Which method is NOT approved for the detection of fluoride in samples?

Gas chromatography is not an approved method for the detection of fluoride in water samples. This is primarily because gas chromatography is typically used for the analysis of volatile organic compounds and some gases, rather than for ionic species like fluoride, which are not volatile and require different analytical techniques.

The approved methods for detecting fluoride include SPADNS, ion-selective electrode, and colorimetric analysis. SPADNS is a colorimetric method that specifically reacts with fluoride ions to produce a measurable color change, making it a reliable choice for fluoride detection. Ion-selective electrodes are also specifically designed to measure the concentration of fluoride ions in a solution, providing accurate and direct measurements. Colorimetric analysis is a broad technique that can encompass various methods, including those specifically developed for fluoride detection, illustrating its versatility and reliability in analyzing this ion in drinking water.

In summary, while there are established techniques for measuring fluoride levels, gas chromatography is not suitable for this purpose, highlighting the importance of choosing appropriate methods based on the chemical nature of the analyte.