AOAC SPIFAN Nutrients ERP (December 7, 2022)

Fluor-06 (SLV) 10-2022 FOR ERP USE ONLY DO NOT DISTRIBUTE

4 Method 4.1 Principle

The method is adapted from literature 2 , and is aimed at inorganic fluoride present in the sample. The method is based on derivatization in which the, involatile and water soluble, fluoride ion is transformed to the volatile trimethylsilyl derivative. Most silylations are performed in non-aqueous solutions and trimethylsilylchlorosilane (TMCS) is often used as a catalyst for other silylating reagents, e.g. MSTFA + 1% TMCS. As this regards, the derivatization used in the current method is slightly different as the sample is dissolved in acidified water and pure TMCS is used. Moreover, water takes part in the chemical reaction that is described in the literature as a two-step process. TMCS is converted to silanol which in turn reacts with fluoride to form trimethylfluorosilane (TMFS) 2-5 (Fig. 1).

Figure 1 . Reaction mechanism behind derivatization of fluoride to TMFS.

The high volatility of TMFS has the advantage that headspace sampling (HS) can be used to introduce the analyte on the gas chromatograph (GC). Sample introduction with HS requires almost no preparation steps. Therefore the whole procedure of inweigh, solution (comparable to reconstitution of the infant formula) and derivatization can all be performed in a single glass 20-mL vial that can be directly placed in the autosampler of the HS-GC system and analyzed. Detection is performed with a flame ionization detector (FID). Similar to several other methods, the sample is acidified to dissolve all fluoride present in the sample. As such, the measured amount will include inorganic fluoride, and fluoride from acid-labile organic (covalent) fluorine compounds 3 . To our knowledge such acid-labile compounds are not present in infant formula. The most important modification compared to literature 2 , is the use of sulfuric acid instead of hydrochloric acid. This minimizes the degradation the column, headspace sampler, and transfer line due to the aggressive HCl fumes.