Quantification of methyl thiocyanate in the headspace of Pseudomonas aeruginosa cultures and in the breath of cystic fibrosis patients by selected ion flow tube mass spectrometry.

Department of Paediatrics
 
Abstract:
Infection by Pseudomonas aeruginosa (PA) is a major cause of morbidity and mortality in patients with cystic fibrosis (CF). Breath analysis could potentially be a useful diagnostic of such infection, and analyses of volatile organic compounds (VOCs) emitted from PA cultures are an important part of the search for volatile breath markers of PA lung infection. Our pilot experiments using solid-phase microextraction, SPME and gas chromatography/mass spectrometric (GC/MS) analyses of volatile compounds produced by PA strains indicated a clear presence of methylthiocyanate. This provided a motivation to develop a method for real-time online quantification of this compound by selected ion flow tube massspectrometry, SIFT-MS. The kinetics of reactions of H₃O⁺, NO⁺ and O₂^+o with methyl thiocyanate at 300 K were characterized and the characteristic product ions determined (proton transfer for H₃O⁺, rate constant 4.6 × 10^–9 cm³ s^–1; association for NO⁺, 1.7 × 10^–9 cm³ s^–1 and nondissociative charge transfer for O₂^+o 4.3 × 10^–9 cm³ s^–1). The kinetics library was extended by a new entry for methylthiocyanate accounting for overlaps with isotopologues of hydrated hydronium ions. Solubility of methyl thiocyanate in water (Henry's law constant) was determined using standard reference solutions and the linearity and limits of detection of both SIFT-MS and SPME-GC/MS methods were characterized. Thirty-six strains of PA with distinct genotype were cultivated under identical conditions and 28 of them (all also producing HCN) were found to release methyl thiocyanate in headspace concentrations greater than 6 parts per billion by volume (ppbv). SIFT-MS was also used to analyze the breath of 28 children with CF and the concentrations of methyl thiocyanate were found to be in the range 2–21 ppbv (median 7 ppbv).