Concentration calibration curve

A calibration curve defines the relation between an analytical signal fP, produced by a definite component of a sample, the analyte, and the amount of this component or its concentration. Calibration curves usually have a linear range - sometimes after the analytical signal has been linearized (Fig. 3.3-14b). If there is a linear relation between the concentration c (or amount) of the sample and the observed signal its slope is called sensitivity 7 ... 

In general, it is preferable to deal with diffusion-controlled currents because they are generally much less affected by experimental variables and usually result in linear current-versus-concentration calibration curves. 

Other substance can be applied in order to get a concentration calibration curve. Indeed, the two parts located from 2900 to 3345 cm and from 3345 to 3600 cm evolve in intensity with concentration and can be used as a signature of NaCl. This approach allows emphasizing the two frequency ranges mentioned above, which have clearly opposite behaviour with concentration. It should be mentioned that the difference spectra show, at high frequency, a third band with a very small intensity. 

If a nonabsorbing spectral line occurs within the band pass of the monochromator along with the absorbing line, useful analytical data can frequently still be obtained. The sensitivity of the determination will be reduced and a nonlinear absorbance vs. concentration calibration curve will result. 

Size exclusion chromatograms and concentration calibration curve of low MW AM/AA copolymer ( = 8,000 g/mole). 

The external standard method is the technique used most frequently to gather quantitative information from a chromatogram. In this case, a pure reference substance (ideally the same compound as the one to be determined in the sample) is injected in increasing concentrations and the peak areas or peak heights obtained are plotted versus the concentration (calibration curve ->Chemo-metrics). These calibration curves should show a constant slope (linear curves), and the intercept should be as close to zero as possible. Since the calibration curves usually show nonlinear behavior and flatten off at higher concentrations (.see Section 12.2.6), the quantification should be carried out only within the linear part of the curve. 

This method is based on the bleaching action of fluoride ion content of the sample. The color of fhe red Zr-solochrome cyanine R (Aldrich Cat. No. 23,406-0, Sigma Prod. No. E2502) [52] complex fades as ZrOp2 is formed in the medium. As a matter of fact, no simple stoichiometric relationship exists between the fluoride and the zirconium complex with the dye. Therefore, in order to obfain reliable results, the reaction conditions need to be controlled very carefully. The absorbance of fhe reaction media is measured at 540 nm. The fluoride concenfrafion is evaluated using an absorbance-fluoride concentration calibration curve prepared with standard solutions. The method can be used for samples containing 0-2.5 pg fluoride. 

Direct potentiometric method The sample and standard solutions are introduced into the potentiometric measuring cell mixed with background electrolyte. The fluoride ion-selective electrode and appropriate reference electrode (saturated calomel electrode or silver/silver chloride electrode with double jimction is recommended) are dipped into the solution and the electromotive force is measured. An EMF versus log (fluoride ion concentration) calibration curve is used for evaluation. 

In the above equation, k is the absorption coefficient and I is the optical path length for the atom cell. Therefore, in AFS the signal size is directly proportional to both the light-source intensity and the atom concentration. Calibration curves for AFS with HCL excitation are linear with a slope of 1 (on a logarithmic plot) at low concentrations and bend back towards the concentration axis with a limiting slope of -0.5 at high concentrations. The curvature at high concentration is related to self-absorption in the atom cell. 

This is in contrast to concentration calibration curves, for which standard and sample solutions should differ in composition as little as possible. This completely unproblematic use of a single calibration curve is counterbalanced by the difficulty of precisely determining the individual activity coefficients of a particular type of ion in the standard solutions. Here again, one starts with concentrations, available by accurately weighing a certain amount of some salt of the corresponding measured ion. All methods for calculating the individual activity coefficients are only approximations. Thus one must use sufficiently accurate values experimentally determined by an independent method, and these are available only for a few ions (see Appendix). When this same problem arose in very precise pH measurements, the value of the activity coefficient of a dilute chloride solution was internationally agreed upon [54]. This lead to the operational definition of pH and to the well-defined pH values of a few standard buffer materials. 

In using concentration calibration curves, it is always assumed that the activity coefficient of the measured ion is the same in the standard and sample solutions. However,... 

This method, use of a concentration calibration curve obtained by dilution with a suitable conditioning solution, spans a broad range (usually 4 to 6 orders of magnitude in concentration) with the same relative accuracy. It is easy to carry out, and in the absence of interferences of the first kind allows analyses with an accuracy of 2% for monovalent and 4% for divalent indicated ions. 

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