Analysis of the influence of interdetector volume on local calibration in size exclusion chromatography with dual multiangle-light-scattering/concentration detection

A comparison of the s- and w-detections of molecular weight, a necessary condition for precise determination of interdetector volume in size exclusion chromatography of polymers with the dual light scattering--concentration detection, is examined. From the theoretical analysis for a polymer with the log-normal molecular weight distribution it follows that the error in determination of the local calibration, i.e., of a logarithmic dependence of molecular weight on elution volume obtained by the s-detection, from the known dependence of radius of gyration on elution volume and of molecular weight on the radius of gyration, remains linear and is slightly shifted with respect to that obtained by the w-detection (dual detection giving the weight-average molecular weight) towards higher molecular weights, the difference being below the experimental error. It also follows that the error in the slope of local calibration found by the w-detection is given by the ratio of the error in interdetector volume and the variance of the elution curve. This is demonstrated on several polymer samples with symmetric elution curves of polymer samples differing in polydispersity indices. The range of this rule depends on the broadness and symmetry of elution curves. The precision of the interdetector volume determination depends therefore strongly on the Mw/Mn ratio of the sample.     

Determination of potassium and sodium in mixtures of their salts by means of cation exchangers

Methods for the determination of two substances in aqueous solutions (or water-soluble mixtures) are described. A sample of the solution is dried and the residue accurately weighed. Another sample of the same volume is passed through a cation exchanger (when cations are to be determined), saturated with one of the ions in the solution. After washing with distilled water the combined effluent and wash-water are dried and the residue carefully weighed. From these weighings the composition of the solution is determined by simple calculations, the maximum relative error being < 2.0%. The method may be used for the determination of anions, if an anion exchanger is used instead of the cation exchanger.A variation of the method consists in passing the solution through a cation exchanger in the hydrogen form. The number of equivalents is determined by standard alkali titration and from this and the weight before the ion-exchange passage the composition of the solution can be determined. In this case, however, the maximum relative error is larger. This method is directly applicable to the determination of anions.     

Manipulations, pM'-pH diagrams and accuracy in determination of metals

Manipulations done with metal ion solutions can influence the accuracy of a determination. During a manipulation, when a non-homogeneous solution in a non-equilibrium state is dealt with, polynuclear hydroxide compounds and/or a hydroxide precipitate may be formed, which generally do not immediately disappear after homogenization of the solution. Deviations from the equilibrium state that is expected after homogenization, may not be negligible during the subsequent analysis. A part of the metal content escapes determination and systematic errors arise. The magnitude of the error depends upon the nature of the particular metal to be determined and the manipulations performed. The influence of the manipulations can be understood and foreseen by studying the behaviour of the metal ions in solution by means of a pM'-pH diagram. In this diagram regions can be indicated where the experimental conditions correspond to the formation of a hydroxide precipitate and polynuclear hydroxide complexes. The momentary state of the solution during a manipulation can be represented by a point. This point changes its position as the manipulation proceeds, describing a locus through the diagram. The locus depends upon the initial conditions and the particular manipulation performed. If the locus penetrates one of the "error" regions, the corresponding compounds will be formed and the error of the determination will increase. Conversely, if the locus is adapted so as to avoid penetration of the regions for polycomplexation and precipitation, the error is found to decrease remarkably.     

The titrimetric determination of carbon dioxide with special reference to the determination of carbon in organic compounds

The titrimetric barium carbonate method for the determination of. carbon in organic compounds has been examined and the various sources of error are discussed. The carbon dioxide from the combustion of the sample is absorbed in excess baryta containing 20% (w/v) barium chloride and the excess is back-titrated with hydrochloric acid in the presence of o-cresolphthalein indicator.The value of this method in conjunction with the previously described determination of water with succinyl chloride, for the simultaneous determination of carbon and hydrogen is assessed     

论如何通过设计性实验提升学生的综合创新能力——以分光光度法同时测定维生素C和维生素E为例

Simultaneous determination of vitamin C and vitamin E by spectrophotometry is a typical experiment of simultaneous determination of multiple components. However, the current experimental method has a large relative error (13.7%) in the determination of vitamin C. In this paper, according to the molecular structural characteristics of vitamin C and vitamin E, we studied the factors affecting the measurement accuracy of vitamin C and improve the experimental method by optimization pf design, and the error is reduced to 1.98%. Results proved that the improved method achieved higher accuracy, better reproducibility, more reliable results and better teaching responses. Through this designed experiment, the students are guided from many aspects such as aiming at practical problems and using theoretical knowledge to optimize the experimental scheme, therefore their comprehensive innovation abilities are improved.     

