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D5475-93 – Standard Test Method Technical Guide
🧪 Scope and Applicability
This gas chromatographic (GC) test method, designated ASTM D5475-93 (Reapproved 2002), is designed for the determination of specific nitrogen- and phosphorus-containing pesticides in ground water and finished drinking water. The method utilizes a GC equipped with a Nitrogen-Phosphorus Detector (NPD).
The analytes listed in Table 1 of the standard have been validated for this method through an interlaboratory study involving 10 volunteer laboratories. Summary statistics for mean recovery, overall method precision, bias, and single-analyst precision were calculated using the Interlaboratory Method Validation Study (IMVS) program.
⚠️ Important Limitations: This test method is restricted to analysts experienced in GC and chromatogram interpretation. Analytes that are not separated chromatographically (i.e., have very similar retention times) cannot be individually identified unless alternative techniques exist. Furthermore, analyte identifications in unfamiliar samples must be confirmed by at least one additional qualitative technique. The collaborative study found the method acceptable for all tested analytes except merphos, which was observed to decompose in the GC injection port.
⚙️ Key Terminology and Method Principles
The standard establishes specific terminology to differentiate between quality control measures applied throughout the analytical process. Understanding these definitions is critical for proper method implementation.
| 🟦 Term | 📏 Definition | 🎯 Purpose |
|---|---|---|
| Internal Standard | A pure analyte(s), not a sample component, added to the final solution in a known amount. | Used to measure the relative responses of other test method analytes and surrogates. |
| Surrogate Analyte | A pure analyte(s), extremely unlikely to be found in a sample, added to the sample aliquot before extraction. | Added to monitor test method performance with each specific sample through the entire extraction and analysis procedure. |
📊 Validation and Quality Control Data
The method’s performance was rigorously evaluated. The interlaboratory validation included both reagent water and finished drinking water matrices. The calculated statistics ensure a high level of confidence in the method’s output for the specified analytes. The table below summarizes key validation parameters extracted from the standard.
| 📐 Parameter | ⚡ Specification / Value |
|---|---|
| Matrix Validated | Reagent Water, Finished Drinking Water |
| Participating Laboratories | 10 |
| Validation Software | Interlaboratory Method Validation Study (IMVS) |
| Critical Analyte Limitation | Merphos (decomposes in GC injection port) |
| Analyst Proficiency | Must be demonstrated using the procedure in Section 12.3 |
💡 Proficiency Tip: Section 12.3 of the standard requires each analyst to demonstrate the ability to generate acceptable results with this test method before analyzing field samples. This typically involves the analysis of a quality control check sample that meets the established precision and bias criteria derived from the interlaboratory study.
❓ Frequently Asked Questions
🔍 What specific types of water samples is this test method applicable to?
This test method is explicitly applicable to the determination of certain nitrogen- and phosphorus-containing pesticides in ground water and finished drinking water.
💡 What is the role of a “Surrogate Analyte” as defined by this standard?
A surrogate analyte is a pure compound added to a sample aliquot before extraction. Since it is extremely unlikely to be naturally found in the sample, measuring it with the same procedures allows the analyst to monitor the overall performance of the test method for that specific sample throughout the entire analytical process.
⚡ Why was the pesticide Merphos considered unacceptable during the collaborative study?
The collaborative study showed the test method to be acceptable for all analytes tested except merphos, which was found to decompose within the GC injection port, leading to inaccurate or unreliable results.
📌 What should an analyst do if an unfamiliar sample contains analytes with very similar retention times?
According to Sections 1.5 and 1.6, analytes that are not chromatography separated cannot be individually identified in the same calibration mixture or water sample unless an alternative identification and quantitation technique exists. Furthermore, identifications of compounds in unfamiliar samples should be confirmed by at least one additional qualitative technique.