Multi-point Calibration Research Articles

Impact of Future Climate and Land Use Changes on Runoff in a Typical Karst Basin, Southwest China

Climate change and land use change are the two main factors affecting the regional water cycle and water resources management. However, runoff studies in the karst basin based on future scenario projections are still lacking. To fill this gap, this study proposes a framework consisting of a future land use simulation model (FLUS), an automated statistical downscaling model (ASD), a soil and water assessment tool (SWAT) and a multi-point calibration strategy. This frameword was used to investigate runoff changes under future climate and land use changes in karst watersheds. The Chengbi River basin, a typical karst region in southwest China, was selected as the study area. The ASD method was developed for climate change projections based on the CanESM5 climate model. Future land use scenarios were projected using the FLUS model and historical land use data. Finally, the SWAT model was calibrated using a multi-site calibration strategy and was used to predict future runoff from 2025–2100. The results show that: (1) the developed SWAT model obtained a Nash efficiency coefficient of 0.83, which can adequately capture the spatial heterogeneity characteristics of karst hydro-climate; (2) land use changes significantly in all three future scenarios, with the main phenomena being the interconversion of farmland and grassland in SSPs1-2.6, the interconversion of grassland, farmland and artificial surfaces in SSPs2-4.5 and the interconversion of woodland, grassland and artificial surfaces in SSPs5-8.5; (3) the average annual temperature will show an upward trend in the future, and the average annual precipitation will increase by 11.53–14.43% and (4) the future annual runoff will show a significant upward trend, with monthly runoff mainly concentrated in July–September. The variability and uncertainty of future runoff during the main-flood period may increase compared to the historical situation. The findings will benefit future water resources management and water security in the karst basin.

An Automatic Calibration Method for Kappa Angle Based on a Binocular Gaze Constraint.

Kappa-angle calibration shows its importance in gaze tracking due to the special structure of the eyeball. In a 3D gaze-tracking system, after the optical axis of the eyeball is reconstructed, the kappa angle is needed to convert the optical axis of the eyeball to the real gaze direction. At present, most of the kappa-angle-calibration methods use explicit user calibration. Before eye-gaze tracking, the user needs to look at some pre-defined calibration points on the screen, thereby providing some corresponding optical and visual axes of the eyeball with which to calculate the kappa angle. Especially when multi-point user calibration is required, the calibration process is relatively complicated. In this paper, a method that can automatically calibrate the kappa angle during screen browsing is proposed. Based on the 3D corneal centers and optical axes of both eyes, the optimal objective function of the kappa angle is established according to the coplanar constraint of the visual axes of the left and right eyes, and the differential evolution algorithm is used to iterate through kappa angles according to the theoretical angular constraint of the kappa angle. The experiments show that the proposed method can make the gaze accuracy reach 1.3° in the horizontal plane and 1.34° in the vertical plane, both of which are within the acceptable margins of gaze-estimation error. The demonstration of explicit kappa-angle calibration is of great significance to the realization of the instant use of gaze-tracking systems.

One sample multi-point calibration curve as a novel approach for quantitative LC-MS analysis: The quantitation of six aflatoxins in milk and oat-based milk as an example

Preparing of calibration curves are critical steps for accurate quantitative LC-MS bioanalysis. Traditional multi-sample external calibration curve (MSCC) is labor-intensive and prone to error. In this study, a novel strategy of one sample multi-point calibration curve (OSCC) using multiple isotopologue reaction monitoring (MIRM) was proposed and validated using LC-MS for the quantitation of six aflatoxins in milk and oat-based milk samples. The developed MIRM-OSCC methodology is comprehensively validated and the results indicated that the established method exhibits good performance in selectivity, sensitivity, accuracy and precision. Furthermore, the OSCC could realize sample dilution by monitoring the MIRM channel with less intensity for samples beyond the upper limit of quantification, without the need of sample dilution, which improves the assay throughput. Considering the advantages of excluding the MSCC preparation and sample dilution in OSCC, this strategy can be widely applied in various fields such as drugs, food safety and environmental analysis.

