Finding the Missing Selenium

Yeast grown on selenium-rich media is used in various ways as a nutritional supplement, and may have a role in treatments for the prevention of prostate and colon cancer. However, the mass balance of the selenium species identified in this material often does not reach 100%, suggesting the presence of unaccounted forms of selenium. In this context, the research team of Joanna Szpunar and Ryszard Lobinski at the Institute of Analytical Sciences and Physico-Chemistry for Environment and Materials (IPREM), affiliated at the French National Research Council (CNRS) at the University of Pau, France, decided to investigate the hypothesis that the “missing” selenium was in the form of biogenic nanoparticles. Dr. Javier Jiménez Lamana, a post-doctoral fellow in the group, spoke to us about his work to overcome the size-detection limitations of existing analytical methods necessary to test that theory.

In a recent study, you examined the use of single-particle inductively coupled plasma–mass spectrometry (ICP-MS) to detect and characterize biogenic selenium nanoparticles (SeNPs) in yeast (1). Why is single-particle ICP-MS a good technique for this kind of study? And what is the significance of biogenic SeNPs in particular?

Biogenic SeNPs synthesized by yeast (or other microorganisms) possess interesting antibacterial, antiviral, and antioxidant properties. However, what made us interested in the presence of biogenic SeNPs was the speciation of selenium in selenium-rich yeast, which has been one of the research topics in our lab for more than 20 years. After all these years working in this field, we have been able to identify hundreds of selenium species, yet the selenium mass balance for the identified species rarely exceeds 90%. In this context, we were wondering whether this “missing” selenium is inorganic selenium in nanoparticle form. The best way to verify this hypothesis was using single-particle ICP-MS, the best analytical technique when it comes to the identification, characterization, and quantification of nanoparticles at low concentrations.

An important aim of the study was to improve the size detection limit. What limit were you able to achieve, and how did the instrumental parameters you chose improve that limit?

Yes, one of the main limitations of single-particle ICP-MS is the size detection limit. In the case of selenium, the theoretical size detection limit had been estimated to be around 200 nm. So, to be able to detect SeNPs in yeast we needed to decrease this limit significantly. However, this limit has been estimated using the isotope ⁷⁶Se, which is relatively low in abundance. In this context, we decided to use the more abundant isotope ⁸⁰Se to gain sensitivity. At the same time, the use of an H₂ reaction cell allowed us to remove the polyatomic interferences associated with isotope ⁸⁰Se and the corresponding background signal. Finally, the use of dwell times in the microsecond range allowed us to reduce the noise and further improve the size detection limit. Under the optimal conditions we were able to achieve a size detection limit of 18 nm, which represented a gain of a factor of 10. 

Is using a collision–reaction cell a better way to deal with interferences in a case like selenium, which is interfered with by argon, than using mathematical corrections?

In our lab we have extensive experience working with selenium in ICP-MS and with the current state-of-the-art ICP-MS instruments we are able to decrease the background signal produced by the argon dimer at m/z 80 down to less than 50 counts per second while at the same time maintaining good sensitivity for ⁸⁰Se. This capability allows us to work with selenium at very low concentrations that otherwise would be impossible to analyze using mathematical corrections.

In your study, you analyzed yeast samples to detect SeNPs and determine their size distribution. What did you find?

Well, I would suggest people to read our article, but we did find SeNPs in the analyzed yeast samples. With our developed method we demonstrated the presence of SeNPs with sizes ranging from 40 to 200 nm. These results revealed that the study of biogenic nanoparticles must be included in metal speciation schemes. 

What are your next steps in this work?

With respect to the analytical method developed, we are implementing it as a routine method for the analysis of SeNPs in yeast and other matrices. As for the speciation of selenium in Se-rich yeast, we want to identify all the species present and achieve a selenium mass balance of 100%. We are very close to achieving that.

Reference

J.J. Jiménez-Lamana, I. Abad-Álvaro, K. Bierla, F. Laborda, J. Szpuar, and R. Lobinski, J. Anal. At. Spectrom. 33, 452–460 (2018).

Javier Jiménez Lamana, PhD, is a post-doctoral fellow in the research group of Joanna Szpunar and Ryszard Lobinski at the Institute of Analytical Sciences and Physico-Chemistry for Environment and Materials (IPREM) at the University of Pau, France.