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D6508-15 – Standard Test Method Technical Guide
📐 Scope and Applicable Matrices
This test method, designated ASTM D6508-15, covers the determination of the dissolved inorganic anions fluoride, bromide, chloride, nitrite, nitrate, orthophosphate, and sulfate in drinking water, wastewater, and other aqueous matrices. The methodology employs capillary ion electrophoresis (CIE) with a chromate-based electrolyte and indirect UV detection at a fixed wavelength of 254 nm. The values stated in SI units are to be regarded as standard.
It is the responsibility of the user to ensure the validity of this test method for untested aqueous matrices or for anion concentrations falling outside the specified ranges. For specific hazard statements, refer to Section 9 of the standard.
⚠️ User Responsibility: This standard does not purport to address all safety concerns. It is the responsibility of the user to establish appropriate safety, health, and regulatory practices prior to use, particularly when handling samples and reagents.
⚙️ Principle and Test Procedure
Capillary Ion Electrophoresis (CIE) separates anions based on their differential electrophoretic mobilities within a fused-silica capillary (50 µm to 75 µm internal diameter). The capillary is filled with a chromate-based electrolyte, which includes an electroosmotic flow (EOF) modifier to control the direction and velocity of the bulk solution. A voltage applied across the capillary drives the electrolyte and the injected sample anions toward the UV detection window.
Detection is achieved through indirect UV absorbance at 254 nm. The chromate ion absorbs strongly at this wavelength, providing a high background signal. When a non-UV-absorbing analyte anion passes through the detector, it displaces the chromate, causing a temporary decrease in absorbance. This negative peak is integrated and quantified against calibration standards. Proper sampling must follow Practices D3370, and overall method quality control must adhere to Practice D5847 and Guide D3856.
✅ Validated Performance: The method has been successfully validated for the specified anion ranges. An inter-laboratory precision and bias study (Note 1) extends the accepted concentration for Chloride up to 93 mg/L, Sulfate to 90 mg/L, Nitrate to 72 mg/L, and Orthophosphate to 58 mg/L.
📊 Analytic Ranges and Method Specifications
The following table summarizes the nominal concentration ranges for the seven target analytes and highlights the highest concentrations accepted during the precision and bias study.
| 🟦 Analyte | 📏 Symbol | 📐 Nominal Range (mg/L) | ⚡ Max Accepted* (mg/L) |
|---|---|---|---|
| Fluoride | F⁻ | 0.1 – 25 | — |
| Bromide | Br⁻ | 0.1 – 50 | — |
| Chloride | Cl⁻ | 0.1 – 50 | 93 |
| Nitrite | NO₂⁻ | 0.1 – 50 | — |
| Nitrate | NO₃⁻ | 0.1 – 50 | 72
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