Identification and determination α and β-acids from Columbus hops using Reverse Phase High Pressure Liquid Chromatography

The present study has demonstrated the HPLC method for identification and determination a and b-acids in Columbus hops. the measurement conditions has been set up included the use of Reverse phase C18 column Chromolith, gradient elution with methanol (A) and Acetonitril acidified with phosphate buffer at pH 2.8. While the flow rate has been applied of 0.8 mL/min, and it is run for 25 minutes. The separation of standard and sample has been monitored using UV variable detector in the rang of 300-340 nm. Pretreatment sample has been done using SPE with the cartridge of C18 similar to the analyte column. The results found that a four predominant peaks for α and β-acids, namely cohumulone (6.6 min), the second peak represents a mixture of humulone and adhumulone (8.9 min), the third peak is colupulone (15.9 min) and the fourth is a mixture of lupulone and adlupulone (16.7 min). The quantity of sample has been determined according to multipoint calibration using ICE2 standard, the sample found are 0,209 g/L cohumulone, 1.087 g/L of humulone and adhumulone, 0,349 g/L of Colupulone and 0.630 g/L of mixture of lupulone and adlupulone. In addition it is high precision measurement that is indicated by the value of RSD less than 1%.

A variable-measurement-space-based method for improving the measurement accuracy of AACMMs

Articulated arm coordinate measuring machines (AACMMs) are usually used to measure mechanical parts of different sizes in practical applications. When the size of parts is smaller than that of the AACMM, a redundant measurement space of the measuring machine is generated, resulting in reduced measurement accuracy of the AACMM. Moreover, due to the series structure of the AACMM, the joint errors would be propagated with the increasing length of the bar, which could amplify the measurement error. Therefore, on the premise of ensuring the measurement space required by the measured parts, this work proposes a new method based on variable measurement space, i.e. locking some movable joints of the AACMM, to improve the measurement accuracy of the AACMM for small mechanical parts. The measurement accuracy of the AACMM is examined under various measurement spaces. Firstly, the Monte Carlo method is used to simulate the measurement space of the AACMM with one or more joints locked. Then, the spatial error of the AACMM and the multipoint calibration with joint locked are simulated and discussed in detail. Finally, the feasibility and effectiveness of the proposed variable-measurement-space method are verified by the standard single-point cone experiments. The results show that the measurement accuracy of the AACMM is substantially improved under three kinds of strategies, i.e. locking joint 1, joint 2, and joint 3, respectively.

Novel one-point calibration strategy for high-throughput quantitation of microcystins in freshwater using LC-MS/MS

Precise quantification of microcystins (MCs) in freshwater is crucial for environmental monitoring and human health. However, the preparation of traditional multi-sample external calibration curve (MSCC) is time consuming and prone to error. Here, a novel one-point calibration strategy including one sample multi-point calibration curve (OSCC) and in sample calibration curve (ISCC) is proposed for the quantitation of eight MCs in freshwater lakes using liquid chromatography tandem mass spectrometry (LC-MS/MS). The multiple isotopologue reaction monitoring (MIRM) of MCs and its 15N-labelled internal standards were used for OSCC and ISCC, respectively. The isotopic abundance of each MIRM channel could be calculated and measured accurately. Additionally, this strategy was comprehensively validated and showed good performance in selectivity, sensitivity, accuracy and precision as the traditional MSCC. Interestingly, OSCC could realize sample dilution by monitoring the less abundant MIRM transitions, while ISCC remove blank matrixes and generate calibration curve in each study samples. Furthermore, the proposed methodology was successfully applied to analyze several freshwater lake samples contaminated by MCs. Considering the advantages of excluding the MSCC preparation, simplified workflows and improved throughput, OSCC and ISCC will be favored for MCs monitoring and as an emerging approach in environmental pollutant control and prevention.

