Pretreatment method for oxygen stable isotope ratio analysis of the sugar-rich fraction in fruit juice via isotope ratio mass spectrometry

Rationale

The oxygen stable isotope ratio (δ¹⁸O) of the sugar-rich fraction of fruit juice is important as a tracer of the geographical origin of raw material. This study sought to minimize the inter-day variation of δ¹⁸O attributable to the influence of water to accurately monitor geographical origin labeling.

Methods

Two drying devices (freeze dryer and vacuum oven) were compared. Then, two humidity levels (normal and low humidity) at which the samples were placed after drying were compared. The low-humidity environment was constructed using a glove bag and pure argon gas. δ¹⁸O was measured using thermal conversion elemental analyzer/isotope ratio mass spectrometry. Improvements were made to the measurement method based on aforementioned analyses results, and the performance of the initial and improved methods was compared.

Results

δ¹⁸O of juice dried in a vacuum oven was 3.30‰ lower than that of juice dried in a freeze dryer. Moreover, δ¹⁸O of juice samples exposed to normal humidity was 3.74‰ lower than that of samples exposed to low humidity. The combined inter-day and intra-day standard deviation was reduced from 1.20‰ in the initial method to 0.42‰ in the improved method.

Conclusions

This study describes a pretreatment method for δ¹⁸O measurement in the sugar-rich fraction of fruit juice with less inter-day variation, and it will be useful for monitoring geographical origin labeling.