Applications, features, and mechanistic aspects of liquid water beam desorption mass spectrometry

In the present study we highlight recent applications of liquid beam desorption mass spectrometry for the analysis of biomolecules. The protonated macromolecules are desorbed from a 10 μm thick liquid jet in vacuum with an IR laser pulse tuned in resonance with the OH stretch vibration of water. Cytochrome c, viscotoxin A3, synthetic analogues of DNA and nucleobase substituted β-peptides (PNA like oligomers), bovine serum albumin (BSA), as well as specifically designed pharmaceutical macromolecules have been investigated. The salt and buffer tolerance has been measured for the desorption of cytochrome c. For the PNA oligomers it has been shown that a mixture can be desorbed and that the relative intensity of the mass peaks reflects relative concentrations in solution. With diluted BSA water solutions it has been demonstrated that the desorption technique provides a quantitative measure of BSA in solution. The gas phase signal of singly protonated BSA desorbed from a series of well defined solution concentrations has been found to be linear over at least three orders of magnitude. This feature appears to be promising for quantitative online monitoring applications of this technique. Beyond applications of this technique mechanistic aspects of the poorly characterized desorption mechanism are discussed. With a field-free-drift time-of-flight approach we were able to monitor features of the desorption process by selecting fractions of the broad velocity distribution of desorbed species using an ion optics acting as an ion gate. The observed features were discussed within a “desorption/ionization” model featuring the interplay of an explosive thermal and a shock wave dispersion of the microfilament controlling the ejection of hot nano-droplets and microsolvated molecules as well as their desolvation.