R&D Focus
Tracing the Pollutant Emission Source via Inorganic Isotope Ratio Analysis By Chun-Ting Kuo
The Multiple Collector Inductively Coupled Mass Spectrometer (MC-ICP-MS) traces the pollutant source via inorganic isotope ratio analysis.
Tracing the source of emission for pollutants is often a complicated issue yet a critical step for eliminating pollution. Although traditional analysis methods can provide precise quantitative information, these results reveal little about the origin of pollutants. To remedy this situation, ITRI has developed a novel forensic method by improving on isotope ratio analysis technology.
ITRI’s inorganic isotope ratio analysis system has been built upon a Multiple Collector Inductively Coupled Mass Spectrometer (MC-ICP-MS). Internally, the isotopes in the sample solution are introduced into the instrument and ionized by ~6000oC Ar plasma. With the control of an electric and magnetic field, ionized isotopes are emitted with different emergence angles in terms of their molar mass and can be collected by multiple collectors. An isotope ratio is considered a useful index representing local climate change and environmental evolution, and therefore can act as a fingerprint for pollutants. With the development of mass spectrometry over the recent decades, a high-precision inorganic isotope analysis can now be realized and put to use in tracking pollution sources from ground water.
The internal structure shows that the isotopes that pass through the Ar Plasma, electric field, and magnetic field are finally collected by multiple collectors. (Figure adapted from Thermo Fisher MC-ICP-MS Manual)
In contrast to common inductively coupled plasma mass spectrometry (ICP-MS), which utilizes only a single collector and relies on a scanning mode to collect various isotopes sequentially, the multiple collectors in ITRI’s MC-ICP-MS achieve simultaneous multi-isotope measurements so that a more precise isotope ratio result can be obtained.
This system was initially built to realize the SI unit – mole (Avogadro constant), in which silicon isotopes are precisely determined with an extremely low measurement uncertainty with the help of isotope dilution mass spectrometry (ID-MS). The extremely low measurement uncertainty is highly useful to distinguish the subtle discrepancies of isotope ratios between different sample sources, allowing the system to trace the source of emission pollutants.
In a recent real case, several underground water samples from chromium (Cr) contaminated areas and nearby effluent sewage in Central Taiwan were collected for the determination of Cr isotope ratios. With a superior mass resolution, the signal of interferences can be separated from the signal of target elements. Through the comparison of obtained Cr isotope ratios in different areas, a regional consistency was found that shows the feasibility of tracing the source of real pollution events via the isotope ratio analysis.
The signal of Cr isotopes measured by MC-ICP-MS.
Efforts have been made to continuously improve the analysis performance, expand the service range to common metal pollutants such as lead (Pb), and build up the database for more efficient tracing of the pollutant source. Comparing to isotope measurement technology worldwide, simultaneous measurement of multiple isotopes and the introduction of ID-MS enables lower measurement uncertainly for ITRI’s technology, greatly assisting in distinguishing the source of pollutants.
In the future, this system will also be able to trace emission pollutants in the air (such as PM2.5 and PM10) via a comprehensive sample pretreatment, widening potential applications of this technology.
Acknowledgments
The author gratefully acknowledges the contribution of the Green Energy and Environment Research Laboratories at ITRI for evaluating the feasibility of this technology for application in real pollution events, especially in the design of field experiments and data interpretation.
About the Author
Dr. Chun-Ting Kuo is a senior scientist at the Center for Measurement Standards at ITRI. He received his PhD degree in the Department of Chemistry from National Taiwan University, and worked as a postdoctoral researcher and senior scientist respectively at the University of Washington and Lamprogen Inc. in the United States. His current research focuses on the determination of inorganic trace elements and isotopes.