Single Cell Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging

Application of mass spectrometry imaging (MS imaging) analysis to single cells was so far restricted either by spatial resolution in the case of matrix-assisted laser desorption/ionization (MALDI) or by mass resolution/mass range in the case of secondary ion mass spectrometry (SIMS). In this study we demonstrate for the first time the combination of high spatial resolution (7 mu m pixel), high mass accuracy (< 3 ppm rms), and high mass resolution (R = 100 000 at m/z = 200) in the same MS imaging measurement of single cells. He La cells were grown directly on indium tin oxide (ITO) coated glass slides. A dedicated sample preparation protocol was developed including fixation with glutaraldehyde and matrix coating with a pneumatic spraying device. Mass spectrometry imaging measurements with 7 pm pixel size were performed with a high resolution atmospheric-pressure matrix-assisted laser desorption/ionization (AP-MALDI) imaging source attached to an Exactive Orbitrap mass spectrometer. Selected ion images were generated with a bin width of Delta m/z = +/- 0.005. Selected ion images and optical fluorescence images of He La cells showed excellent correlation. Examples demonstrate that a lower mass resolution and a lower spatial resolution would result in a significant loss of information. High mass accuracy measurements of better than 3 ppm (root-mean-square) under imaging conditions provide confident identification of imaged compounds. Numerous compounds including small metabolites such as adenine, guanine, and cholesterol as well as different lipid classes such as phosphatidylcholine, sphingomyelin, diglycerides, and triglycerides were detected and identified based on a mass spectrum acquired from an individual spot of 7 mu m in diameter. These measurements provide molecularly specific images of larger metabolites (phospholipids) in native single cells. The developed method can be used for a wide range of detailed investigations of metabolic changes in single cells.