Calibration direct

Direct calibration is the simplest investigational method and is used widely for molten salts. Subsequent calculations of the equilibrium parameters of the 

Oxygen electrodes in ionic melts. Oxide ion donors 

Lux acid-base reactions require data on the dependence of e.m.f. of the cell used on the equilibrium oxide-ion activity (concentration) in the melt. The calibration of the potentiometric cell is performed as described below. 

Potassium and sodium hydroxides and sodium peroxide are conventionally used as strong Lux bases for the calibration of the potentiometric cells. 

Their equivalent masses corresponding to the quantity of the Lux base whose complete dissociation results in the formation of 1 mol of oxide ions, are 80.0, 112.2 and 77.98 g, respectively. Other bases, e.g. BaO and Na2C03, are rarely used for the calibration owing to some specific features of incomplete dissociation, which will be considered in the next part. 

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Moisture measurements are important in the process industries because moisture can foul products, poison reactions, damage equipment, or cause explosions. Moisture measurements include both absolute-moisture methods and relative-humidity methods. The absolute methods are those that provide a primaiy output that can be directly calibrated in terms of dew-point temperature, molar concentration, or weight concentration. Loss of weight on heating is the most familiar of these methods. The relative-humidity methods are those that provide a primaiy output that can be more direc tly calibrated in terms of percentage of saturation of moisture. 

The voltage output of the more common types of thermocouple is of the order of 50V/C and the output is either read on a sensitive moving-coil meter or on a digital voltmeter. The reading is converted to temperature using a calibration chart supplied with the thermocouple. Some commercial units are available in which the thermocouple and instrument is supplied as an integral unit with the scale directly calibrated in temperature. If a separate instrument is to be used then it should be noted that the thermocouple resistance is only of the order of 10 and... 

The trade-offs between direct calibration and standard addition are treated in Ref 103. The same recovery as is found for the native analyte has to be obtained for the spiked analyte (see Section 3.2). The application of spiking to potentiometry is reviewed in Refs. 104 and 105. A worked example for the application of standard addition methodology to FIA/AAS is found in Ref 106. Reference 70 discusses the optimization of the standard addition method. 

Situation Suppose a (monovalent) ionic species is to be measured in an aqueous matrix containing modifiers direct calibration with pure solutions of the ion (say, as its chloride salt) are viewed with suspicion because modifier/ion complexation and modifier/electrode interactions are a definite possibility. The analyst therefore opts for a standard addition technique using an ion-selective electrode. He intends to run a simulation to get a feeling for the numbers and interactions to expect. The following assumptions are made ... 

Although the decomposition of a data table yields the elution profiles of the individual compounds, a calibration step is still required to transform peak areas into concentrations. Essentially we can follow two approaches. The first one is to start with a decomposition of the peak cluster by one of the techniques described before, followed by the integration of the peak of the analyte. By comparing the peak area with those obtained for a number of standards we obtain the amount. One should realize that the decomposition step is necessary because the interfering compound is unknown. The second approach is to directly calibrate the method by RAFA, RBL or GRAFA or to decompose the three-way table by Parafac. A serious problem with these methods is that the data sets measured for the sample and for the standard solution should be perfectly synchronized. 

PCR combines aspects of both CLS and ILS. In common with ILS it is based on the direct calibration of the property of interest from the multivariate predictor, irrespective of the direction of the causal relation. Contrary to ILS and in common with CLS it can use all predictor information even when there are many more... 

For relatively simple matrices, such as pure metallic CRMs synthetic reference materials for direct calibration were prepared and used, for example Bi, Cd, Hg, Pb and T1 in high purity gallium (HUtenkamp and Jackwerth 1988), Ag in copper (Pau-wels et al. 1990) and Au and Pd in silver (Hinds 1993). Direct calibration by solid biological materials with added analyte belongs also to these quite successfully applied techniques (Hofmarm et al. 1992). 

Direct calibration of methods and instrumentation i.e. ensuring that an analytical device is giving a correct reading. For some types of direct soUd sample analysis, sample results can be calibrated using several CRMs with suitable matrices (Kur-furst 1998) see also Section 4.4. 

Only a direct matrix match of sample and CRM, and the CRM s use as a direct calibrant will allow the user to demonstrate accuracy and subsequently traceability close to the uncertainties established during the CRM certification ( note matrixmatching may not be necessary with matrix-independent techniques). This reality places a significant burden on the CRM producers, since large uncertainties in the certified values may degrade the perceived value of the CRM. 

Direct calibration can be applied when the calibration coefficients are known, otherwise - in case of indirect calibration - the calibration coefficients are computed by means of experimentally estimated spectra-concentrations relations. 

General Routes. If a SAXS beamline in normal transmission geometry is used, calibration to absolute intensity is, in general, carried out indirectly using secondary standards. Direct methods require direct measurement of the primary beam intensity under consideration of the geometrical setup of the beamline. On a routine basis such direct calibration was commercially available for the historic Kratky camera equipped with zero-dimensional detector and moving slit device 14. 

