Showing papers on "Calibration curve published in 2004"

The intercept is a poor estimate of a calibrated radiocarbon age

Abstract: Intercept-based methods of generating a point estimate of a calibrated radiocarbon date are very popular, but exhibit undesirable behaviour. They are highly sensitive to the mean of the radiocarbon date and to adjustments of the calibration curve. Other methods give more stable results. The weighted average of the probability distribution function is recommended as the best central-point estimate, but more consideration should be given to using the full probability distribution rather than a point estimate in developing age-depth models.

Quantitative analysis of silicates using LA-ICP-MS with liquid calibration

Abstract: For more than a decade liquid calibration has been proposed and selectively applied as a calibration strategy for laser-generated aerosols. However, matrix independent calibration is not a well-accepted technique for quantification in direct solid analysis using LA-ICP-MS. In this study three synthetic calibration solutions were used for the analysis of three silicate reference materials (BCR-2G, ATHO, and NIST 610). The calibration solutions were introduced into the ICP using an Aridus liquid sample introduction system with aerosol desolvation, while argon was used as the carrier gas. A 193 nm ArF excimer laser ablation system generated sample aerosols using helium as carrier gas. The argon carrier gas containing the solution nebulization aerosols was mixed, using a sheath gas adaptor in front of the ICP, with the laser aerosol carried in helium. Two different calibration strategies were applied for the multielement quantification of three geological reference materials. The first calibration technique used internal standardization with Ca as the internal standard. The second approach used synthetic calibration solutions containing all major, minor, and trace elements, in which the concentration ratios and concentrations of the elements were known. A normalization algorithm was used which calculated the sum of all of the determined element concentrations as oxides to 100%. The data shown for Ca indicate that both procedures are well suited for multi-element determinations. The 100% oxide normalization strategy allowed the calculation of the Ca concentration based on the total matrix used as internal standard. This Ca concentration was then used for the determination of the trace element composition of the sample. The advantage of this calibration is the possibility of element analysis in silicates without knowing the element concentration of at least one internal standard element prior to analysis. However, this study also shows that the composition of the solution used for the calibration can lead to losses of some elements during the desolvation process in the Aridus or during aerosol transfer to the ICP, which will be shown for Cu.

Patient specific optimization of the relation between CT-hounsfield units and proton stopping power with proton radiography.

Abstract: The purpose of this work is to show the feasibility of using in vivo proton radiography of a radiotherapy patient for the patient individual optimization of the calibration from CT-Hounsfield units to relative proton stopping power. Water equivalent tissue (WET) calibrated proton radiographs of a dog patient treated for a nasal tumor were used as baseline in comparison with integrated proton stopping power through the calibrated CT of the dog. In an optimization procedure starting with a stoichiometric calibration curve, the calibration was modified randomly. The result of this iteration is an optimized calibration curve which was used to recalculate the dose distribution of the patient. One result of this experiment was that the mean value of the deviations between WET calculations based on the stoichiometric calibration curve and the measurements was shifted systematically away from zero. The calibration produced by the optimization procedure reduced this shift to around 0.4 mm. Another result was that the precision of the calibration, reflected as the standard deviation of the normally distributed deviations between WET calculation and measurement, could be reduced from 7.9 to 6.7 mm with the optimized calibration. The dose distributions based on the two calibration curves showed major deviations at the distal end of the target volume.

Partly Nonparametric Approach for Determining the Limit of Detection

Abstract: Background: According to recent International Organization for Standardization (ISO) standards, the limit of detection (LoD) of an assay should be estimated taking both type I (α) and II (β) errors into account. The suggested procedure, however, supposes gaussian distributions of both blank and sample measurements and a linear calibration curve. In clinical chemistry, asymmetric, nongaussian blank distributions are common, and the calibration curve may be nonlinear. We present a partly nonparametric procedure that takes these aspects into account. Methods: Using theoretical distribution models and simulation studies, we developed a LoD estimation procedure suitable for the field of clinical chemistry that is partly based on nonparametric statistics. Results: For sample size n, the nonparametrically determined 95th percentile of the blank measurements {obtained as the value of the [n(95/100) + 0.5]th ordered observation} defines the limit for results significantly exceeding zero [limit of blank (LoB)]. The LoD is the lowest value that is likely to yield a result exceeding the LoB. LoD is estimated as: LoB + c β × SDS, where SDS is the analytical SD of a sample with a low concentration; c β = z 1 − β/[1 − 1/(4 × f )]; z 1 − β is the standard normal deviate; and f is the number of degrees of freedom for estimation of SDS. c β is approximately equal to 1.65 for a type II error of 5%. Approaches and needed tabular values for calculation of confidence limits are presented as well as sample size. Worked examples are given to illustrate estimation and verification of the limit of detection. Simulation results are used to document performance. Conclusion: The proposed procedure appears useful for application in the field of clinical chemistry and promotes a standardized approach for estimating LoDs of clinical chemistry assays.

