Atmospheric-pressure Molecular Imaging of Biological Tissues and Biofilms by LAESI Mass Spectrometry

In order to perform a lazy mass spectrometry imaging experiment on a biological tissue or biofilm, the lazy setup is prepared for robust operation. The next step is mounting the sample into a sample stage and identifying experimental conditions required for the sample under investigation. Then the lateral lazy molecular imaging experiment is started and the required data are used to obtain spatial molecular distributions.

The results show the molecular architecture of biological tissues and biofilms. I'm Dr.Peter Nemes from the Vertus Research Laboratory at the George Washington University. The main advantage of the lazy technique over other methods, including confocal, optical, and mass spectrometry imaging methods such as Maldi and sims, is that the sample analysis can be performed directly on the tissue under atmospheric pressure conditions and no sample treatment is necessary.

Mount tissue sections of interest directly onto a flat surface, such as a chemically pre-cleaned glass slide without chemical modifiers. Secure the sample to apelt cooling stage immediately upon mounting if needed, use a heat sink equipped with a low power fan to facilitate heat removal from the pelt stage. Then if working in a humid environment over an extended period of time for about one to two hours, inspect for condensation of water or ice on the tissue surface.

Water condensation on the tissue detrimentally affects the imaging performance in lazy experiments. If needed, use a room dehumidifier or place the called sample in an environmental chamber filled with an inert gas, such as dry nitrogen gas to prevent condensation. After optimizing the conditions for maintaining the sample work to optimize the ion source used the laser ablation, electro spray ionization, or lazy iron source consists of a mid-infrared laser, abbreviated mid ir, and a series of optical elements for light steering and focusing.

The setup also includes additional sample holders, cooling components, translation stages, and an electro spray system. A portion of the particulate matter expelled during mid IR ablation coalesces with the electro spray to yield charged droplets seeded with molecules and ions of the sample. The ions released from these droplets are analyzed and recorded by the mass spectrometer, the orifice of which is shown here.

The lazy setup is configured before starting the experiment. Precise adjustments of the experimental parameters are required to accommodate the sample of choice to optimize the ion source. Start by positioning the sample 15 to 20 millimeters below the orifice of the mass spectrometer sampling cone.

Operate the mid IR laser at 2.94 micrometer wavelength and 10 hertz repetition rate. Attenuate the laser output to around 100 micro joules per pulse energy. Use a combination of gold mirrors and a focusing lens transparent at the laser wavelength to couple the laser light energy into the sample.

At normal incidents position the mid-infrared B axis, five to eight millimeters in front of the orifice of the mass spectrometer sampling cone. Adjust the focusing lens position and the laser pulse energy to achieve tissue removal in the focal spot to the desired depth. The dimensions of the ablated volume.

Determine the pixel size for the imaging application. Position a nanos spray emitter in line with the inlet axis of the mass spectrometer, and at an orifice to emit a tip distance of around 10 millimeters. For the electro spray, prepare a 50%methanol solution with 0.1%acetic acid for positive ion mode or 0.1%ammonium acetate.

For negative ion mode. The stability of the electro spray is crucial for successful imaging. Depending on the solvent selection, the flow rate and the spraying voltage need to be adjusted to achieve stable spray.

For reactive, lazy and imaging applications, the electros spray solution may contain reactants. Use a syringe pump to deliver the electro spray solution through the electro spray emitter at a flow rate of around 300 nanoliters per minute. If the mass spectrometer orifice is kept at a low voltage, for example, below around 500 volts measured against ground, generate electro spray by applying high voltage, for example, 3000 volts directly to the electro spray emitter or through a metal union.

Operate the electro spray source in cone jet spraying mode for the most efficient ion generation by lazy, carefully adjust the relative distances of the lazy setup to optimize for lazy ion yield while keeping the laser beam, the emitter and the orifice axes in the same plane with an optical microscope. Determine the lateral dimensions of the ablation crater on the sample for three dimensional lazy imaging experiments. Perform ablation with individual pulses and determine the depth of a voxel using, for example, the Z stack mode in optical microscopy.

Now that the tissue mounting conditions and the ionization source are optimized, proceed to molecular imaging in the imaging experiment. The tissue sample is moved in the focal plane of the laser in the x and y directions with step sizes larger than or equal to the dimensions of the ablation spot. The spatial resolution is limited by the focusing of the incident laser beam.

Select an area of interest on the sample surface and obtain the XY coordinates of the corresponding boundaries. Choose a gridding algorithm with which to raster the sample surface with selected dwell time at each pixel over the area to be imaged. Use a three XS translation stage and software that is capable of ing the sample according to the predetermined grid.

Then calculate the total time required for imaging. Start the mid-infrared laser source at a repetition rate proper to produce sufficient signal to noise ratio for selected ions in the mass spectrum within the dwell time at each pixel. To perform a lazy lateral imaging experiment, turn on the electro spray source.

Make sure that there is enough solution for the full-time required for imaging. Now simultaneously start the acquisition of mass spectra, the mid IR laser ablation and the surface scanning and collect data. When the imaging run has finished, stop the surface, scanning the mid IR laser and the data acquisition.

Disable the laser source, turn off the high voltage, switch off the syringe pump and set the mass spectrometer to stand by mode. Also, turn off the palt cooling electronics and close the inert gas flow. If used.

Finally, plot the ion intensity signal for a selected MZ value against the absolute coordinates of analysis to obtain lateral or 3D molecular images. These are representative images from lateral lazy imaging of a 100 micrometer thick coronal section of a rat brain. The section was frozen during the experiment and kept in a dry nitrogen gas environment.

The anatomical regions of the brain show good correlation with the molecular image obtained for two plasmodium with MZ 2 78 0.59. This is a molecular imaging experiment in 3D using zebra plant leaf tissue interrogated in the ambient environment among other primary and secondary metabolites. ASCE with MZ 2 85 0.076 was detected at higher ion counts in the yellow sectors of the second and third layers from the top with a homogeneous distribution.

In the others, this distribution agreed with the pattern of the variation seen in the optical image. H After its development. The lazy technique paved the way for researchers in the field of analytical and B chemistry to study molecular composition and spatial organization of tissues and biofilms with appreciable water content.

Don't forget that working with a mid infrared laser and an electric spray source can be extremely hazardous. Always follow a standard laser safety protocols and apply proper shielding to all electrical connections during this procedure.