Matrix-assisted laser desorption electrospray ionization

The current MALDESI source employs endogenous water or a thin layer of exogenously deposited ice as the energy-absorbing matrix where O-H symmetric and asymmetric stretching bonds are resonantly excited by a mid-IR laser.

[2] The IR-MALDESI source can be used for mass spectrometry imaging (MSI), a technique using MS data analyzed from a sample area to detect hundreds to thousands of biomolecules and visualize their spatial distributions.

The computer controlled motorized stage and a charge-coupled device (CCD) camera are placed in a nitrogen purged enclosure where ambient ions and relative humidity can be regulated.

[2] The ESI-like ionization mechanism has been experimentally demonstrated and studied in depth,[8][9][10] showing equivalent softness as ESI and enabling the detection of intact protein complexes.

The formation of ice matrix starts with purging the enclosure to below 12% relative humidity with the dry nitrogen gas source prior to cooling the Peltier stage down to −10 °C.

[13][9][2] In addition to the source geometry optimization, the electrospray solvent composition has an effect on the MALDESI signals (i.e. influencing molecular coverage and ion abundance).

There are many essential functions available in MSiReader, namely heat map generation with high mass measurement accuracy,[7] peak normalization,[16] absolute and relative quantification,[7][17] and polarity switching.

The Rastir software, also built on a Matlab platform, was developed to enable users to visually define a rectangular region-of-interest (ROI) surrounding a tissue section for MALDESI experiments.

The latest software version, denoted RastirX, allows users to draw arbitrary ROI on a live video image of a sample with the computer mouse.

By associating m/z with positions where they are acquired, unique ion maps of analytes can be generated, which provides valuable information for spatial distributions of lipids, peptides, metabolomics, and other small biomolecules from mammalian samples[1][12][21][23][14][24] to plant tissues.

[27] IR-MALDESI has shown the ability to measure selected neurotransmitters in rat brains exposed to flame retardant tetrabromobisphenol A without any chemical derivatization.

IR-MALDESI is an ablation-based technique, which is able to measure and generate 3D heat maps of biomolecules from pharmaceutical tablets[20] and nude mouse skin[21] by sequentially imaging a sample with consecutive ablation events.

In a proof-of-concept study, HIV antiretroviral drugs in incubated cervical tissues were imaged and quantified using IR-MALDESI, and the results were evaluated by a validated LC-MS/MS method.

Another technique, desorption atmospheric pressure photoionization (DAPPI), uses a jet of heated solvent vapor to desorb analytes from the sample surface via a nebulizer microchip.