What’s New in Inductively Coupled Plasma Mass Spectrometry for Environmental Analysis?

In this review article, the editors of Spectroscopy break down the most recent research and trends using inductively coupled plasma mass spectrometry (ICP-MS).

Inductively coupled plasma mass spectroscopy (ICP-MS) is a form of atomic spectroscopy that is used to measure the elemental composition of a sample. It is typically used to analyze samples that are liquid or can be dissolved (1). The technique uses argon (Ar) plasma to convert the sample into ions that are then measured using a mass spectrometer (1).

The technique is versatile and can be used in a variety of applications including environmental analysis. In this review article, the editors of Spectroscopy break down some of the most recent research done using ICP-MS that is being done across the globe.

Analyzing Potassium Isotopic Composition in Plants

In a recent study in Spectrochimica Acta Part B: Atomic Spectroscopy, a group of scientists from multiple universities in China investigated the potassium isotopic composition of different plant reference materials using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) (2). The research team, led by Mao-Yong He from the State Key Laboratory of Loess and Quaternary Geology, studied the potassium isotopic composition (δ⁴¹K) of five certified plant reference materials (BCR 679, GSV-2, GSB-2a, GSB-3, and GSB-6a) with varying potassium concentrations using MC-ICP-MS.

Potassium is crucial for the functioning of living cells. It specifically holds significant importance as a nutrient for plants, comprising up to 10% of plant biomass on a dry weight basis. However, despite being abundant in soil, there is a lack of K available for plants, which places great importance on understanding the K cycle in soil-plant systems. One recent avenue of analysis is developing high-precision stable K isotopic composition analysis, which allows for the study of both K concentration and isotopic fractionation (2).

Three potassium isotopes exist in nature, ³⁹K, ⁴⁰K, ⁴¹K, with ³⁹K and ⁴⁰K being the most stable K isotopes. Using MC-ICP-MS, scientists have been able to improve isotope ratio measurements. This enables precision K isotopic analysis to be routinely achieved with precision better than 0.06%. High-precision K isotopic analysis can be achieved using two different approaches. The first involves using high-resolution mode to segregate Ar hydrides and K after the suppression of Ar through the reduction of radio frequency (RF) forward power using cold plasma. The second approach uses the collision gas strategy to eliminate Ar-based ionic species.

The researchers employed a reduced radio frequency (RF) forward power and low-resolution mode without a collision cell. The δ⁴¹K values of the plant reference materials (BCR 679, GSV-2, GSB-2a, GSB-3 and GSB-6a) were recorded as −2.44 ± 0.03‰, −0.37 ± 0.04‰, −0.37 ± 0.05‰, −1.27 ± 0.04‰ and −0.62 ± 0.01‰, respectively. According to the study, the δ41K values of plants allowed a methodology for pre-treatment and determination of δ⁴¹K in plants to be established. δ⁴¹K is now believed to have potential as a notable tracer in ecosystems’ soil-plant systems, though further research is needed.

Detecting Cadmium in Marine Phytoplankton Using Single-Cell Inductively Coupled Plasma Mass Spectrometry (SC-ICP-MS)

A group of scientists from Kanazawa University in Japan are using single cell inductively coupled plasma mass spectrometry (SC-ICP-MS) to detect cadmium in marine phytoplankton (3). Recent studies have reported an uptake in heavy metals by phytoplankton and their cells. Acid digestion for sample preparation is the most common method for detecting intracellular metals, though data from this method has certain limitations. Namely, these experiments only measure the intracellular cadmium (Cd) content of culture populations, meaning that only the average metal content in all cells can be obtained. Cell changes and their average values do not represent the intracellular value of each individual cell.

To address these issues, the research team, led by Yinghan Zai, used single cell inductively coupled plasma mass spectrometry (SC-ICP-MS). This method has high sensitivity, allowing for the analysis of extremely trace elements (3). SC-ICP-MS has been used to analyze various types of single-cell systems, including bacteria, yeast, and cancer cells. By directly analyzing cell suspension fed by pumps, a single cell can be analyzed internally with short residence time, thus avoiding simultaneous detection of multiple cells.

