Researchers from Canada have developed a method for continuous leaching of soil samples, providing improved data for the bioaccessible lead. The obtained elution profile shows different fractions, indicating multiple Pb sources.
Background: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 modelled 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 behaviour 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 nebulised into the ICP-MS instrument.
Figure 1: Instrumental setup of the on-line leaching test
Except for 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 petrol.
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
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
https://www.epa.gov/sites/production/files/2015-12/documents/1340.pdf US Environmental Protection Agency,
Estimation of Relative Bioavailability of Lead in Soil and Soil-like Materials Using in Vivo and in Vitro Methods, 2007.
https://semspub.epa.gov/work/11/175416.pdf. 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
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D. Beauchemin,
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R.A. Althobiti,
D. Beauchemin,
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DOI: 10.46770/AS.2021.708.
R.A. Althobiti, N.W. Sadiq,
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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,
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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,
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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,
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last time modified: September 19, 2024
