High-Throughput Single-Cell Immunoassay in the Cellular Native Environment Using Online Desalting Dual-Spray Mass Spectrometry

Single-cell mass spectrometry (MS) remains challenging in the analysis of cells in the native environment due to the severe ion suspension from nonvolatile salts. Synchronous desalting and ionization would be ideal to both ensure the native environment and remove the salt interference. Here, a novel dual-spray ionization technique combining electrospray and nanoelectrospray ionization (ESI-nESI) was developed, enabling highly efficient online desalting during the ionization process. In situ detection of cell surface proteins from the intact cells in phosphate buffer saline (PBS) was achieved by dual ESI-nESI MS with the help of an MS-based immunoassay using rhodamine-based mass tags. These mass tags were confirmed to be highly competitive during desalting, which improved the protein detection sensitivity to a single-cell level. Through the combination of the single-cell immunoassay with ESI-nESI MS, the important surface protein markers, cancer antigen 125, in two cancer cell lines (OVCAR-3 and MCF-7) suspended in the PBS buffers were screened in a high-throughput cytometric mode, along with some proposed cellular endogenous lipids. The ESI-nESI MS system is promising for multidimensional organic mass cytometric analysis in the cellular native environment for clinical use and many basic biology researches.