For this study, the scientists used SC-ICP-MS to analyze cadmium content in marine phytoplankton cells. The team removed the supernatant through centrifugal separation, replacing the culture media of three species, P. parvum, O. viridis, and E. gynnastica, with non-metallic salt solutions, namely (NH₄)₂SO₄ and NH₄Cl. Cell images were also studied at different concentrations of replacement solutions, so as to check for ruptured cells stemming from changes in osmotic pressure.

It was observed that several cells ruptured at low concentrations of the non-metallic salt solutions, but they did maintain integrity between 0.6 and 0.8 M solutions. These surviving cells were successfully determined using SC-ICP-MS, and they showed increased presences of high cadmium-containing cells when the phytoplankton were in high-cadmium-concentration mediums. Based on their findings, the scientists concluded that (NH₄)₂SO₄ and NH₄Cl can be suitable replacement solutions for the determination of marine phytoplankton using SC-ICP-MS. However, different species of marine phytoplankton could have different levels of compatibility with these solutions, implying that more research should be done.

TARIM Calcite: A New Benchmark for Laser ICP-MS in situ Calcite U–Pb Dating

It is difficult for scientists to find suitable reference materials for calcite. The mineral is known for its utility in uranium-lead (U–Pb) dating through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A study conducted by researchers at China University of Geosciences, the University of Toronto, and Sichuan Chuangyuan Weipu Analytical Technology, introduced TARIM calcite as a potential solution to this problem (4).

Calcite plays a crucial role in U–Pb dating by serving as a common accessory mineral that can incorporate uranium. When calcite forms, it may contain trace amounts of uranium, including the radioactive isotope uranium-238. Over time, as uranium undergoes radioactive decay to lead, the ratio of uranium to lead isotopes in calcite crystals can be measured, providing valuable information about the age of the geological samples in which calcite is found.

TARIM—also the name of a basin in the Xinjiang region of northwestern China—characterized through electronic probe microanalysis (EPMA) exhibits homogeneity in its low-magnesium (Mg) calcite composition. LA-ICP-MS trace elemental analyses and mapping results confirm the relative homogeneity of TARIM, with an appropriate U content (mean value = 0.48 μg/g) suitable for U–Pb isotopic dating.

The U–Pb analysis performed in this study, which was published in the Journal of Analytical Atomic Spectrometry, employed isotope dilution-thermal ionization mass spectrometry (ID-TIMS) methods and yielded a lower intercept age of 208.5 ± 0.6 Ma (2S, mean squared weighted deviation [MSWD] = 1.04). Further validation through 515 LA-ICP-MS U–Pb isotope analyses on random calcite pieces consistently produced ages with a lower intercept age of 208.0 ± 0.4/3.2 Ma (2S, MSWD = 3.0), aligning with the ID-TIMS age.

In the context of U–Pb dating, the intercept age refers to the age calculated based on the intersection point, or intercept, of the discordia line with the concordia line on a concordia diagram. The concordia diagram is a graphical representation used in radiometric dating to assess the reliability of age determinations. The intercept age provides an estimate of the time at which the minerals in a sample last experienced a common lead-loss event, helping researchers constrain the geological history of the sample.

The significance of TARIM calcite lies in its potential application as a reference material for matrix-matched calibration or as a monitoring standard for calcite LA-ICP-MS U–Pb dating. This study helps to bridge a critical gap in the availability of suitable reference materials for this analytical technique, offering researchers a reliable benchmark for enhancing the precision and accuracy of in situ U–Pb dating using calcite.

This discovery may also open avenues for improved methodologies in geological research, where U–Pb dating plays a crucial role in understanding various geological phenomena. The TARIM calcite’s homogeneous composition and consistent U–Pb isotopic dating results position it as an asset in advancing the reliability of LA-ICP-MS analyses in geological studies.

TARIM calcite emerges as a promising addition to the toolkit of researchers involved in U–Pb dating using LA-ICP-MS, its apparent suitability as a reference material would bring much-needed precision to in situ analyses, contributing to the refinement of geological timelines and enhancing our understanding of Earth’s history.

