ICP/MS picks up many elements, usually with exceptional sensitivity
When a scientist needs a sensitive method that detects a wide range of elements, inductively coupled plasma/mass spectrometry (ICP/MS) is a good choice. That has been so for a couple decades, since Iowa State University chemist R.S. Houk and his colleagues described the technique in 1980.1 With ICP/MS, a researcher can detect many of the elements in the periodic table at single parts per trillion or better.
Just three years after Houk’s group described ICP/MS, Canada-based MDS SCIEX released the first commercial platform. Today, various companies— including Agilent, PerkinElmer, and Thermo Fisher Scientific—make ICP/ MS platforms or accessories.
With this technology, the sample—usually a liquid—goes into an argon laser, and that produces ions. The singly charged ions go to an MS, which is often a quadrupole in modern systems. After the MS separates the ions by their mass-to-charge ratios, they go to a detector.
In the beginning, geochemists latched onto ICP/MS, especially for its rare-earth capabilities. But they are not alone—others find the technique invaluable as well.
Ups and downs of ICP/MS
The ability to identify so many elements at high resolution makes ICP/MS a great analytical tool. It can analyze tens of elements in quick succession. Moreover, it can be combined with other techniques.
Despite the upsides, ICP/MS is not perfect. Some platforms use single-quadrupole MS, which is low in resolution compared to triple-quad MS. ICP/MS can also get a little pricey. The detector, for example, is a consumable, and ions hitting it damage the coating. Eventually, the detector must be replaced. To get the highest levels of detection, scientists put an ICP/ MS in a cleanroom, which adds to the cost of operation.
Some elements are easier to see than others with ICP/MS. Calcium and iron, for instance, can be hard to analyze. Still, scientists find more ways to use this technology every year.
Measuring mine water and more
Like the geochemists who first gravitated toward ICP/MS, many scientists still apply this technology to earth-science questions. Researchers from India used ICP/MS to analyze water being pumped out of coal mines.2 They collected samples before, during, and after the monsoon season. The team analyzed more than 10 elements. They reported that the “results demonstrated that concentrations of the metals showed significant seasonality and most variables exhibited higher levels in the pre-monsoon season.”
Similarly, scientists from Japan and Malaysia applied ICP/MS to heavy metals in the surface water and soil in a river basin.3 The samples came from areas used in different ways, including oil-palm farming, quarrying, and bare land. These scientists also measured the concentrations of several heavy metals in the blood from 136 pregnant women, including samples from the mother and her umbilical cord. The heavy metals varied by location and use. For example, they reported that “nickel and zinc were discharged from sewage and/or industrial effluents, and that lead was discharged from mining sites.” Specific heavy metals behave differently in pregnant women. For example, cadmium and lead levels could be high in a mother’s blood but reduced in the cord blood; arsenic was the same in the mother and the cord. As the scientists note, the variations depend on “the different kinetics of maternal-to-fetal transfer.”
So, ICP/MS can connect earth and health sciences or many other sciences— all depending on the creativity of the researchers.
Upstream additions
Getting the most from ICP/MS or branching out to new applications often requires additional technology. In some situations, upstream variations make a difference, essentially changing the sample preparation.
Geochemists pushed IPC/MS into a wide range of uses, from exploring rare-earth materials and waters of all sorts. (Image courtesy of the author.)A team of scientists in China used magnetic solid-phase extraction (MSPE) and added high-performance liquid chromatography (HPLC) to make MSPE-HPLC-ICP-MS.4 They used this instrumentation to study the forms of mercury in water and fish. With this system optimized, the researchers could detect different forms of mercury down to fractions of a nanogram per liter.
The investigators concluded: “The developed MSPE-HPLC-ICP-MS method was validated by the speciation of mercury in … real-world samples including environmental water, wastewater, tap water, and fish samples, and it has the advantages of simple operation, rapid separation, high sensitivity, [and] high enrichment factor, and is suitable for the analysis of mercury species in samples with a complex matrix.”
Similar technology could be adapted to explore speciation of other elements in different matrices.
Adding immunoassays
ICP/MS can also be paired with biological technologies that allow completely new possibilities. One of those is immunoassays. Here, an antibody indicates the presence of an antigen in a sample. A label added to the antibody can be used to locate it after it binds to its target. Using a label that includes an element that ICP/MS detects opens new opportunities for these assays, from industrial applications to exploring biomarkers in physiology and medicine.
One group of scientists in China wrote that immunoassays are very important methods “for detecting and measuring specific proteins or other substances through their properties as antigens or antibodies,”5 and added, “Conventional immunoassay detection methods, such as radioimmunoassay, fluorescence immunoassay, and chemiluminescent immunoassay, are often challenged by limited dynamic range, low sensitivity, and the overlap of detection signals.”
With labels that can be detected with ICP/MS, scientists develop immunoassays that can identify more targets. In some cases, this allows more labels to be used in the same experiment. Studies show that this works with many targets, including various reproductive and thyroid-stimulating hormones, which could be used in medical research. This technology can also be used in the food industry, because it can detect problem ingredients, such as peanut allergens.
This combination of immunoassays with ICP/MS as the method of detection brings the benefits noted above: high sensitivity, specificity, multiplexing, and more. It also brings the same problems, including high cost. In addition, ICP/MS requires skills that could limit some medical applications, especially in a clinic.
In less than 40 years, ICP/MS went from being an experiment to a commercial technique used far beyond the rare-earth crowd that got this method rolling. By advancing the platforms and creating new combinations of technology, ICP/MS offers scientists new methods of probing the periodic table and beyond.
References
- Houk, R.S.; Fassel, G.D. et al. Inductively coupled argon plasma as an ion source for mass spectrometric determination of trace elements. Anal. Chem. 1980, 52, 2283–9.
- Mahato, M.K.; Singh, G. et al. Assessment of mine water quality using heavy metal pollution index in a coal mining area of Damodar River Basin, India. Bull. Environ. Contam. Toxicol. 2017; doi: 10.1007/s00128- 017-2097-3.
- Sakai, N.; Alsaad, Z. et al. Source profiling of arsenic and heavy metals in the Selangor River basin and their maternal and cord blood levels in Selangor State, Malaysia. Chemosphere 2017; doi: 10.1016/j.chemosphere. 2017.06.070.
- Zhu, S.; Chen, B. et al. Speciation of mercury in water and fish samples by HPLC-ICP-MS after magnetic solid phase extraction. Talanta 2017; doi: 10.1016/j.talanta.2017.04.068.
- Rui, L.; Peng, W. et al. Inductively coupled plasma mass spectrometry based immunoassay: a review. Mass Spectrom. Rev. 2017, 33, 373–93.
Mike May is a freelance writer and editor living in Texas. He can be reached at mike@techtypercom.