Electrospray ionization of volatiles in breath
P. M-L Sinues, J. F. de la Mora
Recent work by Zenobi and colleagues reports that human breath charged by contact with an electrospray (ES) cloud yields many mass peaks of species such as urea, glucose, and other ions, some with molecular weights above 1000Da. All these species are presumed to be involatile, and to originate from breath aerosols by so-called extractive electrospray ionization EESI However, prior work by Fenn and colleagues and by Hill and colleagues have reported the ability of electrospray drops to ionize a variety of low vapor pressure substances directly from the gas phase, without an apparent need for the vapor to be brought into the charging ES in aerosol form.
The Ph.D. Thesis of Martínez-Lozano had also previously argued that the numerous human breath species observed via a similar ES ionization approach were in fact ionized directly from the vapor. Here, we observe that passage of the breath stream through a submicron filter does not eliminate the majority of the breath vapors seen in the absence of the filter. We conclude that direct vapor charging is the leading mechanism in breath ionization by electrospray drops, though aerosol ionization may also play a role.