Simone Mathias

Direct analysis in real time is typically performed using helium as the ionisation gas for the detection of analytes by mass spectrometry (MS). Nitrogen and argon are found with abundance in the air and provide a cheaper and greener alternative to the use of helium as ionisation gas. This study explores the use of helium, nitrogen and argon as ionisation gas for detection of organic compounds. Four illicit drugs, 2 amino acids and 5 explosives were chosen as target analytes to understand selectivity, sensitivity and linearity when helium, nitrogen or argon was used as the ionisation gas with the direct analysis in real time (DART) source. Analysis was carried out on a Waters Acquity QDa single quadrupole mass spectrometer. Calibration curves over the range of 5 - 100 ng were produced for each analyte using the different ionisation gases to assess the instrument response. Nitrogen gave a higher response to concentration than helium or argon, however the lowest limits of detection were observed when helium was used. All the target analytes were detected using DART-MS with helium, nitrogen or argon as the ionisation gas. Whilst helium provides the highest sensitivity, nitrogen produced reasonable limits of detection and had good linearity across the concentration range explored, suggesting it provides a greener and cheaper alternative to helium.

Five different classes of explosives were analysed by ambient ionisation mass spectrometry testing selectivity, sensitivity, and repeatability. We compare the effectiveness of two techniques (ASAP and SESI) for the trace detection of five explosives representative of the most common classes of high explosive: HMTD, RDX, PETN, Tetryl and TNT. Experiments also compared the effectiveness of sample loading via a glass fibre swab or glass rod. All analyses were carried out with a Waters Acquity QDa mass spectrometer, a small format mass spectrometer which can be operated in a transportable mode (using ambient air and a small diaphragm pump). Both ambient ionisation techniques, ASAP and SESI, successfully detected the five different explosives which could make them suitable for a screening method. By directly comparing a calibration range of 0.8–10 ng on both swabs and rods for each explosive, it appears that SESI produces less variability per repeat, particularly at the higher end of the range when compared to ASAP which typically has a lower limit of detection and better linearity.

Polypyrrole (PPy) fibre electrodes were studied to determine their ability to sense paracetamol (as a model drug) in the presence of the interferents dopamine and ascorbic acid. PPy was electropolymerised onto carbon fibres using cyclic voltammetry in the presence of two different counter anions: sodium dodecyl sulfate (SDS) and potassium nitrate (KNO3). The surface of the PPy.SDS and PPy.KNO3 fibre electrodes was characterised using Raman spectroscopy and scanning electron microscopy. The PPy.SDS-coated carbon fibre had a 14-fold larger electrochemical surface area compared to a bare carbon fibre (calculated using the Randles-Sevcik equation). The use of a large counter anion as dopant (dodecyl sulfate) produced fibres with a greater drug sensing response (cf. the use of smaller nitrate anion). The use of the PPy.SDS fibre electrode in differential pulse voltammetry (DPV) allowed sensing of paracetamol with a detection limit (3σ S/N) of 34 µM. For the anodic peak current (0.5 V vs. Ag/AgCl), a linear response range was observed for 50–500 µM. At a paracetamol concentration of 100 µM, the DPV anodic peak current (at 0.5 V vs. Ag/AgCl) was unaffected by the addition of interferents: 100 µM dopamine and 100 µM ascorbic acid. A real-world application of drug sensing was trialled with the anti-psychotic medication clozapine; where the PPy.SDS carbon fibre could sense clozapine with a detection limit of (3σ S/N) of 6 µM and a sensitivity of 15 μA cm−2 μmol−1 L.