Packed backpressure

Several of these points are met by applying the optimization steps discussed above, e.g., using smaller particles, shortening column lengths, and reducing solvent viscosity to reduce backpressure. When we consider a virtual column—a packed bed in a purely theoretical sense— we commonly accept that reducing particle size proportional to column length results in columns with at least the same theoretical efficiency. This is true, but only in the theoretical world. 

Monolithic column — The trend to use shorter columns in liquid chromatography means that the resultant lower separation efficiency is of concern. One way to improve HPLC separation efficiency on a shorter column is to reduce the size of the packing material, but at the cost of increased backpressure. Another approach to improve performance is increasing permeability with a monolithic column. Such a column consists of one solid piece with interconnected skeletons and flow paths. The single silica rod has abimodal pore structure with macropores for through-pore flow and mesopores for nanopores within a silica rod8182 (Figure 12.1). 

The promise of monolith is the achievement of a higher performance at a lower backpressure than a packed bed. While this is true in principle, current implementations are limited by the fact that the external wall to the structure is made from PEEK. At the time of this writing, the commercially available monoliths can only be used up to a pressure of 20MPa (200 atm, 3000 psi), while packed bed steel columns can be used up to double this pressure and higher. Also, the preparation of the monolith appears to be cumbersome. At the current time, the silica-based monoliths are available only with an internal diameter of 4.6mm. The speed is thus also limited by the flow rate achievable by the HPLC instrument. At the same time, the detector of choice today is the mass spectrometer, which can tolerate only much... 

While smaller particles in shorter columns can reduce run times, resolution and efficiency are sometimes compromised. We would like to pack smaller particles in longer columns for better efficiency, but the backpressures generated are outside the capability of current commercial HPLC instrumentation. One way to alleviate the pressure issue is to use high-temperature HPLC. 

From both theory and experimental evidence, raising the temperature by 10°C decreases the retention time by about 20% in isocratic chromatography and decreases the backpressure by 10% to 20% because of a reduction in the viscosity of the mobile phase. This can help to overcome the instrument limitations associated with running shorter columns packed with smaller particles, i.e., the pressure limitations of current HPLC systems. However, since the majority of reversed-phase columns available are silica-based, operating at temperatures above... 

Use of Operating Curve Frequently, it is not possible to assume that y, = 0 as in Example 2, due to diffusional resistance in the liquid phase or to the accumulation of solute in the liquid stream. When the backpressure cannot be neglected, it is necessary to supplement the equations with a material balance representing the operating line or curve. In view of the countercurrent flows into and from the differential section of packing shown in Fig. 14-5, a steady-state material balance leads to the following equivalent relations ... 

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