Showing papers on "Calibration curve published in 2018"

Electrochemical determination of trace mercury in water sample using EDTA-CPE modified electrode

Abstract: The analytical performance of the EDTA-CPE sensor that is designed for the detection of metals was studied. EDTA plays an important role in the complexation of metal ions because of their good and high response to these ions. The modified electrode EDTA-CPE was used for the voltammetric determination of mercury ions (square wave voltammetry). Sodium chloride solution 0.3 mol− 1 was used as the support electrolyte. Using optimal parameters, calibration graph is linear (varying from 5 to 35 ∗ 10− 4 mol/l) for 5 min of pre-concentration time with the detection limit of 16, 6 × 10− 9 mol L− 1. These results indicate that the developed method is simple, with high stability and sensitivity for the determination of low concentration of Hg (II).

Optical pressure nano-sensor based on lanthanide doped SrB2O4:Sm2+ luminescence – Novel high-pressure nanomanometer

Abstract: A new, non-contact optical sensor of pressure based on the SrB2O4:Sm2+ nanoparticles has been successfully synthesized via a simple and low-cost Pechini method. The obtained nanomaterial was thoroughly characterized using powder X-ray diffraction, transmission electron microscopy, and Raman and luminescence spectroscopies, including high-pressure and high-temperature extreme conditions measurements. Compression of the material leads to a significant red-shift of the intraconfigurational 5D0→7FJ (J = 0−3) emission bands, and to an increase of the Raman mode energies, whose shift rates as a function of pressure were determined. The shift of the extremely sharp and the most intense 5D0→7F0 emission band (Δλ ≈0.24 nm/GPa; Γ (FHWM) ≈0.15 nm) has been used for the determination of the pressure calibration curve. The high-temperature luminescence measurements revealed a desirably weak temperature dependence of the peak spectral position, which was included in the determined pressure calibration curve. The SrB2O4:Sm2+ nanomaterial studied exhibits a negligible temperature-induced emission lines broadening, and relative low thermal quenching of luminescence. The use of such Sm2+-based contactless pressure nano-sensor allows a very accurate pressure sensing ( ± 0.01 GPa), in the sub-micro sized regions, both at low and high temperature conditions.

TDR-Based Measurements of Water Content in Construction Materials for In-the-Field Use and Calibration

Abstract: In this paper, a time domain reflectometry (TDR)-based system for measuring water content of raw construction materials is presented. The proposed system relies on the fact that the presence of water leads to an increase of the dielectric permittivity of materials; therefore, from TDR-based permittivity measurements, it is possible to infer the water content value. In practical applications, the proposed system could be used for assessing the intrinsic water content of construction materials before they are poured into the concrete mixture. Knowing the intrinsic water content of the raw materials, in fact, would allow to evaluate the optimal amount of water that should be added to the mixture in order to achieve the desired water-to-cement ratio. This, in turn, would permit to fine-tune and control the mechanical properties of the final concrete structures. For assessing the feasibility of using the proposed system for the intended purpose, water content measurements were carried out on three construction materials, namely, sand, gray cement, and white portland cement. For each of these materials, a calibration curve relating water content to the apparent dielectric permittivity was derived; additionally, through repeated measurements, also a confidence interval was associated with the calibration curves.

Fabrication of a Ga3+ sensor probe based on methoxybenzylidenebenzenesulfonohydrazide (MBBSH) by an electrochemical approach

Abstract: A series of (E)-N′-methoxybenzylidene-benzenesulfonohydrazide (MBBSH) compounds were synthesized using a condensation method from the derivatives of methoxybenzaldehyde and benzenesulfonylhydrazine in good (crystallized) yield in an alcoholic medium. The as-prepared MBBSH compounds were well-characterized using various spectroscopic techniques, such as 1H-NMR, 13C-NMR, FTIR, and UV-Vis. The structure of the MBBSH molecules was confirmed using the single crystal X-ray diffraction (SCXRD) method and used for the selective detection of the gallium (Ga3+) ion by the I–V technique. A thin-layer of MBBSH was prepared by deposition onto a smooth glassy carbon electrode (GCE) with Nafion (Nf) as a coating binder in order to modify the sensitive and selective Ga3+ sensor probe. The sensitivity, the limit of detection (LOD), and the limit of quantification (LOQ) of the modified electrode towards Ga3+ were calculated from the calibration curve as 949.37 pA μM−1 cm−2, ≈84.0 ± 0.2 pM, and 280.0 ± 0.5 mM, respectively. The prospective MBBSH/GCE sensor was applied for the determination of Ga3+ in spiked biological (human, mouse, and rabbit serum) and environmental (industrial effluent, red sea water, well-water, and tap water) samples and acceptable and reasonable results were obtained.

