Accommodating detection limits of multiple exposures in environmental mixture analyses: an overview of statistical approaches

BACKGROUND

Identifying the impact of environmental mixtures on human health is an important topic. However, such studies face challenges when exposure measurements lie below limit of detection (LOD). While various approaches for accommodating a single exposure subject to LOD have been used, their impact on mixture analysis has not been thoroughly investigated. Our study aims to understand the impact of five popular LOD accommodation approaches on mixture analysis results with multiple exposures subject to LOD, including omitting subjects with any exposures below LOD (complete case analysis); single imputations by LOD/ 2 , and by estimates from a censored accelerated failure time (AFT) model; and multiple imputation (MI) with or without truncation based on LOD.

METHODS

In extensive simulation studies with high-dimensional and highly correlated exposures and a continuous health outcome, we examined the performance of each LOD approach on three mixture analysis methods: elastic net regression, weighted quantile sum regression (WQS) and Bayesian kernel machine regression (BKMR). We further analyzed data from the National Health and Nutrition Examination Survey (NHANES) on how persistent organic pollutants (POPs) influenced leukocyte telomere length (LTL).

RESULTS

Complete case analysis was inefficient and could result in severe bias for some mixture methods. Imputation by LOD/ 2 showed unstable performance across mixture methods. Conventional MI was associated with consistent mild biases, which can be reduced by using a truncated distribution for imputation. Estimating censored values by AFT models had a minimal impact on the results. In the NHANES analysis, imputation by LOD/ 2 , truncated MI and censored AFT models performed similarly, with a positive overall effect of POPs on LTL while PCB126, PCB169 and furan 2,3,4,7,8-pncdf being the most important exposures.

CONCLUSIONS

Our study favored using truncated MI and censored AFT models to accommodate values below LOD for the stability of downstream mixture analysis.

Using the fish plasma model to evaluate potential effects of pharmaceuticals in effluent from a large urban wastewater treatment plant

Several pharmaceuticals and personal care products (PPCPs) were evaluated using the fish plasma model (FPM) for juvenile Chinook salmon exposed to effluent from a large urban wastewater treatment plant. The FPM compares fish plasma concentrations to therapeutic values determined in human plasma as an indication of potential adverse effects. We used human Cmax values, which are the maximum plasma concentration for a minimum therapeutic dose. Observed and predicted plasma concentrations from juvenile Chinook salmon exposed to a dilution series of whole wastewater effluent were compared to 1%Cmax values to determine Response Ratios (RR) ([plasma]/1%Cmax) for assessment of possible adverse effects. Several PPCPs were found to approach or exceed an RR of 1, indicating potential effects in fish. We also predicted plasma concentrations from measured water concentrations and determined that several of the values were close to or below the analytical reporting limit (RL) indicating potential plasma concentrations for a large number of PPCPs that were below detection. Additionally, the 1%Cmax was less than the RL for several analytes, which could impede predictions of possible effect concentrations. A comparison of observed and predicted plasma concentrations found that observed values were frequently much higher than values predicted with water concentrations, especially for low log10Dow compounds. The observed versus predicted values using the human volume of distribution (Vd), were generally much closer in agreement. These data appear to support the selection of whole-body concentrations to predict plasma values, which relies more on estimating simple partitioning within the fish instead of uptake via water. Overall, these observations highlight the frequently underestimated predicted plasma concentrations and potential to cause adverse effects in fish. Using measured plasma concentrations or predicted values from whole-body concentrations along with improved prediction models and reductions in analytical detection limits will foster more accurate risk assessments of pharmaceutical exposure for fish.

How many microplastics do you need to (sub)sample?

