Iodine Volatilisation and Analyte Loss in Vegetable Sample Preparation: A Comparative Evaluation of Microwave Digestion, Dry Ashing, and Alkaline Extraction

Abstract

Accurate determination of iodine in vegetable matrices depends strongly on sample preparation because iodine can be lost through volatilisation under unsuitable digestion conditions. This review evaluates iodine volatilisation and analyte loss during three commonly used preparation approaches: microwave acid digestion, dry ashing and alkaline extraction. The discussion focuses on vegetable matrices and examines the chemical conditions that control iodine retention, particularly pH-dependent redox speciation and the formation or suppression of volatile molecular iodine. Microwave acid digestion provides efficient matrix decomposition and can achieve iodine recoveries of about 85-97% when sealed vessels are properly cooled before opening. Its main advantage is the physical containment of volatile iodine species, although native iodine speciation is not preserved under strongly acidic and oxidising conditions. Dry ashing is the least reliable approach for iodine determination because high temperatures, oxidising conditions and open-system operation favour volatilisation. Reported recoveries are highly variable, particularly in the absence of fixation agents, and the method requires careful validation when used. Alkaline extraction with tetramethylammonium hydroxide presents the lowest volatilisation risk because the high-pH medium suppresses molecular iodine formation and helps retain iodine in solution. Reported recoveries of about 92-100% indicate strong suitability for total iodine determination and speciation-oriented analysis. Compatibility with ICP-MS, ICP-OES, IC, HPLC, XRF and spectrophotometric methods is also considered. Overall, alkaline extraction is most appropriate for iodine retention and speciation, while microwave digestion remains suitable for routine total iodine analysis.