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D6350-24 – Standard Test Method Technical Guide
🧪 Scope and Summary of Test Method
ASTM D6350-24 specifies a robust standard test method for the determination of total mercury in gaseous fuels, including natural gas, using gold amalgamation coupled with atomic fluorescence spectrometry (AFS). Approved in 2024, it reinstates the previously withdrawn D6350-14. The method is designed to measure mercury at concentrations as low as 0.001 µg/m³ and is explicitly stated to detect all forms of mercury—elemental, inorganic, and organic—providing a true total gaseous mercury concentration.
The sample gas is depressurized and routed through a speed loop bypass system to ensure a representative sample. Mercury is quantitatively absorbed and preconcentrated onto two gold-coated silica traps arranged in series. The traps are subsequently heated above the dewpoint of the gas stream under reduced sample pressure to release the mercury vapor, which is then swept into the AFS detector. All values are to be regarded in SI units as the standard.
⚠️ Health and Safety Advisory: Mercury is classified by numerous regulatory agencies as a hazardous material that can cause serious medical issues and is corrosive to materials. Users must consult the applicable Safety Data Sheet (SDS) and verify that selling mercury or mercury-containing products is permitted by law in their jurisdiction.
⚙️ Key Terminology, Apparatus, and Performance Data
For definitions of general gaseous fuel terms, the standard references Terminology D4150. Specific terms defined within this method include the detection limit (the statistically derived lowest quantity of analyte distinguishable from background signal) and the limit of quantification (the lowest concentration measurable with a defined error and confidence level). The primary acronyms employed are AFS (Atomic Fluorescence Spectrometry) and PTFE (Polytetrafluoroethylene).
| 🟦 Parameter | 📏 Specification / Value | 🎯 Details |
|---|---|---|
| Analyte | Total Mercury (Hg) | Sum of Elemental, Inorganic, and Organic forms |
| Matrix | Gaseous Fuels (Natural Gas) | Sampled via speed loop bypass for representativity |
| Detection Limit | ≤0.001 µg/m³ | Statistically derived from background signal (13.4) |
| Detection Principle | Atomic Fluorescence Spectrometry (AFS) | Gold amalgamation with thermal desorption |
| Sampling Trains | Two gold-coated silica traps in series | Primary trap + breakthrough check |
| Units | SI Units | Standard per Section 1.2 |
💡 Critical Sampling Note: The use of a speed loop bypass is essential for obtaining a representative sample. Direct sampling from stagnant lines or dead legs can introduce significant negative bias due to mercury wall adsorption and thermal stratification.
📊 Referenced Standards and Supporting Documents
The method is aligned with international best practices and references several key standards for terminology and methodology.
| 📚 Standard / Document | 🔬 Title | ⚙️ Relevance to D6350-24 |
|---|---|---|
| ASTM D4150 | Terminology Relating to Gaseous Fuels | Defines general gaseous fuel terms used in the method |
| ASTM D5954 | Mercury Sampling and Measurement in Gaseous Fuels by AA | Related test method using atomic absorption spectroscopy |
| ISO 6978-2 | Natural gas — Determination of mercury — Part 2 | Sampling of mercury by amalgamation on gold/platinum alloy |
The standard was developed in accordance with internationally recognized principles established by the WTO Technical Barriers to Trade (TBT) Committee, ensuring global applicability and rigor.
❓ Frequently Asked Questions
🔍 What is the specific scope of D6350-24?
The standard specifies a test method for the determination of total gaseous mercury in gaseous fuels (e.g., natural gas) by gold amalgamation atomic fluorescence spectroscopy (AFS) down to concentrations of 0.001 µg/m³.
💡 Why are two gold-coated silica traps required in series?
The first trap serves as the primary sampling media for mercury amalgamation. The second trap acts as a strict breakthrough check; if mercury is detected on the second trap, it indicates the first trap was saturated and the quantitative result for that sample is invalidated.
⚡ What is the difference between the Detection Limit and the Limit of Quantification?
The Detection Limit is a statistically derived value representing the lowest quantity of analyte that can be confidently distinguished from background noise. The Limit of Quantification is a higher threshold defining the lowest concentration at which the instrument can measure reliably with a pre-defined error and confidence level.
📌 Does this standard replace previous methods?
D6350-24 was reinstated in 2024, replacing the withdrawn D6350-14. It is a companion method to D5954 (Atomic Absorption Spectroscopy). While both determine mercury, D6350-24 specifically utilizes Atomic Fluorescence Spectrometry (AFS) for detection and emphasizes speed loop bypass for representative sampling.