Polarity changes: Simultaneous positive and negative modes in DESI

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  • Published: Apr 15, 2013
  • Author: Steve Down
  • Channels: Base Peak
thumbnail image: Polarity changes: Simultaneous positive and negative modes in DESI

Imaging advances

Danish scientists have introduced a dual-mode imaging system for desorption ionisation mass spectrometry, in which images can be acquired in positive and negative mode simultaneously.

The expansion of ambient mass spectrometry methods, in which ionisation takes place in the open air, continues unabated. One of the growth areas is tissue imaging, which can reveal diseased regions in organs and detect the distributions of natural compounds, drugs and metabolites either by imaging the whole body or surveying particular organs.

A tissue imaging experiment can be conducted in positive-ion mode to analyse many drug compounds, or in negative-ion mode for endogenous species like phospholipids. Sometimes the two polarities are switched in the same experiment. In this respect, ambient methods like desorption electrospray ionisation (DESI) MS are not as good as MALDI MS, due to differences in the size of the impacting beam.

In MALDI MS imaging experiments, the laser strikes a small area on the sample, leaving much of it undamaged for the next shot. So, positive-negative ion switching between laser shots is feasible, giving high resolution images. The same is not true for DESI. Here, the spray is much more diffuse than a laser shot and damages a wider area on the sample, making it more difficult to acquire the second image from the same spot at the same quality as the first.

Now, Danish scientists have managed to reconcile the differences by adapting instrumentation which they first reported in 2012. Christian Janfelt and colleagues from the University of Copenhagen had designed a system they called displaced dual-mode imaging (DDI), in which DESI and easy sonic spray ionisation signals were measured alternately. The same system has now been used to take simultaneous positive-ion and negative-ion images, as they described in the Journal of Mass Spectrometry.

Overlapping sprays

The DESI spray solution, comprising aqueous methanol, was directed at the target and scanned in horizontal rows in one polarity, either positive or negative. The spatial resolution during the scans was fixed at a certain value, say 150 µm. At the beginning of the next row, the polarity was switched and this resolution was maintained. However, the distance between each row was just 75 µm, so the data from the positive and negative modes originated from overlapping sites on the sample.

Some operational trickery had to be incorporated for successful experiments. For instance, a delay of 20 seconds before the next scan began was introduced between the consecutive rows to allow for charge equilibration after the polarity was switched. Without this, the signal intensities increased gradually until charge equilibrium was maintained, causing dark shading on the images.

As the solvent flow rates were increased to values around 5 µL/min, the spray geometry became less critical. At these rates, excess wetting of the samples or even damage could occur, but this was minimised by keeping a low proportion of water, typically 5%, in the methanol spray. No acids or bases were added since the same solution was used for both positive and negative modes.

The mass spectra were recorded on a linear ion trap instrument and processed to give images which mapped the locations of the selected compounds in the samples using unique m/z values. For instance, m/z 834.5 for the phospholipid phosphatidylserine or m/z 278.2 for the drug amitriptyline were employed.

Tissue testing

The new system was tested first on a section of mouse kidney. A number of compounds were detected and mapped in negative-ion mode, including phosphatidylserine and phosphatidylinositol and the free fatty acid docosahexaenoic acid. The acid was unexpected and might be an indicator of the post-mortem activity of phospholipase enzymes. In positive-ion mode, the sodium ion adducts of two phosphatidylcholines were observed.

An imprint of a leaf of Hypericum perforatum was also analysed by the dual-polarity system. Leaves of this plant have a series of dark translucent glands which contain a number of secondary metabolites. Both quercitrin and hyperforin were mapped as their deprotonated ions in negative mode and as adducts with protons and/or sodium and potassium ions in positive mode.

Using a linear ion trap mass spectrometer also allowed simultaneous full-scan and MS/MS imaging to be carried out. This capability was illustrated on the kidneys of mice that had been given amitriptyline before sacrifice. In full scan, the phospholipids were detected but the drug and its metabolites were only visible by tandem mass spectrometry.

The DDI technique appears to give a broader picture of samples by simultaneous imaging at both polarities. This will be helpful for mapping the distributions of compounds around a sample, like the biological tissue analysed here, but also for detecting the types of compounds that are found in certain locations within a sample, like glands or follicles.

The ability to co-record MS and MS/MS spectra will be particularly useful for whole body imaging studies of drugs and their metabolites and will be able to place them in their biological context alongside the endogenous species that are also detected.

Related Links

Journal of Mass Spectrometry 2013, 48, 361-366: "Displaced dual-mode imaging with desorption electrospray ionization for simultaneous mass spectrometry imaging in both polarities and with several scan modes"

Article by Steve Down

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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