Analysis of microplastics in the environment requires polymer characterization as a confirmation step for suspected microplastic particles found in a sample. Material characterization is costly and can take a long time per particle. When microplastic particle counts are high, many researchers cannot characterize every particle in their sample due to time or monetary constraints. Moreover, characterizing every particle in samples with high plastic particle counts is unnecessary for describing the sample properties. We propose an a priori approach to determine the number of suspected microplastic particles in a sample that should be randomly subsampled for characterization to accurately assess the polymer distribution in the environmental sample. The proposed equation is well-founded in statistics literature and was validated using published microplastic data and simulations for typical microplastic subsampling routines. We report values from the whole equation but also derive a simple way to calculate the necessary particle count for samples or subsamples by taking the error to the power of negative two. Assuming an error of 0.05 (5 %) with a confidence interval of 95 %, an unknown expected proportion, and a sample with many particles (> 100k), the minimum number of particles in a subsample should be 386 particles to accurately characterize the polymer distribution of the sample, given the particles are randomly characterized from the full population of suspected particles. Extending this equation to simultaneously estimate polymer, color, size, and morphology distributions reveals more particles (620) would be needed in the subsample to achieve the same high absolute error threshold for all properties. The above proposal for minimum subsample size also applies to the minimum count that should be present in samples to accurately characterize particle type presence and diversity in a given environmental compartment.

Configuration of super-fast Cu2+-responsive chemosensor by attaching diaminomaleonitrile to BODIPY scaffold for high-contrast fluorescence imaging of living cells

A highly fluorescent Cu2+-responsive sensor, 2-amino-3-(BODIPYmethyleneamino)maleonitrile (BD) was constructed by attaching diaminomaleonitrile to a BODIPY scaffold. Cu2+ can be selectively recognized on a 2-s time-scale by way of fluorescence emission. When Cu2+ and BD coexist in solution, typical BODIPY emission was observed and the emission intensity could be increased to 334 times that of the blank dye solution. The mechanism of fluorescence increase is based on the generation of highly fluorescent species by Cu2+-triggered oxidative cyclization of the attached diaminomaleonitrile. The absolute fluorescence quantum yield (AFQY) of the cyclization product is 98% determined by integrating sphere. The highly emissive character can be attributed to the imidazole ring and dicarbonitrile on the BODIPY scaffold. It surpasses the meso-phenyl substituted analogue in AFQY and detection limits (DL). The specific Cu2+ recognition behavior was also validated in Hela cells with high-contrast fluorescence images.

Review of scientific literature on available methods of assessing organochlorine pesticides in the environment

Organochlorine pesticides (OCPs) are persistent organic pollutants (POPs) widely used in agriculture and industry, causing serious health and ecological consequences upon exposure. This review offers a thorough overview of OCPs analysis emphasizing the necessity of ongoing work to enhance the identification and monitoring of these POPs in environmental and human samples. The benefits and drawbacks of the various OCPs analysis techniques including gas chromatography-mass spectrometry (GC-MS), gas chromatography-electron capture detector (GC-ECD), and liquid chromatography-mass spectrometry (LC-MS) are discussed. Challenges associated with validation and optimization criteria, including accuracy, precision, limit of detection (LOD), and limit of quantitation (LOQ), must be met for a method to be regarded as accurate and reliable. Suitable quality control measures, such as method blanks and procedural blanks, are emphasized. The LOD and LOQ are critical quality control measure for efficient quantification of these compounds, and researchers have explored various techniques for their calculation. Matrix interference, solubility, volatility, and partition coefficient influence OCPs occurrences and are discussed in this review. Validation experiments, as stated by European Commission in document SANTE/11813/2017, showed that the acceptance criteria for method validation of OCP analytes include ≤20 % for high precision, and 70-120 % for recovery. This may ultimately be vital for determining the human health risk effects of exposure to OCP and for formulating sensible environmental and public health regulations.

Easy and accessible way to calibrate a fluorescence microscope and to create a microplastic identification key

We present here a technique for setting up detection limits on any fluorescent microscope in conjunction with the fluorophore Nile Red for microplastic identification. Our method also describes a rigorous morphology-specific identification key for microplastics to reduce subjectivity between researchers. The detection limits were established for nine common polymer types and five natural substrates which could result in false-positive signals when using Nile Red for microplastic identification. This method was then applied to real freshwater samples and identified particles were validated with micro-FTIR or Raman spectroscopy. This approach may reduce subjectivity in microplastic identification and counting and enhances transparency, repeatability and harmonization within microplastic research community.•Instructions for calibration of detection limits for microplastics on fluorescence microscope systems described•Microplastic identification key developed and tested to reduce false positive detection•Lower subjectivity for microplastic identification obtained using the detection limits & identification key.

