Showing papers on "Calibration curve published in 2019"

The Integrated Calibration Index (ICI) and related metrics for quantifying the calibration of logistic regression models

Abstract: Assessing the calibration of methods for estimating the probability of the occurrence of a binary outcome is an important aspect of validating the performance of risk-prediction algorithms. Calibration commonly refers to the agreement between predicted and observed probabilities of the outcome. Graphical methods are an attractive approach to assess calibration, in which observed and predicted probabilities are compared using loess-based smoothing functions. We describe the Integrated Calibration Index (ICI) that is motivated by Harrell's Emax index, which is the maximum absolute difference between a smooth calibration curve and the diagonal line of perfect calibration. The ICI can be interpreted as weighted difference between observed and predicted probabilities, in which observations are weighted by the empirical density function of the predicted probabilities. As such, the ICI is a measure of calibration that explicitly incorporates the distribution of predicted probabilities. We also discuss two related measures of calibration, E50 and E90, which represent the median and 90th percentile of the absolute difference between observed and predicted probabilities. We illustrate the utility of the ICI, E50, and E90 by using them to compare the calibration of logistic regression with that of random forests and boosted regression trees for predicting mortality in patients hospitalized with a heart attack. The use of these numeric metrics permitted for a greater differentiation in calibration than was permissible by visual inspection of graphical calibration curves.

Determination of 5-nitro-2-furaldehyde as marker residue for nitrofurazone treatment in farmed shrimps and with addressing the use of a novel internal standard

Abstract: We developed a significantly improved ultra-high performance liquid chromatography-tandem mass spectrometry method for determination of 5-nitro-2-furaldehyde (NF) as a surrogate using a novel internal standard for the detection of nitrofurazone. We used 2,4-dinitrophenylhydrazine derivatization and furfural as the internal standard. Derivatization was easily performed in HCl using ultrasonic manipulation for 5 min followed by liquid extraction using ethyl acetate. The samples were concentrated and purified using reverse phase and alumina cartridges in tandem. The derivatives were separated using a linear gradient elution on a C18 column with methanol and water as the mobile phase in negative ionization mode and multiple reaction monitoring. Under the optimized conditions, the calibration curves were linear from 0.2 to 20 μg/L with correlation coefficients >0.999. Mean recoveries were 80.8 to 104.4% with the intra- and inter-day relative standard deviations <15% at spiking levels of 0.1 to 10 μg/kg. The limits of detection and quantification were 0.05 and 0.1 μg/kg, respectively. This method is a robust tool for the identification and quantitative determination of NF in shrimp samples.

Battery-operated portable high-throughput solution cathode glow discharge optical emission spectrometry for environmental metal detection

Abstract: In this study, miniaturized optical emission spectrometry (OES) based on solution cathode glow discharge (SCGD) was developed for the determination of metal elements in samples. By redesigning and integrating the entire device, it can be controlled at the computer end to achieve automatic detection and analysis with battery power. The detection process does not require complex pretreatment of liquid samples, thereby enabling analysis in an environmentally friendly manner. A series of parameters affecting the analysis process were investigated. Without separation or enrichment of the analyte, the proposed SCGD-OES instrument could analyze Cd, Hg and Pb simultaneously, with detection limits (DLs) in the range of 33–253 μg L−1 (flow injection mode) and 7–92 μg L−1 (continuous flow mode). The calibration curves for Cd, Hg and Pb were linear in the concentration range of 2–50 μg mL−1. The instrument was successfully applied to certified reference materials (CRMs), and tap water, and seawater samples. The experimental results demonstrated the accuracy and practicality of the instrument.

Comprehensive Target Analysis for 484 Organic Micropollutants in Environmental Waters by the Combination of Tandem Solid-Phase Extraction and Quadrupole Time-of-Flight Mass Spectrometry with Sequential Window Acquisition of All Theoretical Fragment-Ion Spectra Acquisition.