Primary accelerometer calibration with two-axis automatic positioning stage

In this study, we developed an automated, multipoint primary accelerometer calibration system using a two-axis positioning stage and a heterodyne laser interferometer. The proposed system offers low-cost, convenient, and automated multipoint accelerometer calibration, enabling less calibration lead time. The positioning stage also offers better positioning repeatability of 1μm, which is impossible through manual alignments. We measured the surface deformation of a laser reflection adaptor for a single-ended accelerometer by measuring more than 450 measurement positions. Visualizing the deformation of laser reflection surfaces facilitates understanding the effects of deformation or non-linear motion, which are among the most significant uncertainty components in high-frequency accelerometer calibrations.

The oxygen stable isotope composition of CRM 125-A UO2 standard reference material

While there is a clear need for standardized reference materials for analytical calibrations and for inter-laboratory comparisons, there are not currently any for the oxygen stable isotopic composition of uranium oxides. In this paper we summarize the results from four laboratories by seven different methods of oxygen stable isotope analyses using fluorination techniques of CRM 125-A UO2 Standard Reference Material. We synthesize these data and methods to arrive at a consensus oxygen stable isotope composition for CRM 125-A δ18O = −9.63‰ (±0.29‰) VSMOW. We discuss methodological differences between analytical approaches, including furnace vs laser heating, fluorination using BrF5 or ClF3, as well as calibration strategies. We highlight the potential effects of calibration scale compression from single-point calibrations using reference material with δ18O values having a large relative difference from the sample being analyzed. We demonstrate how calibration scale compression can yield differences in calibrated δ18O values up to ∼2‰ for samples with ∼20‰ difference from a single reference material, if the calibration slope of different analytical systems differs by 0.1. We suggest the use of liquid water calibration standards sealed in silver capillary tubes for multi-point calibrations of fluorination analysis systems.

Quality control of cannabis inflorescence and oil products: Response factors for the cost-efficient determination of ten cannabinoids by HPLC

The quality control of medicinal cannabis should include quantification of as many cannabinoids as practicable in a routine analytical lab, to accurately reflect the quality of the product. However, the cost and availability of some cannabinoid standards is an impediment to their routine use. This work seeks to overcome this obstacle by analysing samples using relative retention times (RRT) and relative response factors (RRF), relative to CBD and CBDA reference standards which are readily available. A high-performance liquid chromatography-photodiode array method was developed to quantify ten cannabinoids (Δ9-THC, Δ8-THC, THCA-A, CBN, CBD, CDBA, CBC, CBDV, CBG, and CBGA) in dried cannabis inflorescence and cannabis oil. This method was validated according to ICH guidelines. The proposed method has detection limits ranging from 20 to 78 µg/g, which provided sufficient sensitivity for the panel of cannabinoids. Non-cannabinoid surrogate matrices were used for spike recovery studies to determine method accuracy – analyte recoveries for the inflorescence and oil ranged from 90.1 to 109.3% (inflorescence mean, 100.9%; oil mean, 99.6%). The RRT and RRF values determined independently by three analysts were comparable, indicating the method is robust. The validity of analysis using RRT and RRF was further confirmed by testing six inflorescence samples, as it was found that concentrations above the order of magnitude of the LoQ agreed satisfactorily (range, 95.0 to 111.9%; mean, 100.0%) with the concentrations obtained through the conventional approach of multipoint calibration using pure standards. The proposed method is therefore suitable for the rapid and simple determination of a panel of ten cannabinoids without having to repeatedly purchase every expensive pure standard. Accordingly, analysts in the medicinal cannabis field may explore the use of RRF and RRT for their methods and instruments.

