Pulsed Blue Laser Diode Thermal Desorption Microplasma Imaging Mass Spectrometry

An
ambient air laser desorption, plasma ionization imaging method
is developed and presented using a microsecond pulsed laser diode
for desorption and a flexible microtube plasma for ionization of the
neutral desorbate. Inherent parameters such as the laser repetition
rate and pulse width are optimized to the imaging application. For
the desorption substrate, copper spots on a copper-glass sandwich
structure are used. This novel design enables imaging without ablating
the metal into the mass spectrometer. On this substrate, fixed calibration
markers are used to decrease the positioning error in the imaging
process, featuring a 3D offset correction within the experiment. The
image is both screened spot-by-spot and per line scanning at a constant
speed, which allows direct comparison. In spot-by-spot scanning, a
novel algorithm is presented to unfold and to reconstruct the imaging
data. This approach significantly decreases the time required for
the imaging process, which allows imaging even at decreased sampling
rates and thus higher mass resolution. After the experiment, the raw
data is automatically converted and interpreted by a second algorithm,
which allows direct visualization of the image from the data, even
on low-intensity signals. Mouse liver microtome cuts have been screened
for dehydrated cholesterol, proving good agreement of the unfolded
data with the morphology of the tissue. The method optically resolves
30 μm, with 30 μm diameter copper spots and a 10 μm
gap. No conventional chemical matrices or vacuum conditions are required.