Use of a Smartphone as a Colorimetric Analyzer in Paper-based Devices for Sensitive and Selective Determination of Mercury in Water Samples

Abstract: A smartphone application, called CAnal, was developed as a colorimetric analyzer in paper-based devices for sensitive and selective determination of mercury(II) in water samples. Measurement on the double layer of a microfluidic paper-based analytical device (μPAD) fabricated by alkyl ketene dimer (AKD)-inkjet printing technique with special design doped with unmodified silver nanoparticles (AgNPs) onto the detection zones was performed by monitoring the gray intensity in the blue channel of AgNPs, which disintegrated when exposed to mercury(II) on μPAD. Under the optimized conditions, the developed approach showed high sensitivity, low limit of detection (0.003 mg L-1, 3SD blank/slope of the calibration curve), small sample volume uptake (two times of 2 μL), and short analysis time. The linearity range of this technique ranged from 0.01 to 10 mg L-1 (r2 = 0.993). Furthermore, practical analysis of various water samples was also demonstrated to have acceptable performance that was in agreement with the data from cold vapor atomic absorption spectrophotometry (CV-AAS), a conventional method. The proposed technique allows for a rapid, simple (instant report of the final mercury(II) concentration in water samples via smartphone display), sensitive, selective, and on-site analysis with high sample throughput (48 samples h-1, n = 3) of trace mercury(II) in water samples, which is suitable for end users who are unskilled in analyzing mercury(II) in water samples.

A comparison of semi-quantitative methods suitable for establishing volatile profiles

Abstract: Full scent profiles emitted by living tissues can be screened by using total ion chromatograms generated in full scan mode and gas chromatography–mass spectrometry technique using Headspace Sorptive Extraction. This allows the identification of specific compounds and their absolute quantification or relative abundance. Quantifications ideally should be based on calibration curves using standards for each compound. However, the unpredictable composition of Volatile Organic Compounds (VOCs) and lack of standards make this approach difficult. Researchers studying scent profiles therefore concentrate on identifying specific scent footprints i.e. relative abundance rather than absolute quantities. We compared several semi-quantitative methods: external calibration curves generated in the sampling system and by liquid addition of standards to stir bars, total integrated peak area per fresh weight (FW), normalized peak area per FW, semi-quantification based on internal standard abundance, semi-quantification based on the nearest n-alkane and percentage of emission. Furthermore, we explored the usage of nearest components and single calibrators for semi-quantifications. Any of the semi-quantification methods based on a standard produced similar or even identical results compared to quantification by a true-standard for a compound, except for the method based on standard addition. Each method beholds advantages and disadvantages regarding level of accuracy, experimental variability, acceptance and retrieved quantities. Our data shows that, except for the method of standard addition to the biological sample, the rest of the semi-quantification methods studied give highly similar statistical results. Any of the methodologies presented here can therefore be considered as valid for scent profiling. Regarding relative proportions of VOCs, the generation of calibration curves for each compound analysed is not necessary.

Metal Contamination Distribution Detection in High-Voltage Transmission Line Insulators by Laser-induced Breakdown Spectroscopy (LIBS).

