Elimination characteristics of post-operative isoflurane levels in alveolar exhaled breath via PTR-MS analysis

Isoflurane (1-chloro-2,2,2-trifluoroethyl difluoromethyl ether), C₃H₂ClF₅O, is a commonly used inhalation anaesthetic. Using a proton transfer reaction mass spectrometer (PTR-MS) we have detected isoflurane in the breath of patients several weeks following major surgery. That isoflurane is detected in the breath of patients so long after being anaesthetised raises questions about when cognitive function has fully returned to a patient. Temporal profiles of isoflurane concentrations in breath are presented for five patients (F/M 3/2, mean age 50 years, min-max 36-58 years) who had undergone liver transplant surgery. In addition, results from a headspace analysis of isoflurane are presented so that the product ions resulting from the reactions of H₃O⁺ with isoflurane in PTR-MS could be easily identified in the absence of the complex chemical environment of breath. Six product ions were identified. In order of increasing m/z (using the ³⁵Cl isotope where appropriate) these are (m/z 51), CHFCl⁺ (m/z 67), CF₃CHCl⁺ (m/z 117), C₃F₄OCl⁺ (m/z 163), C₃H₂F₄OCl⁺ (m/z 165), and C₃F₄OCl⁺ H₂O (m/z 183). No protonated parent was detected. For the headspace study both clean air and CO₂ enriched clean air (4% CO₂) were used as buffer gases in the drift tube of the PTR-MS. The CO₂ enriched air was used to determine if exhaled breath would affect the product ion branching ratios. Importantly no significant differences were observed, and therefore for isoflurane the product ion distributions determined in a normal air mixture can be used for breath analysis. Given that PTR-MS can be operated under different reduced electric fields (E/N), the dependence of the product ion branching percentages for isoflurane on E/N (96-138 Td) are reported.