Fractional exchange

The principal exchangeable bases in soils are the cations Ca, Mg, K and Na (Table 4.3). In a fractional exchange, sodium is released most readily, and Ca least readily. The cation preference of soil clays and silts follows the lyotropic series. Special attention has been given to the K Ca (plant nutrition), Na Ca (saline soils) and Ca Al (acid forest soils) ion pairs. In soil organic compounds, selectivity for multivalent cations such as Ca relates mainly to the disposition of their acidic groups, for example carboxylic groups and phenolic hydroxyl groups. 

The following steps outline the approach taken in developing a relative fractional exchange... 

Finally, the same time point data for each peptide for the reference sample are then connected with a specific colored line. This procedure is carried out for each time point, using a different color. This process is then repeated for the experimental sample using the same color scheme. The resulting plot is a relative fractional exchange comparability plot (which looks like a butterfly or mirror plot, as shown in Figure 13.5a), which allows for quick visual (qualitative) comparative assessments of all the data that is acquired in an HX-MS experiment. 

The associated dififa-ence plot that goes with each relative fractional exchange comparability plot is gena-ated by subtracting the change in mass data for peptide i) at time point (t) for the experimental sample from the change in mass data for the same peptide i) obtained at the same time point... 

With the difference plot and the relative fractional exchange comparahility plot, all of the HX-MS data acquired in the HX-MS experiment are made accessible, and any difference between the two samples should be relatively easy and quickly established. 

Figure 7 shows other results using sputtering rates of about 1 ///hr for samples oxidized at various temperatures. The numbers in parenthesis give the time (in hours unless otherwise stated) in and then in OJ. The mole fraction exchanged at the surface varies markedly with temperature. 

Experiments were also performed at 600° and 910°C, in which the time in Oi was varied for the same initial time in (Figs. 8 and 9). At 600°C the surface fraction exchanged varied from 0.17 to 0.25. With a short Oi exposure the slow sputtering rate of 1 ///hr gives a pronounced decrease in concentration with sputtering time or depth. At longer times the curves become more nearly horizontal. After exposures of 4 and 16 hr to Oi at 910°C the lines become horizontal for the sputtering times used. Extrapolation of... 

To derive the line diffusion coefficient D, from the concentration profile of we change the scale of the ordinate so that the fraction exchanged at the oxide-oxygen interface reads unity, and fit the curve to an error function plot ( ). It is found that A(600°C) = 9.0 X lO and A(910°C) = 7.7 X 10" cm sec ... 

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