Towards single cell analysis using ambient mass spectrometry techniques

Abstract

The metabolomic and lipidomic (i.e., the systematic study of the global set of small molecules and structural lipids) characterisation of cells has been a long-standing objective in obtaining a more profound understanding of biological systems, processes, and different diseases. A novel ambient mass spectrometry technique, Rapid Evaporative Ionisation Mass Spectrometry (REIMS) was developed to perform sample preparation-free MS analysis of tissues for in vivo tissue characterisation during surgical procedures. To achieve single cell analysis, two approaches were investigated. The development of a novel LD-REIMS platform was carried out using several different lasers operating at mid-infrared wavelengths, including a picosecond pulse length Optical Parametric Amplifier (OPA). A novel multi-modal LD-REIMS analysis platform was developed, the sampling/ionisation process and the parameters were optimised, and single cell imaging was demonstrated on mouse brain tissues. To facilitate the high throughput analysis of bulk cell samples, an automated well plate reader platform was designed and developed, allowing the automated analysis of cell lines from industry-standard well plate formats. A custom flow-focusing droplet source was developed, which allows the coating of the cells incoming in liquid form with an organic sheath to make them more compatible with MS analysis. Different cell lines were analysed using these methods, and lipidomic/metabolic fingerprints were acquired from various cell lines. A novel inlet was designed to remove the inlet capillary from the source, enabling higher transmission efficiency. The above-described novel methods provide tools enabling reproducible and sensitive metabolomic and lipidomic profiling of single cells. These methodologies developed herein are particularly pertinent to studying diseases such as cancer, where there is large heterogeneity within the tissue; hence single cell spatial resolution is crucially important.