Dielectric barrier discharges (DBD) are highly versatile
plasma
sources for forming ions at atmospheric pressure and near ambient
temperatures for the rapid, direct, and sensitive analysis of molecules
by mass spectrometry (MS). Ambient ion sources should ideally form
intact ions, as in-source fragmentation can limit sensitivity, increase
spectral complexity, and hinder interpretation. Here, we report the
measurement of ion internal energy distributions for the four primary
classes of DBD-based ion sources, specifically DBD ionization (DBDI),
low-temperature plasma (LTP), flexible microtube plasma (FμTP),
and active capillary plasma ionization (ACaPI), in addition to atmospheric
pressure chemical ionization (APCI) using para-substituted benzylammonium
thermometer ions. Surprisingly, the average extent of energy deposited
by the use of ACaPI (90.6 kJ mol–1) was ∼40
kJ mol–1lower than the other ion sources (DBDI,
LTP, FμTP, and APCI; 130.2 to 134.1 kJ mol–1) in their conventional configurations, and slightly higher than
electrospray ionization (80.8 kJ mol–1). The internal
energy distributions did not depend strongly on the sample introduction
conditions (i.e., the use of different solvents and sample vaporization
temperatures) or the DBD plasma conditions (i.e., maximum applied
voltage). By positioning the DBDI, LTP, and FμTP plasma jets
on axis with the capillary entrance to the mass spectrometer, the
extent of internal energy deposition could be reduced by up to 20
kJ mol–1, although at the expense of sensitivity.
Overall, the use of an active capillary-based DBD can result in substantially
less fragmentation of ions with labile bonds than alternate DBD sources
and APCI with comparably high sensitivity.