Abstract: The fast detection of classical contaminants and their distribution on high-voltage transmission line insulators is essential for ensuring the safe operation of the power grid. The analysis of existing insulator contamination has traditionally relied on taking samples during a power cut, taking the samples back to the lab and then testing them with elemental analysis equipment, especially for sugars, bird droppings, and heavy metal particulates, which cannot be analysed by the equivalent salt deposit density (ESDD) or non-soluble deposit density (NSDD) methods. In this study, a novel method called laser-induced breakdown spectroscopy (LIBS) offering the advantages of no sample preparation, being nearly nondestructive and having a fast speed was applied for the analysis of metal contamination. Several LIBS parameters (laser energy and delay time) were optimized to obtain better resolution of the spectral data. The limit of detection (LOD) of the observed elements was obtained using a calibration curve. Compared to calibration curves, multivariate analysis methods including principal component analysis (PCA), k-means and partial least squares regression (PLSR) showed their superiority in analyzing metal contamination in insulators. Then, the elemental distribution of natural pollution was predicted using LIBS to fully capture information about the bulk elements (Na, Ni, Cu, Mn, Ca, etc.) of entire areas with PLSR. The results showed that LIBS could be a promising method for accurate direct online quantification of metal contamination in insulators.

Optofluidic differential colorimetry for rapid nitrite determination

Abstract: Nitrite detection plays a very important role in environmental monitoring and for industrial purposes. The commonly used colorimetric analysis requires the measurement of a system's calibration curve by asynchronously preparing and detecting a dozen standard samples, leading to time-consuming, slow and cumbersome procedures. Here, we present a differential colorimetry method that determines the nitrite level based on the paired chromaticity gradient, formed by coupling the colour reaction into the microfluidic network. The two gradients reshape each other and contain enough information for the quantitative analysis of the sample being tested, without the need for a calibration curve. The independence of the two gradients of the absorbance change caused by the detecting system and water quality results in a high stability and anti-interference performance, with the assistance of its self-correcting ability. This differential colorimetry method requires little time and energy consumption as only one sample is needed. Standard nitrite solutions of 0.50 mM and 0.33 mM have been determined with an error of 1.16% and 0.50%, respectively. These measurements are advantageous in terms of greater stability by up to 10 times and accuracy by 6 times, compared with the calibration curve approaches. It is foreseeable that this differential colorimetry method will find a wide range of applications in the field of chemical detection.

An LIBS quantitative analysis method for alloy steel at high temperature based on transfer learning

Abstract: The analysis accuracy of laser-induced breakdown spectroscopy (LIBS) in high temperature applications will decrease when certified standard samples used for building calibration curves are insufficient. A novel LIBS quantitative method based on transfer learning is proposed, in which information on the spectra at room temperature is transferred to the spectra at high temperature in order to assist in building a better regression model. An iterative weight adjusting scheme is used for different samples in model training and the concept of ensemble learning is involved when the results of testing samples are predicted. Experiments on certified alloy steel standard samples were conducted to analyze Cr concentrations. The calibration dataset consisted of 15 standard samples at room temperature and 4 standard samples at high temperature. Another 3 samples at high temperature were used for testing. The results showed that the average absolute and relative errors of 3 testing samples were reduced by 1.8% and 20.58%, respectively. The proposed method provides a feasible way for LIBS analysis of samples at high temperature with lower cost and enhances the potentiality of LIBS in online industrial measurement in high temperature production processes, such as iron and steel smelting.

Paper-based headspace extraction combined with digital image analysis for trace determination of cyanide in water samples

Abstract: A simple, cost-effective, instrumental free and user-friendly analytical method based on the combination of paper-based headspace extraction and digital scanning image analysis was developed for the sensitive detection of cyanide in water and wastewater samples. The method relied on chloramine-T/pyridine-barbituric acid reaction, which is a well-known process for measuring cyanide in standard analytical methods. During the reaction, cyanide was converted to cyanogen chloride, which was extracted and collected on a paper impregnated with pyridine-barbituric acid. The color appeared on the paper was used to quantify the analyte using a scanning-assisted image processing software. Experimental parameters affecting the sensitivity of the method such as stirring rate, paper size, chloramine-T and barbituric acid concentration, equilibrium and extraction time were studied. Under optimal conditions, the calibration curve was linear in the range of 3.0–100 μg L−1 with the detection limit of 0.7 μg L−1. The intra- and inter- day relative standard deviations of the method at 10 and 70 μg L−1 concentration levels were less than 6%. Analyte recovery in real samples was in the range of 69–111%.

