Evaluation of Calibration Equations by Using Regression Analysis: An Example of Chemical Analysis

A calibration curve is used to express the relationship between the response of the measuring technique and the standard concentration of the target analyst. The calibration equation verifies the response of a chemical instrument to the known properties of materials and is established using regression analysis. An adequate calibration equation ensures the performance of these instruments. Most studies use linear and polynomial equations. This study uses data sets from previous studies. Four types of calibration equations are proposed: linear, higher-order polynomial, exponential rise to maximum and power equations. A constant variance test was performed to assess the suitability of calibration equations for this dataset. Suspected outliers in the data sets are verified. The standard error of the estimate errors, s, was used as criteria to determine the fitting performance. The Prediction Sum of Squares (PRESS) statistic is used to compare the prediction ability. Residual plots are used as quantitative criteria. Suspected outliers in the data sets are checked. The results of this study show that linear and higher order polynomial equations do not allow accurate calibration equations for many data sets. Nonlinear equations are suited to most of the data sets. Different forms of calibration equations are proposed. The logarithmic transformation of the response is used to stabilize non-constant variance in the response data. When outliers are removed, this calibration equation’s fit and prediction ability is significantly increased. The adequate calibration equations with the data sets obtained with the same equipment and laboratory indicated that the adequate calibration equations differed. No universe calibration equation could be found for these data sets. The method for this study can be used for other chemical instruments to establish an adequate calibration equation and ensure the best performance.

Evaluation of Chemical Quality on Juices and Wine Produced from Mamao Fruit (Antidesma Puncticulatum Miq.) Within Near-Infrared Spectroscopy

The chemical quality of juices and wine produced from Mamao fruit was evaluated by Fourier transform near-infrared (FT-NIR). The calibration equation was created by the cross-validation method to be si+mulated the accuracy. Statistical values composed of correlation coefficient (R), standard error of cross-validation (SECV) and bias were used. Brix values and acidity values of Mao juice and the Brix, acidity, and alcohol values of Mao wine products were evaluated through the standard and cross-validation relation. It was found that was observed with NIR spectrometer to be absorbed in the same IR wavelength (1450 nm) which indicated that the water is the main composition. Based on FT-NIR analysis, the spectrum latices of juices and wine were revealed in the same range of the absorption bands at 1450 nm and 1940 nm to be confirmed the water composition. Also, the FT-NIR spectra from region 2258-2312 nm in Mao wine product have been predicted to the Ethanol functions.

Evaluating the Serum Carbon Isotope Ratio as a Biomarker for Animal Protein Ratio in a Controlled Feeding Study of US Adults

Objectives Recent studies have identified the serum natural abundance carbon isotope ratio (CIR) as a candidate biomarker of animal protein intake in postmenopausal women. Such a biomarker would help clarify the contribution of dietary protein source (animal vs. vegetable) to chronic disease risk. Here we evaluate biomarker performance and develop a biomarker calibration equation in a mixed-age and – gender cohort. Methods We conducted a 15-d feeding study of 100 adults (age 18–70, 55% women) in Phoenix, AZ. Participants were provided individualized diets that approximated habitual food intakes. Total CIR and nitrogen isotope ratio (NIR) were measured in sera collected at the end of the feeding period. We expressed animal protein as a ratio of total protein intake (APratio). We evaluated a model of serum CIR based on APratio, the serum NIR, gender, age and body weight, and the resulting regression equation was inverted to develop an equation for the APratio that we call the calibrated biomarker. We evaluated the association of the calibrated biomarker with actual APratio using Pearson correlation and 5-fold cross validation. Results Animal protein intake in this study was 73 ± 30 g/d (mean ± SD) and the APratio was 0.63 ± 0.13. Our model explained a large proportion of the variation in serum CIR (R2 = 0.77) and APratio was the only significant model effect (coefficient = 6.22, SE = 0.44, P < 0.0001). Inverting that model generated the following biomarker calibration equation: APratio = (CIR – 26.35 – 0.06 (gender) + 0.068 * In age – 0.215 * In body weight – 0.204 * serum NIR)/6.22, where gender = 1,0 (male, female). There was a strong correlation between model-predicted and actual APratio (rP = 0.85, P < 0.0001), with the mean model-predicted APratio differing from mean actual APratio by 0.0015 (SE = 0.0077). The standard deviation of the prediction error was 0.076. The 5-fold cross validation procedure produced very similar model R2, effects, and prediction errors. Conclusions These data suggest that the serum CIR has potential as a predictive biomarker of APratio, providing a useful tool for objectively assessing dietary protein intake patterns. Such a tool could help resolve the contribution of dietary patterns favoring animal protein intake to chronic disease risk. Funding Sources This work was funded by NIH U01 CA197902.

