Mass Cytometry (CyTOF)

Flow cytometry reimagined with rare-earth atoms as the reporter — instead of reading colour, a mass spectrometer weighs the label, freeing 50 protein measurements from the tyranny of spectral overlap.

Mass cytometry (commercial name: CyTOF, for Cytometry by Time-Of-Flight) labels cells with a panel of antibodies, each conjugated to a distinct stable heavy-metal isotope — predominantly from the lanthanide series. The labelled cell suspension is nebulised into single-cell droplets, which are introduced into an inductively coupled argon plasma (ICP) at roughly 7,000 K. The plasma vaporises, atomises, and ionises the entire cell in under one millisecond, converting every metal label to a free ion cloud. A quadrupole high-pass filter removes low-mass biological ions (carbon, nitrogen, oxygen), and the remaining ion cloud — containing only the heavy reporter masses — enters a time-of-flight mass analyser. Because heavier ions travel more slowly through the TOF flight tube, all 135 mass channels (75–209 atomic mass units) are resolved simultaneously without compensation. Each cell thus yields a vector of ~50+ metal abundances that encodes its protein expression profile at single-cell resolution.

Cells are stained with a cocktail of metal-conjugated antibodies, then resuspended in a cell acquisition solution and loaded into the pneumatic sample-introduction system. A concentric glass nebuliser aerosolises the suspension; the heated spray chamber delivers single-cell droplets at approximately 30 µL/min into the ICP torch. In the plasma, cell contents are vaporised and all atoms fully ionised. A quadrupole ion deflector passes only masses above ~75 Da; the high-mass ion clouds (one per cell, lasting ~0.5 ms) enter the push-out TOF analyser where 13-µs pulses of electric potential orthogonally accelerate packets of ions toward the detector. Each packet arrives at the detector in a time window proportional to m/z, yielding isotope counts with 4.5 orders of dynamic range. On the Helios (Fluidigm/Standard BioTools) platform, peak throughput is up to 2,000 events/sec; practical acquisition is typically 200–800 cells/sec to maintain cell detection efficiency above 90%.

In plain terms

Imagine labelling every kind of toy in a toy box with a tiny bead of a different rare metal: gold for cars, silver for trains, copper for dolls. Then you melt each toy and weigh all the tiny beads with an incredibly precise scale. Even if a hundred beads all fall onto the scale at once, the scale tells you exactly how many of each metal there are — because gold, silver, and copper all weigh different amounts. CyTOF does that with cells: each protein on the cell's surface gets its own unique metal label, and the instrument weighs all the labels at once to figure out exactly which proteins are there.

Also called: CyTOF, cytometry by time-of-flight, mass cytometry, ICP-TOF-MS cytometry.

Why it matters

Where it lives

Key numbers

Surprising facts

Open questions

A short history

Mass cytometry grew from the recognition by Scott Tanner's group at the University of Toronto that inductively coupled plasma mass spectrometry (ICP-MS) — already used for trace-metal analysis in geology and environmental science — could be adapted to detect metal-labelled antibodies on single cells. Tanner's group published the first mass cytometer in 2009 (Bandura et al., Analytical Chemistry), and the instrument was commercialised by DVS Sciences as the CyTOF. The landmark biological application was Bendall et al. in Science in May 2011, which used 34 simultaneous parameters to map signalling states across the human haematopoietic hierarchy in bone marrow. DVS Sciences was acquired by Fluidigm Corporation in 2014; Fluidigm rebranded as Standard BioTools in 2022. The Helios, released in 2015, tripled throughput and expanded to 135 channels. Imaging mass cytometry (Hyperion), coupling CyTOF detection to laser ablation of tissue sections, was introduced in 2017, enabling spatially resolved 40-parameter imaging of tumour microenvironments.

Sources

Related