Trace Element Distribution and Arsenic Speciation in Toenails as Affected by External Contamination and Evaluation of a Cleaning Protocol: An Interview with Joerg Feldmann

While inductively coupled plasma mass spectrometry (ICP-MS) and high-performance liquid chromatography ICP-MS (HPLC-ICP-MS) are commonly used to measure trace elements and their speciation in toenails, the impact of the removal of external contamination on trace element quantification has not been thoroughly studied. Researchers have recently investigated the microdistribution of a variety of trace elements in dirty and washed toenails (and the speciation of arsenic specifically in situ) using synchrotron X-ray fluorescence microscopy (XFM) and laterally resolved X-ray absorption near edge spectroscopy (XANES). Joerg Feldmann of the Institute of Chemistry at the University of Graz (Austria), corresponding author of the paper resulting from this research, discussed this investigation with Spectroscopy magazine.

In your paper (1), you state that, in general, the intake of trace elements in the human body can be monitored by measuring their concentrations in different biological matrices, including urine, blood, and hair, as well as fingernails and toenails. What made you decide to specifically test toenails?

Some trace elements especially arsenic, which is a problem in Bangladesh food and water, is known to accumulate in hair, finger, and toenails. Hair can easily be contaminated externally by certain washing processes. Nails are easier to sample and can easily be stored. The epidemiological studies for what that was used was in Bangladesh. So, storage of urine and blood in a freezer at a premium. Every sample would take a lot of space and energy to store. Furthermore, the shipping of those samples is also a bit more cumbersome, hence a decision on taking nails. Toenails are in general not exposed to toxic material which might be handled by the hands. Toenails are maybe also not so much externally contaminated.

You also state that you applied X-ray fluorescence microscopy (XFM) and X-ray absorption near edge spectroscopy (XANES) in your investigations. Why these techniques?

We wanted to evaluate the effect of the washing process on the distribution of the elements and check if external contamination can be identified and can be shown how this is removed by the washing process. For arsenic we knew that it accumulates when incorporated into the nails, it will bind to keratin via a reduced sulfur (thiol group). Arsenic from external sources is more likely to be in a form of an oxo-ion such as arsenite or arsenate. Hence, direct speciation in the nail using XANES would potentially give us the information, what is external contamination and what is accumulated through the ingestion of arsenic containing food and water.

Other than the techniques being used, does your work differ from what has been previously done by yourself or others?

Toenails and fingernails, as well as hair, were occasionally used for measuring the exposure to toxic mercury and arsenic but were always criticized for not excluding external contamination. Subsequently washing processes have been used before analysis, but no evaluation of the washing process has been done before. This here was the first time that the washing process has been evaluated by using elemental mapping using synXRF and elemental speciation for arsenic using XANES.

Briefly state your findings.

It could be seen that external contamination is on the outside of the nails and those were characterized by the high intensities of elements often been associated with soils such as iron, manganese, and so forth, while arsenic showed often higher intensities inside the nail. Hence, we see different profiles of the elements in the cross section. Interesting is that a multi-stage washing process could remove most of the external contamination and much of the arsenic remained in the nail.

Was there anything particularly interesting that stands out from a particular analytical perspective, such as one element searched for versus another, or one type of subject matter versus another?

What was very interesting that the non-essential element rubidium has not been associated with the inner nail but with the contamination from external sources. This could be washed away. This means we have now a surrogate element which could be used as an indicator for the effect of the contamination from external sources. Hence, measuring in addition to the analytes such as arsenic, rubidium could be measured to gain an idea how the arsenic measurement is affected by external sources in the nail.

Were there any limitations or challenges you encountered in your work?

As mentioned previously, toenails are usually not as much exposed to external sources while in closed shoes. Not so in Bangladesh, hence we have had severe external exposure due to the tradition to wear sandals and open shoes.

What best practices that can you recommend in this type of analysis for both instrument parameters and data analysis?

Much of the work was done at the synchrotron. Hence, the sample number is limited and therefore cannot directly be used for epidemiology. Not only is the measurement costly and time consuming, but the data analysis is also very complex. But our study is only an evaluation paper to find out how to wash the specimen, and how to control the washing process using ICP-MS.

Can you please summarize the feedback that you have received from others regarding this work?

I have heard from epidemiologists that the use of toenail would be great when measuring the exposure to arsenic and potentially other elements. Also, the ambiguity of external contamination can now be controlled or at least measured.

Do you imagine these techniques to be adaptable for other elements, or work with different biological matrices that may or may not have been mentioned in my first question?

Well yes for selenium, mercury etc. the techniques should however include elements which are associated with external contamination such as rubidium to quantify the external contamination. 

What are the next steps in this research?

This toenail washing and subsequent ICP-MS method has already been used to investigate the influence of toxic and essential elements on cardiovascular diseases. This study is currently written and will be published soon.

Faidutti, C.; Doolette, C.; Hair, L.; van Daalen, K. R.; Naheed, A.; Lombi, E.; Feldmann, J. Trace Element Distribution and Arsenic Speciation in Toenails as Affected by External Contamination and Evaluation of a Cleaning Protocol. Anal. Chem. 2024, 96, (10), 4039–4047. DOI: 10.1021/acs.analchem.3c03962

Jöerg Feldmann is professor for environmental analytical chemistry and head of the Institute for Chemistry at the University Graz, Austria after leading for 23 years TESLA at University of Aberdeen, Scotland. He is an expert in developing novel methodologies for environmental and biological applications using plasma spectrochemistry and was awarded the RSC Interdisciplinary Medal in 2016 and the European Plasma Spectrochemistry in 2015. He has written more than 350 peer-reviewed papers (h-index 76) mainly using NTS approaches for arsenic, mercury and lately fluorine with the focus on PFAS and supervised successfully more than 50 PhD students.

Jöerg Feldmann © courtesy of interviewee