Abstract: There are many thousands of chemicals in use for a wide range of purposes, and highly efficient analytical methods are required to monitor them for protection of the environment. In order to cope with this difficult task we developed a novel, comprehensive method for 484 substances in water samples. In this method target chemicals were extracted by tandem SPE and then determined by LC-QTOF-MS-SWATH. Targets were unambiguously identified using retention times, accurate masses of a precursor and two product ions, their ion ratios, and accurate MS/MS spectrum. Quantitation was achieved by the internal standard method using a precursor ion. Results of recovery tests at two concentrations (50 and 500 ng L-1) showed average recoveries of 87.5% and 87.0% (RSD, 9.1% and 9.4%), respectively. Limits of detection of one-half of the targets were below 1.0 ng L-1. The method was applied to the influent and effluent of a sewage treatment plant, and around 100 chemicals were detected. Results of examination on matrix effects using their extracts spiked with 209 pesticides showed that the ratios of detected amounts between the extracts and the standard solution were 89.8% (influent) and 91.7% (effluent), respectively. In addition, investigation on the stability of calibration curves by injecting the same standards for 1 year showed that their quantitative results did not change; average accuracy was 103.3% (RSD, 10.0%), indicating that the calibration curves can be used for an extended period of time without calibration, and quantitative retrospective analysis can be done after creating calibration curves for new targets.

Measurement of Pulse Wave Signals and Blood Pressure by a Plastic Optical Fiber FBG Sensor.

Abstract: Fiber Bragg grating (FBG) sensors fabricated in silica optical fiber (Silica-FBG) have been used to measure the strain of human arteries as pulse wave signals. A variety of vital signs including blood pressure can be derived from these signals. However, silica optical fiber presents a safety risk because it is easily fractured. In this research, an FBG sensor fabricated in plastic optical fiber (POF-FBG) was employed to resolve this problem. Pulse wave signals were measured by POF-FBG and silica-FBG sensors for four subjects. After signal processing, a calibration curve was constructed by partial least squares regression, then blood pressure was calculated from the calibration curve. As a result, the POF-FBG sensor could measure the pulse wave signals with an signal to noise (SN) ratio at least eight times higher than the silica-FBG sensor. Further, the measured signals were substantially similar to those of an acceleration plethysmograph (APG). Blood pressure is measured with low error, but the POF-FBG APG correlation is distributed from 0.54 to 0.72, which is not as high as desired. Based on these results, pulse wave signals should be measured under a wide range of reference blood pressures to confirm the reliability of blood pressure measurement uses POF-FBG sensors.

An approach to select linear regression model in bioanalytical method validation

Abstract: The accuracy of any bioanalytical method depends on the selection of an appropriate calibration model. The most commonly used calibration model is the unweighted linear regression, where the response (y-axis) is plotted against the corresponding concentration (x-axis). The degree of association between these two variables is expressed in terms of correlation coefficient (r2). However, the satisfactory r2 alone is not adequate to accept the calibration model. The wide calibration curve range used in the bioanalytical methods is susceptible to the heteroscedasticity of the calibration curve data. The use of weighted linear regression with an appropriate weighting factor reduces the heteroscedasticity and improves the accuracy over the selected concentration range. The present work describes a rapid and simple RP-HPLC method for the estimation of chlorthalidone in spiked human plasma. The calibration curve standards were studied in the concentration range of 100–3200 ng/mL. The chromatography was performed on a C18 column (250 × 4.6 mm, 5 μm) in an isocratic mode at a flow rate of 1 mL/min using methanol:water (60:40%, v/v) as a mobile phase. The detection was carried out at 276 nm. Both the unweighted regression model and weighted regression models with different weighting factors (1/x, 1/√x, and 1/x2) were evaluated for heteroscedasticity. The statistical approach for the selection of a suitable regression model with appropriate weighting factors was discussed and the developed bioanalytical method was further validated, as per US-FDA guidelines. In calibration curve experiments, although the acceptable r2 of 0.998 was obtained, the % residual plot showed that the data were susceptible to heteroscedasticity. When the weighted linear regression was applied to the same calibration curve data set, no significant difference between % relative residual (% RR) was observed. Furthermore, when % relative error (% RE) was calculated for different weighting factors, it was observed that the regression model with 1/x weighting factor gave a minimum % RE. The calibration curve was found to be linear in the range of 100 to 3200 ng/mL. The validation experiments proved good accuracy, and intra- and inter-day variability and acceptable recovery. Stability studies proved that the drug was stable under tested stability cycles. From the statistical reports obtained from the present work, it was observed that the calibration curve in bioanalytical experiments was susceptible to heteroscedasticity using the unweighted linear regression model. Hence, to obtain homoscedasticity in the calibration curve experiments, there is a need for a weighted linear regression model. The appropriate regression model was further selected by evaluating the % RE for different weighting factors.