All-Step-in-One Test Kit for Paraquat Detection in Water and Vegetable Samples

This work presents the first development of an all-steps-in-one test kit for the determination of paraquat in natural water, and vegetable and agricultural samples. A handheld photometer incorporated with a magnetic stirrer was used to complete the steps of extraction, mixing, and detection. Paraquat produces a blue free radical ion via a reduction with sodium dithionite in alkaline conditions. Sodium dithionite powder was investigated for the enhancement of reagent stability duration, which was added directly into sample solution that showed insignificant difference in sensitivity as compared with that of the solution format of sodium dithionite. The developed test kit showed good performance with the linear calibration of 0.5 to 10 mg L−1 with a high coefficient of determination (r2 = 0.9947). The lower limit of quantitation (LLOQ = 3SD of intercept per slope) carried out from the method using the handheld photometer was 0.50 mg L−1. The limit of detection (LOD) by naked eye was 0.30 mg L−1. The recovery study was acceptable in the range of 101–115%. Intraday (n = 3) and interday (n = 3) precision was less than 1%. On the basis of the significance test at the 95% confidence interval, quantitative results of the developed test kit agreed well with those from high-performance liquid chromatography (HPLC). To the best of our knowledge, this is the first report demonstrating an online extraction for vegetables incorporated into a test kit, applicable for on-site analysis. Single-point calibration based on the Beer–Lambert law also demonstrated the measurement of paraquat. In testing with a nominal standard solution of 5.00 mg L−1 paraquat, the reading concentration was 5.09 ± 0.03 mg L−1 paraquat (n = 20) with a K value of 0.0967 (close to the slope of multipoint calibration). This research is a direct benefit to agricultural products and the health of a population for the analysis of pesticides and herbicides.

A novel calibration strategy based on isotopic distribution for high-throughput quantitative analysis of pesticides and veterinary drugs using LC-HRMS

Preparation of calibration curves is a critical step for large-scale quantification. However, this procedure is time-consuming, labor intensive. Herein, a novel isotopologue multipoint calibration (IMC) strategy, was proposed and demonstrated for the simultaneous quantitation of 120 pesticides and 83 veterinary drugs in surface water samples using Liquid Chromatography-High Resolution Mass Spectrometry (LC-HRMS). In this strategy, the natural isotopic distribution was used to generate external calibration curves, eliminating the need of analyst’s adjustment and many sets of chemical standard solutions required in external calibration curves. Additionally, this strategy was comprehensively validated, and the results indicated this strategy had better performance in both accuracy and precision, fully meeting the requirements for the quantitative analysis. Interestingly, for the samples with high concentration beyond the upper limit of quantitation, the IMC strategy could avoid samples dilution by monitoring the less abundant isotopic channels. Furthermore, the IMC method was successfully applied in the surface water samples collected from Anhui province, China. Among which, sulfamethoxazole and imidacoprid were the main contributors. In conclusion, we present a promising LC-HRMS strategy for the accurate quantitation of small molecules, which has a potential application in food and environmental analysis.

Improving precision and trueness in the quantification of cadmium using square wave anodic stripping voltammetry and bismuth film electrodes

Single-point standard addition, along with internal standard calibration (SSA-IS), is investigated as a new protocol in cadmium quantification by square wave anodic stripping voltammetry (SWASV) and using bismuth film/glassy carbon electrode (Bi/GCE). The bismuth oxidation signal used as an in situ internal standard compensates for random variations between measurements. Moreover, the SSA-IS calibration method is proposed as a practical solution to the passivation of the working electrode. SSA-IS improves the results (RSD = 6.77 %, bias = 5.16 %), compared to the multi-point external standard calibration without internal standard (ESC), (RSD = 29.71 %, bias = 58.80 %) and with internal standard (ESC-IS), (RSD = 6.88 %, bias = 29.23 %). Also, the uncertainty and trueness of the proposed method were estimated and compared with the NIST 3108 reference material. An average concentration of 9.77 ± 0.96 mg g−1 was obtained, which statistically agrees with the certified value of the NIST reference material, 10.007 ± 0.027 mg g−1. Using the SSA-IS method improves the metrological qualities of the analytical technique, making the methodology ready to be tested in real samples such as cocoa.

Plasma Lysosphingolipid Biomarker Measurement by Liquid Chromatography Tandem Mass Spectrometry.