Thread/paper‐ and paper‐based microfluidic devices for glucose assays employing artificial neural networks

Abstract: This paper describes the fabrication of and data collection from two microfluidic devices: a microfluidic thread/paper based analytical device (μTPAD) and 3D microfluidic paper-based analytical device (μPAD). Flowing solutions of glucose oxidase (GOx), horseradish peroxidase (HRP), and potassium iodide (KI), through each device, on contact with glucose, generated a calibration curve for each platform. The resultant yellow-brown color from the reaction indicates oxidation of iodide to iodine. The devices were dried, scanned, and analyzed yielding a correlation between yellow intensity and glucose concentration. A similar procedure, using an unknown concentration of glucose in artificial urine, is conducted and compared to the calibration curve to obtain the unknown value. Studies to quantify glucose in artificial urine showed good correlation between the theoretical and actual concentrations, as percent differences were ≤13.0%. An ANN was trained on the four-channel CMYK color data from 54 μTPAD and 160 μPAD analysis sites and Pearson correlation coefficients of R = 0.96491 and 0.9739, respectively, were obtained. The ANN was able to correctly classify 94.4% (51 of 54 samples) and 91.2% (146 of 160 samples) of the μTPAD and μPAD analysis sites, respectively. The development of this technology combined with ANN should further facilitate the use of these platforms for colorimetric analysis of other analytes.

CF-LIBS analysis of frozen aqueous solution samples by using a standard internal reference and correcting the self-absorption effect

Abstract: The aim of this paper is to present an innovative procedure to determine the composition of a liquid sample using Laser Induced Breakdown Spectroscopy without calibration curves, or the so-called CF-LIBS (Calibration Free-Laser Induced Breakdown Spectroscopy). Our results of CF-LIBS for the elemental composition were compared with the sample standard concentration determined by Microwave Plasma-Atomic Emission Spectroscopy (MP-AES). In this work, a controlled amount of an internal standard, an extra chemical element, is placed into a liquid sample that is afterward frozen to produce a solid target. The plasma of the target material is produced at atmospheric pressure by focusing a Nd:YAG laser on the frozen target surface. Time-resolved emission spectra are acquired and used for quantitative analysis. An extraordinary increase in the accuracy of elemental concentration is achieved when the spectroscopic data are corrected through an analytical elimination of the self-absorption effect. The corrected data plotted in a Boltzmann diagram show that CF-LIBS can provide good analytical sensitivity and accurate and reliable elemental concentrations when the local thermodynamic equilibrium conditions are experimentally fulfilled and the self-absorption effect on the line intensities is corrected. In the case analyzed here, the concentration results obtained by the improved CF-LIBS procedure give relative deviations of 3.3% maximum when compared with those of MP-AES.

Potential use of surface-assisted LIBS for determination of strontium in wines.

Abstract: Laser-induced breakdown spectroscopy (LIBS) is a well-established technique for elemental analysis and has been widely used for qualitative and quantitative analysis of different solid samples. LIBS is also well-known for not requiring sample preparation, but the analysis of liquids is actually a great challenge. In the present work, a novel approach of elemental analysis of liquids with an organic matrix has been performed, to the best of our knowledge, making a liquid-to-solid matrix conversion by drying wine samples on aluminum and silicon wafers, which have demonstrated an increase in the analytical performance of LIBS. A red wine from Slovakia (not blended with any other variety or wine from other regions or adulterants) was prepared according to the procedure consisting of drying 2 ml of wine dropped on a solid wafer having a flat surface area of about 25 cm2. Surface-assisted LIBS in combination with the conversion of liquid into solid avoids the difficulties and limitations of working with liquid samples by LIBS, improving the limit of detection (LOD), repeatability, and sensitivity. The quantification of strontium (Sr) has been realized by addition of the known concentration of SrCl2·6H2O in wine before the drying process. Sr is an important element among those usually used as markers for identification of the soils on which the vines grow. Two ionic (407.771 nm and 421.552 nm) and two neutral (460.733 nm and 481.188 nm) Sr lines were used to plot the calibration curves in order to study the LODs and the matrix effects for the analysis of Sr in the tested wines and for different wafer materials. This direct surface-assisted LIBS measuring method has been successfully applied for the determination of Sr in a red wine sample from Slovakia, and the obtained results with two kinds of substrates (Al and Si) were compared. Finally, a validation sample has been employed to test the accuracy of the established calibration curves.

