Introduction:
Mass spectrometry imaging (MSI) has been successfully used for spatial biomarker discovery, drug biodistribution and for clinical research applications. Most of the studies were conducted using a high resolution TOF mass spectrometers, starting with MALDI and more recently using DESI ionisation techniques. Once the targeted molecules have been identified (drug or biomarker), there is a need for a more sensitive and faster MSI technique.

Objectives:
Here we are proposing a system combining a DESI source with a TQ MS applied to MSI application for increase in sensitivity and acquisition speed.

Methods:
MSI experiments were carried out using a DESI XS source mounted on a TQ MS in positive and negative ionisation mode. All experiment were performed in MRM mode. DESI High-Performance sprayer conditions were set at 2 µL/min, 95:5 MeOH: water, with a N2 nebulising gas pressure was set at 10-15 psi and capillary voltage set between 0.55 to 0.8 kV allowing a focused spray. The samples were imaged at different pixel sizes from 15 up to 75 µm and a range of acquisition speeds from 5 Hz up to 50 Hz.

Results:
Pharmaceutical compound, chloroquine, was spotted on a control porcine liver tissue section in different concentrations from 0.003 µM up to 30 µM in a 10 fold dilution series. One series was imaged at a speed of 10 Hz with two MRM transitions for chloroquine (320.2 > 142.17 and 320.2 > 274.1) and MRM transitions for protonated PC (34:6) (782.55 > 184) which led to a dwell time of 21 ms per MRM transition per pixel. A second series was acquired with only two of the MRM transitions at 50 Hz, leading to a dwell time of 6 ms. For both acquisition speeds, chloroquine was detected from the 0.3 µM spot with s S/N=14.
Further pharmaceutical compound dilution series such as a mix solution of propranolol, olanzapine, erlotinib, moxifloxacin, terfenadine and irinotecan was DESI imaged at 10 Hz. LODs were determined between 0.02 µM and 0.25 µM. Calibration curves were generated by drawing regions of interest (ROIs) for each spot and average intensities were plotted vs. amount per spot. For all compounds the calibration curves shown linearity superior as R2 of 0.99.
Additional experiments were performed for drugs administrated in tissue to visualise drugs and their metabolites as well as endogenous molecules at different acquisition speeds to demonstrate the efficacity of the target imaging system for clinical research applications.

Conclusion:
We have demonstrated drug and biomarkers with increased level of detection and acquisition speed using a targeted DESI MRM TQ system.