Plasma lysosphingolipids are highly elevated in patients with Gaucher, Krabbe, Fabry, and Niemann-Pick diseases and tend to accumulate to a greater extent than their respective primary sphingolipids in the plasma of affected patients. In this chapter, we describe two liquid chromatography tandem mass spectrometry (LC-MS/MS) methods to measure plasma concentrations of four lysosphingolipids species. The first method described measures glucosylsphingosine (lyso-GL1) and galactosylsphingosine (psychosine), biomarkers that accumulate in Gaucher and Krabbe diseases, respectively. The second method measures globotriaosylsphingosine (lyso-Gb3) and sphingosylphosphorylcholine (lyso-SPM), biomarkers for Fabry and Niemann-Pick diseases, respectively. Each method utilizes isotope-labeled internal standards and multipoint calibration curves to quantify the analytes of interest. Briefly, plasma samples are mixed with five volumes of LC-MS grade methanol containing internal standard, and protein is removed via centrifugation. Supernatant is dried and resuspended in initial mobile phase. Samples are separated by liquid chromatography using either a BEH amide column (lyso-GL1+psychosine) or a C18 column (lyso-Gb3+lyso-SPM). Protonated analytes are measured by selected reaction monitoring (SRM) in positive electrospray ionization mode. Using these methods, we have observed elevations of these lyso- species in Gaucher, Fabry, and Niemann-Pick and successfully distinguished different subtypes reflecting the disease severity.

A highly accurate mass spectrometry method for the quantification of phenylalanine and tyrosine on dried blood spots: Combination of liquid chromatography, phenylalanine/tyrosine-free blood calibrators and multi-point/dynamic calibration

BackgroundMeasurement of quantitative levels of phenylalanine and tyrosine in blood is an essential test for the diagnosis of and monitoring genetic disorders associated with phenylalanine metabolism, such as phenylketonuria (PKU), tyrosinemia, and defects of tetrahydrobiopterin synthesis and recycling. We developed a highly accurate and fast liquid chromatography with tandem mass spectrometry (LC-MS/MS) method for the quantification of phenylalanine and tyrosine on dried blood spot (DBS). We also designed a performance score system to evaluate various calibration methods in matrix matched material. MethodsPhenylalanine/tyrosine-free whole blood was used to make accurate and stable DBS calibrators. Six calibrators cover the range of 0–1000 µmol/L. Underivatized phenylalanine and tyrosine were extracted and measured by LC-MS/MS. Precision, accuracy, limit of quantification, recovery and carryover were validated. External quality assurance materials were also used to evaluate performance of multi-point calibrations and single-point calibrations. ResultsThe run time was 4.5-minute. Accuracy analysis showed good agreement with reference materials. Precision, recovery, and the lower and upper limit of quantification met the criteria. When phenylalanine and tyrosine concentrations were less than 150 µmol/L, the 5-point calibration without the calibrator of 1000 µmol/L had the best performance. When the concentrations were > 250 µmol/L, the single-point calibration of 500 µmol/L had the best performance. ConclusionWe developed a simple, fast and highly accurate method for the detection of phenylalanine and tyrosine on DBS, with chromatographic separation and underivatized analysis. Based on the calibration performance, a 6-point calibration method is satisfying for this test. An optional dynamic calibration method, which includes 6-point calibration, 5-point calibration and single-point calibration, can further increase test reliability.

A Portable Device for Methane Measurement Using a Low-Cost Semiconductor Sensor: Development, Calibration and Environmental Applications.

Methane is a major greenhouse gas and a precursor of tropospheric ozone, and most of its sources are linked to anthropogenic activities. The sources of methane are well known and its monitoring generally involves the use of expensive gas analyzers with high operating costs. Many studies have investigated the use of low-cost gas sensors as an alternative for measuring methane concentrations; however, it is still an area that needs further development to ensure reliable measurements. In this work a low-cost platform for measuring methane within a low concentration range was developed and used in two distinct environments to continuously assess and improve its performance. The methane sensor was the Figaro TGS2600, a metal oxide semiconductor (MOS) based on tin dioxide (SnO2). In a first stage, the monitoring platform was applied in a small ruminant barn after undergoing a multi-point calibration. In a second stage, the system was used in a wastewater treatment plant together with a multi-gas analyzer (Gasera One Pulse). The calibration of low-cost sensor was based on the relation of the readings of the two devices. Temperature and relative humidity were also measured to perform corrections to minimize the effects of these variables on the sensor signal and an active ventilation system was used to improve the performance of the sensor. The system proved to be able to measure low methane concentrations following reliable spatial and temporal patterns in both places. A very similar behavior between both measuring systems was also well noticeable at WWTP. In general, the low-cost system presented good performance under several environmental conditions, showing itself to be a good alternative, at least as a screening monitoring system.

