Abstract
M.Sc. (Chemistry)
Rooibos tea is gaining importance as an herbal drink and product enhancer in the beverage, food, nutraceutical and cosmetic markets. Popular food products such as rooibos tea can be a target of fraudulent scheme, which consists of deliberate misrepresentation with the aim to mislead buyers with regard to the true product or any of its ingredients. Determination of the provenance of rooibos tea (fingerprinting) may play a key role in ensuring consumer safety, decreasing damage to producers and in creating awareness about potential use of such tool as deterrent to misleading traders. Among the fingerprinting techniques, inorganic mass spectrometry based on stable isotope ratios such as 87Sr/86Sr is used for determining the origin of various agricultural products.
The purpose of this work was to evaluate the potential use of 87Sr/86Sr isotope ratio as a tool for determining the origin of the South African Rooibois tea. The ratio was determined by single quadrupole inductively coupled plasma mass spectrometry. Rooibos tea leaves and soil samples collected from seven tea growing farms in Clanwilliam district and the neighboring Northern Cape region were mineralized and total concentrations of the isotopes determined using internal standardization and external calibration. It was found that scandium (Sc) cannot be used as internal standard since it is present in tea leaves. Hence, throughout this work 69Ga, 115In, and 175Lu were used for instrument drift correction.
The evaluation of 87Sr/86Sr ratio was difficult due to isobaric interference of 87Rb on 87Sr. To overcome this challenge, the two isotopes were separated successfully on Dowex 50W-X8 resin by metal-EDTA and metal-DCTA complex elution prior to isotopes ratio determination. Accuracy of the method was checked using tea leaves and sandy soil certified reference materials INCT-TL-1 and CRM-SA-C, respectively. The limits of detection (LOD) and of quantification (LOQ) were between 0.13-0.82 μg L-1 and 0.44-2.75 μg L-1, respectively. The lowness of the LOD and LOQ showed that analysis could be repeated. It was found that DCTA elution method was more efficient than the EDTA one because it complexes metal ions more strongly. The results obtained for 87Sr/86Sr isotope ratio in all the seven farms showed lack of a good correlation between the soil and the tea leaf samples.