Differentiation of heavy metal ions by fluorescent quantum dot sensor array in complicated samples

Abstract: The interference of co-existing substances often causes trouble in the analysis of trace metal ions in complex samples, even for highly selective sensors. For this reason, an instant and inexpensive fluorescence sensor array for discrimination of heavy metal ions in complex samples was developed. The sensor array was constructed using four kinds of Mn doped ZnS quantum dots (Mn: ZnS QDs), which were facilely modified with N-Acetyl-cysteine, citric acid, mercaptopropionic acid and triammonium-N-dithiocarboxyiminodiacetate, respectively. For each metal ion, quantitative calibration curves were obtained with the concentration level ranging from 10 to 100 pg/mL (R2 > 0.96). Due to the different quenching effect for Cu, Hg, Ag and Cd on various QDs, the sensor array exhibited a unique pattern of fluorescence variations at a low concentration of 30 pg/ml and can be discriminated successfully by principal component analysis (PCA). The contribution of individual sensors within the array was demonstrated and the obtained information was used to design sensor arrays with two and three sensor elements. The sensor array was also applied to identify the metal ions in unknown mixtures, containing single, binary, ternary and quaternary constitutions. Linear discriminant analysis (LDA) showed that the samples could be well recognized and distinguished. The sensor array was finally applied in water samples with three different concentrations, indicating the co-existing substances rarely influenced discriminatory capacity of the sensor array. Compared to instrumental analysis, this fluorescence sensor array-based method has proven to be more convenient since the nanoparticles can be prepared flexibly according to the property of the target.

Pd@CeO2-SnO2 nanocomposite, a highly selective and sensitive hydrogen peroxide electrochemical sensor

Abstract: In this study, Pd@CeO2-SnO2 nanocomposite was synthesized and various methods such as FE-SEM, TEM, EDS, FT-IR, and XRD were used to characterize the nanocomposite. The electrocatalytic behaviour of the nanocomposite was studied for the reduction of H2O2. After optimization of various experimental parameters including pH, scan rate, rotation rate and potential, the calibration curve was plotted for the reduction reaction of the H2O2 at pHs 7.0 and 8.0. At pH 7.0, the linear range and detection limit were obtained as 0.15–1920 μM and 44 nM, respectively. Also, at pH 8.0, the linear range and detection limit were obtained as 0.12–2170 μM and 36 nM, respectively. In addition, the detection sensitivity of the sensor for the determination of H2O2 at pH 7.0 and 8.0 was 472.40 and 533.07 μA.mM−1. cm-2, respectively. Further, the sensor response to the H2O2 was studied in the presence of various types of potential interference species. The results confirmed the high selectivity of the designed sensor for the reduction of H2O2. Finally, the accuracy and applicability of the H2O2 electrochemical sensor in three different milk samples were evaluated with satisfactory results.

D-Glucose sensor based on ZnO·V2O5 NRs by an enzyme-free electrochemical approach

