What New Methods to Measure Isotope Ratios Can Reveal About the Environment, Humans, and the Earth

Multiple-collector inductively coupled plasma–mass spectrometry (MC-ICP-MS) has some specific advantages over traditional ICP-MS instruments. Because of its ability to measure high-precision isotope ratios, MC-ICP-MS shows promise for applications as diverse as tracing the transport of heavy metals into bivalves (oysters and mussels), studying isotopic variations of metals in the human body, and identifying the source of volcanoes. Spectroscopy recently spoke with Dominique Weis, PhD, a Canadian Research Chair, Tier I, and Director at the Pacific Centre for Isotopic and Geochemical Research (PCIGR), in the Department of Earth, Ocean, and Atmospheric Sciences at the University of British Columbia, about her research on this technique.

How do methods using MC-ICP-MS differ from traditional inductively coupled plasma-mass spectrometry (ICP-MS) methods? What are the advantages and disadvantages of MC-ICP-MS?

Weis: There are several main differences between the methods using MC-ICP-MS and those using traditional ICP-MS. First, MC-ICP-MS instruments are mostly used to measure high-precision isotope ratios whereas ICP-MS instruments are used to determine elemental concentrations. Second, MC-ICP-MS instruments measure within a narrow range of masses (the isotopes of the element of interest and spectral interferences) whereas ICP-MS analyses typically cover a wide to full mass range. Third, high-precision isotope analyses by MC-ICP-MS require minimum matrix load in samples, whereas concentration measurements by ICP-MS have a much higher matrix tolerance.

The advantages of MC-ICP-MS versus conventional ICP-MS (sector field inductively coupled plasma–mass spectrometry [SF-ICP-MS] or quadrupole inductively coupled plasma–mass spectrometry [Q-ICP-MS]) are the ability to simultaneously collect masses as a result of the presence of up to 16 collectors as well as a wider mass dispersion because of larger geometry.

The disadvantages of MC-ICP-MS versus conventional ICP-MS (SF-ICP-MS or Q-ICP-MS) are that the instruments are too slow to scan over a wide mass range, and to achieve better precision the detectors are Faraday cups, which have better linearity yet lower sensitivity than ion counters (ICs) typically used for SF-ICP-MS or Q-ICP-MS.

What strategies are used in MC-ICP-MS to avoid analytical artifacts from spectral interferences and matrix effects in Earth science studies, and in isotope ratio measurements in particular (1,2)?

Weis: The main strategies to reduce or remove spectral and matrix effects include

• chemical separation of the analyte from the matrix;
• using desolvating systems (reducing oxide interference with the partial removal of the water in the sample);
• using retardation filters (for ICs);
• monitoring possible interfering masses in addition to the masses of interest and correcting offline;
• spiking the sample and the standard with a similar matrix ;
• different tuning settings in ion optics or subtle variations at the front end, such as torch position, nebulizer selection, and introduction of additional gases; and
• using high resolution including the pseudo-high resolution option.

The most difficult interferences to deal with in MC-ICP-MS measurements are isobaric interferences. The separation of the isotope of interest in MC-ICP-MS is achieved according to the principle that ions are bent into different radius of curvature passing through the magnetic field according to their mass-to-charge ratios (m/z). The only way to remove isobaric interferences is to use the higher resolution feature of the instrument to either completely separate the peak of the interferent from that of the analyte (for example, Nu1700) or to partially separate the adjoining peaks to reveal a sufficient mass width of the analyte peak to be used for the measurement (for example, Finnigan Neptune, or Nu Plasmas I and II). Technically, the higher resolution is achieved by reducing the width of the slits to narrow down the ion beam, which in turn reduces the sensitivity. There is generally a trade-off to make between the sensitivity and resolution adopted.

In your research with MC-ICP-MS on natural and anthropogenic sources of metals in bivalves (3), how important is the ionization efficiency provided by the technique in obtaining meaningful data? What other techniques have been used to look at the small variations in the isotopic composition of heavy metals in the environment?

