Evaluation of measurement uncertainties for the determination of total metal content in soils by atomic absorption spectrometry

In this work estimation of measurement uncertainties associated with the total metal content in soils was done by an intralaboratory approach based on method validation and quality control data, and using two certified reference materials (CRM). CRM and soil samples were analyzed following procedures based on the methods that are applied to silicate materials. All elements were determined by atomic absorption spectrometry following a quality assurance program previously established. Quality control actions were implemented in order to provide reliable data. The precision under within-laboratory reproducibility conditions was estimated from triplicate analysis. The trueness component was determined as recovery of the analyte from CRMs: soil sample, SO-2 and river clay sediment, LGC 6139. Combined measurement uncertainty was expressed in terms of precision and recovery uncertainties and the later further split on CRM replicate analysis and uncertainty of the certified value components. The results obtained are critically discussed on the basis of the different contributions. For the selection of the reference material, the CRM dependent terms are critically compared in order to fulfill specific requirements. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The utilization of modified BCR three-step sequential extraction procedure for the fractionation of Cd, Cr, Cu, Ni, Pb and Zn in soil reference materials of different origins

A modified three-step sequential extraction procedure for the fractionation of heavy metals, proposed by the Commission of the European Communities Bureau of Reference (BCR) has been applied to the Slovak reference materials of soils (soil orthic luvisols, soil rendzina and soil eutric cambisol), which represent pedologically different types of soils in Slovakia. Analyses were carried out by flame or electrothermal atomic absorption spectrometry (FAAS or ETAAS). The fractions extracted were: exchangeable (extraction step 1), reducible-iron/manganese oxides (extraction step 2), oxidizable-organic matter and sulfides (extraction step 3). The sum of the element contents in the three fractions plus aqua-regia extractable content of the residue was compared to the aqua-regia extractable content of the elements in the origin soils. The accuracy obtained by comparing the determined contents of the elements with certified values, using BCR CRM 701, certified for the extractable contents (mass fractions) of Cd, Cr, Cu, Ni, Pb and Zn in sediment following a modified BCR-three step sequential extraction procedure, was found to be satisfactory.     

What’s new with the NRC certified reference material program

The environmental CRMs currently available from NRC Ottawa comprise four natural waters for trace elements, four biological tissues for trace elements and methylmercury, three marine sediments for major and trace elements (one sediment has certified values for butyltins) and one fish tissue for organochlorines (PCBs, dioxins). Although significant effort has been expended to maintain this suite of materials, several new initiatives have also recently been completed or are in progress. The certified value for arsenobetaine in Dogfish Muscle CRM DORM-2 has been established. The certification approaches, as well as the studies to determine extraction efficiency of this organoarsenic compound, are presented. A stability study has been completed for Hg in a natural water sample and plans are underway for production of a CRM. Assessment of micronutrient stability in seawater for the purposes of CRM production is continuing and progress on this initiative is discussed.     

What's new with the NRC certified reference material program

The environmental CRMs currently available from NRC Ottawa comprise four natural waters for trace elements, four biological tissues for trace elements and methylmercury, three marine sediments for major and trace elements (one sediment has certified values for butyltins) and one fish tissue for organochlorines (PCBs, dioxins). Although significant effort has been expended to maintain this suite of materials, several new initiatives have also recently been completed or are in progress. The certified value for arsenobetaine in Dogfish Muscle CRM DORM-2 has been established. The certification approaches, as well as the studies to determine extraction efficiency of this organoarsenic compound, are presented. A stability study has been completed for Hg in a natural water sample and plans are underway for production of a CRM. Assessment of micronutrient stability in seawater for the purposes of CRM production is continuing and progress on this initiative is discussed.     

Sequential extraction of phosphorus in soil and sediment using a continuous-flow system

Distribution and mobility of phosphorus in soil and sediment are usually studied by sequential extraction. In the extraction procedure, a sample is treated with a series of reagents to distinguish the phases to which phosphorus is associated such as carbonate and iron or manganese oxides, etc. There have been a number of extraction schemes presented for phosphorus. At present, all of the existing schemes are carried out batchwise. Phosphorus contents derived from all sequences are operationally defined and depend on experimental conditions.An extraction procedure, which is a continuous-flow-based technique, was recently proposed by our group for metals in soils and sediments. The extraction is carried out in a closed chamber through which extractants are passed sequentially. In this paper, the system was investigated using the extraction scheme of Hieltjes and Lijklema to study distribution of phosphorus in three certified reference materials (CRMs). A number of fractions were collected for each reagent for subsequent colorimetric determination. The results are compared with those obtained from a batch extraction. The summation of phosphorus contents of all phases were compared with the certified values and with the values obtained from total digestion. These results have demonstrated that the continuous extraction system developed is also applicable for fractionation of phosphorus. Advantage and disadvantage are discussed.     

