Direct Quantitative Analysis of n-Pentane in Exhaled Breath Using NO₂⁺ Chemical Ionization Time-of-Flight Mass Spectrometry

The presence of n-pentane in breath mainly originates from lipid peroxidation within the body, a process triggered by free radicals attacking phospholipids in cell membranes. When the level of reactive oxygen species (ROS) increases in the body, it can further exacerbate lipid peroxidation, leading to an elevated release of lipid peroxidation products, including n-pentane. Therefore, the concentration of n-pentane in exhaled breath can serve as an effective biomarker for reflecting the degree of oxidative stress and lipid peroxidation, and its changes are closely related to various disease states. Additionally, the concentration of n-pentane in exhaled breath rises significantly after alcohol consumption, which is closely related to the reactive oxygen species generated during alcohol metabolism. Thus, the development of a direct and rapid method for quantitatively analyzing n-pentane in exhaled breath holds significant scientific value and practical significance. This study introduced a novel chemical ionization source with NO₂⁺ (NO₂CI) as the reactant ion based on a vacuum ultraviolet (VUV) lamp. By directly ionizing NO₂ molecules with VUV photons, NO₂⁺ reactant ions with high intensity were generated, which then underwent ion-molecular reactions with n-pentane molecules to produce the characteristic product ion C₅H₁₁⁺, achieving a branching ratio of 88.8%. This study also systematically investigated the effects of the humidity and sample flow rate on detection of n-pentane, and the dehumidification performance of a semiconductor cooler was validated. The experimental results showed that the optimized sample flow rate was 23 mL/min and sample humidity can significantly affect production of the reactant ion NO₂⁺ and characteristic ion C₅H₁₁⁺ of n-pentane. The moisture in the sample can be effectively removed using the semiconductor cooler, thereby avoiding the interference on the detection of n-pentane. Under optimal experimental conditions, a good linearity of n-pentane in the concentration range of (10-1 000) ×10⁻⁹ (V/V) with a linear correlation coefficient (R²) of 0.9954 was achieved. The limit of detection (LOD) was 0.5×10⁻⁹, while the quantification limit was 1.7×10⁻⁹ under the average measurement condition of 1 min, relative standard deviations (RSDs, n=5) ranged from 2.3% to 6.3%. Finally, this method was applied to directly dynamic quantification of n-pentane in the exhaled breath of two healthy non-smokers before and after alcohol consumption. The findings indicated that the proposed method has the advantages of high sensitivity, speed, and reliability, showcasing promising application potential and broad prospects in the trace-level detection of n-pentane in exhaled breath and related disease studies.