posted on 2023-11-20, 18:41authored byTeodora Zagorac, Hugo Andrés López Peña, Jason M. Gross, Katharine Moore Tibbetts, Luke Hanley
Imipramine class tricyclic antidepressants
have low ionization
efficiencies that make them difficult to detect by using secondary
ion mass spectrometry. Ultraviolet picosecond laser desorption postionization
(ps-LDPI-MS) is examined here for the detection of four tricyclic
antidepressants: imipramine, desipramine, amitriptyline, and clomipramine.
About 30 ps laser pulses at either 213 nm (5.8 eV) or 355 nm (3.5
eV) are used for desorption of samples under vacuum, 7.9 eV (157 nm)
fluorine laser pulses are used for post-ionization, and the ions so
formed are detected by time-of-flight mass spectrometry. Detection
of imipramine by 213 nm ps-LDPI-MS shows less fragmentation than either
355 nm ps-LDPI-MS or prior results from 800 nm fs-LDPI-MS. Ionization
energies of imipramine, desipramine, amitriptyline, and clomipramine
are predicted using density functional theory calculations and used
to explain the corresponding ps-LDPI-MS data for these four compounds
as resulting from single-photon ionization. The experimental observation
of low-mass amine-containing fragments with calculated ionization
energies below 7.9 eV is attributed mostly to dissociation during
laser desorption, followed by single-photon ionization of the neutral
fragments rather than the more traditional mechanism of unimolecular
dissociation following single-photon ionization of the parent molecule.