关键词: 离子色谱, 有机酸, 阴离子, 呼出气冷凝液

Abstract:

A non-invasive condensation collection-ion chromatography method was established for the determination of organic acids and anions including lactic acid, formic acid, acetic acid, pyruvic acid, chloride, nitrate, nitrite, and sulfate in the exhaled breath of humans. The breath exhaled was condensed and collected using a home-made exhaled breath condensation equipment. This equipment included a disposable mouthpiece as a blow-off port, one-way valve and flow meter, cold trap, disposable condensate collection tube placed in the cold trap, and gas outlet. A standard sampling procedure was used. Before collection, the collection temperature and sampling volume were set on the instrument control panel, and sampling was started when the cold-trap temperature dropped to the set value, while maintaining the balance. Subjects were required to gargle with pure water before sampling. During the sampling process, the subjects were required to inhale deeply until the lungs were full of gas and then exhale evenly through the air outlet. When the set volume was collected, the instrument made a prompt sound; then, the collection was immediately ended, the expiration time was recorded, and the average collection flow was calculated according to the expiration time and sampling volume. After collection, the disposable condensation collection tube was immediately taken out, sealed, and stored in the refrigerator at -20 ℃ away from light, and immediately used for further testing. The organic acids and anions in exhaled breath condensation (EBC) were filtered through a 0.22 μm membrane filter before injection and detected by ion chromatography with conductivity detection. Factors such as collection temperature and collection flow rate during condensation collection were optimized. The optimal cooling temperature was set at -15 ℃, and the optimal exhaled breath flow rate was set at 15 L/min. The mobile phase consisted of a mixture of sodium carbonate (1.5 mmol/L) and sodium bicarbonate (3 mmol/L). The flow rate was 0.8 mL/min, and the injection volume was 100 μL. An IC-SA3 column (250 mm×4.0 mm) was used, and the temperature was set at 45 ℃. An ICDS-40A electrodialysis suppressor was used, and the current was set at 150 mA. The linear ranges of the eight organic acids and anions were 0.1-10.0 mg/L; their correlation coefficients (r) were ≥0.9993. The limits of detection (LODs) for the eight organic acids and anions were 0.0017-0.0150 mg/L based on a signal-to-noise ratio of 3, and the limits of quantification (LOQs) were 0.0057-0.0500 mg/L based on a signal-to-noise ratio of 10. The intra-day precisions were 5.06%-6.33% (n=5), and the inter-day precisions were 5.37%-7.50% (n=5). This method was used to detect organic acids and anions in the exhaled breath of five healthy subjects. The contents of organic acids and anions in the exhaled breath were calculated. The content of lactic acid was relatively high, at 1.13-42.3 ng/L, and the contents of other seven organic acids and anions were 0.18-11.0 ng/L. During a 10 km-long run, the majority of organic acids and anions in the exhaled breath of five subjects first increased and then decreased. However, due to abnormal metabolism, the content changes of lactic acid, acetic acid, pyruvic acid and chloride in one subject were obviously different from others during exercise, showing a continuous rise. This method has the advantages of involving a simple sampling process and exhibiting good precision, few side effects, and no obvious discomfort or risk to the subjects. This study provides experimental ideas and a theoretical basis for future research on human metabolites.

Key words: ion chromatography (IC), organic acids, anions, exhaled breath condensation (EBC)