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D3371-95 – Standard Test Method Technical Guide
🔬 Scope, Significance, and Sample Preservation
ASTM D3371-95 provides a standard test method for identifying and quantifying specific organic nitriles in aqueous solution using gas-liquid chromatography (GLC). This method is applicable to nitriles that can be separated and detected at a limit of approximately 1 mg/L via direct aqueous injection. The procedures and general precautions outlined in Practice D 2908 are integral to this method.
Nitriles, even at concentrations of a few milligrams per litre, pose a potential toxicity risk to aquatic life. This GLC technique is crucial for detecting and determining mixtures of nitriles in wastewater discharges where traditional wet chemical procedures fall short of the required sensitivity and specificity.
⚠️ Critical Precaution – Sample and Standard Stability: For accurate quantitation, samples that cannot be analyzed immediately should be quick frozen. Furthermore, samples must be neutralized to pH 7 at the time of collection to minimize the hydrolysis of the nitrile groups. Pure nitriles used for standards are inherently unstable and must be stored in a freezer.
💡 Operator Guidance – Instrument Optimization: It is not always practical to directly transfer operating conditions from one GLC instrument to another. Operators must carefully optimize their specific instrument, including injection port temperature, detector temperature, and carrier gas flow rates, to achieve the required separation and sensitivity.
⚙️ Instrument Parameters and Chromatographic Procedure
The method specifies a packed column configuration with a flame ionization detector to achieve the necessary separation of nitrile compounds. The following table details the typical instrument parameters used to obtain the standard reference chromatograms.
| 🟦 Parameter | 📏 Specification / Condition |
|---|---|
| Column Material & Dimensions | ⅛ in. OD stainless steel, 8 ft (2.44 m) length |
| Column Packing | Porous styrene divinylbenzene polymer (e.g., Chromosorb 101, 50/60 mesh) |
| Detector | Flame Ionization Detector (FID) |
| Injection Port Temperature | 240°C |
| Detector Temperature | 240°C |
| Oven Temperature (Isothermal) | 130°C |
| Oven Temperature (Programmed) | 110°C to 200°C at 10°C/min |
| Carrier Gas | Helium at 25 mL/min |
| Sample Size (Isothermal) | 5 µL |
| Sample Size (Programmed) | 3 µL |
| Recorder Settings | ¾ in./min chart speed, 1 mV full-scale response |
📊 Key Measured Properties and Kovats Index Values
Compound identification is based on the Kovats retention index system and relative retention times. These values serve as the definitive benchmarks for identifying the specific nitriles present in a sample. The following table presents the standard data for the target analytes.
| 🧪 Compound | 🔄 Relative Retention | 📐 Kovats Index |
|---|---|---|
| Acetonitrile | 1.00 | 470 |
| Acrylonitrile | 1.25 | 512 |
| Proprionitrile | 1.67 | 570 |
| Methoxyacetonitrile | 2.21 | 635 |
| Butyronitrile | 2.50 | 678 |
| Isovaleronitrile | 3.04 | 740 |
| Valeronitrile | 3.38 | 783 |
| Hexanenitrile | 4.25 | 905 |
| Benzonitrile | 5.42 | 990 |
| Note: Kovats index values marked with a footnote in the standard (e.g., Methoxyacetonitrile, Isovaleronitrile, Hexanenitrile) are estimated from relative retention data per ASTM AMD25A (1967) because standard compounds were not readily available. | ||
The precision and bias of this test method were established by an interlaboratory study conducted in 1972, which involved four laboratories and seven operators, providing a validated basis for its use in regulatory and environmental applications.
❓ Frequently Asked Questions
🔍 What is the detection limit for nitriles using this method?
This test method is capable of detecting and quantitatively measuring nitriles in aqueous solution at a limit of approximately 1 mg/L using the specified GLC column and conditions.
💡 How should water samples containing nitriles be preserved?
According to the standard, samples that cannot be analyzed immediately should be quick frozen. It is also critical to neutralize the samples to a pH of 7 at the time of collection to prevent the hydrolysis of the nitrile functional groups.
⚡ What are the standard instrument conditions for this test?
The standard specifies an 8 ft x ⅛ in. OD stainless steel column packed with a porous styrene divinylbenzene polymer. A Flame Ionization Detector (FID) is employed with an injection port and detector temperature of 240°C. Helium is used as the carrier gas at a flow rate of 25 mL/min.
📌 Why is it important to monitor nitriles in wastewater?
Nitriles at concentrations of just a few milligrams per litre are potentially toxic to aquatic life. This GLC method is specifically designed to detect and determine mixtures of nitriles at these critical levels where traditional wet chemical procedures are not sensitive or selective enough.