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) detection limits for blood on fabric: Orientation and coating uniformity effects

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to analyze four types of forensically relevant fabrics coated with varying dilutions of blood. The blood was applied in two manners, dip coating with a smooth and uniform layer and drip coating with droplets from pipettes. Spectra of neat and dip coated fabrics were acquired using controlled orientations, and these were compared to spectra collected on samples with random orientations. The improved reproducibility seen in visual inspection of the spectra is confirmed by principal component and linear discriminant projections of the spectra, as well as by statistical hypothesis testing. Principal component regression (PCR), using the regions of the IR spectra associated with the amide A/B, I, II, and III vibrational bands (3500-2800, 1650, 1540, and 1350 cm^-1), was employed on the more uniform dip coated spectra to estimate limits of detection for blood on two of the four fabrics - acrylic and nylon. These results demonstrate that detection limits for blood on fabrics can be decreased significantly by controlling for the orientation and face of the fabric samples while collecting spectra. Limits of detection for acrylic and nylon were found to be 196 × and 227 × diluted blood, respectively.

Chemical footprints of harmattan dust and traffic corridor particulates monitored at two environmentally distinct geopolitical zones in Nigeria

Harmattan dust and traffic-related pollution have been a serious environmental concern in the West African sub-region. In order to further contribute to the understanding of ambient levels of atmospheric pollution and chemical composition in the region, this study monitored harmattan dust and traffic-related particulate matter at four locations across southwestern (Ile-Ife) and north-western (Zaria) geo-political zones of Nigeria. The collected samples were characterized for their chemical composition using Energy-Dispersive X-Ray Fluorescence spectrometer equipped with an optimized secondary target x-ray excitation conditions (Al, CaF₂, Fe, Ge, Zr, Mo, Ag, Al₂O₃). The objectives are to assess spatio-temporal mass concentrations, chemical footprints, enrichment factors, elemental correlations, and ratios at all locations. The X-ray analytical method was validated with a NIST SRM 2783 air particulate standard, and detection limits for each chemical specie were determined. Validation results showed good reproducibility of the certified reference material with relative standard deviations of the elements much lower by about 1-13% than the corresponding reference values. Mass concentrations reached up to 2200 μgm^-3 in the north and 1500 μgm^-3 in the south. The range of mean concentration of crustal marker elements were Al (5-27 μgm^-3), Si (5-856 μgm^-3), Ca (0.78-13 μgm^-3), and Fe (2-13 μgm^-3), and were most abundant during the harmattan particularly in the southwestern region. Highest mean concentration values of 380, 810, and 420 ngm^-3 were recorded for Cr, Cu, and Pb respectively at the traffic corridor which also recorded the highest enrichment factors. Black carbon and elemental concentrations contributed between 1 to 54% and 9 to 94% across the locations respectively. Backward trajectories of atmospheric flow over the locations showed two dominant sources; dust laden source from the Sahara desert and maritime flow over the Gulf of Guinea. This study found that chemical footprints (Al, Si, K, Ca, Ti, and Fe) of harmattan-related dust were more correlated (r² between 0.88 and 0.99) than those attributed to dust re-suspension at the traffic location.

Sensitivity limits of biosensors used for the detection of metals in drinking water

Even when present in very low concentrations, certain metal ions can have significant health impacts depending on their concentration when present in drinking water. In an effort to detect and identify trace amounts of such metals, environmental monitoring has created a demand for new and improved methods that have ever-increasing sensitivities and selectivity. This paper reviews the sensitivities of over 100 recently published biosensors using various analytical techniques such as fluorescence, voltammetry, inductively coupled plasma techniques, spectrophotometry and visual colorimetric detection that display selectivity for copper, cadmium, lead, mercury and/or aluminium in aqueous solutions.

Quantitative framework for validating two methodologies that are used to enumerate viable organisms for type approval of ballast water management systems

