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D3921-96 – Standard Test Method Technical Guide
📖 Scope and Definitions
This standard test method, designated ASTM D3921-96 (Reapproved 2011), is under the jurisdiction of ASTM Committee D19 on Water and has been approved for use by agencies of the Department of Defense. It provides a procedure for determining fluorocarbon-extractable substances in water and wastewater, serving as an estimation of the combined oil and grease and petroleum hydrocarbon contents. The method is applicable across a range from 0.5 to 100 mg/L. The values stated in SI units are to be regarded as the standard.
Key definitions established by the method include:
- Oil and Grease: The organic matter extracted from water or waste water and measured by infrared absorption in this test method.
- Petroleum Hydrocarbons: The fraction of oil and grease that remains in solution after the extract is contacted with silica gel and is subsequently measured by infrared absorption.
It is the user’s responsibility to validate the standard for untested types of water.
⚙️ Extraction and Quantitative Procedure
The analytical procedure is based on liquid-liquid extraction followed by infrared spectroscopic measurement.
⚠️ Critical Note on Volatility: Section 1.3 explicitly notes that low-boiling organic materials are lost by evaporation during manipulative transfers. However, these evaporative losses are generally much lower than those experienced with traditional gravimetric procedures that require complete solvent evaporation before the residue is weighed. Analysts must execute transfers efficiently to minimize losses.
- Sample Acidification: The water or wastewater sample is acidified prior to the extraction process.
- Serial Extraction: The acidified sample is extracted serially with three 30-mL volumes of 1,1,2-trichloro-1,2,2-trifluoroethane (referred to in the method as the solvent).
- Extract Preparation: The combined serial extracts are diluted to a final volume of 100 mL.
- Infrared Analysis (Oil & Grease): A portion of the extract is examined by infrared (IR) spectroscopy to measure the total amount of oil and grease removed from the original sample, following the general techniques of Practices E168.
- Silica Gel Treatment (Petroleum Hydrocarbons): A major portion of the remaining extract is contacted with silica gel to remove polar substances. This treated extract, now representing the petroleum hydrocarbon fraction, is then similarly examined by IR spectroscopy.
📊 Method Specifications and Key Parameters
| 🟦 Key Parameter | 📐 Specification from Standard |
|---|---|
| Working Range (Oil & Grease) | 0.5 to 100 mg/L |
| Extraction Solvent | 1,1,2-Trichloro-1,2,2-trifluoroethane |
| Sample Condition | Acidified prior to extraction |
| Extraction Procedure | Serial extraction (3 x 30 mL portions) |
| Final Extract Volume | 100 mL |
| Quantitative Measurement | Infrared Spectroscopy (Per Practices E168) |
| Oil & Grease Definition | Total fluorocarbon-extractable matter measured by IR |
| Petroleum Hydrocarbons Definition | Oil & grease not adsorbed by silica gel, measured by IR |
🧪 Essential Quality Assurance Framework: The standard relies on a comprehensive suite of companion standards. Sample collection must follow Practices D3370 (Sampling from Closed Conduits) and preservation per Practice D3325. Reagent water must comply with Specification D1193. Laboratories should structure their management systems according to Guide D3856 and apply QC specifications following Practice D5847. Precision and bias determinations should be conducted in accordance with Practice D2777.
❓ Frequently Asked Questions
🔍 What is the fundamental difference between Oil & Grease and Petroleum Hydrocarbons in this method?
Oil and Grease is defined as the total matter which is extractable by the solvent (1,1,2-trichloro-1,2,2-trifluoroethane) and measurable by infrared absorption. Petroleum Hydrocarbons are defined specifically as the fraction of that extracted Oil and Grease which is not adsorbed by silica gel during the procedure. The silica gel removes polar organic substances, leaving the non-polar petroleum hydrocarbons in solution for subsequent IR measurement.
💡 What is the specified analytical range of the method?
The test method standard covers the determination of these substances in the range from 0.5 to 100 mg/L. It is noted in the scope that this method may not address all safety concerns, and it is the user’s responsibility to establish safety practices and validate the method for specific untested types of water.
⚡ How is the extraction procedure performed?
The analysis begins with an acidified sample. This sample is then extracted serially with three 30-mL volumes of the specified fluorocarbon solvent. The extracts are combined and diluted to a final volume of 100 mL before infrared analysis. For petroleum hydrocarbon determination, a major portion of the remaining extract is treated with silica gel prior to IR measurement.
📌 What are the key referenced companion standards relied upon by D3921?
D3921 integrates several other ASTM standards. Key ones include Terminology D1129, Reagent Water Specification D1193, Precision and Bias Practice D2777, Water Sampling Practices D3370, Oil Sample Preservation D3325, Laboratory Management Guide D3856, and Infrared Quantitative Analysis Practices E168.