Abstract: A simple wet-chemical technique was used to prepare zinc oxide-doped vanadium pentaoxide nanorods (ZnO·V2O5 NRs) in an alkaline environment. The synthesized ZnO·V2O5 NRs were characterized using typical methods, including UV-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (XEDS), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD). The D-glucose (D-GLC) sensor was fabricated with modification of a slight coating of nanorods (NRs) onto a flat glassy carbon electrode (GCE). The analytical performances, such as the sensitivity, limit of quantification (LOQ), limit of detection (LOD), linear dynamic range (LDR), and durability, of the proposed D-GLC sensor were acquired by a dependable current–voltage (I–V) process. A calibration curve of the GCE/ZnO·V2O5 NRs/Nf sensor was plotted at +1.0 V over a broad range of D-GLC concentrations (100.0 pM–100.0 mM) and found to be linear (R2 = 0.6974). The sensitivity (1.27 × 10−3 μA μM−1 cm−2), LOQ (417.5 mM), and LOD (125 250 μM) were calculated from the calibration curve. The LDR (1.0 μM–1000 μM) was derived from the calibration plot and was also found to be linear (R2 = 0.9492). The preparation of ZnO·V2O5 NRs by a wet-chemical technique is a good advancement for the expansion of nanomaterial-based sensors to support enzyme-free sensing of biomolecules in healthcare fields. This fabricated GCE/ZnO·V2O5 NRs/Nf sensor was used for the recognition of D-glucose in real samples (apple juice, human serum, and urine) and returned satisfactory and rational outcomes.

Improved Analysis of Manganese in Steel Samples Using Collinear Long-Short Double Pulse Laser-Induced Breakdown Spectroscopy (LIBS)

Abstract: A long-short double pulse laser-induced breakdown spectroscopy (long-short DP-LIBS) method was employed to improve the performance of LIBS for the measurement of manganese in steel samples The long pulse was generated by a Nd:YAG laser which was operated at free runing (FR) mode To investigate the detection ability without sample preparation, the steel washers were tested using SP-LIBS and long-short DP-LIBS, respectively The measurement results show that the long-short DP-LIBS method was able to record clear spectra from the rusty steel washers The steel washers were also measured after the polishing process The measurement results show that the signal intensity was enhanced by long-short DP-LIBS Through the observation with scanning electron microscope (SEM) on the laser craters, the results suggest that the improvement of detection ability can be attributed to the pre-radiation effect of long-pulse laser beam Next, the analytical performance for quantitative measurement of manganese was evaluated by employing ten standard steel samples The results show that the linearty fit (R2) of calibration curve is 0988 for long-short DP-LIBS, whereas, R2 is only 0810 for SP-LIBS under the same measurement conditions The five times repeated measurement results show that the average Relative Standard Deviation (RSD) of the tested samples is 293% for SP-LIBS and is 105% for long-short DP-LIBS The prediction results also show that the average Relative Error of Prediction (REP) is 949% for SP-LIBS and it 49% for long-short DP-LIBS The experimental results in current work demonstrate that long-short DP-LIBS is promising for the on line measurement of steel in the steelmaking plant

A high-temperature electrochemical sensor based on CaZr0.95Sc0.05O3–δ for humidity analysis in oxidation atmospheres

Abstract: A way of in situ water vapor partial pressure determination in oxidizing atmospheres and high temperatures is proposed in this work, utilizing a new and simple amperometric-type sensor design made of ZrO2- and CaZrO3-based electrolytes. These electrolyte membranes allow conduction of oxygen-anions and protons, resulting in full water decomposition on account of the applied potential through an external electrical circuit. At a certain range of applied voltages, a limiting current condition is observed. By plotting the limiting current against temperature and water vapor partial pressure, the functional dependences are obtained, which could be used as the calibration curves for analytical reasons. The proper operation of the developed sensor is confirmed by changing water vapor partial pressures (0.003–0.110 atm) in air and temperatures (675–750 °C). It is found that the sensor’s reading is stable, reproducible, and corresponds to the theoretically predicted values, confirming the test success. The obtained results allow extending the field of possible applications of oxide materials with proton conduction nature.