The Calibration of Size Exclusion Chromatography Columns: Molecular Mass Distributions of Heavy Hydrocarbon Liquids

Abstract: The aim of the study was to obtain calibration curves for a pair of size exclusion chromatography (SEC) columns operating with 1-methyl-2-pyrrolidinone (NMP) as eluent. The dependence of the calibrations on sample chemical structures has been examined. The calibrations have been compared with elution times of several sets of standards. The level of agreement between SEC and MALDI-mass spectrometry has been evaluated. Molecular mass distributions of several complex samples have been examined in terms of these calibrations. The polystyrene (PS) and poly(methyl methacrylate) (PMMA) calibration curves were close, while a set of polysaccharides (PSAC) and other oxygenates eluted much earlier. However, numerous other samples eluted closer to the PS−PMMA line. To a first approximation, deviations between the PSAC and PS−PMMA lines may be treated as an upper limit to errors arising from structure-dependent variations in this SEC system. Below 15 000 u, MMs of oxygenated samples could be estimated to within a fact...

Bayesian analysis of serial dilution assays.

Abstract: In a serial dilution assay, the concentration of a compound is estimated by combining measurements of several different dilutions of an unknown sample. The relation between concentration and measurement is nonlinear and heteroscedastic, and so it is not appropriate to weight these measurements equally. In the standard existing approach for analysis of these data, a large proportion of the measurements are discarded as being above or below detection limits. We present a Bayesian method for jointly estimating the calibration curve and the unknown concentrations using all the data. Compared to the existing method, our estimates have much lower standard errors and give estimates even when all the measurements are outside the "detection limits." We evaluate our method empirically using laboratory data on cockroach allergens measured in house dust samples. Our estimates are much more accurate than those obtained using the usual approach. In addition, we develop a method for determining the "effective weight" attached to each measurement, based on a local linearization of the estimated model. The effective weight can give insight into the information conveyed by each data point and suggests potential improvements in design of serial dilution experiments.

Determination of refractory elements in used lubricating oil by ICPOES employing emulsified sample introduction and calibration with inorganic standards.

Abstract: The quantification of trace elements in used lubricating oil is useful for evaluating the wearing of specific components of engines. In the present work, inductively coupled plasma optical emission spectrometry (ICPOES) was used for determining five refractory elements (Ni, Mo, Cr, V and Ti) in lubricating-oil samples. The methodology was developed while aiming at the introduction of such organic samples into the ICP as emulsions. Several nebulization systems were tested with clear advantage for Meinhard K3 coupled with a cyclonic spray chamber. The carbon deposition on the injector tip as well as the plasma background was minimized through careful optimization of the Ar and O2 gas mixture flows into the plasma. The optimization of instrumental and experimental parameters allowed quantification using calibration curves prepared with analyte inorganic standards. An internal standard (Sc) was used to correct the matrix effects and signal fluctuations. The limits of detection (3Sb/m), in the ng g-1 range were obtained for all five elements. The methodology was validated through an analysis of standard reference materials (SRM 1084a, 1085a and 1085b). Good analyte recoveries (from 92.6 to 104.7%) were achieved. Comparison studies against established ICPOES methodologies (sample acid decomposition or sample direct dilution in an organic solvent) have shown that the proposed methodology present clear advantages in terms of simplicity of sample preparation, overall analysis time, and the use of inorganic standards for calibration instead of expensive metallorganic standards.

Formal statistical models for estimating radiocarbon calibration curves.