A greener microwave-assisted extraction method for rapid spectroscopic determination of selected metals in river and freshwater sediment certified reference materials

The current study describes development of novel, cost-effective, rapid and greener microwave-assisted extraction (MW-AE) method using diluted hydrogen peroxide (H₂O₂) and 400 µL of concentrated aqua-regia for the extraction of 10 selected metals (Ba, Cr, Cu, Co Fe, Mn, Ni, Pb, V and Zn) in sediment certified reference materials (CRMs), prior to inductively coupled plasma-optical emission spectroscopic analysis. The optimum extraction conditions were achieved when 0.2 g of sediment was extracted with 1 mol L^?1 of H₂O₂ (12.00 mL) and 400 µL of concentrated aqua-regia, at 180°C microwave extraction temperature for 30 min. Accuracy of the proposed MW-AE method was evaluated by using river (LGC6187) and freshwater (CRM015-050) sediment CRMs. Quantitative extraction recoveries of 80–120% in LGC6187 and 74–124% in CRM0150-050 were obtained with acceptable precision (0.4–1.0%). The proposed extraction method has a clear advantage over classical approaches as 1 mol L^?1 H₂O₂ was used instead of notorious acidic mixture (Hydrochloric acid/ nitric acid/ hydrofluoric). The method detection limits (MDLs) of 0.03–0.9 μg g^?1 were obtained for all the investigated metals and were fairly comparable with the MDLs of the published literature reports. Therefore, the proposed MW-AE method can be routinely used for monitoring of metal ions in various river and fresh water sediments.     

Use of INAA in the preparation of a set of soil reference materials with certified values of total element contents

A set of certified Reference Materials was prepared consisting of four natural agricultural soils with normal (n) and elevated (e) levels of element contents: CRM 7001 Light Sandy Soil (n), CRM 7002 Light Sandy Soil (e), CRM 7003 Silty Clay Loam (n), and CRM 7004 Loam (e). In these materials, certified and/or information values of the total contents of the elements As, Ba, Be, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, V and Zn, and their fractions extractable by aqua regia, boiling and cold 2M nitric acid were derived from an interlaboratory comparison in which 28 laboratories participated. Highly precise and accurate procedures of instrumental neutron activation analysis (INAA) were employed for homogeneity testing and also for certification of the total element contents. For comparation purposes, NIST SRM-2704 Buffalo River Sediment was analyzed by INAA, as well. The INAA results obtained compared very well with the certified and/or information values for four soil CRMs and also with NIST values for SRM-2704. From this agreement, a very high reliability of the new soil CRMs can be inferred.     

Determination of bioavailable fractions of Zn, Cu, Ni, Pb and Cd in soils and sludges by atomic absorption spectrometry

The single extraction procedures validated by the standards, measurement and testing programme (formerly BCR), extraction with 0.05 mol l⁻¹ EDTA and 0.43 mol l⁻¹ acetic acid, have been applied to reference materials of soils and sludges with certified total values of elements, in order to determine bioavailable contents of Cd, Cu, Ni, Pb and Zn. These soils, which represent uncontaminated pedologically different types of soils from Slovakia and sludges from city water treatment are characterized for the bioavailable fraction of the metals using the procedures followed by SM&T Programme. Concentrations of the elements under the study in the extracts were determined by flame (FAAS) using calibration curves in appropriate extractants and by electrothermal (ETAAS) atomic absorption spectrometry, using technique of standard additions for the evaluation of the results. The accuracy of the extraction procedures and determinations of the elements in the extracts was controlled using CRM 483 certified for EDTA- and acetic acid-extractable contents of Cd, Cu, Ni, Pb and Zn in sewage sludge amended soil.     

Certification of a low value electrolytic conductivity solution using traceable measurements

Reliable measurement results of electrolytic conductivity (EC), in particular for low values, must be metrologically traceable and be based on a realistic measurement uncertainty budget. The use of certified reference materials (CRMs) can help to achieve this goal. This paper presents results from all stages of the certification of an EC CRM with a conductivity of 1.5 μS cm^?1, including the preparation of the batch solution and the evaluation of the homogeneity and stability of the bottled CRM. An uncertainty budget is presented for the CRM, including the main contributions from each of these sources. The CRM batch remained stable within its certified uncertainty for more than 1 year.     

Determination of Al, Cu, Fe, Mn, Pb and Zn in certified reference materials using the optimized BCR sequential extraction procedure

Sequential extraction procedures are widely used to characterize the fractionation of metal species in solid media. With the variety of different sequential procedures used in environmental and geochemical exploration studies, it is difficult to compare results between studies. Thus, harmonization and standardization are required to provide greater inter-study comparability for fraction-specific metals. In this study, the optimized BCR three-step sequential extraction procedure is applied to five certified reference materials (SRM 2710, SRM 2711, CRM 483, CRM 601 and CW 7). Four fractions are reported, acid extractable, reducible, oxidizable, and residual for Al, Cu, Fe, Mn, Pb and Zn. The objectives of this study were to characterize experimental precision and/or accuracy and to establish baseline data of fraction-specific element concentrations for future studies applying the optimized BCR three-step extraction procedure. The optimized procedure was found to be precise (typically <5%) for all metals in all fractions. Accuracy was acceptable (typically ±15% relative to published indicative values for Cu, Pb and Zn for CRM 483 and CRM 601) for all individual fractions. Detailed fraction-specific concentration data are presented, based on five replicates, for the first time using the optimized procedure for Al, Fe and Mn in CRM 483 and CRM 601, and for Al, Cu, Fe, Mn, Pb and Zn in SRM 2710, SRM 2711 and CW 7.     

Sediment certified reference materials for the determination of polychlorinated biphenyls and organochlorine pesticides from the National Metrology Institute of Japan (NMIJ)

Two marine sediment certified reference materials, NMIJ CRM 7304-a and 7305-a, have been issued by the National Metrology Institute of Japan in the National Institute of Advanced Industrial Science and Technology (NMIJ/AIST) for the determination of polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs). The raw materials of the CRMs were collected from a bay near industrial activity in Japan. Characterization of these CRMs was conducted by NMIJ, where the sediments were analyzed using multiple analytical methods such as pressurized liquid extraction (PLE), microwave-assisted extraction (MAE), saponification, Soxhlet extraction, supercritical fluid extraction (SFE), and ultrasonic extraction; the target compounds were determined by one of the primary methods of measurements, isotope dilution–mass spectrometry (ID-MS). Certified values have been provided for 14 PCB congeners (PCB numbers 3, 15, 28, 31, 70, 101, 105, 138, 153, 170, 180, 194, 206, 209) and 4 OCPs (γ-HCH, 4,4′-DDT, 4,4′-DDE, 4,4′-DDD) in both CRMs. NMIJ CRM 7304-a has concentrations of the contaminants that are a factor of 2–15 greater than in CRM 7305-a. Both CRMs have information values for PCB homolog concentrations determined by collaborative analysis using a Japanese official method for determination of PCBs. The total PCB concentrations in the CRMs are approximately 920 and 86 μg kg⁻¹ dry mass respectively. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Estimating the uncertainty of stability for matrix CRMs.

The new version of ISO Guide 34 requires producers of certified reference materials (CRMs) to include contributions of possible instability to the overall CRM uncertainty, to obtain a value for the uncertainty in compliance with the Guide to the Expression of the Uncertainty in Measurement (GUM). A pragmatic approach to estimating the uncertainty of stability is presented. It relies on regression analysis of stability data with subsequent testing of the slope of the regression line for significance. If the slope is found to be statistically insignificant, a shelf life is chosen and the uncertainty connected with this time is estimated via the standard deviation of the slope. This uncertainty is included in the overall uncertainty of the CRM. This approach is explained with examples showing its applicability to matrix CRMs.     

Electrothermal atomic absorption spectrometric determination of vanadium in extracts of soil and sewage sludge certified reference materials after fractionation by means of the Communities Bureau of Reference modified sequential extraction procedure

A modified three-step sequential extraction procedure proposed by the Commission of European Communities Bureau of Reference (BCR) was applied to certified reference materials of three different soil groups (rendzina, luvisol, cambisol) and sewage sludge of different composition originating from a municipal water treatment plant in order to assess potential mobility and the distribution of vanadium in the resulting fractions. Analysis of the extracts was carried out by electrothermal atomic absorption spectrometry with Zeeman background correction using transversely heated graphite atomizers. Extracts showed significant matrix interferences which were overcome by the standard addition technique. The original soil and sludge certified reference materials (CRMs) and the extraction residue from the sequential extraction were decomposed by a mixture of HNO₃–HClO₄–HF in an open system. The content of V determined after decomposition of the samples was in very good agreement with the certified total values. The accuracy of the sequential extraction procedure was checked by comparing the sum of the vanadium contents in the three fractions and in the extraction residue with the certified total content of V. The amounts of vanadium leached were in good correlation with the certified total contents of V in the CRMs of soils and sewage sludge. In the soils examined, vanadium was present almost entirely in the mineral lattice, while in the sewage sludge samples 9–14% was found in the oxidizable and almost 25% in the reducible fractions. The recovery ranged from 93–106% and the precision (RSD) was below 10%.     

Direct automatic screening of soils for polycyclic aromatic hydrocarbons based on microwave-assisted extraction/fluorescence detection and on-line liquid chromatographic confirmation

An integrated screening-confirmation system for PAHs in soils is presented. The sample screening configuration comprises on-line microwave-assisted extraction of the selected pollutants, followed by continuous preconcentration and sample clean-up on RP-C18. Those samples for which the total concentration is close to or above the threshold limit established (10 microg/g) are subjected to liquid chromatographic separation for confirmation. An evaluation of the qualitative data obtained was also carried out, by calculating the unreliability zone as well as the false positive and false negative rates. The whole method (extraction/determination/confirmation) was validated using industrial soil and harbour sediment certified reference materials (IRMM, European Commission CRM 524 and CRM 535). Application to the screening of solid environmental samples with subsequent confirmation of the results is also presented.     

Estimating the uncertainty of stability for matrix CRMs

The new version of ISO Guide 34 requires producers of certified reference materials (CRMs) to include contributions of possible instability to the overall CRM uncertainty, to obtain a value for the uncertainty in compliance with the Guide to the Expression of the Uncertainty in Measurement (GUM). A pragmatic approach to estimating the uncertainty of stability is presented. It relies on regression analysis of stability data with subsequent testing of the slope of the regression line for significance. If the slope is found to be statistically insignificant, a shelf life is chosen and the uncertainty connected with this time is estimated via the standard deviation of the slope. This uncertainty is included in the overall uncertainty of the CRM. This approach is explained with examples showing its applicability to matrix CRMs. Received: 12 October 2000 / Revised: 2 January 2001 / Accepted: 3 January 2001     

Utilization of optimized BCR three-step sequential and dilute HCl single extraction procedures for soil-plant metal transfer predictions in contaminated lands

The prediction of soil metal phytoavailability using the chemical extractions is a conventional approach routinely used in soil testing. The adequacy of such soil tests for this purpose is commonly assessed through a comparison of extraction results with metal contents in relevant plants. In this work, the fractions of selected risk metals (Al, As, Cd, Cu, Fe, Mn, Ni, Pb, Zn) that can be taken up by various plants were obtained by optimized BCR (Community Bureau of Reference) three-step sequential extraction procedure (SEP) and by single 0.5 mol L(-1) HCl extraction. These procedures were validated using five soil and sediment reference materials (SRM 2710, SRM 2711, CRM 483, CRM 701, SRM RTH 912) and applied to significantly different acidified soils for the fractionation of studied metals. The new indicative values of Al, Cd, Cu, Fe, Mn, P, Pb and Zn fractional concentrations for these reference materials were obtained by the dilute HCl single extraction. The influence of various soil genesis, content of essential elements (Ca, Mg, K, P) and different anthropogenic sources of acidification on extraction yields of individual risk metal fractions was investigated. The concentrations of studied elements were determined by atomic spectrometry methods (flame, graphite furnace and hydride generation atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry). It can be concluded that the data of extraction yields from first BCR SEP acid extractable step and soil-plant transfer coefficients can be applied to the prediction of qualitative mobility of selected risk metals in different soil systems.     

Kinetic speciation of BCR reference materials

The accurate study of heavy metal speciation is important in environmental monitoring. There has been much work developing various operationally defined speciation methods for soil and sediment, but there is a need to compare the different approaches by evaluating them for the same sample. In this article, a kinetic method was applied for the heavy metal speciation of the two BCR reference materials, CRM601 and BCR701, which have been specifically developed as materials to evaluate the validated BCR three-step sequential extraction method. When EDTA was used as an extractant, 81.0% of Cd, 68.0% of Cu, 21.5% of Ni, 80.3% of Pb and 71.9% of Zn was extracted from CRM601. For BCR701, the removal ratios were 92.0, 52.3, 18.7, 50.6 and 67.5% with EDTA and 95.7, 25.2, 20.0, 52.4 and 68.5% with hydroxylamine hydrochloride as an extractant, for Cd, Cu, Ni, Pb and Zn respectively. A two-component kinetic model was applied to the extraction curve and the extractable metals were readily classified into two categories, namely, labile fraction and non-labile fractions. The rate constants obtained from the regression model were found to be useful in quantifying the lability of an element. The rate constants obtained from the labile fractions in BCR701 were higher than that of obtained from CRM601, which indicated the high lability of metals in BCR701. When compared with the sequential extraction data, it seemed that the lability of an element was positively correlated to the first step extraction fraction.     

The certified reference materials CRM 142 R light sandy soil, CRM 143 R sewage sludge amended soil and CRM 145 R sewage sludge for quality control in monitoring environmental and soil pollution

The determination of various heavy metals in soils and sludges provides an indication for their degree of contamination. Therefore the reliability of such determinations is of utmost importance. Quality assessment can be performed by using adequate certified reference materials. Because of their success, the present supplies of the BCR CRM 142, CRM 143 and CRM 145 are exhausted. The Measurement and Testing Programme (BCR) of the Commission of the European Communities decided to replace them. After a careful preparation procedure of materials sufficiently similar to the exhausted ones and a homogeneity and stability study, the new materials were certified for a series of trace elements also including their aqua regia soluble fraction. Indicative values for other elements are given as well.     

Certified reference material for the quality control of EDTA- and DTPA-extractable trace metal contents in calcareous soil (CRM 600)

Single extraction tests are commonly used to study the eco-toxicity and mobility of metals in soils, e.g. to assess the bioavailable metal fraction (and thus to estimate the related phyto-toxic and nutritional deficiency effects) and the environmentally accessible trace metals upon disposal of e.g. sediment on to a soil (e.g. contamination of ground waters). However, the lack of uniformity in the different procedures does not allow the results to be compared worldwide nor the procedures to be validated. This paper describes the interlaboratory testing of EDTA- and DTPA-extraction procedures for soil analysis, followed by the preparation of a calcareous soil reference material (CRM 600), the homogeneity and stability studies and the analytical work performed for the certification of the EDTA- and DTPA- extractable contents of some trace metals (following the standardized extraction procedures). Received: 21 May 1997 / Revised: 4 July 1997 / Accepted: 9 July 1997     

Certified reference material for the quality control of EDTA- and DTPA-extractable trace metal contents in calcareous soil (CRM 600)

Single extraction tests are commonly used to study the eco-toxicity and mobility of metals in soils, e.g. to assess the bioavailable metal fraction (and thus to estimate the related phyto-toxic and nutritional deficiency effects) and the environmentally accessible trace metals upon disposal of e.g. sediment on to a soil (e.g. contamination of ground waters). However, the lack of uniformity in the different procedures does not allow the results to be compared worldwide nor the procedures to be validated. This paper describes the interlaboratory testing of EDTA- and DTPA-extraction procedures for soil analysis, followed by the preparation of a calcareous soil reference material (CRM 600), the homogeneity and stability studies and the analytical work performed for the certification of the EDTA- and DTPA- extractable contents of some trace metals (following the standardized extraction procedures). Received: 21 May 1997 / Revised: 4 July 1997 / Accepted: 9 July 1997