Performance of a Handheld Chlorophyll-a Fluorometer: Potential Use for Rapid Algae Monitoring

Chlorophyll-a measurements are an important factor in the water quality monitoring of surface waters, especially for determining the trophic status and ecosystem management. However, a collection of field samples for extractive analysis in a laboratory may not fully represent the field conditions. Handheld fluorometers that can measure chlorophyll-a in situ are available, but their performance in waters with a variety of potential light-interfering substances has not yet been tested. We tested a handheld fluorometer for sensitivity to ambient light and turbidity and compared these findings with EPA Method 445.0 using water samples obtained from two urban lakes in Tucson, Arizona, USA. Our results suggested that the probe was not sensitive to ambient light and performed well at low chlorophyll-a concentrations (<25 µg/L) across a range of turbidity levels (50–70 NTU). However, the performance was lower when the chlorophyll-a concentrations were >25 µg/L and turbidity levels were <50 NTU. To account for this discrepancy, we developed a calibration equation to use for this handheld fluorometer when field monitoring for potential harmful algal blooms in water bodies.

DETERMINATION OF TOTAL PHENOLIC CONTENT FROM PLANT EXTRACTS USED IN COSMETIC PURPOSE

Spectrometric analysis belongs to a group of measurement methods used for their simplicity and large selectivity to solve various problems of analytes. The total phenolic content of different plant extracts used for cosmetic purpose was determined by a slightly modified version of traditionally Folin-Ciocalteau method. The objective of this work was the validation of UV-Vis spectrometric method, investigation of uncertainty sources when measuring gallic acid concentration and evaluation of the calibration equation effect on measurement uncertainty of UV– Vis spectrophotometer. Validation was performed by studying analytical curve linearity (R2=0.9995) and range (37.5 – 225 mg L-1), estimated limit of detection (LOD, 0.11mg L-1) and limit of quantification (LOQ, 0.37 mg L-1), precision (%RSD, 0.14 – 1.34), recoveries (83-110%) and stability (%RSD, 0.8 – 2.83). To obtain more representative values for precision, recoveries and stability simultaneous replicates at different times, on different matrices including plant (marigold, chamomile and lavender) were performed during the study period. The validated method was successfully applied to determine TPC in marigold extracts. For chamomile and lavender extracts, the spectrometric method presented only acceptable precision, among all the performance parameters studied. The sources of the gallic acid concentration measurement uncertainty include purity, volume of flasks, mass and the calibration equation. The results indicated that the uncertainty components from purity were the smallest. An important source of the uncertainty was the mass.The volumes of the volumetric flasks had only modest effect on the uncertainty. The contribution of calibration equation is the greatest from all sources.

In vivo calibration of the T2* cardiovascular magnetic resonance method at 1.5 T for estimation of cardiac iron in a minipig model of transfusional iron overload

Background Non-invasive estimation of the cardiac iron concentration (CIC) by T2* cardiovascular magnetic resonance (CMR) has been validated repeatedly and is in widespread clinical use. However, calibration data are limited, and mostly from post-mortem studies. In the present study, we performed an in vivo calibration in a dextran-iron loaded minipig model. Methods R2* (= 1/T2*) was assessed in vivo by 1.5 T CMR in the cardiac septum. Chemical CIC was assessed by inductively coupled plasma-optical emission spectroscopy in endomyocardial catheter biopsies (EMBs) from cardiac septum taken during follow up of 11 minipigs on dextran-iron loading, and also in full-wall biopsies from cardiac septum, taken post-mortem in another 16 minipigs, after completed iron loading. Results A strong correlation could be demonstrated between chemical CIC in 55 EMBs and parallel cardiac T2* (Spearman rank correlation coefficient 0.72, P < 0.001). Regression analysis led to [CIC] = (R2* − 17.16)/41.12 for the calibration equation with CIC in mg/g dry weight and R2* in Hz. An even stronger correlation was found, when chemical CIC was measured by full-wall biopsies from cardiac septum, taken immediately after euthanasia, in connection with the last CMR session after finished iron loading (Spearman rank correlation coefficient 0.95 (P < 0.001). Regression analysis led to the calibration equation [CIC] = (R2* − 17.2)/31.8. Conclusions Calibration of cardiac T2* by EMBs is possible in the minipig model but is less accurate than by full-wall biopsies. Likely explanations are sampling error, variable content of non-iron containing tissue and smaller biopsies, when using catheter biopsies. The results further validate the CMR T2* technique for estimation of cardiac iron in conditions with iron overload and add to the limited calibration data published earlier.

