Biochar-based “in-vial” stir bar sorptive microextraction for a greener determination of steroid hormones in saliva

Abstract

This work presents a novel application of biochar as a new-generation sorbent to bioanalytical sample preparation. The carbonaceous material produced by pyrolysis of vegetal wastes (orange and pumpkin peels) at 650°C was easily attached (∼ 4 mg) to a small magnetic stir bar (8 × 3 mm), enabling a novel stir bar sorptive microextraction protocol to be performed directly in the HPLC-MS vial. The device was first tested in synthetic saliva samples (1 mL) observing sorption equilibrium in 1 h contact time and full elution was achieved using 300 µL 1 % v/v acetic acid ethanol solution after 30 min. Under the selected conditions, recovery in real saliva, spiked with each analyte in the range 0.5-5 ng mL^-1, was in the range 88%-112%, with good inter-day inter-batch device-to-device precision (RSD < 18%, n=3), and no need for additional clean-up. Reusability tests confirmed that the device retains its sorption/desorption efficiency over at least 20 consecutive cycles. Additionally, the greenness of this biochar-based “in-vial” approach as a new stir bar sorptive microextraction was proven by using different dedicated software. The practical applicability of the procedure was demonstrated in real saliva samples from healthy volunteers.