The Stain-Motility (S-M) and Most Probable Number (MPN) methodologies may be used for enumerating viable organisms≥ 10 μm to <50 μm in discharge water for administrative type approval of a ballast water management system (BWMS). Only MPN is suitable for assessing the efficacy of disinfection using UV radiation - a chemical-free treatment technology - but the U.S. Coast Guard has not approved an MPN-based approach as an alternate to their required S-M method. Approval depends on a demonstration of equivalence, but a framework for comparative validation is not well established. The purpose of this study is to provide such a framework. It is shown that the requirement for 5 consecutive successful results in BWMS type approval testing fundamentally changes the relationship between a method's precision and its effectiveness in ensuring compliance with regulations. False approval due to random measurement error is effectively eliminated for both methods because it requires 5 consecutive underestimates, and false rejection due to a single erroneously high measurement is more likely for the method with wider confidence limits, imposing an extra margin of safety for MPN. These results reverse conventional interpretations of efficacy based on method precision alone. Sources of systematic error (bias) are reviewed and methods for estimating the errors are described. If combined bias is positive (overestimation), either method would yield type-approval results fully compliant with regulations. Subject to similar negative bias, the less precise method (generally, MPN) would be more protective of the environment. An illustrative analysis of 64 paired MPN and S-M counts from BWMS trials indicates that neither method was significantly biased relative to the other. Considered in the framework for method validation described here, available evidence strengthens arguments that in a BWMS type approval testing regime, the efficacy of the MPN method is equivalent to that of Stain-Motility.

Hallmarks in prostate cancer imaging with Ga68-PSMA-11-PET/CT with reference to detection limits and quantitative properties

BACKGROUND

Gallium-68-labeled prostate-specific antigen positron emission tomography/computed tomography imaging (Ga68-PSMA-11-PET/CT) has emerged as a potential gold standard for prostate cancer (PCa) diagnosis. However, the imaging limitations of this technique at the early state of PCa recurrence/metastatic spread are still not well characterized. The aim of this study was to determine the quantitative properties and the fundamental imaging limits of Ga68-PSMA-11-PET/CT in localizing small PCa cell deposits.

METHODS

The human PCa LNCaP cells (PSMA expressing) were grown and collected as single cell suspension or as 3D-spheroids at different cell numbers and incubated with Ga68-PSMA-11. Thereafter, human HCT116 cells (PSMA negative) were added to a total cell number of 2 × 10⁵ cells per tube. The tubes were then pelleted and the supernatant aspirated. A whole-body PET/CT scanner with a clinical routine protocol was used for imaging the pellets inside of a cylindrical water phantom with increasing amounts of background activity. The actual activity bound to the cells was also measured in an automatic gamma counter. Imaging detection limits and activity recovery coefficients as a function of LNCaP cell number were obtained. The effect of Ga68-PSMA-11 mass concentration on cell binding was also investigated in samples of LnCaP cells incubated with increasing concentrations of radioligand.

RESULTS

A total of 1 × 10⁴ LNCaP cells mixed in a pellet of 2 × 10⁵ cells were required to reach a 50% detection probability with Ga68-PSMA-11-PET/CT without background. With a background level of 1 kBq/ml, between 4 × 10⁵ and 1 × 10⁶ cells are required. The radioligand equilibrium dissociation constant was 27.05 nM, indicating high binding affinity. Hence, the specific activity of the radioligand has a profound effect on image quantification.

CONCLUSIONS

Ga68-PSMA-11-PET detects a small number of LNCaP cells even when they are mixed in a population of non-PSMA expressing cells and in the presence of background. The obtained image detection limits and characteristic quantification properties of Ga68-PSMA-11-PET/CT are essential hallmarks for the individualization of patient management. The use of the standardized uptake value for Ga68-PSMA-11-PET/CT image quantification should be precluded.

Detection limit of Mycobacterium chimaera in water samples for monitoring medical device safety: insights from a pilot experimental series

BACKGROUND

A growing number of Mycobacterium chimaera infections after cardiosurgery have been reported by several countries. These potentially fatal infections were traced back to contaminated heater-cooler devices (HCDs), which use water as a heat transfer medium. Aerosolization of water contaminated with M. chimaera from HCDs enables airborne transmission to patients undergoing open chest surgery. Infection control teams test HCD water samples for mycobacterial growth to guide preventive measures. The detection limit of M. chimaera in water samples, however, has not previously been investigated.

AIM

To determine the detection limit of M. chimaera in water samples using laboratory-based serial dilution tests.

METHODS

An M. chimaera strain representative of the international cardiosurgery-associated M. chimaera outbreak was used to generate a logarithmic dilution series. Two different water volumes, 50 and 1000mL, were inoculated, and, after identical processing (centrifugation, decantation, and decontamination), seeded on mycobacteria growth indicator tube (MGIT) and Middlebrook 7H11 solid media.

