Measuring Isotopic Compositions with Multiple-Collector ICP-MS: The Only Limit Is the Imagination

Multiple-collector inductively coupled plasma–mass spectrometry (MC-ICP-MS) is a powerful technique for measuring isotopic ratios in various areas of research. Michael Wieser, who is an associate professor in the Department of Physics and Astronomy at the University of Calgary, uses MC-ICP-MS to measure isotopic compositions at trace levels in applications ranging from geological studies to protein research. He recently spoke to us about this work.

A significant part of your research has involved the use of multiple-collector inductively coupled plasma–mass spectrometry (MC-ICP-MS) for measuring isotopic ratios. Can you please briefly describe the basic components of the system and its fundamental principles of operation?

Ions are generated by an argon plasma. The high temperatures generated in the plasma enable many elements to be ionized with high efficiency. This is very attractive for investigating the isotopic compositions of many metals, which have high first ionization potentials and have been challenging to measure using older methods such as thermal ionization mass spectrometry. The ions produced by the plasma have a relatively large spread in kinetic energy and some velocity focusing is required to achieve acceptable levels of mass resolution. Therefore, in the multiple-collector instrument, the ions are accelerated and focused into an electrostatic analyzer (ESA). The ions that emerge from the ESA enter a magnetic field region, which acts to disperse the ions according to their mass-to-charge ratios. The ions are brought to focus along the focal plane of the instrument where several detectors (typically nine) may be positioned by the operator to intercept specific ions, which correspond to the isotopes of interest. The multiple collectors are a significant reason why the plasma ion source has become such an important interest in research because it is possible to measure the isotopes simultaneously such that the flicker of the plasma will not result in large uncertainties in isotope amount ratio measurements.

What are the technique’s advantages compared with other methods for isotopic studies?

Definitely the plasma ion source is the advantage. The ability to ionize metals easily and efficiently is an important step forward from our previous methods, which involved thermal ionization methods. The plasma ion source has enabled us to improve our sensitivity by orders of magnitude for metals including Mo and Cu. This means that we can explore processes affecting these elements in environments where the elements are found in very low amounts. For example, elements like Cu and Zn are essential for many life processes and some investigators are now exploring isotope abundance variations in single cells or small regions. Another significant advantage is the flexibility of the plasma ion source to accept a variety of sample preparation devices. For example, one can very easily connect a laser-ablation system or a gas or liquid chromatography system. The innovation and creativity in the development of peripherals coupled to the plasma ion source is very exciting. Previously, bulk samples were converted to a measurable form by acid digestion techniques. This would necessarily homogenize the sample. However, there is an incredible amount of information encoded in the isotopic composition of the individual components of a sample-for example, the elements associated with specific proteins or metals in organic versus inorganic compounds. The thoughtful use of high performance liquid chromatography (HPLC) could enable one to unravel processes recorded by the isotopic patterns of metal compounds in a sample.

Another important advantage of the plasma-based ion source is the relatively stable mass bias. While the uncertainty resulting from mass-dependent effects that occur during the transfer of the ions through the instrument is significant, this systematic offset is typically quite stable. Therefore, it is possible to monitor and correct for these effects.

What are the limitations of the technique?

Probably the most significant limitation is the identification and monitoring of spectral interferences. The plasma source tends to result in the production of many molecular ions that can interfere with the isotopes of interest. For example, in the measurement of molybdenum isotopes, one has to be aware of possible ⁴⁰Ar⁵⁶Fe+ interferences occurring at mass ⁹⁶Mo.

Current multiple-collector ICP mass spectrometers are still complex instruments. They are expensive to purchase and require a fair amount of maintenance and upkeep.

Otherwise, the only limitation is one’s imagination. This is a really exciting tool to use and is enabling some research that crosses disciplines and brings together scientists and experts in diverse fields.

How do you use the technique to your advantage in your research? What are the primary applications you have targeted?

