Particle Size and Back Pressure

Selection of columns and mobile phases is determined after consideration of the chemistry of the analytes. In HPLC, the mobile phase is a liquid, while the stationary phase can be a solid or a liquid immobilised on a solid. A stationary phase may have chemical functional groups or compounds physically or chemically bonded to its surface. Resolution and efficiency of HPLC are closely associated with the active surface area of the materials used as stationary phase. Generally, the efficiency of a column increases with decreasing particle size, but back-pressure and mobile phase viscosity increase simultaneously. Selection of the stationary phase material is generally not difficult when the retention mechanism of the intended separation is understood. The fundamental behaviour of stationary phase materials is related to their solubility-interaction... 

The principles of how to operate an HPLC column under optimal isocratic conditions were outlined a long time ago by Guiochon and coworkers [4]. The maximum column performance at the lowest pressure is always achieved around the minimum of the van Deemter curve. If we reduce the particle size, the back-pressure increases - at equal velocity - inversely proportionally to the square of the particle diameter. At the same time, the velocity at the minimum of the van Deemter curve increases with decreasing particle diameter. Thus, the pressure at the optimum rises with the third power of the reduction in particle diameter dp. 

Data collected for each run Included cation concentration using ICP and H concentration by titration. Filtering characteristics were determined using solid and liquid yield rates, as well as back pressures during the filtration cycle. The filter cake was characterized by moisture content and particle size. Selected samples of the cake were analyzed using SEM to determine average particle size and shape. 

Test 10 Silica-H was recirculated in MLC pump at 5000 rpm, 11 psi back pressure, 31.7turnovers/h, 41pm, for 24h (761 turnovers). LPC and PSD measurements (Figs. 18.31a-c) show results very similar to Test 8, with slight increase in mean particle size and an increase in LPC for size 0.56-0.60 pm. 

The pressure experienced by the pump as it forces mobile phase through the column is known as the column back pressure. This will vary of course, from column to column (according to dimensions and particle size) and with the flow rate and viscosity of the mobile phase used. The relationship between these parameters is expressed in the following equation ... 

Particle size and size distribution define the quality of the support material and are the key determinants of efficiency and back-pressure of the column.1-3 The effect of dp on H is discussed in Chapter 2. For a well-packed column, Hmin is approximated to 2-2.5 dp. Also, since the van Deemter equation C term is proportional to dp2, columns packed with small particles have much less efficiency loss at high flow rates.14 However, since column back-pressure is inversely proportional to dp2, columns packed with sub-3-pm particles can easily exceed the pressure limit of most HPLC instruments at 6,000 psi. Note that decreasing particle size while keep the L constant can increase column efficiency and peak resolution (Figure 3.5A) and also increase peak height and sensitivity (Figure 3.5B). 

Fig. 10.16 shows some fictional pressure/densification plots of high-pressure agglomeration, which illustrate the above and some additional characteristics of this technology, particularly the influence of particle size and distribution on degassing during and spring-back after compaction. It was assumed that all materials reach a final density of 90% (residual porosity 10%) at maximum pressing force. 

Particle size was relatively insensitive to solute concentration in the liquid according to some authors [17,19], while others observed that backing away from saturation conditions may be abetter choice to prepare more uniform particles. A marked particle size increase and PSD enlargement with increasing concentration was observed by Reverchon et al. [18] in the SAS precipitation of yttrium, samarium, and neodimium acetates. This result is well illustrated in Figure 24.9a—c that are referred to samarium acetate precipitation from DMSO at the same pressure and temperature but at 10,40 and 65 mg/ml concentration, respectively. From these SEM images the large increase of samarium acetate particle size and PSD is evident. 

Since the publication of the third edition in 2004, considerable effort has been focused on the development of monolithic separation materials for use in ion chromatography. Monolithic media offer the potential benefit of faster analysis or improved resolution with comparable analysis speed, thus following the trend toward shorter analysis times observed in conventional liquid chromatography. While method speedup in conventional liquid chromatography (UHPLC) is achieved by utilizing smaller particle sizes and smaller column formats, this pathway can be followed only to a certain extent in ion chromatography due to the limited back pressure tolerance of metal-free components in the fluidic... 

Microfiltration (MF) is a membrane filtration in which the filter medium is a porous membrane with pore sizes in the range of 0.02-10 pm. It can be utilized to separate materials such as clay, bacteria, and colloid particles. The membrane structures have been produced from the cellulose ester, cellulose nitrate materials, and a variety of polymers. A pressure of about 1-5 atm is applied to the inlet side of suspension flow during the operation. The separation is based on a sieve mechanism. The driving force for filtration is the difference between applied pressure and back pressure (including osmotic pressure, if any). Typical configurations of the cross-flow microfiltration process are illustrated in Fig. 2. The cross-flow membrane modules are tubular (multichannel), plate-and-frame, spiral-wound, and hollow-fiber as shown in Fig. 3. The design data for commercial membrane modules are listed in Table 1. 

The advantages of monosized chromatographic supports are as follows a uniform column packing, uniform flow velocity profile, low back pressure, high resolution, and high-speed separation compared with the materials of broad size distribution. Optical micrographs of 20-p,m monosized macroporous particles and a commercial chromatography resin of size 12-28 p,m are shown in Fig. 1.4. There is a clear difference in the size distribution between the monodispersed particles and the traditional column material (87). 

A trend in chromatography has been to use monosized particles as supports for ion-exchange and size-exclusion chromatography and to minimize the column size, such as using a 15 X 4.6-mm column packed with 3-/rm polymer particles for size exclusion chromatography. The more efficient and lower back pressure of monosized particles is applied in the separation. 

Several compromises are involved in the selection of the correct particle size. On one hand, one desires the highest possible resolution in the shortest amount of time. Therefore, the smallest particle size should be chosen that still gives resolution of the polymer without causing excessive column back pressure. On the other hand, there are constraints on both the strength of the particle and the strength of the polymer. This section discusses the selection of the best particle size. 

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