High-throughput Nano-DESI Mass Spectrometry Imaging of Biological Tissues Using an Integrated Microfluidic Probe

Nanospray desorption electrospray mass spectrometry imaging (nano-DESI MSI) enables quantitative imaging of hundreds of molecules in biological samples with minimal sample pretreatment and a spatial resolution down to 10 µm. We have recently developed an integrated glass microfluidic probe (iMFP) for nano-DESI MSI, which simplifies the experimental setup and enables imaging with a spatial resolution of 25 µm. Herein, we describe an improved design of the iMFP for the high-throughput imaging of tissue sections. We increased the dimensions of the primary and spray channels and optimized the spray voltage and solvent flow rate to obtain a stable operation of the iMFP at both low (0.04 mm/s) and high (0.4 mm/s) scan rates. We demonstrate the performance of the high-throughput iMFP by imaging mouse uterine and brain tissue sections. We observe that the sensitivity, molecular coverage, and spatial resolution obtained using the iMFP do not change to a significant extent as the scan rate increases. Using a scan rate of 0.4 mm/s, we obtained high quality images of mouse uterine tissue sections (scan area: 3.2 mm×2.3 mm) in only 9.5 minutes and of mouse brain tissue (scan area: 7.0 mm×5.4 mm) in 21.7 minutes, which corresponds to a 10-15-fold improvement in the experimental throughput. We have also developed a quantitative metric for evaluating the quality of ion images obtained at different scan rates. Specifically, by defining regions of interest (ROI) in both a representative ion image and optical image of the tissue section, we calculated the spatial deviation between the two ROIs by counting the number of mismatched pixels. Using this metric, we demonstrate that the percent deviation increases slightly from 6.7% to 10.2 % with an increase in the scan rate from 0.02 to 0.4 mm/s. The maximum experimental throughput achieved in this study is limited by the acquisition rate of a mass spectrometer necessary to achieve the desired spatial resolution and may be further improved using a faster instrument. The ability to image biological tissues with high throughput using iMFP-based nano-DESI MSI will substantially speed up tissue mapping efforts.