Separation of fluoride in the presence of large amounts of organic acids by ion chromatography

ABSTRACT

An ion chromatography (IC) method for the separation of fluoride in presence of large concentrations of organic acid anions was developed with a view to determine trace level concentration of fluoride in plant samples. Present study deals with determination of fluoride in dry black gram seeds. Pyrohydrolysis was employed for extracting fluoride from the seeds. Organic acid anions are produced during pyrohydrolysis of seeds/plant materials and these organic anions show similar retention behavior of fluoride in IC. Separation of fluoride from organic acids and common anions was investigated. An IC method involving the appropriate dilution of pyrohydrolysis distillate of the sample followed by a large volume injection through a pre-concentrator column is proposed to circumvent the interference due to organic acids. Fluoride recovery of the pre-concentrator column is found to be more than 90%. The method provides a precision better than 7% (RSD). The limit of detection of the method is 0.5ppm_w whereas the LOD for fluoride by IC method alone is 10 ppb. Results obtained are validated by analyzing the samples by using fluoride ion selective electrode method.

KEYWORDS:

• Pyrohydrolyis
• ion chromatography
• fluoride
• organic acids
• anion trace concentrator column
• black gram seeds

Acknowledgements

Authors thank Dr. S. Kannan, Director, RC&IG, and BARC for his kind support and encouragement.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Novelty statement

• Ion chromatography separation of fluoride in presence of large amounts of organic acids having similar retention behavior of fluoride is reported for the first time.

• A new approach based on dilution of sample, pre-concentration of fluoride and separation with dual columns of small dimension is proposed.

• Pyrohydrolysis, a simple combustion technique, is successfully employed for the initial separation of fluoride inseeds.

• Controlled pyrohydrolysis of sample with U₃O₈ as blanket provided more than 95% of fluoride recovery.