Capillary HDC

Recent developments in fused-silica capillary technology has led to such columns being used for HDC. The microcapillaries are normally 1-10 pm diameter with column volumes not exceeding 20 nl. The corresponding flow rates required for such systems are thus much lower than those of packed-column HDC. This technology has recently been commercialised with the 

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Figure 1. Comparison between capillary HDC model calculations (-) and ejxperimental data. (Reproduced with permission from Ref. 13. Copyright 1980, Plenum Publishing Corp-...

The utility of HDC is somewhat limited by the relatively poor resolution and particle-size discrimination of the method, which restrict the precision of HDC in silica sol characterization. In principle, accurate particle-size distributions of silica sols also are possible with the HDC method. However, for such characterizations special software with corrections for the extensive band dispersion in HDC is required, along with a suitable band deconvolution method (28). Commercial HDC apparatus with this sophisticated software package apparently is no longer available. Standards are generally required, although quantitative retention relationships have been reported for capillary HDC systems in characterizing polymers (37). As with all of the other separation methods, careful selection of the mobile phase is required in HDC. Mobile phases generally are the same as those used for the FFF methods and SEC. 

The experimental set-up used for porous chromatography is virtually identical, to that used for HDC as described elsewhere (i6,1T). The use of stainless steel columns for the LEG work required 3l6 stainless steel column end-fittings and l/l6 O.D. capillary tubing. 

A number of publications (6-10) have demonstrated that the size separation mechanism In HDC can be described by the parallel capillary model for the bed Interstices. The relevant expression for the separation factor, Rj., (ratio of eluant tracer to particle mean residence times) Is given by. 

Figure 9 shows the data fit obtained by use of Equation 26 with () = = 0.5. These results Illustrate that by Including all three mechanisms (HDC in small and large capillaries, and particle partitioning) an Improved fit results. At this point. It must be emphasized that although i and are computational parameters with arbitrarily chosen values, they represent physically meaningful quantities with regard to the separation process. 

Sedimentation techniques are also utilized for particle size distribution for particles on the order of 0.1-50 pm. Capillary hydrodynamic chromatography (HDC) gives particle size distributions for particles of about 0.005-0.7 pm. 

Capillary hydrodynamic chromatography (CHDC), in which a long narrow capillary replaces the packed bed, is an extension of HDC [58]. 

Another extension of HDC is to replace the packed bed with a long capillary (39). Capillary particle chromatography requires 30 kPa pressure and has a separating range of 0.2-200 pm. In such techniques, the particle transit time is a logarithmic function of particle size. 

This separation method HDC was developed within the Dow Chemical Company in the early 1970s [27]. This method has been utilized for determining the particle size of many polymer lattices [27,28], but it also can be used for characterizing a wide range of silica sols. Separation by the hydrodynamic effect is illustrated in Figure 40.12 [29]. Colloids flowing between particles in a packed bed or within a capillary are subjected to different velocities... 

Equipment and technique for HDC of silica sols are essentially the same as for SEC. The difference is that the packed bed of the separating column is composed of nonporous, rather than porous, particles. Typically, these particles are polystyrene-based beads, but glass or dense silica beads also are effective. Alternatively, a long, narrow capillary can be used as the separating medium [30]. 

In the first approximation, the column packed with small particles can be considered an array of capillaries. Hydrodynamic processes augment the exclusion based separation (see section 11.7.2, Retention Mechanisms and Accompanying Processes in SEC) and form the ground of a liquid chromatography-like method called hydrodynamic chromatography, HDC. HDC found application in separation of veiy large macromolecules, particles and dispersions. Its separation selectivity is rather low and it was partially substituted by the group of methods termed fieldflow fractionation (see seetion 11.3.3, Molar Mass Dispersity). 

Figure 10.16 Rp — Dp curves calculated from HDC capillary bed model at low ionic strength (1 mM), illustrating universal calibration behaviour. Values shown are for the assumed...

Hydrodynamic Chromatography (HOC) - Fractionation and Sizing (packed-column HDC 0.03-2 pm, capillary HOC 0.02-50 pm)... 

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