Lipid imaging with time-of-flight secondary ion mass spectrometry (ToF-SIMS)

Abstract

Fundamental advances in secondary ion mass spectrometry (SIMS) now allow for the examination and characterization of lipids directly from biological materials. The successful application of SIMS-based imaging in the investigation of lipids directly from tissue and cells are demonstrated. Common complications and technical pitfalls are discussed. In this review, we examine the use of cluster ion sources and cryogenically compatible sample handling for improved ion yields and to expand the application potential of SIMS. Methodological improvements, including pre-treating the sample to improve ion yields and protocol development for 3-dimensional analyses (i.e. molecular depth profiling), are also included in this discussion. New high performance SIMS instruments showcasing the most advanced instrumental developments, including tandem MS capabilities and continuous ion beam compatibility, are described and the future direction for SIMS in lipid imaging is evaluated.

Introduction

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a surface analysis technique capable of producing high resolution chemical images and is a well-suited platform for the analysis of lipids directly from the surface of biological materials. With this technique the sample surface is bombarded with a focused high energy primary ion beam (1–40 keV), causing desorption of secondary ions. A mass spectrometry-based image is then produced by rastering the ion beam across the sample surface. The high lateral resolution and sensitivity attributed to SIMS allows for the detection of lipid molecules at the nanometer scale and at attomolar concentrations [1], [2]. The ToF detection scheme also offers parallel detection of multiple lipid species, ideal for the analysis of complex biological samples.

In addition to ToF-SIMS, matrix-assisted laser desorption ionization (MALDI) and desorption electrospray ionization (DESI) are imaging mass spectrometry (IMS) techniques utilized in the analysis of biological materials. Like ToF-SIMS, MALDI [3], [4], [5], [6] and DESI [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] have proven to be particularly successful in the detection and analysis of lipids. The pitfalls, advantages and successful applications of each technique are reviewed in detail elsewhere and are only briefly discussed in this review [19], [20], [21], [22], [23], [24]. In terms of spatial resolution, MALDI and DESI are capable of resolving features as small as 20 μm and 100 μm, respectively. In many cases for tissue imaging, ToF-SIMS offers a complementary perspective to these alternative IMS techniques since the lateral resolution of ToF-SIMS can be below 1 micron (Fig. 1). Various efforts have been made to improve the spatial resolution of MALDI imaging, including oversampling [25], laser modulation [26] (i.e. smart beam technology) and solvent-free sublimation matrix application techniques [27]. Despite these efforts, the technique has not achieved the spatial resolution of ToF-SIMS.

In terms of chemical specificity, however, MALDI and DESI techniques cover a broader range of biomolecules—including proteins, peptides and nucleotides. The ability to detect proteins and peptides directly correlates to the techniques’ success in bio-analytical chemistry and biomedicine. Currently, MALDI is the prominent IMS method utilized in medical and bioanalytical research with applications in clinical diagnostics [28], [29], [30], [31], [32], pharmaceutical research [33] and biomarker discovery [34]. Although proteins represent only 20 % (by weight) of a cell, proteomics has traditionally been at the heart of biomedical investigations. However, a recent trend among system biologists from proteomics to lipidomics [35] raises the question: Will SIMS, with its higher spatial resolution and equivalent sensitivity to lipids, be more readily accepted into the biochemical and biomedical community?