ICP-MS: Taking the “Gold Standard” to the Next Level

Cells are the basic unit of all living organisms – “the building blocks of life” – so it isn’t surprising that analyzing cells and their constituents is central to life science research. One challenge for researchers is that cells vary considerably – even cells of the same culture behave differently at a chemical level because of their different life cycles (or cell status). ICP-MS is the only method that provides quantitative metal data in single cells with multi-element coverage and high sensitivity, which has led to its rise as the gold-standard tool for measuring how cells interact with their environment.

Elemental single-cell analysis comprises both the assessment of biomarkers through metal tagging of antibodies (denoted as mass cytometry or imaging mass cytometry), as well as the determination of endogenous/toxic metal accumulation and the up-take of nanoparticles from cell suspensions. Isolated cells and cell agglomerates in tissue samples have been investigated with laser ablation (LA)-ICP-MS in an imaging mode, which has focused on detecting metals – either directly or with metal tagged antibodies. 

Though ICP-MS has been on the market since 1983, mass cytometry is a relatively young technology (commercially available since 2009, launched by DVS – Dimitry, Vladimir and Scott – under the brand name CyTOF Mass Cytometer). The basic idea is similar to fluorescence-based flow cytometry. Instead of using fluorescence to tag antibodies, single-enriched isotopes of rare earth elements (REE) are used. This approach overcomes the problem of “bleaching” or high-background interference associated with autofluorescence, while also allowing the use of a greater number of isotopes in a single assay. Then, to bind metal isotopes to antibody polymers, tags are applied, which mostly bind to surface receptors of single cells or, in the case of phenotyping, the antibodies bind to intracellular proteins after cells are permeabilized (the approach for cell functional assays). Because each polymer can bind up to 100 atoms, it is possible to amplify a single protein/receptor by the same factor. In fact, a cell can have up to 1,000 surface receptors of the same kind, which can really increase the amplification for extremely high sensitivity! 

After incubation of the target cells with different antibodies, which are tagged individually by enriched nuclides from lanthanide elements, the cell suspension is transported to a pneumatic nebulizer. Stochastically spaced cells in the laminar flow are then delivered concentrically to the plasma core, where the cells get ionized. Since each cell generates ion signals of a few hundred µs peak width, a very fast mass spectrometer (a time-of-flight instrument) is required to measure each cell time resolved. 

There have been a number of improvements in ICP-MS over the years. For example, higher sensitivity, higher time resolution, and dedicated sample introduction systems. For laser ablation systems, lasers have been developed with higher repetition rates (up to 200 Hz), the ability to wash out aerosols more quickly to achieve better lateral resolution at the sub-micrometer level.

With novel LA-ICP-MS devices, sub-micrometer resolution has been achieved, which is needed to compete with light microscopy! Early results have demonstrated multiparametric analysis of cancer tissues with up to 40 parameters measured in the imaging mode by laser ablation. Indeed, such analysis has revealed the role of the neighboring community of cells in cancers – going some way to explaining why patients respond differently to cancer treatments. 

There have been thousands of papers published on the use of ICP-MS, covering nano-particle interactions, metallo-drug research, various types of cancer research, and many more areas. However, some big challenges remain. For example, at the cellular level, we don’t have reference materials, which holds true for imaging and mass cytometry. And that means we must use our own standards to calibrate devices – not an ideal situation. Validation reference materials and inter-lab comparisons are urgently needed. And concerning mass cytometry, we still wait for it to be accepted as a routine diagnostic tool for detection of cancer, inflammation or other diseases.

In the future, through improvements in reagents, it may be possible to detect single viruses or even single molecules. Currently, we can measure the end-point of a disease, such as a cancer cell, or the toxification of a cell leading to cell death, but we cannot measure these dynamic processes as they occur (time resolved analysis). We would need multimodal spectroscopies to measure – in a time resolved manner – the complex chemistry underpinning living processes. 

In fact, we know very little about the complex machinery of life, which is why we need more scientists – especially younger scientists – applying analytical chemistry to the complex reactions taking place within cells at the nano-meter level and at time scales of µ- or milli-seconds. Single cell analysis – including ICP-MS – can take us a long way, but we need to measure living processes in real time to truly understand what’s going on. Who will step up to the plate?!

Norbert will be chairing a session on elemental imaging and mass cytometry at the Winter Conference, Tucson, Arizona, USA, in January 2022