Abstract: We report on the development and implementation of a model-based statistical method for the estimation of radiocarbon calibration curves using diverse data. The method takes account of uncertainty on both the (super 14) C and calendar scales, coherently integrating data, the calendar age estimates of which arise from different dating methods. It also allows for correlation between observations, if they have particular sources of uncertainty in common. We adopt an approach based on a random walk model, tailoring it to take account of possible calendar age offsets between different data sources by adding a random effect component. The latter allows us to use the same modeling framework for constructing the new calibration curve IntCal04, the comparison curve NotCal04, the Southern Hemisphere curve SHCal04, and the marine calibration curve Marine04.

Determination of gabapentin in human plasma by capillary electrophoresis with laser-induced fluorescence detection and acetonitrile stacking technique

Abstract: Chemical vapour generation has been used in combination with atomic fluorescence spectrometry to determine mercury at ultratrace concentrations down to 0.1 ng L−1. A time-based injection of 1 mL of solution for measurement was sufficient to generate a steady-state detector response in the direct mode of measurement. The detection limit calculated from a ten-point calibration curve according to DIN 32645 was 0.26 ng L−1. Instrument noise is limited by reflected radiation from the light source rather than by the dark current of the photomultiplier. The detection limit is directly influenced by the reagent blank which was 2 ng L−1 in the experiments described. Focusing by amalgamation and subsequent thermal desorption generates a detector response which is about eight times higher in peak intensity and about twice as large in integrated intensity. The detection limit under these conditions is 0.09 ng L−1 which can be further improved by preconcentration of larger volumes of solution for measurement. The cycle time for one individual reading is about 40 s without amalgamation and 125 s with amalgamation. The linear dynamic range of the system is five orders of magnitude with a single photomultiplier gain setting. The carry-over is less than 0.3% in direct measurement mode. Reference water samples and a surface water containing approximately 5 ng L−1 were used to prove the validity of the method for real samples. Good accuracy and recoveries of 103% were calculated using the fast direct determination technique.

Ultratrace determination of mercury in water following EN and EPA standards using atomic fluorescence spectrometry

Abstract: Chemical vapour generation has been used in combination with atomic fluorescence spectrometry to determine mercury at ultratrace concentrations down to 0.1 ng L−1. A time-based injection of 1 mL of solution for measurement was sufficient to generate a steady-state detector response in the direct mode of measurement. The detection limit calculated from a ten-point calibration curve according to DIN 32645 was 0.26 ng L−1. Instrument noise is limited by reflected radiation from the light source rather than by the dark current of the photomultiplier. The detection limit is directly influenced by the reagent blank which was 2 ng L−1 in the experiments described. Focusing by amalgamation and subsequent thermal desorption generates a detector response which is about eight times higher in peak intensity and about twice as large in integrated intensity. The detection limit under these conditions is 0.09 ng L−1 which can be further improved by preconcentration of larger volumes of solution for measurement. The cycle time for one individual reading is about 40 s without amalgamation and 125 s with amalgamation. The linear dynamic range of the system is five orders of magnitude with a single photomultiplier gain setting. The carry-over is less than 0.3% in direct measurement mode. Reference water samples and a surface water containing approximately 5 ng L−1 were used to prove the validity of the method for real samples. Good accuracy and recoveries of 103% were calculated using the fast direct determination technique.

Carbon-14 wiggle-match dating of peat deposits: advantages and limitations

Abstract: Carbon-14 wiggle-match dating (WMD) of peat deposits uses the non-linear relationship between C-14 age and calendar age to match the shape of a series of closely spaced peat C-14 dates with the C-14 calibration curve. The method of WMD is discussed, and its advantages and limitations are compared with calibration of individual dates. A numerical approach to WMD is introduced that makes it possible to assess the precision of WMD chronologies. During several intervals of the Holocene, the C-14 calibration curve shows less pronounced fluctuations. We assess whether wiggle-matching is also a feasible strategy for these parts of the C-14 calibration curve. High-precision chronologies, Such as obtainable with WMD, are needed for studies of rapid climate changes and their possible causes during the Holocene. Copyright (C) 2004 John Wiley Sons, Ltd.

Direct fluorescence quantification of chromium(VI) in wastewater with organic nanoparticles sensor.

Abstract: Under ultrasonic irradiation, organic fluorescence nanoparticles have been prepared by a reprecipitation method. Compared with single organic fluorophores, these nanoparticles are brighter, more stable against photobleaching and more water-soluble. They also have high room-temperature fluorescence quantum yields (∼20%) and a long fluorescence lifetime (∼0.2 µs). Based on the fluorescence quenching of nanoparticles by chromium(VI), a method for the selective determination of chromium(VI) without the separation of chromium(III) in water was developed. Under the optimal conditions, the linear range of the calibration curve was 7.0 × 10-6 - 1.0 × 10-4 mol L-1. The detection limit was 2.8 × 10-6 mol L-1. The method is characterized by a short reaction time, stable fluorescence signals, simplicity and high selectivity. The present assay has been applied to the selective quantification of Cr(VI) in wastewater with satisfactory results.

Comparison of Several Calibration Procedures for Glass Electrodes in Proton Concentration

Abstract: Several reported procedures for calibrating glass electrodes in proton concentration are compared. Some recommendations for non-experts are also given. The examined procedures can be classified into two broad categories, namely: those based on direct potential difference measurements of solutions of known proton concentration and those that use one or several pH standards to calibrate the electrode and subsequently measure the pH of solutions containing known proton concentrations. With a single buffer, the two types of procedures lead to equivalent results. However, if two pH buffers are used, the slope of the calibration graph in proton concentration will differ from the real electrode slope to an extent proportional to the difference between the liquid junction potentials of the two buffers. Therefore, any other method is preferable under these circumstances.

Direct Determination of Carbon Dioxide in Aqueous Solution Using Mid-Infrared Quantum Cascade Lasers

Abstract: A method for the direct determination of carbon dioxide in aqueous solutions using a room-temperature mid-infrared (MIR) quantum cascade laser at 2330 cm–1 is reported. The absorption values of different carbon dioxide concentrations were measured in a 119 μm CaF2 flow-through cell. An optical system made of parabolic mirrors was used to probe the flow cell and to focus the laser beam on the mercury cadmium telluride (MCT) detector. Aqueous carbon dioxide standards were prepared by feeding different mixtures of gaseous N2 and CO2 through wash bottles at controlled temperature. The concentration of the dissolved CO2 was calculated according to Henry's law, taking into account the temperature and the partial pressure of CO2. The carbon dioxide standards were connected via a selection valve to a peristaltic pump for subsequent, automated measurement in the flow-through cell. A calibration curve was obtained in the range of 0.338 to 1.350 g/L CO2 with a standard deviation of the method sxo equal to 19.4 mg/L CO2. The limit of detection was calculated as three times the baseline noise over time and was determined to be 39 mg/L.

Using nonequilibrium capillary electrophoresis of equilibrium mixtures for the determination of temperature in capillary electrophoresis.

Abstract: Until now, all methods for temperature sensing in capillary electrophoresis (CE) relied on molecular probes with temperature-dependent spectral/optical properties. Here we introduce a nonspectroscopic approach to determining temperature in CE. It is based on measuring a temperature-dependent rate constant of complex dissociation by means of a kinetic CE method known as nonequilibrium capillary electrophoresis of equilibrium mixtures (NECEEM). Conceptually, a calibration curve of "the rate constant versus temperature" is built using NECEEM and a CE instrument with a reliable temperature control or, alternatively, a nonelectrophoretic method, such as surface plasmon resonance. The calibration curve is then used to find the temperature during CE in the same buffer but with another CE apparatus or under otherwise different conditions (cooling efficiency, length and diameter of the capillary, electrical field, etc.). In this proof-of-principle work, we used the dissociation of a protein-DNA complex to demonstrate that the NECEEM-based temperature determination method allows for temperature determination in CE with a precision of 2 degrees C. Then, we applied the NECEEM-based temperature determination method to study heat dissipation efficiency in CE instruments with active and passive cooling of the capillary. The nonspectroscopic nature of the method makes it potentially applicable to nonspectroscopic detection schemes, e.g. electrochemical detection. A "kinetic probe" can be coloaded into the capillary along with a sample for in situ temperature measurements. Higher order chemical reactions can also be used for temperature sensing, provided a kinetic CE method for measuring a corresponding rate constant is available.