Research on the calibration method of a new multifunctional calibrator based on AACMM

Calibrators can effectively improve the calibration accuracy of articulated coordinate measuring machine (AACMM). However, the traditional calibration method of a calibrator is usually cumbersome. To study the influence of sampling strategies of various calibration methods on the calibration effect of AACMM, we design a new multifunctional calibrator for AACMM kinematics parameters, and propose three calibration methods with high accuracy and high efficiency. In the multifunctional calibrator, 84 different calibration points are provided by changing the rotation angle of calibration bar and the inclination angle of calibration plate. Compared with previous calibrators, the calibrator can obtain more positions and directions. The length calibration, multi-point calibration and virtual center distance calibration methods are proposed to calibrate kinematic parameter of AACMM. In addition, the sampling strategy for the three calibration methods is explored. Finally, the consistency of single-axis coordinate measurement is verified based on the three calibration methods. The results show that the three calibration methods can effectively improve the measurement accuracy of AACMM. Especially, the length calibration method has the highest accuracy of the three methods. The multi-point calibration method can significantly improve the calibration efficiency.

Characterization & categorization of garnet samples for major and minor constituents by energy dispersive X-ray fluorescence spectroscopy

An EDXRF based methodology was developed for the characterization of major and minor elemental constituents in Garnet samples. The method was optimized using multi-point calibration in high purity graphitic materials for the analytes: Ca, Mg, Si, Al, V, Zr, Fe, Mn and Cr. The method optimization was done based on evaluation of analytical performance including, detection limits, sensitivity, and precession. The actual garnet samples were analysed and based on the concentration of the major constituents, four different types of Garnet were identified: Pyrope (Mg 33 %, Si 41 % and Al 24%), Almandine (Si 19 %, Al 14 %, and Fe 53 %), Spessartine (Si 21%, Al 17 % and Mn 48 %) and Grossular (Ca 54%, Si 22% and Al 15 %). The analytical results were compared with AES. The results were found to be in good agreement with each other. Student t-test and F-test were performed with 95 % confidence level and ten degrees of freedom. The estimated t-values and F-values were found to be lower than tc= 2.228 and Fc= 2.98. This analyses revealed that the data are reliable and the within statistical fluctuation, even the associated precision values are also accepted

Multi-point calibration method for articulated arm coordinate measuring machine based on an observability index

Kinematic calibration is an effective way to improve the accuracy of articulated arm coordinate measuring machine (AACMM). In this paper, a multi-point calibration method for AACMM based on observation index is proposed. First, the kinematic model is established according to the D–H method, and the measurement error model is established on the basis of the D–H model. Then, the feasibility of using the observation index to evaluate the posture of AACMM is analyzed, and simulations are carried out to verify the correlation between the level of the observation index and the quality of the calibration results. In the simulation, the influence of the number of measurement positions and the number of measurement points on the calibration results are studied in combination with the observability index, and on this basis, a multi-point calibration method is proposed. Finally, experiments are carried out to verify the superiority of the multi-point calibration method. The results show that, compared with the single point calibration method, the multi-point calibration method can improve the measurement accuracy by 31.58%, and the average measurement error of the AACMM after the multi-point calibration method is 0.0091 mm.

