Single-cell analysis has become an method of importance in immunology. Fluorescence flow cytometry has been a major player. However, due to issues such as autofluorescence and emission spillover between different fluorophores, alternative techniques are being developed. In recent years, mass cytometry has emerged, wherein antibodies labeled with metal ions are detected by ICP-MS. In order for a cell to be seen, a metal in the mass window must be present; there is no analogous parameter to

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... d or side scatter. The current mass window selected is approximately AW 103-196, which includes the lanthanides used for most antibody labeling, as well as iridium and rhodium for DNA intercalators. In this protocol, we use a cocktail of antibodies labeled with MAXPAR metal-chelating polymers to surface-stain live PBMC that have been previously cryopreserved. Many of these markers were taken from a standard fluorescence phenotyping panel (Maecker et al., 2012) . No intracellular antibodies are used. We use a CyTOF TM (Cytometry by Time-Of-Flight) mass cytometer to acquire the ICP-MS data. Subsequent analysis of the dual count signal data using FlowJo software allows for cell types to be analyzed based on the dual count signal in each mass channel. The percentage of each cell type is determined and reported as a percent of the parent cell type.