Column impedance

It is difficult to compare separation techniques in any general way. Comparison may be based on the traditional figures of merit, such as resolution Rs (including column efficiency N, selectivity, retention, and peak capacity), chromatographic speed, sample capacity, sensitivity, detection and column impedance, as well as breadth of application. Usually a tradeoff between these attributes is found. