Certified Reference Soil Research Articles

X-ray fluorescence spectroscopic analysis of trace elements in soil with an Adaboost back propagation neural network and multivariate-partial least squares regression

Poisonous elements such as lead (Pb), arsenic (As), cadmium (Cd), and chromium (Cr) are commonly observed in polluted soil but hard to detect due to low concentration. Currently, the quantitative content of elements can be determined by an x-ray fluorescence (XRF) analyzer if the limit of detection (LOD) is known. However, to ensure that elements with extremely low content can be detected, the LOD needs to be reduced and determined. Firstly, the authors utilized the traditional method detection limit (MDL) formula to calculate the LOD for benchmark purposes. Secondly, in order to improve the signal to noise ratios of the characteristic peaks, the wavelet threshold denoising method and iterative discrete wavelet transform algorithm were utilized for spectral denoising and background noises deduction of XRF spectra based on 57 certified reference soil samples. Thirdly, a novel Adaboost back propagation neural network model was proposed to reduce and determine the values of LOD. The results showed that the LODs of trace elements determined from the proposed algorithm decreased greatly compared to the traditional MDL method. Finally, the calibration curve of elemental concentration versus counting rate was established and regressed using the proposed multivariate partial least squares model. The goodness of fit (R 2) was significantly increased. Therefore, the proposed algorithm is effective to determine the LOD and can yield optimal performance in instrument calibration of trace elements in soil.

Impact of Soil Water on the Spectral Characteristics and Accuracy of Energy-Dispersive X-ray Fluorescence Measurement.

Soil water is a major interference in the on-site analysis of soil by energy-dispersive X-ray fluorescence. Apparent consequences of this interference include lowered readings for elemental concentrations and significant changes in spectral characteristics in wet soils compared with dry soils. A rigorous interpretation on this issue remains unresolved. Thus, this study evaluated the impact of soil water on the detection of Ca, Ti, Mn, Fe, Cu, Zn, As, Rb, Sr, and Pb. Specimens were prepared from 11 certified reference soils and 3 field soils with water contents from 0 to ∼40 wt %. Results from three commercial models revealed that the readings were subjected to respective internal quantification algorithms; therefore, they could not provide a fundamental perspective of this issue. We analyzed the spectra to examine the mechanism underlying this phenomenon. The spectra of wet soils feature elevated baseline, increased Compton and Rayleigh scatter peaks, and lowered characteristic peaks of elements. Previous studies attributed the lowered characteristic peaks to the absorption of fluorescent X-rays by water and considered soil water and dry soil as separate layers in the calculation. This work argues that wet soils should be treated as mixtures. Water becomes part of the soil matrix and leads to lower attenuation capability, which could be explained by the matrix effect. Meanwhile, the mass fraction of analytes is lowered because of dilution. Results confirmed that dilution lowers the characteristic peaks, whereas the matrix effect heightens them. When estimating the elemental concentrations on a wet weight basis, the matrix effect becomes the major interference. The Compton compensation method provided satisfying results on correcting the matrix effect caused by soil water on Zn, As, Rb, Sr, and Pb.

Spectrophotometric methods for the measurement of soil pH: A reappraisal

The pH of aqueous soil extracts is generally measured potentiometrically by glass electrode (GE). Spectrophotometric methods have also been used till around the '80s, and then they have apparently been abandoned. However, the use of microplates and spectrophotometers able to read absorbance values in them can significantly increase the speed of the data collection (saving analysis time), thus justifying their reappraisal. Three spectrophotometric methods are proposed in this work: a one-indicator (OISM), a separated three-indicator (STISM) and a mixed three-indicator (MTISM) spectrophotometric method. They are based on the addition of one or three colorimetric indicators (methyl red, bromocresol violet, and bromothymol blue) to the aqueous extract. The pH is measured through its absorption properties in the visible region. The analysis of 60 soil samples showed that STISM and MTISM results correlate well with the ones obtained by GE. The STISM method, being more simple and general than the other two methods, is proposed for quick routine analyses. The repeatability, reproducibility and accuracy of STISM (and of GE, for comparison) were evaluated by measuring several times the pH of a series of soil samples and of a certified reference soil. The standard deviations of STISM results were slightly worse than those of GE, whereas the accuracy was slightly better, indicating that STISM and GE have overall similar performances. STISM method is much faster than GE one: the analysis time saving is around 2 min per sample, and it becomes very high when hundreds of samples have to be analysed. It follows that STISM can represent an advantageous alternative to GE for rapid and accurate soil pH measurements.

Challenges in the quantification of nutrients in soils using laser-induced breakdown spectroscopy – A case study with calcium

The quantification of the elemental content in soils with laser-induced breakdown spectroscopy (LIBS) is challenging because of matrix effects strongly influencing the plasma formation and LIBS signal. Furthermore, soil heterogeneity at the micrometre scale can affect the accuracy of analytical results. In this paper, the impact of univariate and multivariate data evaluation approaches on the quantification of nutrients in soil is discussed. Exemplarily, results for calcium are shown, which reflect trends also observed for other elements like magnesium, silicon and iron. For the calibration models, 16 certified reference soils were used. With univariate and multivariate approaches, the calcium mass fractions in 60 soils from different testing grounds in Germany were calculated. The latter approach consisted of a principal component analysis (PCA) of adequately pre-treated data for classification and identification of outliers, followed by partial least squares regression (PLSR) for quantification. For validation, the soils were also characterised with inductively coupled plasma optical emission spectroscopy (ICP OES) and X-ray fluorescence (XRF) analysis. Deviations between the LIBS quantification results and the reference analytical results are discussed.

Matrix-effect-free determination of BTEX in variable soil compositions using room temperature ionic liquid co-solvents in static headspace gas chromatography mass spectrometry

Concerns about the environmental impact of shale energy exploration (i.e., hydraulic fracturing and other well stimulation techniques) have risen due to its rapid expansion in the United States and other countries. Soils in shale basins engaged in unconventional oil and gas development can become contaminated by volatile organic compounds (VOCs), such as benzene, toluene, ethylbenzene, and xylenes (BTEX) through mishandling of chemical additives, products, and/or waste fluids. In this study, room temperature ionic liquids (RTILs) were evaluated as solvents in an effort to increase sensitivity and to reduce and normalize matrix effects associated with varying soil compositions during analysis. Headspace gas chromatography mass spectrometry (HS-GC-MS) experiments demonstrated that hydrophilic RTILs 1-ethyl-3-methylimidazolium ethyl sulfate [EMIM][ESO4], 1-ethyl-3-methylimidazolium diethyl phosphate [EMIM][DEP], and tris(2-hydroxyethyl) methylammonium methylsulfate [MTEOA][MeOSO3] normalized the response for BTEX compounds between 2 different soils, sandy loam and sandy clay loam. Furthermore, the optimization of the HS equilibration time to 30 min resulted in the reduction of matrix effects in certified reference soils of sand, clay, and loam textures. Limits of detection and limits of quantification were in the sub- to mid- pg g−1 level in soil. For determination at 1 g of certified BTEX reference soil, the relative standard deviation was within 10% and percent recoveries were above 80% for toluene, ethylbenzene, and xylenes. This method reduces the need to characterize and matrix-match soil texture for calibration purposes. It also reduces the analysis time and increases precision and accuracy for the quantification of BTEX in variable soil matrices relative to standard methods.

Application of portable X-ray fluorescence spectrometry in environmental investigation of heavy metal-contaminated sites and comparison with laboratory analysis

In this study, portable X-ray fluorescence spectrometry (pXRF) was used to measure the heavy metal contents of As, Cu, Cr, Ni, Pb and Zn in the soils of heavy metal-contaminated sites. The precision, accuracy and system errors of pXRF were evaluated and compared with traditional laboratory methods to examine the suitability of in situ pXRF. The results show that the pXRF analysis achieved satisfactory accuracy and precision in measuring As, Cr, Cu, Ni, Pb, and Zn in soils, and meets the requirements of the relevant detection technology specifications. For the certified reference soil samples, the pXRF results of As, Cr, Cu, Ni, Pb, and Zn show good linear relationships and coefficients of determination with the values measured using the reference analysis methods; with the exception of Ni, all the measured values were within the 95% confidence level. In the soil samples, the coefficients of determination between Cu, Zn, Pb, and Ni concentrations measured laboratory pXRF and the values measured with laboratory analysis all reach 0.9, showing a good linear relationship; however, there were large deviations between methods for Cr and As. This study provides reference data and scientific support for rapid detection of heavy metals in soils using pXRF in site investigation, which can better guide the practical application of pXRF.

Background removal in soil analysis using laser- induced breakdown spectroscopy combined with standard addition method.

The matrix effect of powder samples, especially for soil samples, is significant in laser-induced breakdown spectroscopy (LIBS), which affects the prediction accuracy of the element concentration. In order to reduce this effect of the soil samples in LIBS, the standard addition method (SAM) based on background removal by wavelet transform algorithm was investigated in this work. Five different kinds of certified reference soil samples (lead (Pb) concentrations were 110, 283, 552, 675, and 1141 ppm, respectively) were used to examine the accuracy of this method. The root mean square error of prediction (RMSEP) was more than 303 ppm by using the conventional calibration method. After adoption of SAM with background removal by wavelet transform algorithm, the RMSEP was reduced to 25.7 ppm. Therefore, the accuracy of the Pb element was improved significantly. The mechanism of background removal by wavelet transform algorithm based on SAM is discussed. Further study demonstrated that this method can also improve the predicted accuracy of the Cd element.

Embedment of Ag(I)-organic frameworks into silica gels for microextraction of polybrominated diphenyl ethers in soils

Three Ag(I)-organic frameworks, [Ag5(pydc)2(CN)]n, {[Ag4(pydc)2]CH3CN}n, and [Ag(4,4′-bpy)NO3]n, were synthesized and embedded into silica gels to form metal-organic-framework (MOF)-embedded gels for the microextraction of polybrominated diphenyl ethers (PBDEs) in soils. Despite the great differences in the structures of the organic ligands, all three Ag(I)-organic frameworks were found to effectively accumulate and concentrate PBDEs from sample solutions prepared with contaminated soil and purified water, indicating the important roles of Ag centers in PBDE extraction. Under the optimal experimental conditions (MOF mass, water volume, temperature, extraction time, and back-extraction time) for PBDE extraction from sample solutions, the detection limits of seven PBDEs (BDE-28, 47, 99, 100, 153, 154, and 183) ranged from 0.01 to 2.6ngg−1 for [Ag5(pydc)2(CN)]n, 0.20–0.64ngg−1 for {[Ag4(pydc)2]CH3CN}n, and 0.60–3.08ngg−1 for [Ag(4,4′-bpy)NO3]n. The reproducibilities of the three methods were all satisfactory with relative standard deviations (RSDs) in the range of 2.2–9.6%, 5.3–10.4%, and 6.9–9.4% for [Ag5(pydc)2(CN)]n, {[Ag4(pydc)2]CH3CN}n, and [Ag(4,4′-bpy)NO3]n, respectively. The use of Ag(I)-organic frameworks for the microextraction of PBDEs was validated using both certified reference soils and field-contaminated soils, and the proposed methods are recommended as rapid and environmentally friendly alternatives for the extraction and determination of PBDEs in soils.

Portable X-ray fluorescence analysis of mineral and organic soils and the influence of organic matter

Portable X-ray fluorescence (PXRF) spectrometers with tube X-ray source are being used to determine the elemental composition of soils in the laboratory and the field. Most studies with PXRF have been with mineral soils and there is a need to assess PXRF applications to organic soils and peats. Using a commercial PXRF instrument and the manufacturer's soils programme we assessed performance by the analysis of various soils with a range of organic matter concentrations up to those of low ash peats. We first analysed seven certified reference mineral soils and a certified, low ash ombrotrophic peat. The data obtained for the certified reference mineral soils was definitive for Ti, Cr, Mn, Fe, Ni, Cu, and As, quantitative for K, Ca, Zn and Sr, and qualitative for Pb. Portable X-ray fluorescence analysis of the ombrotrophic peat gave satisfactory results for Cu (4.00±1.00mgkg−1, certified 5.28±1.04mgkg−1) and Pb (184±3mgkg−1, certified 174±8mgkg−1) but overestimated the concentrations of Ca, Ti, Cr, Ni and Zn by 2–3 times, and Fe by 5 times. To extend comparison beyond the only available low ash certified reference soil and further exemplify the differences between mineral soils and peat soils we analysed 183 Scottish topsoils that had a wide range of organic carbon (OC) concentrations (1.23 to 48.8% by weight) and compared the concentrations of K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, As, Sr and Pb determined by PXRF with pseudo total concentrations determined by aqua regia extraction and inductively coupled plasma spectrometry. The results demonstrate that to maximise the potential of the PXRF instrument for use beyond mineral soils, to organo-mineral and peats, modifications to the manufacturer's calibrations should be made. We recommend validation using soils on a continuum of OC concentrations from those of mineral soils to peats covering the range required.

