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 (δ 18 O) 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 δ 18 O 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. δ 18 O 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 δ 18 O of juice dried in a vacuum oven was 3.30‰ lower than that of juice dried in a freeze dryer. Moreover, δ 18 O 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 δ 18 O measurement in the sugar‐rich fraction of fruit juice with less inter‐day variation, and it will be useful for monitoring geographical origin labeling.