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Identification of lead contamination sources in soil reference materials using the continuous on-line leaching method and inductively coupled plasma mass spectrometry


When discussing the health risks from the exposure to heavy metals it is often argued that not the total amount of the metals but only the bioaccessible fraction should be evaluated.
The bioaccessible fraction is the amount of matter that is extracted during digestion and is often modeled via a leaching test using artificial gastrointestinal fluids. The standard methods for assessing the bioaccessibility recommended by US EPA, the United States Pharmacopeia (USP) or the European Unified Bioaccessibility Method (UBM) are based on a batch approaches. These batch tests take place during hours in which the samples are leached with the extractants warmed to 37°C being constantly shaken, rotated or stirred. After such sample pretreatment, the extracted metal fraction is determined by atomic spectrometric techniques such as AAS, ICP-AES or ICP-MS. The time consuming procedure provides only limited information, namely a single value named the bioaccessible fraction.

The new study:

Researchers from Canada aimed at improving the information value of the leaching test by investigating the possibility to differentiate fractions having different contamination sources. Instead of the batch procedure they made use of the continuous on-line leaching method (COLM). The COLM technique is using a kind of hyphenated HPLC-ICP-MS system, in which the column is not used for analyte separation but for containing the sample to be leached. The HPLC pump is pumping the different artificial gastrointestinal fluids warmed at 37°C through the sample holding column and feeding the eluate directly into the ICP-MS (see the figure 1). Since the elution profile is detected as a transient signal like a chromatogram, multiple fractions having different leaching behavior can be differentiated. The authors used the information from these leaching profiles for the identification of different contamination sources for lead in reference soils.

As a proof of the concept, the certified reference materials NIST 2710, NIST 2710a, NIST 2711a and BGS 102 were used as the test samples. 1 g of sample material was placed in HPLC guard columns of 6 cm length and 0.6 cm inner diameter. Depending on the sample and the fluids used,  elution times of up to 15 min, were recorded. Mini-column eluents were combined with an internal standard (Indium), using a Y-connector and nebulized into the ICP-MS instrument.

Figure 1: Instrumental setup of the on-line leaching test

With the exception of NIST 2710, all other samples showed two different fractions. For each of the two peaks within the elution profile, the isotopic ratios were calculated over the top half of the peak. To investigate the potential source of the Pb contamination, the Pb isotope ratios for each of the peaks were compared using a Student's t-test. Comparison of these ratios with background Pb concentrations indicated that the Pb source in NIST 2710a and NIST 2711a were geogenic. However, for BGS 102, the second peak from both the US EPA leaching test and the USP gastric extraction had a significantly different Pb isotope ratio than the first peak. These results indicate, that the second peak comes from Tetraethyl-lead historically added to gasoline.

The authors concluded that the additional information obtained by the COLM method allowed to differentiate lead contamination sources and therefore might lead to more robust and informed bioaccessibility studies in the future.

The original publication

Alastair Kierulf, Iris Koch, Diane Beauchemin, Source apportionment of bioaccessible lead in soil reference materials using the continuous on-line leaching method and inductively coupled plasma mass spectrometry, Anal. Chim. Acta, 1189 (2022) 339214. DOI: 10.1016/j.aca.2021.339214

Instrumentation used:

Varian Inc. - 820 ICP-MS

Related Information (standard procedures)
US Environmental Protection Agency, Standard operating procedure for an in vitro bioaccessibility assay for lead and arsenic in soil, Washington D.C. 2017

US Environmental Protection Agency, Estimation of Relative Bioavailability of Lead in Soil and Soil-like Materials Using in Vivo and in Vitro Methods, 2007.

US Environmental Protection Agency, Validation assessment of in vitro arsenic bioaccessibility assay for predicting relative bioavailability of arsenic in soils and soil-like materials at superfund sites, Office of Land and Emergency Management, 2017, pp. 1-12. OLEM 9355.4-29, https://semspub.epa.gov/work/11/175338.pdf.

The United States Pharmacopeia, Dissolution, in: United States Pharmacop. Natl. Formul., 32nd ed., 2014, p. 344.

International Organization for Standardization, Soil Quality - Assessment of Human Exposure from Ingestion of Soil and Soil Material - Procedure for the Estimation of the Human Bioaccessibility/bioavailability of Metals in Soil, ISO 17924:2018

Related studies (newest first)

R.A. Althobiti, D. Beauchemin, Pragmatic method based on on-line leaching and inductively coupled plasma mass spectrometry for risk assessment of the impact of short-term pollution, J. Anal. At. Spectrom. 36 (2021) 622-629, DOI: 10.1039/d0ja00447b.

R.A. Althobiti, D. Beauchemin, An isotopic study of bio-accessible lead in wheat, miswak toothbrush and miswak fruit using the continuous on-line leaching method with inductively coupled plasma mass spectrometry, At. Spectrosc. 42 (2021) 271-277, DOI: 10.46770/AS.2021.708.

R.A. Althobiti, N.W. Sadiq, D. Beauchemin, Realistic risk assessment of arsenic in rice, Food Chem. 257 (2018) 230-236. DOI: 10.1016/j.foodchem.2018.03.015

R.A. Althobiti, D. Beauchemin, Inductively coupled plasma mass spectrometry with on-line leaching to assess the maximum bio-accessibility of toxic and essential elements in wheat from Saudi Arabia, J. Anal. At. Spectrom. 33 (2018) 642-648, DOI: 10.1039/c8ja00047f.

G.L. Diamond, K.D. Bradham, W.J. Brattin, M. Burgess, S. Griffin, C.A. Hawkins, A.L. Juhasz, J.M. Klotzbach, C. Nelson, Y.W. Lowney, K.G. Scheckel, D.J. Thomas, Predicting oral relative bioavailability of arsenic in soil from in vitro bioaccessibility, J. Toxicol. Environ. Health Part A. 79 (2016) 165-173, DOI: 10.1080/15287394.2015.1134038.

R.P. Lamsal, D. Beauchemin, Estimation of the bio-accessible fraction of Cr, As, Cd and Pb in locally available bread using on-line continuous leaching method coupled to inductively coupled plasma mass spectrometry, Anal. Chim. Acta, 867 (2015) 9-17, DOI: 10.1016/j.aca.2015.02.047.

A. Leufroy, L. Noel, D. Beauchemin, T. Guérin, Use of a continuous leaching method to assess the oral bioaccessibility of trace elements in seafood, Food Chem. 135 (2012) 623-633, DOI: 10.1016/j.foodchem.2012.03.119

A. Leufroy, L. Noel, D. Beauchemin, T. Guérin, Bioaccessibility of total arsenic and arsenic species in seafood as determined by a continuous online leaching method, Anal. Bioanal. Chem. 402 (2012) 2849-2859, DOI: 10.1007/s00216-012-5774-4.

M. Chu, D. Beauchemin, Simple method to assess the maximum bioaccessibility of elements from food using flow injection and inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 19 (2004) 1213-1216, DOI: 10.1039/b403215b.


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