We are exploiting the sensitivity of MC-ICP-MS for elements such as Mo and Cu. We are applying the mass spectrometer to study how biogeochemical processes may have altered isotopic composition. Our samples are derived from geological samples, where the element may be only found in trace (from parts-per-billion to parts-per-million) levels or biological tissues where not only is the element found in trace amounts, but also the amount of material is extremely limited.

In one recent study, you used MC-ICP-MS to perform a fully calibrated measurement of the isotopic composition of molybdenum reference materials (1). What is the significance of this study with respect to the method developed to measure the isotopic composition?

The instrumentation available today can provide extremely precise measurements of isotopic composition. The challenge is to realize data that are in some way accurate or at the very least can be compared among different laboratories involved in similar research. One of the biggest challenges investigators are facing now is the availability of calibrated isotopic reference materials, particularly for many of the elements that are now being analyzed by MC-ICP-MS and associated so-called “hyphenated” techniques (laser-ablation ICP-MS or HPLC–ICP-MS). In our study we provided an absolute isotopic composition for a molybdenum solution (SRM 3134) distributed by the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. The goal was to provide researchers working with Mo isotopes a reliable set of isotope amount ratios to enable calibration of their mass spectrometers and measurements and facilitate the sharing of data among different laboratories. With the incredible interest developing in “new” elements (at least new from the point of view of isotope abundance investigations), it is critical to have many laboratories participate in careful measurements of materials that are widely available from reliable sources (the emphasis on widely available and reliable sources).

Another important aspect of this paper was the implementation of the double-spike approach to calibrate the measured isotope amount ratios. The double-spike method has its origins in thermal ionization mass spectrometry measurements for uranium and lead dating experiments. It has experienced a renaissance with a broad number of users over the past decade because of the need to determine isotope amount ratios with high precision and accuracy and the availability of efficient computational tools to reduce data from the measurements. We use double-spiking methods extensively in our measurements of molybdenum and zinc and this paper was an opportunity to promote the tool.

Another recent paper from your group discussed the study of how the distribution of copper isotopes in the body is affected by the cellular prion protein, a copper-binding protein that is highly expressed in the brain and whose misfolded isoform is required for the development of neurological diseases such as scrapie and Creutzfeldt-Jacob’s disease (2). How was MC-ICP-MS used in this research? Did you face any difficulties in dealing with the biological samples?

This work is in its early stages, but we are very optimistic that the inclusion of isotopic composition data can help to identify the sources of Cu in the body as well as processes that are responsible for the movement of Cu in the body. MC-ICP-MS was the best tool to meet the goals of the research project because of its high sensitivity for the measurement of copper. We could obtain very reproducible results efficiently using MC-ICP-MS.

Working with biological materials came with several important challenges. To achieve the best possible results, it is necessary to isolate copper from the sample matrix. This helps to minimize possible spectral interferences as well as keep the mass bias as constant as possible from sample to sample. At the moment, there are some very exciting developments with highly selective ion-exchange resins. Companies such as Triskem and EiChrom offer ion-exchange materials that are able to remove Cu from complex matrices with very low levels of blanks.

What are the next steps in your research?

We are working enthusiastically to develop methods that can analyze Cu, Mo, and S associated with specific molecules-for example, the copper in proteins such as CTR1, ceruloplasmin, and so forth. We want to explore how the isotopic composition of specific organic molecules is impacted by changes to normal expression of copper proteins. We want to participate with colleagues in medicine to explore how Cu is processed by the body and to use changes in isotopic composition to understand how reactions involving copper may be affected by disease. We are now working on zinc and sulfur to complement our work with Mo and Cu. MC-ICP-MS is the hub of these activities because of the sensitivity we can achieve for these elements, the reliability of the data we can realize, and the opportunities to interface sample preparation systems, such as HPLC.

References

• A.J. Mayer and M.E. Wieser, J. Anal. At. Spectrom. 29, 85–94 (2014).

• K.A. Miller, C.M. Keenan, G.R. Martin, F.R. Jirik, K.A. Sharkey, and M.E. Wieser, J. Anal. At. Spectrom. 31, 2015–2022 (2016).