Adsorbates, breakthrough volumes

The application of pressure causes a considerable decrease in the retention or breakthrough volume for an adsorbate transversing down a sorbent column. This trend is amply illustrated in Figure 9 where the retention volume for benzene in C02 has been plottted as a function of pressure for the crosslinked styrene/ divinylbenzene resin at A0°C. In this figure, there is a considerable decrease initially in V over a small pressure interval and the breakthrough volume appears to become constant... 

Two macroscopic methods to design adsorption columns are the scale-up and kinetic approaches. Both methods rely on breakthrough data obtained from pilot columns. The scale-up method is very simple, but the kinetic method takes into account the rate of adsorption (determined by the kinetics of surface diffusion to the inside of the adsorbent pore). The scale-up approach is useful for determining the breakthrough time and volume (time elapsed and volume treated before the maximum allowable effluent concentration is achieved) of an existing column, while the kinetic approach will determine the size requirements of a column based on a known breakthrough volume. 

Collection of the retention data was initially taken on Tenax and XAD-2 adsorbents at 150, 250, and 350 atmospheres. As experimental work progressed, additional data was taken at closer pressure intervals to better define the trend in breakthrough volume with fluid pressure. Measurement of retention volumes below 100 atmospheres was difficult, due to the "threshold pressure" solubility limit (as defined by the sensitivity of the UV detector) of the test solute probes (23). Most of the generated retention data were measured at three temperatures 40, 60 and 80 C. 

The breakthrough volxame trends for many sorbate types on the porous polymeric sorbents indicate a limited trapping capacity in the supercritical fluid CO2 above 200 atmospheres. Fractionation and selective retention on these sorbents seems only possible below this specified pressure limit for the odoriferous solutes examined in this study. Adsorbent surface area appears to be the most significant factor contributing to the retention of sorbates on these sorbents as well as activated carbon. For certain synthetic adsorbents (Tenax, XAD-2) employed in this study, pressure-induced morphological changes in the polymer matrix lead to an increase in the sorption capacity, and hence to an increase in breakthrough volumes at intermediate pressures. 

Flow Rate. The effect of flow rate varies with the sorbent. If the flow rate is too high, a nonequilibrium condition occurs due to poor vapor contact, and poor collection efficiency will result. In most cases, once the equilibrium flow rate is reached, no increase in breakthrough volume is observed with reduction in flow rate. In some sorbents, such as silica gel, a high flow rate will cause a heating effect due to adsorbent water. This heating effect may decrease breakthrough volume. 

Breakthrough volume depends on the retention power of the SPE adsorbing material and determines the volume of the water sample, which can be percolated through the pre-column until the analyte arrives at the pre-column outlet. Thereby, the breakthrough volume determines the extent of analyte enrichment. Obviously, to achieve a high extent of pre-enrichment, the retention of the analyte should be a maximum at the sample loading step (but it should be a minimum at its elution step). 

A thermostated chromatographic system should be used. The column, packed with the studied stationary phase, is first equilibrated by passing the aqueous phase, pure water or buffer or water and organic modifier, without any surfactant molecule. The eluent is then switched to a mixture made of the same mobile phase containing a known concentration, C, of surfectant. A surfactant concentration front thus moves down the column. If the surfactant is adsorbed by the packing material, the breakthrough volume of the front, Vb, will exceed the dead or void volume, Vq. The amount of surfactant adsorbed will then be (Vb-Vo)C [8]. This amount should be expressed preferably in pmol/m for convenient comparison betw n phases. 

For accurate results, it is advisable to control the surfactant adsorption performing a titration. The effluent leaving the column is collected until the detector response reaches a plateau corresponding to the absorbance value of the initial surfactant solution. The surfactant concentration in die collected effluent is determined by titration. The missing surfactant mass is adsorbed on the stationary phase in the column. The mass obtained by titration should corroborate the one obtained using the breakthrough volume. 

Non-Micellar Mobile Phases. Figure 4.2A shows the low concentration part of the adsorption isotherm. From an experimental point of view, the breakthrough volume of mobile phase can be very large. The 1.9 pmol/m SDS amount adsorbed on the ODS phase with a 0.001 M SDS mobile phase corresponds to 0.0002 moles of SDS because the ODS surface area is 105 m /g (Table 4.1). This amount of SDS is contained in 200 mL of mobile phase (=breakthrough volume). It means the duration of the experiment at this point is at least 4 hours at 1 mL/min. 

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