Touching band

Investigators dealing with optimization of chromatography systems for preparative separation on laboratory or larger scales in the first stage using TLC selected stationary and mobile phases to obtain a resolution Rj > 1.5 for each pair of touching bands. Such a resolution permits the introduction of a 3 mg/g adsorbent. 

Based on touching bands between the product and the limiting impurity, the loading factor for the second component is estimated in Eq. (7.11). 

If a compound is the major component in a mixture, the production rate increases if the impurities are eluted first. However, it is preferable for the product to be eluted prior to any closely retained impurity if a compound is not the main component in the mixture. Linder self-displacement conditions, the product can actually be separated in a more concentrated solution than in a touching-band separation, especially if a later-retained impurity is in high concentration. The production rate in this case can be improved by an increase in loading, despite some decrease in recovery. 

Direct scale-up of an analytical HPLC method at touching-band conditions is feasible only when there is a need for small (mg) amounts of the isolated material and the mobile phase does not contain nonvolatile additives. Using a touching-band optimization technique, the amount of material loaded may be increased until visible peak broadening occurs and the first sign of overlapping between the product peak and closely retained impurities is observed. The amount of product that may be loaded onto a column under touching-band conditions can be estimated as [5]... 

Optimizing load until elution profiles start to overlap (touching bands)... 

A resolution of 1.5 corresponds to a baseline separation at touching band situation. At a resolution of 1.0 there is still an overlap of 3% of the peaks. 

No re-equilibration time necessary (touching band-methods as well as closed-loop recycling and SMB-chromatography can be applied)... 

Minimization of the total elution and cycle time results in higher productivity and lower eluent consumption. Optimizing the selectivity can be contradictory to these parameters. However, if the cycle time is minimized the productivity for touching band operation increases. 

Case 4 Touching Bands, Second Component Plateau Present, Zk < L < Zl or... 

The corresponding value of fj is given by combining Eqs. 8.39 and 8.42. We have the touching band case. In this case, is equal to Cf, and the second component plateau is intact. For still lower values of tp, the width of the plateau at C decreases. Band interaction ceases to have an irifluence on the profile of the second component band when the second shock is eluted at time t[ (Eq. 8.29). This situation corresponds to the point K and coliunn length z = Zr, as discussed later. 

Case 4. The two bands are separated, there is no mixed zone left, but there is still a plateau on the top of the second band profile, residual of the interaction between the two bands. This is the beginning of the "touching bands" situation (Eqs. in Table 8.4). 

The separation exceeds touching band resolution but a plateau subsists on the top of the second component band. 

Optimization for Touching Bands Using the Ideal Model.871... 

The maximum production rate is reached at the limit of touching bands, and it remains constant when the loading factor is increased further. 

Knox and Pyper [17] made the first systematic study of the optimization of the experimental conditions in preparative liquid chromatography based on the use of a simple chromatographic model. They considered touching band separation (i.e., in the ideal model, tR 2 — 1e (Figure 8.6), or approximately a imit resolution between the two bands) with the following assumptions ... 

Golshan-Shirazi and Guiochon have investigated the optimization of the experimental conditions using the analytical solution of the ideal model [20-24]. In the case of touching bands, the recovery yield is practically total ( 100%). Therefore, the same experimental conditions assure the maximum production rate for both components. Their assumptions are limited to the following two ... 

Using this model, several optimization problems have been discussed—the touching band problem (this section) and the overlapping band problems, with or without a recovery yield constraint (next two sections). 

In the case of touching bands [20], this approach differs from the one of Knox and Pyper [17] essentially by the fact that the competition between the mixture components is now taken into accoimt. Also, since the Langmuir isotherm model... 

Figure 18.6 Comparison of the true band profiles at the optimum sample size predicted by the noncompetitive model of Knox and Pyper (dotted lines) and by the ideal model with competitive Langmuir isotherms (solid lines) in the case of touching bands, fcg j = 6. a = 1.2. Isotherm coefficients = 2.4 2 = Column length 25 cm. Phase ratio F = 0.25. Mobile phase velocity 0.6 cm/s. (a) Feed composition 1 9. (b) Feed composition 3.6 1. (c) Feed composition 9 1. Reproduced with permission from S. Golshan-Shirazi and G. Guiochon, /. Chromatogr., 517 (1990) 229 (Figs. 3 to 5).

As explained in Chapter 8, in the discussion of the solutions of the ideal model, touching bands take place whenever the loading factor of the second component is equal to... 

---