Phase-height mapping and coordinate calibration simultaneously in phase-measuring profilometry

Abstract: A new method for both the phase-height mapping and the 3-D coordinate calibration in phase-measuring profilometry (PMP) is presented. A calibration gauge containing four virtual calibration planes is designed. The application of the calibration gauge avoids the translation of the standard plane during the calibration process. Furthermore, circular steps are designed to calibrate lateral coordinates automatically. With this calibration gauge, both the phase-height mapping and lateral coordinates calibration could be done simultaneously through only one measurement. Additionally, to recover a continuous phase distribution of all the steps with the same height, an iterative procedure is proposed. The effectiveness of the proposed method is demonstrated by experimental results.

Application of a contactless conductivity detector to the determination of inorganic ions in ion chromatography

Abstract: The contactless conductivity detector was connected to ion chromatographs operating in both, the non-suppressed and suppressed modes and the analytical parameters of the contactless and the conventional commercial contact detectors were compared by using solutions of standards The performance of the contactless detector in terms of reproducibility, linearity of calibration curve and detection limit was found to be largely identical to the commercial detectors in both systems The usefulness of the new detector was further demonstrated with some real samples (tap, mineral and rain water) The contactless conductivity detector, which does not show electrode fouling and is simple in design and inexpensive, was thus found to be a suitable substitute for conventional detectors in ion chromatography

Experimental aspects and modeling for quantitative measurements in scanning capacitance microscopy

Abstract: In this article we discuss the reliability of a quantification method for scanning capacitance microscopy (SCM) measurements based on the calculation of a calibration curve. We demonstrate that an accurate control of the conductive tip coating stability, low temperature oxidation and Si–SiO2 interface microroughness allows one to fabricate a nanometric metal-oxide-semiconductor device (nanoMOS), whose dC∕dV‐V characteristics measured on a set of different concentration levels can be reproduced by simulation of an ideal nanoMOS with the realistic three-dimensional geometry. We also studied the impact of tip coating (metal and conductive diamond coated tips) and oxidation method (wet and UV∕ozone oxides) on the reproducibility of the measured SCM signal for different concentration levels both on p- and n-type Si staircase calibration samples, and we demonstrated that the UV∕ozone oxidation associated with the use of a diamond tip is the best solution. The experimental calibration curve obtained by this choi...

Manual sampling and tank size effects on the calibration curve of plot sediment storage tanks

Abstract: In many experimental soil erosion plots, runoff is collected and carried by a conveyance system to a sequence of storage tanks. If the soil loss is measured by collecting, after mixing, samples of the stored suspension, then a calibration curve between the actual mean concentration (C) and the measured concentration (Cm) in the storage tank occurs. The aim of this article was to evaluate experimentally the factors affecting the relationship between C and Cm. For a sandy loam soil, the replicated measurements of Cm (20 samples) for two values of the actual concentration (C = 5 and 25 g/L) showed that the variability of the measurements of Cm is low and confirmed the reliability of a calibration curve obtained by a single series of runs. Results from experiments carried out with a clay soil to compare the calibration curves obtained by four field workers suggested that the maximum uncertainty in the soil loss measurement due to the choice of the calibration curve should not exceed 100% of the true value. Moreover, the slope of the calibration curve was independent of both the water level in the tank and the field worker. Finally, a comparison among the calibration curves of a prototype tank and some model tanks was carried out for both a sandy loam and a clay soil in order to establish a scaled-up relationship among tanks of different size. Soil-specific and theoretically based scaled-up relationships were deduced.

Multicomponent quantitative resolution of binary mixtures by using continuous wavelet transform.

Abstract: Continuous 1-dimensional wavelet transform (WT) was applied to the quantitative analysis of a vitamin combination of thiamine hydrochloride (THI) and pyridoxine hydrochloride (PYR) with strongly overlapping signals. Absorbance data from the UV-Vis absorption spectrum of width 1150 were subjected to Gauss1 and Gauss2 WTs. Because of its flexibility, data processing, and its high signal amplitude, the continuous WT method is a powerful tool for analysis of multicomponent mixtures. By measuring the amplitude signals corresponding to the selected zero-crossing points of the transformed signal, we obtained the calibration curve. The validation of the calibration graphs was confirmed with different mixtures of THI and PYR at various concentration ratios. A brief explanation of the continuous wavelet method is given. MATLAB 6.5 software was used to perform the calculations. The results of our study were compared with those obtained by spectroscopic, chemometric, and liquid chromatographic methods, and good agreement was found.