Determination of total and electrolabile copper in agricultural soil by using disposable modified-carbon screen-printed electrodes

The objective of the study is to evaluate modified-carbon screen-printed working electrodes (SPE) combined with square wave anodic stripping voltammetry (SWASV) to determine electrolabile and total copper in soils with the perspective to assess the environmental hazard resulting from copper anthropogenic contamination. The voltammetric method was investigated using a mineralized certified reference soil such that it can be assumed that the copper was totally under electrolabile form in the solution of mineralized soil. In optimal conditions, a copper recovery of 97% and a relative standard deviation (RSD) of 9% were found. The limits of detection and quantification for copper were 0.4 and 1.3 μg L(-1), respectively. Finally, the method was applied on soil leachates, which allowed evaluating the cupric transfer from the soil to the leachates and quantifying the electrolabile copper part in leachates.

Determination of polycyclic aromatic hydrocarbons in solid matrices using automated cold fiber headspace solid phase microextraction technique

The extraction efficiency of analytes in a cold fiber headspace solid-phase microextraction (CF-HS-SPME) system was investigated both theoretically and experimentally. The system was applied for quantitative extraction of polycyclic aromatic hydrocarbons (PAHs) from solid matrices. In order to achieve better extraction efficiency for PAHs, a method with programmed coating temperature was considered and optimized, leading to higher extraction efficiency for most studied analytes when compared with traditional methods. After optimization of extraction with the programmed coating temperature method, the recoveries were above 90% for the majority of tested compounds from the sand matrix, resulting in exhaustive extraction. Certified reference soil was used to evaluate the cold fiber SPME system. Several organic solvents were used to improve the extraction efficiency. Diethylamine was used successfully to realize the exhaustive extraction for volatile compounds and enhance the recoveries of 60–75% for semi-volatile PAHs. Obtained results indicate that the automated cold fiber extraction is a convenient approach to facilitate high throughput, solventless sample preparation

Online multichannel ultrasonic extraction for high throughput determination of arsenic in soil by sequential injection slurry hydride generation atomic fluorescence spectrometry

A new analytical method was developed to determine trace arsenic in soil by coupling sequential injection multichannel ultrasonic extraction and online pre-reduction with slurry hydride generation atomic fluorescence spectrometry. Significantly enhanced sampling throughput, reduced sample and reagent consumption, and minimized potential contamination and analyte loss were achieved. Slurry samples spiked with different concentrations of As(III) and hydrochloric acid were pumped into the ultrasonic extraction chamber for 20-minute irradiation to efficiently extract As, which were subsequently merged with a mixture of 1% (m/v) thiourea and 0.5% (m/v) ascorbic acid to pre-reduce As(V) species to As(III), followed by the generation of AsH3via using KBH4. Upon optimization of experimental parameters, limits of detection (LODs) ranging from 30 to 70 ng g−1 were obtained, and the precisions (RSDs) of this method were better than 2.0%. The proposed method was used for the determination of trace arsenic in several Certified Reference soil samples, with the obtained results in excellent agreement with certified values based on a simple and fast standard addition method.