FINDINGS

MGIT consistently showed a lower detection limit than 7H11 solid media, corresponding to a detection limit of ≥1.44 × 10⁴cfu/mL for 50mL and ≥2.4cfu/mL for 1000mL water samples. Solid media failed to detect M. chimaera in 50mL water samples.

CONCLUSION

Depending on water volume and culture method, major differences exist in the detection limit of M. chimaera. In terms of sensitivity, 1000mL water samples in MGIT media performed best. Our results have important implications for infection prevention and control strategies in mitigation of the M. chimaera outbreak and healthcare water safety in general.

Surface plasmon resonance based spectrophotometric determination of medicinally important thiol compounds using unmodified silver nanoparticles

The determination of thiol based biological molecules and drugs, such as cysteine (Cys) (I), α-lipoic acid (II), and sodium 2-sulfanylethane sulphonate (Mesna (III)) in human plasma are becoming progressively more important due to the growing body of knowledge about their essential role in numerous biological pathways. Herein we demonstrate a sensitive colorimetric sensor for the determination of medicinally important thiol drugs based on aggregation of the citrate capped silver nanoparticles (Ag NPs). This approach exploited the high affinity of thiols towards the Ag NPs surface which could tempt replacement of the citrate shell by the thiolate shell of target molecules, resulting in aggregation of the NPs through intermolecular electrostatic interaction or hydrogen-bonding. Because of aggregation, the plasmon band at around 400nm decreases gradually, along with the appearance of a new band connoting a red shift. The calibration curves are derived from the intensity ratios of A₅₃₀/A₄₀₀, which display a linear relation in the range of 1μM-150μM, 5μM-200μM and 10μM-130μM, respectively. The obtained detection limits (3σ) were found to be 1.5μM, 5.6μM and 10.2μM for compound I-III, respectively. The proposed method has been successfully applied for the detection of thiol compounds in real samples.

Practical implementation of ISO 11929: 2010

The determination of characteristic limits in the measurement of radioactive materials is an important aspect of low-level measurements and the underlying principles are well understood by scientists working on such measurements. ISO 11929:2010 provides a rigorous basis for the systematic calculation of detection limits, but may be difficult to interpret for the routine user. This paper attempts to simplify some of the more arcane aspects of this standard.

Application of EELS and EFTEM to the life sciences enabled by the contributions of Ondrej Krivanek

The pioneering contributions of Ondrej Krivanek to the development of electron energy loss spectrometers, energy filters, and detectors for transmission and scanning transmission electron microscopes have provided researchers with indispensible tools across a wide range of disciplines in the physical sciences, ranging from condensed matter physics, to chemistry, mineralogy, materials science, and nanotechnology. In addition, the same instrumentation has extended its reach into the life sciences, and it is this aspect of Ondrej Krivanek's influential contributions that will be surveyed here, together with some personal recollections. Traditionally, electron microscopy has given a purely morphological view of the biological structures that compose cells and tissues. However, the availability of high-performance electron energy loss spectrometers and energy filters offers complementary information about the elemental and chemical composition at the subcellular scale. Such information has proven to be valuable for applications in cell and structural biology, microbiology, histology, pathology, and more generally in the biomedical sciences.

An improved matrix separation method for characterization of ultrapure germanium (8N)

An improved matrix separation method has been described to characterize ultrapure germanium of 8N (99.999999%) purity. In this method, temperature of the reaction vessel in which in-situ generated chlorine gas reacts with germanium solid material directly is optimized to quantitatively remove Ge matrix from all its impurities. Optimized reaction temperature has been found to be 230±5°C. Recovery studies on more than 60 elements have been carried out at the optimized temperature. Recoveries of all the analytes except As, Se, Sn, Hg, Tl are found to be quantitative. The method has been examined for various amounts of Ge material and found to be suitable even for 10g of Ge sample and provides low parts per billion and trillion levels of process blanks. Determination of concentrations of impurities has been done by inductively coupled plasma quadrupole mass spectrometer (ICP-QMS) and high resolution continuum source graphite furnace atomic absorption spectrometer (HR-CS-GFAAS). In the absence of certified reference materials for ultrapure germanium, accuracy of the proposed method is established by spike recovery tests. Precision of this method is found to vary from 7% to 50% for concentrations between 4 and 0.004ngg(-1). Limits of detection (LOD) for the target analytes are found to be between 6 and 0.011ngmL(-1) or 1.8-0.003ngg(-1) for the proposed procedure. The method has been successfully applied for that characterization of ultrapure germanium material of 8N purity.

