Laser desorption postionization mass spectrometry of antibiotic-treated bacterial biofilms using tunable vacuum ultraviolet radiation.

Laser desorption postionization mass spectrometry (LDPI-MS) with 8.0-12.5 eV vacuum ultraviolet synchrotron radiation is used to single photon ionize antibiotics and extracellular neutrals that are laser desorbed both from neat and intact bacterial biofilms. Neat antibiotics are optimally detected using 10.5 eV LDPI-MS but can be ionized using 8.0 eV radiation, in agreement with prior work using 7.87 eV LDPI-MS. Tunable vacuum ultraviolet radiation also postionizes laser desorbed neutrals of antibiotics and extracellular material from within intact bacterial biofilms. Different extracellular material is observed by LDPI-MS in response to rifampicin or trimethoprim antibiotic treatment. Once again, 10.5 eV LDPI-MS displays the optimum trade-off between improved sensitivity and minimum fragmentation. Higher energy photons at 12.5 eV produce significant parent ion signal, but fragment intensity and other low mass ions are also enhanced. No matrix is added to enhance desorption, which is performed at peak power densities insufficient to directly produce ions, thus allowing observation of true VUV postionization mass spectra of antibiotic treated biofilms.