(15) Lipids can further be observed as multiple adducts (e.g., addition of H +, Na +, K +, OAc вЂ“, Cl вЂ“, or loss of H +) that are entangled with isotopes and other isobaric species. In comparison to other biological molecules, resolving lipid complexity is further complicated by their relatively narrow mass range, with the majority of signals observed between m/ z 700вЂ“900. For lipids that are arguably the most widespread analyte class studies with MSI, (11в€’14) high mass resolving power and accuracy can facilitate true identification of sum-composition formula (i.e., lipid class and the combined number of carbons and double bonds across both fatty acyl chains), when sufficient to allow separation from isobaric interferences. (10) This combination allows mass resolution of many isobaric ion species and direct assignment of elemental composition, thereby providing insight into the specific identities of the detected molecules.

The most popular approach to begin addressing this complexity is the coupling of high mass resolving power and high mass accuracy analyzers with MSI ion sources, most commonly matrix-assisted laser desorption/ionization (MALDI). The instrument described allows for future innovative, such as high-end studies to unravel the complexity of biological, geological, and engineered organic material surfaces with an unsurpassed detail.

The unique molecular detail, accuracy, sensitivity, and dynamic range combined in a 21T MALDI FT-ICR MSI experiment enable researchers to visualize molecular structures in complex tissues that have remained hidden until now. These features allow for the separation and identification of lipids to the underlying structures of tissues. Molecular features with m/ z differences as small as 1.79 mDa were resolved and identified with high mass accuracy. Here, MALDI MSI at 21T is demonstrated for the first time: mass resolving power in excess of 1вЂЇ600вЂЇ000 (at m/ z 400), root-mean-square mass measurement accuracy below 100 ppb, and dynamic range per pixel over 500:1 were obtained from the direct analysis of biological tissue sections. Higher magnetic field strength improves all performance characteristics of FT-ICR mass resolving power improves linearly, while mass accuracy and dynamic range improve quadratically with magnetic field strength. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) offers the highest mass spectral performance for MALDI MSI experiments, and often reveals molecular features that are unresolved on lower performance instrumentation. Detailed characterization of complex biological surfaces by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) requires instrumentation that is capable of high mass resolving power, mass accuracy, and dynamic range.

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