Multispecies calibration: a novel application for inductively coupled plasma tandem mass spectrometry

Abstract: Inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) has significantly expanded the reach of analytical atomic spectrometry. In this work, we take advantage of the gas-phase chemistry available in ICP-MS/MS to propose a novel method of calibration. Rather than employing several standard solutions and a single mass-to-charge ratio, multispecies calibration (MSC) uses only one standard reference concentration and several chemical species of a monoisotopic element for calibration. In this work, multiple oxide and ammonia species generated in an ICP-MS/MS collision/reaction cell are used to determine As, Co and Mn in rice and liver samples. Only two calibration solutions are required per sample: S1 is a 1 : 1 mixture of sample and standard solution; S2 has the same 1 : 1 volume ratio of sample and blank. They are run separately, and the analytical calibration curve is built by plotting signal intensities from several ions containing the analyte. S1 and S2 signals are plotted on the x-axis and y-axis, respectively, and each point in the calibration plot corresponds to a different analyte species. The instrumental limits of detection calculated for As, Co and Mn were 0.07, 0.03, and 0.07 μg L−1, with RSDs estimated as 7.8, 3.1 and 1.8%, respectively (n = 10). Certified and MSC-determined values of As, Co and Mn in Tomato Leaves (NIST 1573a) and Bovine Liver (NIST 1577b) presented no statistically significant differences (Student's t-test, 95% confidence level, n = 3). The MSC results were comparable and sometimes better than values determined by the traditional external standard, internal standard and standard additions calibration methods.

All-solid-state paper based potentiometric potassium sensors containing cobalt(II) porphyrin/cobalt(III) corrole in the transducer layer

Abstract: Disposable ion-selective electrodes of high reproducibility of recorded potentials between different sensors have been prepared using carbon-nanotubes modified paper as the transducer and electrical lead, and a poly(vinyl chloride) ion-selective membrane. The conducting paper was obtained using carboxymethylcellulose stabilized carbon nanotubes suspension – combination found to result in high stability of conventional sensors. It was shown for the first time that change of the amount of conducting material used to prepare sensor, from conventional (where it is used as transducer) to disposable sensors (where it is used as transducer and electrical lead), requiring application of much higher amount of carbon nanotubes significantly affects performance of the sensors. Presence of ions in the suspension used results in occurrence of super-Nernstian range on the potentiometric calibration curve of potassium-selective sensor, below 10−5 M KCl. Highly reproducible potential values, between sensors as well as between different calibrations, were achieved introducing mixture of cobalt(II) porphyrin/cobalt(III) corrole to the carbon nanotubes layer, at the back side of the membrane. Presence of porphyrinoids not only stabilizes potential values obtained for the sensors but also tailors ion transport through the membrane, resulting in stable potential of the sensors, although a super-Nernstian region is present on the calibration line.

A Simple Approach to Determine a Curve Fitting Model with a Correct Weighting Function for Calibration Curves in Quantitative Ligand Binding Assays.

Abstract: In ligand binding assays (LBA), the concentration to response data is a nonlinear relationship driven by the law of mass action. Four parameter logistic (4PL) and five parameter logistic (5PL) curve fitting models are two widely accepted and validated models for LBA calibration curve data. Selection of the appropriate regression model and weighting function are key components of LBA development. Assessment of selected model and weighting function should be performed during assay development and confirmed later during validation. There has been limited published work on practical approaches to determining an appropriate weighting function and selection of a regression model for ligand binding assays. Herein, a structured scheme is presented to determine both. By applying commonly available software, assay performance data were analyzed to determine weighting functions and associated choice of a curve fitting model in three presented case studies. As a result, assay ranges of quantification were improved by reducing lower limit of quantification (from 1.00 to 0.317 ng/mL in one case study and from 2.06 to 1.37 ng/mL in another) or extending both low and upper limits of quantification(e.g., 1.04 to 48.3 ng/mL improved to 0.602 to 145 ng/mL). In addition, assay calibration curve performance demonstrated improved assay accuracy (%RE) and precision (%CV). Recommendations on decision flow when determining appropriate weighting function and curve fit model are presented.