Evaluation of different quantification modes for a simple and reliable determination of Pb, Zn and Cd in soil suspensions by total reflection X-ray fluorescence spectrometry

Abstract: In view of the large number of soil analyses in environmental studies dealing with metal contamination, the use of simple, fast, sustainable and cost-effective methodologies is desired. In this sense, the role of total reflection X-ray fluorescence spectrometry (TXRF) has to be highlighted due to the possibility of direct analysis of solid suspensions without the need for a digestion process. Internal standardization is successfully used as a quantification approach in multielement TXRF analysis of liquid samples, including soil digests. However, its application for the analysis of soil suspensions is not always possible. In the present manuscript we found that although acceptable results were obtained for TXRF quantification by internal standardization when considering soil certified reference materials (recoveries: 80–120%), a systematic underestimation of the concentration values was found when using the same approach for real soil sample analysis (recoveries <70%). In view of these problems, we explored the possibilities of different quantification approaches, including external calibration, which can be used in combination with soil suspension preparation and TXRF analysis to obtain reliable results for metal determination in soils. Data obtained showed that the best strategy was the use of internal standardization with correction of the TXRF results using a calibration curve obtained in the analysis of a set of soil samples with a matrix similar to the unknown ones by using a reference technique (digestion and ICP-OES analysis). With this analytical approach quantitative results were obtained for all the studied metals with recovery values around 95%. As a study case, the proposed TXRF method has been applied for the specific determination of Pb, Zn and Cd concentrations in soils from an abandoned metal mining area but the developed methodology could be extended to other environmental studies dealing with the determination of other metals and soil types. Taking into account the selectivity and sensitivity limitations when determining high Z elements such as Cd using TXRF systems equipped with Mo X-ray tubes, we compare the analytical performance of TXRF systems with different sources (Mo and W) as well.

Determination of major to trace elements in metallic materials based on the solid mixing calibration method using multiple spot-laser ablation-ICP-MS

Abstract: We have developed a new analytical method for the determination of major to trace elements in metallic materials using multiple spot laser ablation (msLA)-ICP-MS The concentration values of Cr, Co, Ni, and Cu in metallic materials were calculated from calibration curves obtained using the solid-mixing method achieved by the combination of a high-repetition rate laser and galvanometric optics With the multiple-spot ablation procedure, two or more materials can be ablated nearly at the same time, and the calibration curve could be defined by mixing two standard materials of different concentrations Another important feature of the method is that large amounts of samples can be ablated with the high-repetition rate lasers within short time durations, ie it results in a better signal to noise ratio In this study, concentrations of Cr, Co, Ni, and Cu in 13 metallic materials (iron meteorite, stainless steels, tool steels, and low alloy steels) were measured from calibration curves obtained by aerosol mixing using the msLA-ICP-MS technique The resulting signal intensity data exhibited good correlation with the calculated concentrations The calculated concentration data for the analytes showed good agreement with the reference value within 10% for most of the metallic materials with analyte concentration covering dozens of % down to μg g−1 The data obtained here demonstrate clearly that calibration based on the solid sample mixing can become a powerful method to determine the amounts of major to trace elements in solid materials

Determination of antimony in water samples by hydride generation coupled with atmospheric pressure glow discharge atomic emission spectrometry

Abstract: A low power (∼10 W), miniaturized atmospheric pressure glow discharge (APGD) source sustained between a tungsten cathode and a titanium tube anode was coupled with a hydride generation (HG) system for sensitive determination of antimony in water samples with atomic emission spectrometry (AES). Sb ions were converted into volatile hydrides through the reaction with KBH4 firstly and then were introduced into the APGD source for excitation and detected using a compact CCD (charge-coupled device) microspectrometer. A univariate approach was used to achieve optimized conditions and derive analytical figures of merit. Under optimal conditions, the limits of detection and quantification for Sb were 0.14 and 0.5 μg L−1, respectively. Moreover, the HG-APGD-AES system offered good repeatability (relative standard deviation < 1.5%). The calibration curve was linear in the range between 0.5 and 100 μg L−1, with a correlation coefficient of R2 = 0.9996. The proposed method was successfully applied to the determination of certified reference materials (GSB 07-1376-2001, GBW07305a, GBW07307a and GBW073066) and some groundwater samples. Recoveries of Sb added to these water samples were within 90.9–100.7%, proving the good accuracy of the HG-APGD-AES method.

Absolute calibration of GafChromic film for very high flux laser driven ion beams.

