Selected ion flow tube mass spectrometry, SIFT-MS, is a non-separative method for direct quantitative analyses of volatile compounds, VOCs, in air and humid breath based on chemical ionization. Selected reagent ions, either H₃O⁺, NO⁺ or O₂⁺ (non-reactive with major components of air), ionize analyte molecules during a defined time in a flow tube by ion-molecule reactions thus producing analyte ions that are characteristic of the neutral analyte VOCs. Concentrations can be calculated in real-time from the ion count rates. Direct on-line analysis of single or multiple breath exhalations or off-line analysis of breath samples collected into bags can be performed. Several volatile breath metabolites have been quantified by SIFT-MS, including ammonia, acetone, hydrogen cyanide, alcohols, pentane, acetic acid, methane, and sulphur compounds. Their potential as biomarkers is discussed.

选择离子流管质谱法 SIFT-MS 是一种基于化学电离直接定量分析空气和潮湿呼吸中的挥发性化合物、VOC 的非分离方法。选定的反应离子，H ₃ O ⁺、NO ⁺或 O ₂ ⁺（不与空气的主要成分发生反应），在流管中的规定时间内通过离子-分子反应使分析物分子电离，从而产生具有中性分析物 VOC 特征的分析物离子。可以从离子计数率实时计算浓度。可以对单次或多次呼气进行直接在线分析，或者对收集到袋子中的呼气样本进行离线分析。SIFT-MS 对几种挥发性呼吸代谢物进行了量化，包括氨、丙酮、氰化氢、醇、戊烷、乙酸、甲烷和硫化合物。讨论了它们作为生物标志物的潜力。