Heart-cutting two-dimensional liquid chromatography combined with isotope ratio mass spectrometry for the determination of stable carbon isotope ratios of gluconic acid in honey

Liquid chromatography/isotope ratio mass spectrometry (LC/IRMS) is used to analyze various types of samples, including foodstuffs, to determine their authenticity and trace their origin on the basis of their stable carbon isotope ratios (delta C-13). However, multicomponent samples are difficult to analyze. For example, determining the delta C-13 values of the organic acids in honey is complicated by the presence of large amounts of carbohydrates. Herein, we present a heart-cutting two-dimensional LC/IRMS method for analysis of honey samples. In this method, the organic acids in the samples were first separated from the carbohydrates by a size-exclusion column, and then the organic acids were separated from each other by a reverse-phase column connected to the first column via a switching valve. By means of this method, the delta C-13 values for three organic acids in high-carbohydrate-content simulated honey samples could be determined with high accuracy and precision (<= 0.3 parts per thousand and <= 0.1 parts per thousand, respectively). In addition, the gluconic acid delta C-13 values for 25 honey samples were determined with high precision and found to range from -31.7 to -28.5%. (mean: -30.0 +/- 0.7 parts per thousand). These values shed some light on the mechanism of gluconic acid production. Taken together, our results suggest that this two-dimensional LC method has the potential to be more effective than one-dimensional LC for use in isotopic research. (C) 2019 Elsevier B.V. All rights reserved.