An expanded framework for Swab Touch Spray-Mass Spectrometry towards the detection of allergenic protein residues on food preparation surfaces

Background Ambient Mass Spectrometry (AMS) encompasses a group of techniques that have emerged as powerful strategies for direct, in-situ and high-throughput analysis, also in compliance with the principles of green analytical chemistry. Swab Touch Spray-Mass Spectrometry (Swab TS-MS) is a home-made AMS technique that involves the use of a medical swab as sampling tool and electrospray probe. To date, Swab TS-MS has been applied only for the analysis of small molecules, especially in forensic and medical fields, leaving the analysis of peptides and proteins still unexplored. Results In the present study, the application framework of Swab TS-MS was expanded towards the detection of proteins, focusing on residues of allergenic ingredients as contaminants of food preparation surfaces. Lysozyme from chicken egg white was selected as case study of allergenic protein. Since none of the experimental conditions explored allowed the detection of intact lysozyme, a bottom-up procedure based on tryptic digestion for lysozyme detection by Swab TS-MS, operating in MS/high resolution (HR)MS tandem mode, was investigated. In parallel, liquid chromatography-tandem mass spectrometry was used to develop and characterize an in-situ digestion/swabbing strategy, allowing to reach a LOD value of 0.003 μg/cm2 of egg white powder consistent with that of lateral flow immunoassay technique. Finally, the developed in-situ digestion/swabbing procedure was coupled to the Swab TS-MS/HRMS method, permitting the direct analysis of egg white powder residues on stainless steel surfaces, reaching a LOD of 68 μg/cm2 egg white powder, corresponding to 2.4 μg/cm2 of lysozyme. Significance This study should be intended as a first step for the application of Swab TS-MS technique in protein analysis: the developed in-situ digestion and sampling strategy suitable for Swab TS-MS direct analysis of food allergen has a valuable impact towards the availability of MS-based multiplexed detection tool useful for the development of efficient surface cleaning procedures.