Regional calibration and electromagnetic conductivity imaging for assessing the dynamics of soil salinity

&lt;p&gt;Electromagnetic conductivity imaging (EMCI) is a state-of-the-art methodology for soil salinity assessment over large areas. It involves the following rationale: (1) use of the electromagnetic induction (EMI) geophysical technique to measure the soil apparent electrical conductivity (EC&lt;sub&gt;a&lt;/sub&gt;, mS&amp;#160;m&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;) over an area; (2) inversion of EC&lt;sub&gt;a&lt;/sub&gt; to obtain EMCI, which provides the spatial distribution of the soil electrical conductivity (&amp;#963;, mS&amp;#160;m&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;); (3) calibration process consisting of a regression between &amp;#963; and the electrical conductivity of the saturated soil paste extract (EC&lt;sub&gt;e&lt;/sub&gt;, dS&amp;#160;m&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;), used as a proxy for soil salinity; and (4) conversion of EMCI into salinity maps using the obtained calibration equation.&lt;/p&gt;&lt;p&gt;In this study, we applied EMCI and a regional calibration in Lez&amp;#237;ria Grande de Vila Franca de Xira, located in Portugal. The study area is an important agricultural system where soil faces the risk of salinization due to climate change, as the level and salinity of groundwater are likely to increase as a result of the rise of the sea water level and consequently of the estuary. These changes can also affect the salinity of the irrigation water which is collected upstream of the estuary.&lt;/p&gt;&lt;p&gt;EMI surveys and soil sampling were carried out between May 2017 and October 2018 at four locations with different salinity levels across the study area. A regional calibration was developed and its ability to predict EC&lt;sub&gt;e&lt;/sub&gt; from EMCI was evaluated. The validation analysis showed that EC&lt;sub&gt;e&lt;/sub&gt; was predicted with a root mean square error of 3.14 dS&amp;#160;m&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; in a range of 52.35 dS&amp;#160;m&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;, slightly overestimated (&amp;#8722;1.23 dS&amp;#160;m&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;), with a strong Lin&amp;#8217;s concordance correlation coefficient of 0.94 and high linearity between measured and predicted data (R&lt;sup&gt;2&lt;/sup&gt; = 0.88). It was also observed that the prediction ability of the regional calibration is more influenced by spatial variability of data than temporal variability of data.&lt;/p&gt;&lt;p&gt;Because of the transient nature of data, it was also possible to perform a preliminary qualitative analysis of soil salinity dynamics in the study area, revealing salinity fluctuations related to the input of salts and water either through irrigation, precipitation, or level and salinity of groundwater.&lt;/p&gt;

Ratiometric Sensor Based on PtOEP-C6/Poly (St-TFEMA) Film for Automatic Dissolved Oxygen Content Detection

A ratiometric oxygen sensor based on a platinum octaethylporphyrin (PtOEP)–coumarin 6 (C6)/poly (styrene-trifluoroethyl methacrylate) (poly (St-TFEMA)) film was developed for automatic dissolved oxygen (DO) detection. The oxygen-sensing film according to the dynamic quenching mechanism was prepared by embedding platinum octaethylporphyrin (PtOEP) and coumarin 6 (C6) in poly (styrene-trifluoroethyl methacrylate) (poly (St-TFEMA)). The optical parameter (OP) was defined as the ratio of the oxygen-insensitive fluorescence from C6 to the oxygen-sensitive phosphorescence from PtOEP. A calibration equation expressing the correlation between the OP values and DO content described by a linear function was obtained. A program based on the Labview software was developed for monitoring the real-time DO content automatically. The influence of the excitation intensity and fluctuation on the OP values and the direct luminescence signal (integration areas) was compared, verifying the strong anti-interference ability of the sensor. The detection limit of the sensor was determined to be 0.10 (1) mg/L. The switching response time and recovery time of the sensor were 0.4 and 1.3 s, respectively. Finally, the oxygen sensor was applied to the investigation of the kinetic process of the DO content variation, which revealed an exponential relationship with time.