Quantitative fuzzy neural network for analytical determination

A quantitative fuzzy neural network (Q-FNN) for pattern recognition in analytical determination is reported in this paper. The fuzzy neural network (FNN) combines a fuzzy logic system with an artificial neural network (ANN) so that it has both advantages of a high training speed and strong anti-interference. Importantly, the analytical concept of relative error (RE) in quantitative determination has been integrated into FNN so that the Q-FNN provides a very good quantitative capability in chemical analysis, and prevents the system from an over-fitting problem. The logarithm curve with noise in terms of analytical response versus concentration is calibrated by trained FNN and a close approximation to the ideal one without noise is obtained. The Q-FNN has been applied to the concentration determination of freon in the presence of interference gases. The prediction error for a test set in quantification is less than 10% while no qualitative mistake is observed, implying that the quantitative FNN has sustained the feature of pattern recognition. The results indicate that the Q-FNN has obvious advantages not only in converging speed, but also in the quantitative accuracy over the ANN.     

Investigation of equilibria in solution. Determination of equilibrium constants with the HYPERQUAD suite of programs

A new suite of 10 programs concerned with equilibrium constants and solution equilibria is described. The suite includes data preparation programs, pretreatment programs, equilibrium constant refinement and post-run analysis. Data preparation is facilitated by a customized data editor. The pretreatment programs include manual trial and error data fitting, speciation diagrams, end-point determination, absorbance error determination, spectral baseline corrections, factor analysis and determination of molar absorbance spectra. Equilibrium constants can be determined from potentiometric data and/or spectrophotometric data. A new data structure is also described in which information on the model and on experimental measurements are kept in separate files.     

Determination of the main component in high-purity organic substances: Current status and prospects

The current status of the determination of the purity of high-purity organic substances using direct and indirect chromatographic methods is discussed. It is shown that the results of the chromatographic determination of the main component in high-purity organic materials are generally ambiguous and unreliable. The possibility of determining the purity of organic compounds using an elemental analyzer of C, H, and N without involving standard samples of analytes is examined; the advantages and drawbacks of this approach are discussed. It is demonstrated experimentally by the example of several nitrogenated compounds that the main component can be determined with an error no worse than 0.6 rel. %, whereas the error of element (nitrogen) determination in the molecule does not exceed the permissible error of elemental analysis.     

Hydrogenation enthalpimetry--I: microcomputer-assisted calibration and data-acquisition

A calorimetric device is described which permits enthalpimetric determination of unsaturated hydrocarbons through the enthalpy change for their catalytic hydrogenation. Samples with weights from < 1 to about 20 mg can be analysed with a mean error of about 2%. The method makes extensive use of digital electronics and is well suited to routine automated determination of unsaturation. The principal drawback is lack of specificity.     

SYSTEMATIC ERRORS DUE TO EXCITATION PULSE WIDTH IN FAST KINETIC TECHNIQUES

Abstract— Convolution of the decay function with the excitation pulse function leads to systematic error in both kinetic and spectral analysis of transients. The error in rate constant determination for a first order decay can be kept to about 1% if the decay half-life is equal to the pulse half-width and to well below 1% for half-lives equal to or longer than twice the half-width. The errors in transient absorbance measurements are much greater. Only if the half-life approaches 200 times the pulse half-width is the error reduced to 1%. It is suggested that spectral measurements should be corrected for this error.     

Determination limits in high-performance liquid chromatography of plant phenolic compounds with an ultraviolet detector

The capabilities of high-performance liquid chromatography (h.p.l.c.) for the determination of phenolic compounds in 80% ethanol extracts from plant material are described. A reversed-phase column was used and elution was done with a linear gradient from 0.01 M phosphoric acid up to methanol. The efficiency of the method was studied via determination limits, defined as the minimum concentration of a compound (μg of compound per gram of extracted dry plant material) necessary to provide 90% probability that the relative error on the determination of the compound in an extract from a plant sample taken at random is < 10%. These limits take into account matrix interferences as a source of error, and were calculated with a minicomputer for the determination of 19 phenolic compounds in plant extracts. For good determinations, the concentrations of the components should be in the range 1–10 mg g^?1 of dry plant material. Separating the extracts into different chemical groups (on ion-exchange materials) prior to h.p.l.c. decreases the determination limits about five times. The dependence of determination limits on the u.v. characteristics of the compound, the sample clean-up, and the column characteristics are discussed quantitatively by means of a simple empirical equation.     

Gas-chromatographic determination of 1,1-dimethylhydrazine in water

A gas-chromatographic procedure for the determination of 1,1-dimethylhydrazine in water was developed on the basis of its reaction with 4-nitrobenzaldehyde yielding the corresponding hydrazone, the extraction of the latter from water with an organic solvent, its subsequent preconcentration by evaporation, and the determination on a gas chromatograph with a nitrogen-phosphorus detector. The determination limit of 1,1-dimethylhydrazine is 0.03 μg/L. The relative error of the determination is no larger than 22% in the concentration range 0.06–0.60 μg/L and 33% at a level of 0.03 μg/L.     

The Application of A Three Dimensional Calibration Model to Nickel Determination in Copper - Based Materials

Abstract

The interferences in the determination of Ni by flame atomic absorption spectrometry due to the presence of copper and fluoboric acid have been studied according to a previously proposed calibration method ¹ that allows the investigation and quantification of the matrix effect. The ability of traditional calibration methods, like standard addition (SAM) or direct calibration approach, to overcome matrix effects has been studied. Such investigations have shown that direct calibration could provide as far as a -42% error in nickel determination, whereas SAM error could be about +17%. However this large error could be decreased to ±2% when the mathematical model here proposed is applied.

