated with potential dermal carcinogenicity (risk of skin cancer).

In Europe, under the REACH/CLP regulations, obtaining extractable substances with less than 3% dimethyl sulfoxide using the IP 346 method allows the substance not to be classified as carcinogenic. This threshold is legally binding for certain petroleum streams, including other lubricating base oils, foot oils, and processed distillate aromatic extracts.

In general, the IP 346 method follows these steps:

• Sample preparation: A weighed quantity of oil (typically 4 grams) is dissolved in cyclohexane.
• Extraction: The solution is extracted twice with dimethyl sulfoxide, which selectively attracts polar aromatic compounds (especially potentially carcinogenic ones).
• Back extraction and processing: Brine is added to precipitate the extracted substance, then it is back extracted into cyclohexane, dried, and the solvent is evaporated.
• Measurement: The residue is weighed to determine the percentage of dimethyl sulfoxide-extractable substance (the original method also includes refractive index for verification). The result is expressed as a percentage by mass of dimethyl sulfoxide-extractable substance.

Dimethyl sulfoxide extraction preferentially extracts higher-ring, more polar polycyclic aromatics (those linked to carcinogenicity) while leaving behind less harmful, highly alkylated lower-ring aromatics common in well-refined oils. This makes the test a good indicator of true biological carcinogenicity, as corroborated by long-term mouse skin staining studies.

The importance of this method in industry is as follows:

• Refining control: Modern base oil production (solvent extraction or intensive hydrogenation process) aims to keep the values ​​obtained using the IP 346 method well below 3% to ensure the oil is not carcinogenic.
• Safety data sheets: Many lubricant suppliers report IP 346 method results to demonstrate regulatory compliance.
• Predictive value: Studies show a strong correlation between low IP 346 scores and adverse results in animal carcinogenicity tests.

This method has the following limitations:

• This applies primarily to unused, additive-free lubricating base oils and asphaltene-free fractions.
• Not suitable for finished lubricants (those containing additives) or oils containing asphaltene.
• This is a screening method; further tests may be required for a complete toxicological assessment.

The IP 346 standard remains one of the most important and practical tools for ensuring the safety of mineral-based lubricating oils used in motors, machinery, and industrial applications worldwide. It represents a successful example of industry-developed science directly supporting chemical regulation.

In short, the test method described in the IP 346 standard is an important petroleum industry test method used to determine the amount of polycyclic aromatic compounds or polycyclic aromatics present in unused lubricating base oils and asphaltene-free petroleum fractions.

The IP 346 standard is widely referenced in lubricant production, toxicological assessments, REACH compliance, rubber oil classification, and petroleum product safety evaluations.

The primary purpose of the IP 346 standard is to assess the concentration of potentially carcinogenic polycyclic aromatic compounds found in petroleum-derived oils.

Polycyclic aromatic compounds are naturally occurring components in crude oil. Certain polycyclic aromatic compounds, particularly those containing multiple aromatic rings, are associated with carcinogenicity and other health concerns. The IP 346 standard provides a practical screening method for predicting these harmful components.

This test is especially important for:

• Lubricating base oils
• Process oils
• Rubber fillers
• White fats
• Oil waxes
• Refinery intermediate flows

In principle, the IP 346 standard combines the dimethyl sulfoxide extraction technique with refractive index measurements. The petroleum sample is mixed with dimethyl sulfoxide. Dimethyl sulfoxide selectively extracts aromatic compounds from the oil. The extracted fraction is separated. The concentration of the extracted aromatics is determined gravimetrically and through refractive index correlation. This method is designed to selectively isolate aromatic compounds with the highest carcinogenic potential.

The IP 346 standard applies to the following:

• Unused, additive-free lubricating base oils
• Petroleum fractions that do not contain asphalt
• Samples with atmospheric boiling points above 300 degrees.
• Typically, polycyclic aromatic concentrations range from 1-15 percent by mass.

Although this method can sometimes be used outside these limits, sensitivity data were initially validated within this range.

The IP 346 standard is considered important for the following reasons:

• Health and safety assessment: The test is closely linked to the toxicological evaluation of petroleum products. Oils with high polycyclic aromatic content may pose carcinogenic risks during prolonged skin exposure. Studies have shown a strong correlation between IP 346 results and conventional mouse skin carcinogenicity studies.
• Compliance with regulations: In Europe, it is widely used in chemical regulations such as IP 346, REACH and CLP classification systems. The frequently referenced threshold is: dimethyl sulfoxide extract at a maximum of 3 percent. Products below this limit are generally considered to have a lower carcinogenic potential. This threshold is particularly important for:
  • Rubber processing oils
  • Tire manufacturing oils
  • Lubricant base stocks
  • Petroleum-derived process oils
• Quality control in refineries: Refineries use this standard to monitor the effectiveness of refining processes such as solvent extraction, hydrocracking, hydrotreatment, and hydrogenation. More intensive refining generally reduces polycyclic aromatic content and improves product safety.
• Environmental and occupational health protection: This standard helps to limit hazardous aromatic compounds by:
  • To reduce worker exposure.
  • Improving workplace safety
  • To support environmentally safer petroleum products.
  • Facilitating more secure sub-applications.

Its long-standing acceptance in the lubrication, refining, and rubber industries has made the IP 346 standard a significant standard in petroleum toxicological assessment and quality control.

For many years, our organization, with its competent and experienced team, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting in the establishment, implementation, and improvement of management systems. Within this framework, we also provide IP 346 standard compliant testing services to businesses.