Gradient exchange

BLYP Becke correlation functional with Lee, Yang, Parr exchange Gradient-corrected... 

Fig. 2. HPLC ion exchange gradient separation of samples (25 ug of uronic acid) of smaller (G7) citrus pectin oligomers (a), larger (G12) citrus pectin oligomers (b), and the B. fruit extract (c). Detection was by PAD. For each sample the peak co-eluting with the standard galacturonate oc-tamer was designated peak 8, and the remaining peaks were numbered consecutively.

FIGURE 8.13 Total ion current chromatogram for a UHP-RPLC-MS separation of one anion exchange fraction (fraction numher 6 from the 30-min anion-exchange gradient). 

Finally, I am unaware of reasons why any of the above possible candidates for the exchange gradient coefficient should be correct when applied to a bounded system. The semiempirical procedures based on fitting to the nonlocal exchange energies of atoms are more appropriate in this situation, even though they lack a complete, theoretical foundation at present [35, 36,37, 38]. 

Two ion-exchange gradient runs with either to or F varying can be used to derive values of ATand Z(25), much as in the use of Eqs. (20)-(22) for RPLC. Thus Eq. (18) can be written for tlte case of Iq varying ... 

The gradient-steepness parameter b for ion exchange is given by Eq. (16), which involves the ion-exchange gradient parameter r (Eq. (17)]. Oilcula-tion of r (needed in order to calculate b and requires a value of c. 

Lysozyme and ribonuclease were separated by cation-exchange gradient elution (both strong and weak ion dtange columns) as a function of gra dient time and flow rate (2S). Values of Z were obtained from isocratic data (Fig. 8) and used to predict Ox values for these gradient runs. There data are summarized in Table XI. Overall agreement is acceptable, with 0.98 and SD 0.17 for 14 ta points. 

There workers reported 19 c, values for the anion-exchange gradient elution separation of ovalbumin (M- 44,000), while varying experimental conditions widely column lengths of 7.5 and 15 cm, 30 s to 3 min, and 0.25 F 2 ml/min. A value of Z 8.8 was obtained using the data from Fig. 11 and the approach summarized in Table VI, permitting the prediction of (T, values (as in Table VIII). Good f reement was observed (27), with iLr-1.07 and SD- 0.11. 

This result is general for aU HPLC gradient methods e.g., in ion-exchange gradient du-tipn, PC is proportioiMl to R, when (cje it held constant... 

The following examples illustrate the application of the model for re-versed-phase and ion-exchange gradient elution, and isocratic separation by SEC. The examples correspond to actual experimental runs reported here or earlier. 

Cf In ion-exchange gradient elution, final vahietrfc in the giadieot... 

FIGURE 14.7 Chromatographic analysis of a nucleotide mixture. Anion exchange gradient method. 

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