Detection limits should be a thing of the past in gamma-ray spectrometry in general as well as in neutron activation analysis

In gamma-ray spectrometry with high-resolution detectors, full-energy peaks are often to be detected by a peak-search algorithm, with a threshold for detection. Detection limits can be derived from this. Detection limits are often computed along with measured activities or concentrations. When an analyte is not detected, the detection limit remains as the only available information. This leads to inhomogeneous datasets that are difficult or impossible to process correctly without introducing artefacts or biases. Here, it is proposed to determine peak areas at predetermined energies. An unbiased result with its uncertainty always results, obviating the “detection limit” concept.

Evaluation of in vivo and in vitro dose detection limits for different radionuclides and measurement techniques

Personal monitoring programs for workers handling radioactive materials are influenced by numerous factors as the measurements of radioactivity in tissues or/and in excreta can be carried out using different techniques. This paper summaries the basic procedures needed for accurate and fast measurement of different radionuclides like (235)U, (234)U, (238)U, (226)Ra, (210)Po, (131)I, (99m)Tc, (134)Cs, (137)Cs, (57)Co, (58)Co, and (60)Co. Overviews of in vitro and in vivo monitoring methods are provided as well as methods used to calculate detection limits and internal radiation dose. For the radionuclides of interest, in vivo and in vitro detection limits were converted into committed effective doses to evaluate the applicability and limitations of the systems used at the laboratory. The results proved that the systems' sensitivity is suitable for use in routine monitoring of workers subject to risk of internal exposure from such radionuclides. Consequently, monitoring programs suggested by the Syrian internal dosimetry laboratory are suitable to detect committed effective doses even below 1mSv in most cases.

Modified matrix volatilization setup for characterization of high purity germanium

Modified matrix volatilization (MV) method has been described to characterize high purity germanium material of 7 N (99.99999%) purity. Transport of both, the chlorine gas generated in-situ in this method and the argon gas (carrier) is fine controlled by means of a mass flow controller. This enabled both uniform reaction of chlorine gas with the germanium matrix and smooth removal of germanium matrix as its chloride. This resulted in improvement in the reproducibility of the analytical results. The use of quartz reaction vessel has lead to the reduction in the process blank levels. The combined effect of these modifications in the MV setup has resulted in very consistent and low process blanks and hence improved detection limits of this method. Applicability of the method has been expanded to rare earth elements and other elements after examining their recoveries. The quantification is done by using inductively coupled plasma quadrupole mass spectrometer (ICP-QMS) and continuum source graphite furnace atomic absorption spectrometry (CS-GFAAS). In the absence of certified reference materials for high pure germanium, the accuracy of the method is established by spike recovery tests. The precision of the method has been found to vary from 1 to 30% for concentrations between 1 and 30 ng g(-1). The limits of detection (LOD) for the target analytes are found to be between 18 and 0.033 ng g(-1).

Application of chitosan and its N-heterocyclic derivatives for preconcentration of noble metal ions and their determination using atomic absorption spectrometry

Chitosan and its N-heterocyclic derivatives N-2-(2-pyridyl)ethylchitosan (2-PEC), N-2-(4-pyridyl) ethylchitosan (4-PEC), and N-(5-methyl-4-imidazolyl) methylchitosan (IMC) have been applied in group preconcentration of gold, platinum, and palladium for subsequent determination by atomic absorption spectroscopy (AAS) in solutions with high background concentrations of iron and sodium ions. It has been shown that the sorption mechanism, which was elucidated by XPS, significantly influences the sorption capacity of materials, the efficiency of metal ions elution after preconcentration, and, as a result, the accuracy of metal determination by AAS. We have shown that native chitosan was not suitable for preconcentration of Au(III), if the elution step was used as a part of the analysis scheme. The group preconcentration of Au(III), Pd(II), and Pt(IV) with subsequent quantitative elution using 0.1M HCl/1M thiourea solution was possible only on IMC and 4-PEC. Application of IMC for analysis of the national standard quartz ore sample proved that gold could be accurately determined after preconcentration/elution with the recovery above 80%.