Direct calibration to absolute intensity is not a usual procedure at synchrotron beamlines. Nevertheless, the technical possibilities for realization are improving. Therefore the basic result for the total scattering intensity measured in normal transmission geometry is presented. At a synchrotron beamline point-focus can be realized in good approximation and the intensity /(s) is measured. Then integration of Eq. (7.19) results in... 

Protection of the Detector. With all direct calibration methods the primary beam intensity must be measured. If the primary beam itself is attenuated, shape of the beam and spectral composition of the radiation may be altered. This problem is avoided if the load of the detector is reduced by scanning the beam using a slit or a perforated disc. On the other hand, in order to be useful at a powerful synchrotron beamline these devices should have very tiny and well-defined slits or holes. 

Bond et al. [791 ] studied strategies for trace metal determination in seawater by ASV using a computerised multi-time domain measurement method. A microcomputer-based system allowed the reliability of the determination of trace amounts of metals to be estimated. Peak height, width, and potential were measured as a function of time and concentration to construct the database. Measurements were made with a potentiostat polarographic analyser connected to the microcomputer and a hanging drop mercury electrode. The presence of surfactants, which presented a matrix problem, was detected via time domain dependent results and nonlinearity of the calibration. A decision to pretreat the samples could then be made. In the presence of surfactants, neither a direct calibration mode nor a linear standard addition method yielded precise data. Alternative ways to eliminate the interferences based either on theoretical considerations or destruction of the matrix needed to be considered. 

In contrast to MLR, CLS is a direct calibration method that was designed specifically for use with spectroscopic data, and whose model formula is a reflection of the classical expression of the Beer-Lambert Law for a system with mnltiple analytes ... 

Although the CLS model can be considered rather rigid and limited in its scope of application, its advantages can be considerable in cases where it is applicable. Furthermore, recent work [47-51] has shown that extensions of the CLS model can reduce some of this rigidity , thus enabling the power of direct calibration methods to be applied to a wider scope of practical applications. Such extensions of CLS will be discussed in the following section. 

Rogak, S. N., S. I. Green, and U. Pott, Use of Tracer Gas for Direct Calibration of Emission-Factor Measurements in a Traffic Tunnel, J. Air Waste Manage. Assoc., 48, 545-552 (1998a). 

It can be seen that the accuracy of the method is increased by increasing zo. The stress can then be deduced either from knowledge of the third-order elastic constants (Husson 1985), or by perturbation theory (Husson and Kino 1982), or from direct calibration. 

In Table IV, the values of Mn, Mw, [77], and Mw/Mn for the PMMA fractions D and H and whole polymers 122-7 and 122-9 calculated from the direct Mv vs. PRV calibration curve are compared with the available corresponding true values. In general, the calculated values of [ ] from the direct calibration curve are larger than the true values at low molecular weights as indicated in the values for fraction D and tend to converge to the true values as the molecular weight increases as indicated... 

Methyl esters were identified by comparison with authentic compounds on both the Carbowax 20M column and a XE-60 (a General Electric cyanosilicone gum) column. Some of the earlier identifications were made with a Carbowax 20M + H3P04 column and confirmed on l,2,3-tris(2-cyanoethoxy) propane. Also, some of the analyses of free acids were run on a Carbowax 20M-TPA(terephthalic acid terminated) column rather than the FFAP. Quantitative analysis of free acids and methyl esters was carried out using internal standards. Acetophenone was used with the acids, and veratrole was used with the methyl esters. Formic acid in the nitrobenzene and nitric acid liquors was analyzed quantitatively by direct calibration. 

The deformations of the isomers were also deduced from the spectroscopic quadrupole moment obtained from the hfs constant B. Since no quadrupole moments have been measured previously in Tl, hence no direct calibration could be made, we relied on the calculations of Lindgren and Rosen [LIN75]. Their relativistic calculations were used with our measured B s to give the values of Qs listed in Table I. 

The cost of the experiment (in this case preparation and analysis of 8 model solutions) is only slightly increased in comparison with the cost of direct calibration, so the described procedure can be used for routine water analysis. 

The availability of an on-line radioisotope detector for CE is especially appealing for several reasons. First, state-of-the-art radiation detection technology offers extremely high sensitivity. Second, radioisotope detection affords unrivaled selectivity because only radiolabeled sample components yield a response at the detector. Third, the radiolabeled molecule possesses the same chemical properties as the un-labeled molecule, thereby permitting tracer studies. Fourth, radioisotope detection can be directly calibrated to provide a measurement of absolute concentration of the labeled species. Finally, a capillary electrophoresis system in which radioactivity detection is coupled with more conventional detectors adds extra versatility to this new separation technique. 

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