MALDI imaging mass spectrometry (IMS) links the universal detection capability of mass spectrometry with the positional information of molecular histology, generating mass spectra correlated to known locations within the tissue. MALDI-IMS can reveal the distribution of hundreds of ion signals ranging in size up to 20,000 Daltons. This information can be used to determine tumor margins, drug distribution throughout a tissue or organism, and proteomic profiles under different experimental or therapeutic conditions.

Protein profiles are obtained directly from frozen tissue sections. The Labcyte Portrait^® 630 spotter deposits MALDI matrix, enzymes and other reagents in a user-defined pattern directly onto the tissue (Figure 1). The Portrait 630 user interface allows complete flexibility for the user to control reagent deposition parameters to optimize tissue penetration, reaction conditions, crystal formation, and positional resolution for different tissue types and applications.

Figure 1. Left: H&E stained rat brain coronal section imaged on light microscope (10X). Right: Adjacent tissue section spotted with 25 mg/ml sinapinic acid in 1:1 acetonitrile/TFA (aqueous) using a Labcyte Portrait 630 spotter. (Tissue sections and microscopic image courtesy of Dr. Pierre Chaurand and Dr. Richard Caprioli, Vanderbilt University.)

The coordinates of each crystalline matrix spot are transferred to the MALDI mass spectrometer, and spectra are collected for each spot location (Figure 2.)

Figure 2: Sum spectrum of cumulative signal from all pixels at each m/z. Mass spectrometry was done using a Bruker Daltonics autoflex MALDI-TOF. Inset spectra are amplifications of the higher mass regions. (Spectrum courtesy of Dr. Pierre Chaurand and Dr. Richard Caprioli, Vanderbilt University.)

Molecules can be mapped directly to their locations on the tissue (Figure 3.)

Figure 3. MALDI tissue maps constructed using BioMap image analysis software (www.maldi-msi.org). Each image represents the distribution of a single ion species in the tissue section. Left: m/z 4965 (thymosin beta-4); Middle: m/z 11306-11347 (histone H4); Right: m/z 12133 (cytochrome c). (Tissue maps courtesy of Dr. Pierre Chaurand and Dr. Richard Caprioli, Vanderbilt University.) 

Case Study - Portrait Spotter Enables On-Tissue Enzyme Digests
The Portrait spotter accommodates a wide range of different reagents, including enzyme buffers and surfactants in addition to standard MALDI matrix solutions. Vanderbilt University used the Portrait spotter to deposit four different enzymes (trypsin, chymotrypsin, endoproteinase glu-C, and subtilisin) onto rat brain sections, followed by MALDI matrix deposited onto the same spot locations. In these experiments, each enzyme digest generated multiple unique peptide fragments, corresponding to myelin basic protein (Figure 4), tubulin, and synapsin.

Figure 4 – Sum spectra images of peptide fragments from myelin basic protein. Images were created using BioMap software (www.maldi-msi.org.)

Using MS/MS, peptide sequencing was performed to identify protein fragments directly from the tissue section. A peak at m/z=1603 in the chymotrypsin digest was identified with confidence as tubulin α-1 chain (Figure 5.)

Figure 5 – Peptide sequence analysis of m/z=1603 in the chymotrypsin digest.