The
relative contributions to ionization efficiency by three molecular
chemical properties have been examined for field-free and field-enabled
capillary vibrating sharp-edge spray ionization (cVSSI) using mass
spectrometry (MS) analysis. Ion intensities have been recorded for
model compounds under each operational ionization mode as well as
for aqueous and nonaqueous (methanol) solvent systems. Multiple regression
analysis suggests that for field-free cVSSI, ion intensity is mostly
associated with the log of the base dissociation constant (pK
b) and proton affinity (PA) for both aqueous
and methanol solutions. Comparatively, for field-enabled cVSSI using
aqueous solutions, the dominant factor correlated with ion intensity
is the log of the partition coefficient (log P).
To a lesser degree, this is observed for methanol solutions as well.
For ESI, pK
b is the dominant factor associated
with ion signal levels from methanol and aqueous solutions. These
results are supported by studies conducted on two different mass spectrometers
employing different cVSSI emitter tips. The relationship of ion intensity
and pK
b in ESI is supported by multiple
studies; however, the shift to other chemical properties with the
addition of cVSSI suggests the possibility that a different (or combinations
of) ionization mechanism(s) may be operative for these ionization
modes. These results are briefly considered in light of the different
ESI mechanisms.
The relationship between separate molecular physicochemical properties and ionization efficiency has been investigated for the new ionization technique capillary vibrating sharp-edge spray ionization (cVSSI). Intensity values have been recorded for both positively- and negatively-charged ions arising from various compounds in the aprotic, polar acetonitrile (ACN) solvent environment. These have been recorded for field-free cVSSI as well as field-enabled cVSSI and compared to results obtained from standard ESI. In general, the strongest correlating factors include the logarithm of the octanol/water partition coefficient (log P) and the compound proton affinity (PA) in both positive and negative ion mode. This is contrasted with results for the polar, protic solvents water and methanol where the log of the base dissociation constant (pKb) often produced the strongest correlation. The results suggest that, in the absence of abundant protonating reagent, pre-formed ions do not govern the ionization process for samples in the ACN solvent systems. Another notable result is the increased ion signal levels observed for the majority of the ions in positive ion mode upon their production by a field-free source; that is, remarkably, the application of a DC voltage to the solution serves to decrease the overall ion signal level. Overall, it appears that, regardless of whether or not ions are produced by the charged residue model or the ion evaporation model, gas-phase proton transfer reaction is the major process by which they are produced.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.