Laser desorption/ionization droplet delivery mass spectrometry for live single cell analysis and imaging

Abstract

We have developed a new high-resolution ambient ionization mass spectrometry techniques named laser desorption/ionization droplet delivery mass spectrometry (LDIDD-MS) (Anal Chem, 2016). A pulsed UV laser beam (266 nm) is focused on a surface covered with target analytes to induce their desorption and ionization (Fig. 1A). A spray of liquid droplets is simultaneously directed onto the laser-focused surface region to capture the ionized analytes and deliver them to a mass spectrometer. This approach of rapid and effective capturing of molecules after laser desorption/ionization allows the limit of detection for the amino acid lysine to be as low as 2 amol under ambient ionization conditions. A high spatial resolution around 3 µm for mass spectrometric imaging of a mouse brain tissue was achieved. The LDIDD-MS was employed for single-cell analysis of cellular apoptosis. A significant differences in the profiles of fatty acids and lipids between normal healthy cells and apoptosis-induced HEK cells was observed (Fig. 1B). We observed upregulation of phosphatidylcholine lipid with a relatively shorter carbon chain length and downregulation of phosphatidylcholine with a relatively longer carbon chain length, suggesting a possible new mechanism for apoptotic morphological changes. This technique also allows for a direct measurement of liquid-phase samples including amino acids, peptides, and proteins. A real-time detection of exocytosed neurotransmitters from live PC12 cells, which has been a significant challenge for mass spectrometric analysis, was achieved without damaging cells.