Reference material linear

Quantitative XRF analysis has developed from specific to universal methods. At the time of poor computational facilities, methods were limited to the determination of few elements in well-defined concentration ranges by statistical treatment of experimental data from reference material (linear or second order curves), or by compensation methods (dilution, internal standards, etc.). Later, semi-empirical influence coefficient methods were introduced. Universality came about by the development of fundamental parameter approaches for the correction of total matrix effects... 

Requirements for standards used In macro- and microspectrofluorometry differ, depending on whether they are used for Instrument calibration, standardization, or assessment of method accuracy. Specific examples are given of standards for quantum yield, number of quanta, and decay time, and for calibration of Instrument parameters. Including wavelength, spectral responslvlty (determining correction factors for luminescence spectra), stability, and linearity. Differences In requirements for macro- and micro-standards are considered, and specific materials used for each are compared. Pure compounds and matrix-matched standards are listed for standardization and assessment of method accuracy, and existing Standard Reference Materials are discussed. 

ISO Guide 32 (1993) Linear calibration using reference materials. International Organization for Standardization, Geneva, Switzerland, 31 pp. 

Calibration of the apparatus is necessary usually water and air or nitrogen are the reference materials. Vibrating tube densimeters designed to operate close to atmospheric pressure can achieve repeatability of parts in 10s. If the reciprocal of the frequency is linear in density, accuracies of 1 part in 105 are readily achievable. 

Matrix-matched Certified Reference Materials or spiked samples should be used to determine the linearity of a method. 

Multidimensional Data Intercomparisons. Estimation of reliable uncertainty intervals becomes quite complex for non-linear operations and for some of the more sophisticated multidimensional models. For this reason, "chemometric" validation, using common, carefully-constructed test data sets, is of increasing importance. Data evaluation intercomparison exercises are thus analogous to Standard Reference Material (SRM) laboratory intercomparisons, except that the final, data evaluation step of the chemical measurement process is being tested. 

Calibration Most process analyzers are designed to monitor concentration and/or composition. This requires a calibration of the analyzer with a set of prepared standards or from well-characterized reference materials. The simple approach must always be adopted first. For relatively simple systems the standard approach is to use a simple linear relationship between the instrument response and the analyte/ standard concentration [27]. In more complex chemical systems, it is necessary to adopt either a matrix approach to the calibration (still relying on the linearity of the Beer-Lambert law) using simple regression techniques, or to model the concentration and/or composition with one or more multivariate methods, an approach known as chemometrics [28-30]. 

NBS 706 broad distribution polystyrene, NBS 705 narrow distribution polystyrene, and NBS 1478 narrow distribution polystyrene reference materials were used as samples in the evaluation of the proposed linear calibration method in this study. These reference materials have the following reported values ... 

Based upon studies with the NBS standard reference materials, the linear calibration method appears to give equivalent accuracy compared to a peak position method irrespective of sample disperslty provided that the sample elute over the elution volume range covered by the polydlsperse standard and low dispersion SEC column are utilized. 

A reliable way of identifying the existence of micro- and mesoporosity and of estimating pore volume is the t-plot analysis of de Boer et al. (1966). The amount of gas adsorbed by the test material is related to the amount adsorbed by a non porous reference material. For the latter, there is a linear relationship between the amount adsorbed, V, and the average thickness of the adsorbed layer, t, i. e. 

Diffuse reflectance R is a function of the ratio K/S and proportional to the addition of the absorbing species in the reflecting sample medium. In NIR practice, absolute reflectance R is replaced by the ratio of the intensity of radiation reflected from the sample and the intensity of that reflected from a reference material, that is, a ceramic disk. Thus, R depends on the analyte concentration. The assumption that the diffuse reflectance of an incident beam of radiation is directly proportional to the quantity of absorbing species interacting with the incident beam is based on these relationships. Like Beer s law, the Kubelka-Munk equation is limited to weak absorptions, such as those observed in the NIR range. However, in practice there is no need to assume a linear relationship between NIRS data and the constituent concentration, as data transformations or pretreatments are used to linearize the reflectance data. The most used linear transforms include log HR and Kubelka-Munk as mathemati-... 

To establish linearity and the linear range, a blank solution, plus at least 6, but preferably 10, independent solutions should be prepared from a traceable reference material and presented to the instrument at least as duplicates and in random order. The range of these solutions then defines the linear range, assuming, of course, statistical analysis of the calibration data supports this contention. How is the range known without doing experiments As stated above, there may be enough prior information to have a... 

The next polymer Standard Reference Materials, issued in 1970, were a linear and a branched polyethylene. They are representative of crystalline olefin polymers, which have assumed great commercial and scientific importance. The linear material was kindly provided by the Dupont Co. and the branched by Union Carbide Corp. 

Standard Reference Material 1475, Linear Polyethylene (Whole Polymer)... 

Precision and Accuracy. Table I shows the results of tests on the linear polyethylene NBS Standard Reference Material No. 1475. This reference material has a pellet-to-pellet coefficient of variation of 3% in the limiting viscosity number according to the NBS Certificate. At least 50 pellets are recommended for a representative sample on which limiting viscosity number is obtained. Nine analyses were performed over a period of about one month on the pellets, using approximately three pellets per determination. The analysis of these data is shown in Table I along with a comparison with the NBS data and their estimate of precision. 

Figure F2.3.3 Gradient HPLC separation of the food reference material carotenoids using Protocol 3. Conditions 3-nm x 250-mm x 4.6-mm Waters C30, column, 1.0 ml/min flow rate, visible detection at 450 nm, column temperature 35°C, solvent A = 50 mM ammonium acetate in methanol, B = isopropyl alcohol, C = tetrahydrofuran (all solvents contain 0.1% TEA). Flow program 90% A/10% B linear gradient, 54% A/35% B/11% C over 24 min, linear gradient to 30% A/35% B/35% C over 11 min, hold 8 min, then return to initial conditions over 10 min.

The mechanical properties of poly(methyl methacrylate), PMMA, have been studied for quite a long time and, in addition to its industrial interest, PMMA constitutes a kind of reference material. Indeed, among the amorphous linear polymers it represents an intermediate between the very brittle polystyrene and the tough bisphenol A polycarbonate considered in Sect. 4. Furthermore, as shown in [1] (Sect. 8.1), the molecular motions responsible for its large p transition are precisely identified, as well as the nature of the cooperativity that develops in the high temperature range of the p transition. 

Method validation is important to ensure that the analytical method is in statistical control. A method may be validated by the so-called method evaluation function (MEF) (Christensen et al., 1993), which is obtained by linear regression analysis of the measured concentrations versus the true concentrations. A true concentration in a solution can be obtained by use of a high purity standard obtained from another manufacturer or batch than the one used for calibration. Both the high purity standard and the solvent are weighed using a traceable calibrated balance. If certified reference material is available this is preferred. The method evaluation includes the most important characteristics of the method as the following elements (see Figure 2.7) ... 

---