Requirements for accurate quantification of nitrate and nitrite in molasses: Insights from an interlaboratory comparison

Abstract

The quantification of nitrate and nitrite in molasses is an emerging requirement for the sugar industry. The establishment of a validated analytical methodology is necessary to comply with the EU-guidelines on maximum permissible levels currently under discussion. In the present study, validation of an anion exchange chromatographic method to determine nitrate and nitrite in molasses was performed determining key validation parameters via several stages of an interlaboratory comparison for the first time. Since a stable, homogeneous sample material was necessary for establishing quantitative accuracy, a nitrite-spiked molasses material was prepared and evaluated independently across two laboratories in a parallel study. This preliminary study revealed that quantification of nitrate and nitrite content in molasses could be achieved with excellent precision under repeatability conditions of measurement (1.21% RSD for nitrate; 1.26% RSD for nitrite) in independent laboratories but requires preservation of samples at −20 °C for long-term (>several days) storage prior the analysis. A subsequent stage involving five different participating laboratories was then undertaken to evaluate anion exchange chromatography with conductivity detection for this application within a round robin study. The first phase focused on the analysis of nitrite-spiked molasses to optimize the individual analytical practices of laboratories and to elaborate a harmonized sample preparation protocol. In the second phase the participating laboratories analyzed seven molasses samples with different pH-value, dry substance, factory origin, with nitrate levels ranging from 2931 mg/kg – 6436 mg/kg and nitrite levels ranging from 25.3 mg/kg – 109 mg/kg. The determination of precision of reproducibility conditions of measurement revealed average relative interlaboratory standard deviations of 2.5% for nitrate and of 13% for nitrite. The present study clearly demonstrates the necessity of harmonization and standardization regarding measurement and data evaluation procedures applied for controlling nitrate and nitrite in molasses. • The challenging determination of nitrite with IC in molasses is addressed. • Storage of molasses at −20 °C is necessary to ensure sample stability. • Interlaboratory comparison with five laboratories is performed. • Harmonization and standardization of methods are identified to be necessary.