Feasibility of multifunction calibration of H+-responsive glass electrodes in seawater (IUPAC Technical Report)

Abstract Seawater pH values are of the highest relevance in marine chemistry studies, not only through being acidity indicators but also due to the control provided by H+(aq) over the various simultaneous equilibria occurring in seawater. Although the concept of p H = − l g a H + = − lg ( m H + γ H + / m 0 ) $\mathrm{p}\mathrm{H}=-\mathrm{l}\mathrm{g}{\mathit{a}}_{{\mathrm{H}}^{+}}=-\mathrm{lg}\left({\mathit{m}}_{{\mathrm{H}}^{+}}{\mathit{\gamma }}_{{\mathrm{H}}^{+}}/{\mathit{m}}^{0}\right)$ , where m H + ${m}_{{\text{H}}^{+}}$ is the relative (molality basis) activity, γ H + ${\gamma }_{{\text{H}}^{+}}$ is the molal activity coefficient of the hydrogen ion H+ at molality m H + ${m}_{{\text{H}}^{+}}$ , and m 0 is the standard molality, was introduced in 1910 and reaffirmed on successive occasions by relevant bodies, different conceptual definitions and alternative measurement procedures have been adopted and are in use by some, namely among oceanographers, often leading to confusion. This leads to major difficulties with the use of data, e.g., on what concerns comparison of results in space and time. Primary pH values, the highest quality level in terms of the metrological chain, have been assigned to primary reference pH buffer solutions of low ionic strength, by a primary method based on measurements of the Harned cell potential in association with the Nernst equation, as well as on the adoption of extra-thermodynamic model assumptions for electrolyte solutions. Although equivalent types of recommendations dealing with standards and procedures based on metrological traceability are still lacking for higher ionic strength media, as it is in the case of seawater, reference Tris–Tris·HCl buffer solutions in artificial seawater have been suggested for use in the calibration of pH meter systems. In this work, Tris–Tris·HCl buffer saline solutions of three different molality ratios mTris:mTris.HCl, m/mol kg−1 H2O, have been assigned reference values for free p H = − lg a H + $\mathrm{p}\mathrm{H}=-\mathrm{lg}\,{a}_{{\text{H}}^{+}}$ and total pH T = − lg ( m H + * / m 0 ) ${\mathrm{pH}}^{\mathrm{T}}=-\text{lg}\left({m}_{{\text{H}}^{+}}^{\text{{\ast}}}/{m}^{0}\right)$ , where m 0 = 1 mol kg−1 and m H + * = lim m → m SW [ m ( H + ) + m ( HSO 4 − ) ] ${m}_{{\text{H}}^{+}}^{\text{{\ast}}}=\underset{m\to {m}_{\text{SW}}}{\mathrm{lim}}\left[m\left({\mathrm{H}}^{\mathrm{+}}\right)+m\left({\mathrm{HSO}}_{4}^{\mathbf{-}}\right)\right]$ . Multi-point calibration of pH meters in terms of either pH or pHT is thus possible and supports measurement of their respective values under routine conditions at a high metrological level.

Isotopic Distribution Calibration for Mass Spectrometry.

Mass spectrometry (MS) is widely used in science and industry. It allows accurate, specific, sensitive, and reproducible detection and quantification of a huge range of analytes. Across MS applications, quantification by MS has grown most dramatically, with >50 million experiments/year in the USA alone. However, quantification performance varies between instruments, compounds, different samples, and within- and across runs, necessitating normalization with analyte-similar internal standards (IS) and use of IS-corrected multipoint external calibration curves for each analyte, a complicated and resource-intensive approach, which is particularly ill-suited for multi-analyte measurements. We have developed an internal calibration method that utilizes the natural isotope distribution of an IS for a given analyte to provide internal multipoint calibration. Multiple isotope distribution calibrators for different targets in the same sample facilitate multiplex quantification, while the emerging random-access automated MS platforms should also greatly benefit from this approach. Finally, isotope distribution calibration allows mathematical correction for suboptimal experimental conditions. This might also enable quantification of hitherto difficult, or impossible to quantify, targets, if the distribution is adjusted in silico to mimic the analyte. The approach works well for high resolution, accurate mass MS for analytes with at least a modest-sized isotopic envelope. As shown herein, the approach can also be applied to lower molecular weight analytes, but the reduction in calibration points does reduce quantification performance.