Determination of Hg, As, Pb, and Cd in Orchard Soils by Sequential Injection Vapor Generation Atomic Fluorescence Spectrometry

An analytical procedure was developed for determination of Hg, As, Pb, and Cd in soil samples using sequential injection vapor generation atomic fluorescence spectrometry (SI-VG-AFS) with sample preparation by microwave digestion system. The effects of analytical conditions on the fluorescence intensity were investigated and optimized. Under the optimized condition, the detection limits of the method were 5, 68, 40, and 3 ng L−1 for Hg, As, Pb, and Cd, respectively. The accuracy of method was verified by the determination of the certified reference soil, and the recoveries for Hg, As, Pb, and Cd were in the range of 98.7–106.1%. The proposed method has been successfully applied to determine the heavy metals in navel orange orchard soils in Ganzhou, and the environmental quality of the orchard soils was assessed based on the content of the heavy metals. According to the second criterion of environmental quality standard for soils, Pb content in all soil samples collected from the orchards of 18 districts were within safe limits, but a few orchards were slightly contaminated with Hg, As, and Cd. Comparison of heavy metal mean concentrations with the safe limits of environmental quality standard for soils, the quality of Ganzhou navel orange orchard soils were in line with the request of the green food production base.

The Spatial Heterogeneity of Polycyclic Aromatic Hydrocarbons in Soil Depends on Their Physico-chemical Properties

The present study investigates the heterogenic distribution of nine polycyclic aromatic hydrocarbons (PAHs) in soil on a microscale. For this purpose, we developed a method allowing for the detailed analysis of PAHs in minute amounts of soil (5 to 25 mg). A certified reference soil with a known PAH content and a diffusely polluted Danish surface soil from central Copenhagen were used in the study. In order to separate soil heterogeneity from analytical variation, we attempted to establish the least amount of soil in which homogeneous and reproducible results could be obtained for the PAHs in question. The results demonstrated that for fluoranthene, analytical results in accordance with the certified reference values could be obtained in quantities of 10 mg of soil or more. For phenanthrene and pyrene, certified reference values could be obtained in quantities of 25 mg of soil or more. Similar results were obtained with the Nyboder soil, using the reference soil for quality and quantity assurance. For quantification of all nine PAHs, a minimum of 100 mg of soil was needed for both soils. In order to obtain an expression for the degree of heterogeneity of PAHs in the soils, a subsample variation was calculated based on overall sample variation and analytical measurements variation. The results demonstrate that the PAHs in the Nyboder soil are much more heterogeneously distributed than the PAHs in the reference soil due to much larger subsample variations. Furthermore, strong relationships between different physico-chemical properties and subsample variation were found. These included molecular weight, water solubility, log octanol/water partition coefficient, and soil–water distribution coefficient of the investigated PAHs, demonstrating that the heterogeneity of PAHs in the Nyboder soil is significantly influenced by such parameters.

Contaminant concentration in environmental samples using LIBS and CF-LIBS

The present paper deals with the detection and quantification of toxic heavy metals like Cd, Co, Pb, Zn, Cr, etc. in environmental samples by using the technique of laser-induced breakdown spectroscopy (LIBS) and calibration-free LIBS (CF-LIBS). A MATLABTM program has been developed based on the CF-LIBS algorithm given by earlier workers and concentrations of pollutants present in industrial area soil have been determined. LIBS spectra of a number of certified reference soil samples with varying concentrations of toxic elements (Cd, Zn) have been recorded to obtain calibration curves. The concentrations of Cd and Zn in soil samples from the Jajmau area, Kanpur (India) have been determined by using these calibration curves and also by the CF-LIBS approach. Our results clearly demonstrate that the combination of LIBS and CF-LIBS is very useful for the study of pollutants in the environment. Some of the results have also been found to be in good agreement with those of ICP-OES.

A simple and efficient method for the determination of pollutant phenols in soils with high levels of organic matter

A well-reproducible method to concentrate and determine Environmental Protection Agency (EPA) priority pollutant phenols from soils is developed. Alkaline extraction with 0.1 M NaOH of the phenolic compounds from soil with a high organic content is followed by their concentration in a highly cross-linked polystyrene–divinylbenzene sorbent (Bakerbond SDB-1) and HPLC-UV analysis. Detection limits ranged from 13 to 64 µg kg−1. The effect of the amount of soil and analyte concentration is evaluated and quantitative recoveries are obtained in all the conditions tested. The proposed methodology is validated by comparing the results obtained with those achieved by applying the official EPA Method 3540C (Soxhlet extraction) and by the analysis of a certified reference soil without significant differences being observed with regards to recovery rates, although the new method proves to be faster. The method is applied to the analysis of spiked soils and to the evaluation of the stability of the analytes in these soils.

