Dielectric breakdown-based ambient ionization mass spectrometry assisted by a cotton swab wrapped with aluminum foil

Ambient ionization mass spectrometry (MS) is widely used due to its simplicity. Dielectric breakdown-based ionization has emerged as a promising technique, where air molecules like nitrogen are ionized, producing N^+• and electrons. These initiate ion-molecule reactions that generate protons from water molecules, leading to analyte ionization via protonation. This study explores using a cotton swab wrapped in aluminum foil as an ionization substrate to assist dielectric breakdown for gas-phase analyte ionization without additional electric devices, offering a simple and effective approach for ambient ionization MS.

A cotton swab wrapped with aluminum foil, placed close (∼1 mm) to the metal inlet of a mass spectrometer applied with a high voltage (-6000 V), can readily induce the dielectric breakdown of the air molecules in between, leading to the generation of protons. Volatiles or semi-volatiles evaporating from the sample in the condensed phase were ionized via protonation. The limit of detection of our method for benzoic acid was ∼18 nM. Using the developed method as the interface, MS coupled with a hand-held UV-Vis spectroscope enabled simultaneous qualitative and quantitative analysis. Additionally, our method facilitated and monitored fast reactions, such as imine formation. It also allowed the analysis of low-volatility analytes by forming a Taylor cone for ion acquisition through ESI-like processes. Both volatiles and benzo[a]pyrene (BaP) in a sesame oil sample spiked with BaP were effectively analyzed.

Cotton swabs are commonly used as sampling probes to collect analytes from surfaces. This method demonstrates the potential of using the sampling probe (i.e., a cotton swab) directly as an ionization substrate for ambient ionization MS analysis, making it potentially suitable for the rapid screening of target analytes from various samples.