New Insights into Ancient Greek Coinage: Exploring Gold Purification Techniques

Attica and Cyclades coins from ancient Greece are famous for their historical and cultural significance. These coins are unique because of their low gold content, and a team of researchers is using ICP-MS to analyze them (5). The team from the Ecole Normale Supérieure de Lyon looked at platinum-group elements (PGEs) and gold in 72 silver coins from various ancient civilizations, including Greece, Rome, India, medieval Europe, and colonial Spanish Americas (5).

ICP-MS was used to measure the concentrations of PGEs and gold in silver coins, providing valuable insights into the coinage and metallurgical techniques of the past. The results of this research offer a fresh perspective on the composition and origin of ancient Greek and Hellenistic coins, as well as those from other ancient cultures. The findings reveal that the behavior of PGEs and gold in these coins is closely aligned with their respective positions in the periodic table, shedding light on the coin production processes employed by various civilizations.

The researchers used quadruple ICP-MS to analyze platinum-group elements such as platinum (Pt), iridium (Ir), ruthenium (Ru), palladium (Pd), and rhodium (Rh). They found distinct patterns in these silver coins. For example, the most volatile elements, such as Rh, were found at or below the detection level, possibly because of evaporation during the smelting and cupellation processes. Meanwhile, Ru and Ir demonstrated variations in coinage consistent with these properties. The soluble elements, Pd and Pt, exhibited patterns in the coins that align with their solubility characteristics.

One intriguing discovery was the dichotomy of the Ir:Au ratios, which challenged preconceived notions about salt cementation and its effect on gold. Iridium was believed to be lost in gold during salt cementation, but the ratios indicated that there could be regional variations in ore genesis conditions, such as the chlorinity of hydrothermal solutions (5).

The study pioneers a comprehensive approach to understanding ancient coin metallurgy, providing insight into how elements were selectively incorporated into coinage. The results challenge long-standing assumptions about the composition of ancient coins, particularly in the case of Athenian coinage, where salt cementation of gold was considered a source of silver.

This research deepens our understanding of ancient metallurgical techniques. By analyzing silver coins from various civilizations, the researchers showed through using ICP-MS offers valuable insights into the historical and cultural factors that shaped the coinage of the past.

References

(1) An Introduction to the Fundamentals of Inductively Coupled Plasma–Mass Spectrometry (ICP-MS). https://www.agilent.com/en/product/atomic-spectroscopy/inductively-coupled-plasma-mass-spectrometry-icp-ms/what-is-icp-ms-icp-ms-faqs (accessed 2024-01-04).

(2) He, M-Y.; Ren, T. X.; Jin, Z. D.; Deng, L.; Liu, H. J.; Cheng, Y. Y.; Li, Z. Y.; Liu, X. X.; Yang, Y.; Chang, H. Precise Analysis of Potassium Isotopic Composition in Plant Materials By Multi-Collector Inductively Coupled Plasma Mass Spectrometry. Spectrochim. Acta Part B At. Spectrosc. 2023, 209, 106781. DOI: 10.1016/j.sab.2023.106781

(3) Zai, Y.; Wong, K. H.; Fujizawa, S.; Ishikawa, A.; Li, M.; Mashio, A. S.; Hasegawa, H. Development of a New Detection Method for Cadmium in Marine Phytoplankton Based on Single Cell Inductively Coupled Plasma Mass Spectrometry. Spectrochim. Acta Part B At. Spectrosc. 2023, 209, 106801. DOI: 10.1016/j.sab.2023.106801

(4) Zhang, L.-L.; Zhu, D.-C.; Xie, J.-C.; et al. TARIM Calcite: A Potential Reference Material for Laser ICP-MS in situ Calcite U–Pb Dating. J. Anal. At. Spectrom. 2023, 11, 2302–2312. DOI: 10.1039/D3JA00222E

(5) Albarede, F.; Malod-Dognin, C.; Telouk, P. Platinum-Group Elements and Gold in Slver Coinage and the Issue of Salt Cementation. J. Anal. At. Spectrom. 2023, 38, 2159–2166 .DOI: 10.1039/D3JA00112A