Abstract: We report on the calibration of GafChromic HD-v2 radiochromic film in the extremely high dose regime up to 100 kGy together with very high dose rates up to 7 × 1011 Gy/s. The absolute calibration was done with nanosecond ion bunches at the Neutralized Drift Compression Experiment II particle accelerator at Lawrence Berkeley National Laboratory (LBNL) and covers a broad dose dynamic range over three orders of magnitude. We then applied the resulting calibration curve to calibrate a laser driven ion experiment performed on the BELLA petawatt laser facility at LBNL. Here, we reconstructed the spatial and energy resolved distributions of the laser-accelerated proton beams. The resulting proton distribution is in fair agreement with the spectrum that was measured with a Thomson spectrometer in combination with a microchannel plate detector.

Calibration of Portable Particulate Matter-Monitoring Device using Web Query and Machine Learning.

Abstract: Background Monitoring and control of PM2.5 are being recognized as key to address health issues attributed to PM2.5. Availability of low-cost PM2.5 sensors made it possible to introduce a number of portable PM2.5 monitors based on light scattering to the consumer market at an affordable price. Accuracy of light scattering–based PM2.5 monitors significantly depends on the method of calibration. Static calibration curve is used as the most popular calibration method for low-cost PM2.5 sensors particularly because of ease of application. Drawback in this approach is, however, the lack of accuracy. Methods This study discussed the calibration of a low-cost PM2.5-monitoring device (PMD) to improve the accuracy and reliability for practical use. The proposed method is based on construction of the PM2.5 sensor network using Message Queuing Telemetry Transport (MQTT) protocol and web query of reference measurement data available at government-authorized PM monitoring station (GAMS) in the republic of Korea. Four machine learning (ML) algorithms such as support vector machine, k-nearest neighbors, random forest, and extreme gradient boosting were used as regression models to calibrate the PMD measurements of PM2.5. Performance of each ML algorithm was evaluated using stratified K-fold cross-validation, and a linear regression model was used as a reference. Results Based on the performance of ML algorithms used, regression of the output of the PMD to PM2.5 concentrations data available from the GAMS through web query was effective. The extreme gradient boosting algorithm showed the best performance with a mean coefficient of determination (R2) of 0.78 and standard error of 5.0 μg/m3, corresponding to 8% increase in R2 and 12% decrease in root mean square error in comparison with the linear regression model. Minimum 100 hours of calibration period was found required to calibrate the PMD to its full capacity. Calibration method proposed poses a limitation on the location of the PMD being in the vicinity of the GAMS. As the number of the PMD participating in the sensor network increases, however, calibrated PMDs can be used as reference devices to nearby PMDs that require calibration, forming a calibration chain through MQTT protocol. Conclusions Calibration of a low-cost PMD, which is based on construction of PM2.5 sensor network using MQTT protocol and web query of reference measurement data available at a GAMS, significantly improves the accuracy and reliability of a PMD, thereby making practical use of the low-cost PMD possible.

Surfactant-assisted graphene oxide/methylaniline nanocomposites for lead ionic sensor development for the environmental remediation in real sample matrices

Abstract: Graphene oxide/methylaniline (GO/MA) nanocomposites (NCs) were synthesized by an in situ inverse microemulsion polymerization method. The characterization of nanocomposites is done by scanning electron microscope (SEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) methods. In addition, an efficient chemical sensor probe was fabricated with flat glassy carbon electrode (GCE) by coating with thin layer of synthesized GO/MA NCs. Then the sensor was applied for the selective detection of lead (Pb+2) ion in an aqueous phase. The sensor performance such as sensitivity (20.56 µA mM−1 cm−2) was calculated from the slope of the calibration curve. The linear dynamic range (LDR: 0.1 nM–0.01 M) was calculated on the maximum linearity line from the calibration plot. The detection limit (DL: 50.0 ± 2.5 pM) was also calculated from the slope of the calibration plot by considering of 3 N/S (signal-to-noise ratio of 3). This novel electrochemical approach introduced a reliable and efficient route to detect the heavy metal ions as selective sensor for the safety of medical, biochemical and environmental fields in broad scales.