Weis: In our study using isotopic analyses of Cd (and Zn) in bivalves to trace the fate of these metals in the environment, the precision and the ionization of the MC-ICP-MS technique were critical, as the variations were too small to be resolved with the precision of thermal ionization instruments.

The fate and the transport of heavy metals in the environment are a function of their speciation, a key parameter to keep in mind when interpreting isotopic results for such studies. Bulk analyses, as produced in many laboratories around the world for environmental purposes, will be progressively outdated. However, it is so far extremely challenging to analyze the species-specific isotope composition for a metal. A lot of development work remains to be done to allow coupling classical on-line speciation techniques with MC-ICP-MS as those techniques generate a lot of "noise" in terms of matrix in the plasma, as well as transient signals (few seconds), totally different than those usually measured (10 min) with traditional isotope ratio techniques.

Tremendous developments are being made in the field of in situ analyses, with the coupling of a laser ablation system to an MC-ICP-MS system, allowing researchers to narrow down the scale of observation to micrometers or tens of micrometers.

What other research areas do you think MC-ICP-MS would be applicable to?

Weis: The medical world has yet to discover and integrate the tremendous potential of the isotopic variability of metallic elements in the human body. A few milestone studies have recently been published, showing Cu, Fe, Zn, or Ca variations in tissues, blood, and urine that reflect isotopic variations during cellular fractionation, aging, homeostasis, or immobilization.

The sky is the limit in terms of applications of MC-ICP-MS, although through meteorites and other early earth products, we can already go back up to almost 4.6 billion years.

What are the next steps in your research?

Weis: Modern geochemistry is concerned with quantifying and explaining the transfer of elements within and between the major geological reservoirs of the Earth, including the mantle, the crust, the oceans, and the atmosphere. Accomplishing this task requires the precise analysis of isotopic ratios and elemental concentrations in rocks, minerals, solutions, and organic matter. The development of MC-ICP-MS has opened up many new important avenues for producing groundbreaking research in isotopic geochemistry. The higher ionization efficiency for nearly all elements provided by MC-ICP-MS has dramatically expanded the potential isotopic systems that can be measured (such as isotopes of Mg, Cd, Zn, Cu, and Fe) and has allowed for unprecedented sensitivity and precision in the analysis of isotopic systems previously determined by thermal ionization mass spectrometry (TIMS) (such as isotopes of Hf and Pb). Over the next 10 years, the next steps in my research will cover four main themes of geochemical applications: Earth’s mantle and solid earth, environmental problems, medical applications, and MC high resolution ICP-MS analytical developments. A couple of more specific examples follow.

Very little is known about the sources of the Canadian Cascades volcanoes. A systematic geochemical study of the main volcanoes will allow identification of the sources, modeling of the role of sediments in the compositional variations, and comparison with the United States part of the Cascade Arc. The Cascade Arc is the only subduction zone present in Canada and extends in the United States, all along the west coast.

Hawaii is the most studied mantle plume, and yet some fundamental questions are still to be resolved (4). High-precision isotopic studies are providing an unprecedented perspective on the mantle plume structure and composition. They reveal the existence of binary mixing trends with end-members whose origins remain to be identified. We aim to establish the source characteristics of the Hawaiian plume (components involved, depth), to model its variations in space and time (that is, to create a geochemical-isotopic map), and to determine when the Loa and Kea plume trends became independent. This will ultimately lead to a better understanding of the physical and chemical structure of the mantle (84% of the Earth’s volume) and will allow for an unmatched view of the geochemical evolution of hotspot-related volcanism.

Finally, my main objective in the coming years is to collaborate with medical doctors and to use the analytical pool available at PCIGR to decipher fundamental questions relevant to the understanding, and ultimately cure, of the human body.

References
(1) J. Barling and D. Weis, J. Anal. At. Spectrom.23, 1017–1025 (2008).
(2) A.E. Shiel, J. Barling, K.J. Orians, and D.Weis, Analytica Chimica Acta633, 29–37 (2009).
(3) A.E. Shiel, D. Weis, and K.J. Orians, Geochimica et Cosmochimica Acta76, 175–190 (2012).
(4) D. Weis, Research Media/Innovation International July 14, 116–119 (2012).