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API Publ 347-1998: Performance Evaluation of EPA Method 1664 for Oil and Grease Determination
Critical Insights from API’s Assessment of the 1998 Gravimetric Method for Water Quality Monitoring
Scope and Purpose of API Publ 347-1998
API Publ 347-1998, titled Assessment of the Performance of the 1998 EPA Method 1664 for the Determination of Oil and Grease, is a comprehensive evaluation conducted by the American Petroleum Institute (API). This publication provides critical interlaboratory data on the precision, accuracy, and method detection limits (MDL) of EPA Method 1664, which uses n-hexane extraction followed by gravimetric determination. The scope encompasses the analysis of oil and grease in aqueous matrices, including reagent water, municipal wastewater effluents, industrial wastewaters, and surface waters. The study involved multiple laboratories analyzing spiked samples and real effluents, with the goal of validating the method as a replacement for earlier Freon-113-based procedures mandated by the Clean Water Act.
Technical Requirements of EPA Method 1664
The method specifies a rigorous set of procedural steps to ensure reliable quantification of oil and grease. Samples are collected in glass containers, preserved at pH ≤2 with hydrochloric acid, and stored at 4°C. Analysts must use high-purity n-hexane that has been distilled to remove non-volatile residues. The extraction is carried out in a separatory funnel by serially extracting a 1 L sample with three 30 mL portions of n-hexane. The combined extract is dried over sodium sulfate and concentrated using rotary evaporation at 40°C. The final residue is dried to constant weight in a desiccator and reported as mg/L of n-hexane extractable material.
Key quality control measures include analysis of laboratory blanks, matrix spikes, duplicate samples, and a continuing calibration verification using a reference oil. The publication details the standard operating procedures (SOPs) adopted by participating laboratories during the interlaboratory study, emphasizing the need for solvent purity, careful pH adjustment, and minimization of evaporative losses.
Tip: For best results, ensure that the n-hexane used for extraction is of sufficient purity (e.g., pesticide residue grade) and that fresh distillations are performed if prolonged storage is required. Any non-volatile residue in the solvent will directly increase the measured mass and bias the result upward.
Implementation Highlights in Environmental Monitoring
The study demonstrated that EPA Method 1664 produces consistent recoveries across a range of oil and grease concentrations from 5 to 100 mg/L. The method is now widely used for compliance monitoring under the National Pollutant Discharge Elimination System (NPDES) permits and is the standard test method for wastewater discharge limits in many jurisdictions. Key implementation highlights from the API publication include:
- Comparability with prior methods: Results using n-hexane show no statistical bias compared to Freon-113 extraction, making the method a direct replacement without changing permit limits.
- Environmental benefit: Substituting n-hexane for Freon-113 eliminates the use of ozone-depleting substances while maintaining equivalent analytical performance.
- Matrix robustness: Acceptable recoveries were obtained for both clean and complex matrices, though samples with high surfactant content required additional precautions.
- Precision data: Repeatability relative standard deviations (RSDr) ranged from 4.7% to 8.0%, while reproducibility RSDR varied from 11.3% to 16.7% across participating laboratories.
| Matrix | Spiked Concentration (mg/L) | Mean Recovery (%) | Repeatability RSD (%) | Reproducibility RSD (%) |
|---|---|---|---|---|
| Reagent Water | 10.0 | 95.2 | 4.7 | 11.3 |
| Municipal Wastewater Effluent | 10.0 | 91.5 | 6.2 | 14.1 |
| Industrial Wastewater | 50.0 | 93.8 | 5.5 | 12.8 |
| Surface Water | 5.0 | 88.4 | 8.0 | 16.7 |
Success: The interlaboratory study in API Publ 347-1998 confirms that EPA Method 1664 is a robust and reliable method for oil and grease analysis when properly implemented. Formal adoption of the method by regulatory agencies has streamlined compliance efforts across the water sector.
Compliance Notes and Quality Assurance
To achieve defensible data under regulatory programs, laboratories must adhere to the quality assurance protocols outlined in the publication. The method detection limit (MDL) established during the study was approximately 1.4 mg/L, though individual laboratory MDLs may vary depending on instrumentation and technique. Practitioners should be aware of common interferences:
- Emulsifiers and surfactants: These can increase the apparent oil and grease concentration by stabilizing fine droplets that are co-extracted. Addition of a demulsifier or performing a silica gel cleanup can mitigate this bias.
- High levels of dissolved organic matter: Humic and fulvic acids may be partially extracted by n-hexane, leading to positive bias. A separate silica gel treatment is recommended for samples with high organic content.
- Non‑target residue: Any non‑volatile material that extracts into n‑hexane (e.g., certain plasticizers, vegetable oils) will be included in the measurement. Reporting should clearly state the determinand as “n‑hexane extractable material (oil and grease).”
The publication also stresses the importance of blank contamination control, regular calibration verification using a certified reference oil, and participation in interlaboratory proficiency testing. Reporting units must be expressed as mg/L, and all results should be accompanied by the method used (EPA 1664).
WARNING: Samples containing heavy emulsification may require additional extraction steps or the use of demulsifiers. Incomplete phase separation can lead to erroneously high recoveries. Always check for emulsion breakthrough during the extraction process.
DANGER: n‑Hexane is a highly flammable liquid (flash point –23°C) and is classified as a neurotoxin. All handling must be performed in a properly functioning fume hood. Sources of ignition (sparks, open flames, hot surfaces) must be eliminated. Wear appropriate personal protective equipment, including solvent‑resistant gloves and safety goggles.
Frequently Asked Questions
Q: Is EPA Method 1664 equivalent to previous methods that used Freon‑113?
A: Yes. API Publ 347-1998 demonstrates that the n‑hexane‑based method provides statistically equivalent performance to Freon‑113 methods in terms of recovery and precision, making it a suitable replacement without requiring changes to permit limits.
A: Yes. API Publ 347-1998 demonstrates that the n‑hexane‑based method provides statistically equivalent performance to Freon‑113 methods in terms of recovery and precision, making it a suitable replacement without requiring changes to permit limits.
Q: What is the method detection limit reported in the study?
A: The collaborative study established an MDL of approximately 1.4 mg/L for oil and grease in aqueous matrices. Individual laboratory MDLs may differ based on equipment and technique.
A: The collaborative study established an MDL of approximately 1.4 mg/L for oil and grease in aqueous matrices. Individual laboratory MDLs may differ based on equipment and technique.
Q: Can this method be used for saline waters such as seawater or produced water?
A: Yes, the method is applicable to saline waters, including estuarine, marine, and produced water samples. However, matrix effects (e.g., high ionic strength) should be evaluated through matrix spike recoveries. Silica gel cleanup may be necessary if interferences from dissolved organic matter are suspected.
A: Yes, the method is applicable to saline waters, including estuarine, marine, and produced water samples. However, matrix effects (e.g., high ionic strength) should be evaluated through matrix spike recoveries. Silica gel cleanup may be necessary if interferences from dissolved organic matter are suspected.
Q: Why is silica gel treatment recommended?
A: Silica gel adsorption removes polar organic compounds that are co‑extracted by n‑hexane, such as carboxylic acids, alcohols, and esters. This allows the measurement to more closely represent petroleum‑derived hydrocarbons, if desired.
A: Silica gel adsorption removes polar organic compounds that are co‑extracted by n‑hexane, such as carboxylic acids, alcohols, and esters. This allows the measurement to more closely represent petroleum‑derived hydrocarbons, if desired.
Reference: API Publ 347-1998 – Assessment of the Performance of the 1998 EPA Method 1664 for the Determination of Oil and Grease. This article was prepared for informational purposes in 2026.