Mass spectrometric imaging for biomedical tissue analysis.

1.1. Mass Spectrometry A mass spectrometer is described as the smallest weighing scale in the world ever used.1 Mass spectrometry (MS) is a unique technique that has an interdisciplinary nature, which freely crosses the borders of physics, chemistry and biology. Mass spectrometry makes a great scientific tool due to its capabilities to determine the mass of large biomolecular complexes, individual biomolecules, small organic molecules as well as single atoms and their isotopes. Right from the time of its invention in the first decade of the 20th century, mass spectrometry has undergone tremendous improvements in terms of its sensitivity, resolution and mass range. It currently finds applications in all scientific disciplines such as chemistry, physics, biology, pharmacology, medicine, biochemistry and bio-agro-based industry. Introduction of “soft” ionization sources such as electrospray ionization (ESI) by J.B. Fenn et al.2 and matrix-assisted laser desorption/ionization (MALDI) by M. Karas et al.3 in 1980s revolutionized mass spectrometry as it offered the capability to analyze large intact biomolecules. As such MS became an irreplaceable tool for the biological sciences. The development of both ESI and MALDI made possible the ionization of smaller biomolecules such as drugs and metabolites as well larger biomolecules such as lipids, peptides and even proteins.2,4 The molecular weight (Mw) ranges we use in this review are defined as follows. The low Mw range includes elements and molecules from 1 to 500 Da. Molecules with Mw between 500 and 2000 Da fall in the medium Mw range. All molecules with Mw > 2000 Da are considered to be part of the high molecular weight class. It goes beyond the scope of this review to cover all developments in MS. Rather this review focuses on one of the latest, rapidly developing innovations in MS namely mass spectrometric imaging (MSI). This young technique takes benefit from all methodological and technological developments in general MS over the last decades. Over the last twenty years MSI has transformed from an esoteric, specialist technology studied by few researchers only to a technique that now finds itself at the center stage of mainstream MS. Over the last years the technology has matured to find applications in many different areas, with instrument developments taken up by all MS instrument manufacturers resulting in a rapid rise of the number of research groups active in this area. A thorough review of the area is therefore timely and needed to offer a starting point for all newcomers to the field. In this paper we describe and review approximately 20 years of MSI developement from the perspective of its application to biomedical imaging. We will emphasize the key research steps and pitfalls that determine the outcome of biological applications of this relatively new label free biomolecular imaging technique in life sciences.