Non-zero intercept

There are any number of variations that can be applied to the CLS technique. Here we will only consider the most important one non-zero intercepts. If you are interested in some of the other variations, you may wish to consult the references in the CLS section of the bibliography. 

Referring to equation [40], we can see that we require the absorbance at each wavelength to equal zero whenever the concentrations of all the components in a sample are equal to zero. We can add some flexibility to the CLS calibration by eliminating this constraint. This will add one additional degree of freedom to the equations. To allow these non-zero intercepts, we simply rewrite equation [40] with a constant term for each wavelength ... 

CLS calibration with non-zero intercept from Al/Cl predicts concentrations for AI... 

CLS with non-zero intercept estimated pure component spectra... 

Typically, at least two different values of m1 (besides ml = 0) are used because there are invariably phase shifts that arise from various factors that do not depend on the gradient moments, resulting in a non-zero intercept of c > versus mx. Thus, a velocity image is time consuming because each set of measurements with a value of m1 is an image in itself. 

A graphical method such as this is usually superior to a simplistic calculation of k2 = k -l [NaOH] (e.g. in the Worked Example 8.16), because scatter and/or chemical back or side reactions will not be detected by a single calculation. Also, the involvement of a back reaction (see next section) would be seen most straightforwardly as a non-zero intercept in a plot of k (as y ) against [reagent in excess] (as V). 

Figure 6.17 A Randles-SevCik plot of 7p against Data refer to the oxidation of aqueous ferrous ion at a stationary platinum wire electrode. The non-linearity at the higher scan rates represents the demand for flux at the working electrode being too great since i is too fast, while the non-zero intercept is caused by non-faradaic currents contributing...

Figure 6.17 broadly follows the Randles-Sevcik equation insofar as most of the plot of p against v is linear. The figure does, however, have a non-zero intercept, and is not linear at the highest scan rates. 

The simplest reason for a non-zero intercept is non-faradaic current, the causes of which are discussed in Section 6.8.2. 

Figure 2. Mechanism of dihydroxyacetone/arsenate reaction with FDP aldolase. Both dihydroxyacetone and inorganic arsenate are not the inhibitor of the aldolase reactions. The rate constant for the arsenate ester formation is determined enzymatically (a plot of 1/v vs 1/E gives a non-zero intercept which is attributed to the rate at infinite enzyme concentration and that rate corresponds to the rate of nonenzymatic formation of the arsenate ester).

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