Direct Analysis of Fatty Acid Vapors in Breath by Electrospray Ionization and Atmospheric Pressure Ionization-Mass Spectrometry

Real time analysis of human breath is achieved in an atmospheric pressure ionization mass spectrometer (API-MS) by negatively charging exhaled vapors via contact with an electrospray cloud. The spectrum observed is dominated by a wide range of deprotonated fatty acids, including saturated chains up to C14. Above C14, the background from cutaneous sources becomes dominant. We also tentatively identify a series of unsaturated fatty acids (C7-C10), ketomonocarboxylic acids (C6-C10), and a family of aldehydes. The ionization probability of large fatty acids increases drastically when the humidity changes from 20% to 95%. Accordingly, distinguishing lung vapors (humid) from those in the background (dry) requires special precautions. Estimated fatty acid vapor concentrations in breath based on our measurements (similar to 100 ppt) are in fair agreement with values expected from blood concentrations in the range for which data are available (C3-C6).