Solid phase extraction of gold(III) on silica gel modified with benzoylthiourea prior to its determination by flame atomic absorption spectrometry

A new method has been developed for the determination of gold based on separation and preconcentration using silica gel modified with benzoylthiourea. The optimum experimental parameters for preconcentration of gold, such as acid concentration, sample flow rate, eluent and matrix ions, have been investigated. Gold could be quantitatively retained in the 0.25-2.0 mol L(-1) HCl and HNO(3) concentrations, and then eluted completely with 0.5 mol L(-1) thiourea in 1.0 mol L(-1) HCl. The sorption capacity of gold(III) is 0.92 +/- 0.04 mmol g(-1) with a high enrichment factor of 267. The relative standard deviation of the method, RSD%, was found as 1.2% for 0.1 microg mL(-1). The detection limit for gold was 1.4 microg L(-1). The validation of the proposed method was checked by the analysis of certified reference soil materials. The presented procedure was applied to the determination of gold in some environmental samples.

Selective pressurized liquid extraction for multi-residue analysis of organochlorine pesticides in soil

A selective pressurized liquid extraction (SPLE) procedure capable of performing simultaneous extraction and clean-up has been demonstrated for multi-residue analysis of organochlorine pesticides (OCPs) in soil. The final method was performed at 100 °C for 3 × 10 min using acetone/ n-heptane (1:1, v/v). Florisil was placed inside the extraction cell downstream the sample to remove interfering compounds. Extraction of two soil samples by SPLE gave a recovery which was over 80% for β-endosulfan, endosulfan sulfate, p, p′-DDT and p, p′-DDE compared to PLE with off-line clean-up. The same trend was observed when applying the SPLE method to a certified reference soil sample (CRM 811-050) containing 13 OCPs, where the SPLE method gave 80–90% recovery vis-à-vis the PLE method with off-line clean-up. Feasibility of the SPLE method was further demonstrated by applying it to five real soil samples collected in Ethiopia.

Speciation analysis of inorganic vanadium (V(IV)/V(V)) by graphite furnace atomic absorption spectrometry following ion-exchange separation

A sensitive and simple method of ion-exchange resin separation and graphite-furnace atomic absorption spectrometry (GFAAS) detection was proposed for the determination of inorganic vanadium species. Methylene Blue (MB) was used as a chelating agent of V(V) for ion-exchange separation. The complex of V(V) and MB could be trapped by ion-exchange resin at pH 3.0 and eluted by 1.0 mol L−1 NaOH. The vanadium species was determined subsequently by GFAAS. The concentration of V(IV) was calculated by subtracting the V(V) concentration from the total concentration of vanadium. Under the optimized experimental conditions, the detection limit of V(V) is 0.48 µg L−1 with RSD of 2.6% (n = 5, c = 2.0 µg L−1). In order to verify the accuracy of the method, a certified reference soil sample was analyzed, and the results obtained were in good agreement with the certified values. The range of recovery for V(IV) and V(V) was 97.8–99.3% and 101.7–103.6%, respectively. The proposed method was applied to the speciation analysis of vanadium in lake-water samples.

Determinação das formas de nitrogênio e nitrogênio total em rochas-reservatório de petróleo por destilação com arraste de vapor e método do indofenol

Several extraction procedures are described for the determination of exchangeable and fixed ammonium, nitrate + nitrite, total exchangeable nitrogen and total nitrogen in certified reference soils and petroleum reservoir rock samples by steam distillation and indophenol method. After improvement of the original distillation system, an increase in worker safety, a reduction in time consumption, a decrease of 73% in blank value and an analysis without ammonia loss, which could possibly occur, were achieved. The precision (RSD < 8%, n = 3) and the detection limit (9 mg kg-1 NH4+-N) are better than those of published procedures.