Green Analytical Techniques for Strontium Elemental & Isotope Analysis in Honey: Advancing Sustainable Sample Preparation with HR-ICP-MS

Strontium has been considered an excellent proxy for some important geological processes and it is considered a key marker to understand the provenance, mobility and diet habits in disciplines such as archaeology, paleoecology, and environmental sciences or forensics among others. Besides, whereas Sr is not considered as an essential micronutrient, it shares similar mobility than other cations like radium, barium or calcium and thus could be used as biotracer.

Combining isotopic ratios with trace element analysis offers significant advantages due to its simplicity and precision. Recent studies have highlighted the utility of bees as biosamplers of local geochemical signatures, with honey emerging as a promising biomonitor for trace elements and isotopic geochemistry.

In this work, a green analytical chemistry methodology has been applied to prepare the honeybee samples to be measured by ICP-MS: for major and trace element composition, 1 gram of honey only was mixed with 0.5 M HNO₃, and warmed-up to 50^oC during 10 min to facilitate homogenization. Previously, 100 µL of bismuth as internal standard was included in the mixture. Finally, an aliquot was diluted in 0.5M HNO₃ in order to obtain around 1 mg.L^-1 of potassium. The instrumental analysis comprised i) ICP-OES quantification of the main elements, ii) HR-ICP-MS quantification of the trace elements, including the strontium. To explore the strontium isotope ratios an additional aliquot was processed by the common strontium crown-ether separation and purification. In this stage, the HR-ICP-MS was used to measure the isotopic variations in order to test the sample processing performance. For this purpose, a sample of honeybee was isotopically strengthened with a certain amount of NIST-SRM strontium carbonate isotopic standard.

The method was applied to some organic and completely characterized honeybee from four different locations in north Spain, sampled during the same season. Initial results reveal (a) no significant differences between the standard acid digestion methods and the simple dilution approach; (b) results obtained confirm the biogeochemical behavior found when the physical-chemical properties of the honey were considered; and (c) whereas the results are promising still some methodological improvements have to been applied in order to obtain highly precise isotope ratio values by HR-ICP-MS.

Further research will include crosschecking the isotope honeybee signature with the bioavailable strontium in the corresponding soil.