Once the method is verified to be able to solve matrix effects, it was applied to the determination of nickel in a real sample of copper fluoborate. The theory of experimental design was set as an useful tool in order to study matrix effects.     

Determination of Iridoids in Gentiana rigescens by Infrared Spectroscopy and Multivariate Analysis

Gentiana rigescens is a famous herbal medicine in China for treatment of convulsion, rheumatism, and jaundice. Here, the infrared determination of gentiopicroside, swertiamarin, sweroside, and loganic acid in G. rigescens from different areas and varieties was presented for the first time. Reference information for the iridoids were obtained by high-performance liquid chromatography. Partial least squares was used to characterize the relationship between spectra matrix and concentration vector for the determination of the analytes. For determination of gentiopicroside, the appropriate performance of partial least squares model was acquired with coefficient of determination of calibration and coefficient of determination of prediction values of 0.965 and 0.868. The root mean square error of estimation (RMSEE), root mean square error of cross validation (RMSECV), root mean square error of prediction (RMSEP), and residual predictive deviation (RPD) values were 2.612, 5.292, 5.239?mg g^?1, and 2.701, respectively, based on the first derivative and multiplicative scatter correction. For determination of the total iridoids, the best results were obtained using the coefficient of determination of calibration and coefficient of determination of prediction of 0.943 and 0.834, RMSEE, RMSECV, RMSEP and RPD of 3.896, 7.536, 6.543?mg g^?1 and 2.438, respectively, based on the first derivative. Both models were reliable and robust. The results demonstrated that infrared spectroscopy provided a rapid, low-cost tool to monitor the quality of G. rigescens by the determination of the iridoids.     

偶氮氯膦Ⅰ光度法测定稀土镁硅铁合金中氧化镁

采用重铬酸钾分离氧化镁体系,试验了大量重铬酸钾存在下,镁(Ⅱ)与偶氮氯膦Ⅰ的显色条件,研究了制样条件对测定氧化镁的影响。试验结果表明,测定误差主要来源于取样、制样及氧化镁的分离。建立了稀土镁硅铁合金中氧化镁的偶氮氯膦Ⅰ光度测定法。方法的RSD为6．67％。     

Limits of determination and calibration in HPLC

Summary In HPLC calibration the expressions lowest calibration limit and determination limit are defined in statistical terms. The lowest calibration limit is the minimum mass in the measured series of calibration points. It is calculated from the confidence interval of the inverse of the calibration function as the lowest mass limit that may be differentiated from zero mass with a preset probability of error. If the calculated lowest calibration limit is lower than the actual data, points at lower concentration may be measured. The determination limit is the smallest concentration of an analysis that is differentiated from the concentration zero or an apparent blind value in the calibration curve with a given probability of error.Using two different UV-detectors (variable wavelength and photodiode-array) the lowest calibration limit is experimentally evaluated and compared with specific data for the detectors.Dedicated to Prof. Dr. E. Bayer, Tübingen on ocassion of his 60th birthday.     

Iodimetric determination of carbon disulphide study of the effect of pH

The effects of pH and time on the error (due to decomposition of xanthate) involved in the iodimetric determination of carbon disulphide have been studied. A pH range of 5.4 to 5.8 (acetatebuffered) is recommended, Carbon disulphide varying in amount from 0.0060 to 0.3240 g can be determined with an error of ± 1.0%.     

三异辛胺-硅胶柱高效液相色谱分离与测定矿石中微量金

研究用三异辛胺（ＴＩＯＡ）－硅胶柱反相萃取色谱分离、孔雀绿萃取光度法测定金（Ⅲ）、金回收率在９６％以上。应用于矿石中微量金匠测定,结果满意,测定的最大相对误差（ｎ＝５）为６．３％。     

Determination of naphthalene and total methylnaphthalenes in gasoline using direct injection-mass spectrometry

A high-speed determination of naphthalene and total methylnaphthalenes using a non-separative method based on direct injection into the mass spectrometer was performed. The results obtained for total methylnaphthalenes were very similar to those provided with fast gas chromatography-mass spectrometry (GC-MS). However, the non-separative method afforded higher concentrations in the determination of naphthalene than those found when fast GC-MS was used. We propose a correction that removes this error very satisfactorily and allows the same results to be obtained with both methodologies. The non-separative method is rapid, simple and - in view of the results - highly suitable for the determination of naphthalene and total methylnaphthalenes in gasoline samples.     

铜合金中铅锡极谱催化波同时测定

铅、锡的极谱分析有多种方法,但各有不足之处。本文报道,在醋酸铵-铜铁试剂体系中,铅、锡可分别在峰电位为-0.82V及-0.92V处产生良好的导数极谱催化波,大量铜的存在不干扰测定。此法应用于铜合金中铅、锡的同时测定,获得了满意的结果,检测下限可达10ppb,回收率相对误差在10%以内。