Direct human breath analysis by secondary nano-electrospray ionization ultrahigh-resolution mass spectrometry: Importance of high mass resolution and mass accuracy
Xue Li, Lei Huang, Hui Zhu and Zhen Zhou.
RATIONALE: Direct mass spectrometry (MS)-based methods make it possible to monitor the molecular compositions of hundreds of volatile organic compounds (VOCs) in exhaled human breath in real time. Mass resolution and mass accuracy play important roles for direct MS analysis, especially for the low-concentration isobaric compounds in non-target research.
METHODS: Direct detection of VOCs in exhaled breath of four healthy subjects (3 males and 1 female aged between 25 to 35 years old) has been performed by using secondary nano-electrospray ionization mass spectrometry (Sec-nanoESIUHRMS) t resolutions (R) of 15,000, 30,000, 60,000 and 120,000.RESULTS: For some low-intensity isobaric ions, they could be distinguished only when R ≥ 60,000, e.g., signals at m/z 6.9591 (sulfate/sulfuric acid), m/z 96.9687 (phosphate/phosphoric acid) and m/z 96.9756 ([C4H2O7S]-), m/z 234.1161 ([C10H20O3NS]+) and m/z 234.1338 ([C10H20O5N]+), m/z 119.0686 (isotope of indole) and m/z 119.0705 (an interfering signal), respectively. At R 120,000, the mass errors were obtained from a set of reference ions, and the values were ≤0.6 mDa for ions detected in positive detection mode and in the range of –1.0–1.1mDa for the negative mode. These mass errors were used to exclusively identify unknown compounds detected in the breath samples. By utilizing the present setup, besides the normal VOCs reported previously, we detected non-volatile species (sulfate/sulfuric acid, silicate/silicic acid, phosphate/phosphoric acid and nitrate/nitric acid), dichlorobenzene and an ammonium adduct ([(C2H6SiO)6+NH4]+),whichwere ascribed to exhaled particles, indoor air pollution and an endogenous source, respectively.
CONCLUSIONS: For direct breath analysis, high mass resolution of ≥60,000 and mass errors of 1.0 mDa (absolute value)covering the mass range of interests (e.g., m/z 50-500) are necessary for the exploration and accurate identification of lowintensity unknown isobaric compounds in non-target research.