Boronic Acid-Functionalized Covalent Organic Frameworks Coated Steel Sheet as an Ambient Mass Spectrometry Ion Source for Rapid Determination of Macrolides in Milk

Macrolides are widely-used antibiotics that have raised continuous concerns due to their potential toxicity to humans. Determination of macrolides in complex food matrices by ambient mass spectrometry (AMS) remains a challenge because the lack of selectivity of AMS ion source. Herein, we proposed a novel boronic acid-functionalized covalent organic frameworks coated steel sheet (SSS@COF@BA) serving both as an extraction substrate and an AMS ion source for macrolide determination. The boronic groups on SSS@COF@BA surface can specifically capture the macrolides containing cis-diols via boronate affinity and thereby greatly enhance the extraction selectivity. The adsorption studies revealed that SSS@COF@BA exhibited high selectivity with enrichment factors of 62.26-191.11 compared to bare SSS and satisfactory recoveries for macrolide extraction (>80%) in the presence of interfering antibiotics, large adsorption capacities (6.18-28.58 μg/piece), and good reusability (≥six cycles). The SSS@COF@BA after extraction process was directly subjected to high voltages for electrospray ionization mass spectrometry (ESI-MS) detection. The proposed SSS@COF@BA-ESI-MS method was validated, and proven to be rapid (analysis time < 1 min), sensitive (limits of detection: 0.01-0.09 μg/kg) and accurate (recoveries: 81.84-110.23%) for trace macrolide determination in milk. Overall, this study introduces an efficient approach for the rapid detection of trace food contaminants by the self-developed selective AMS ion source.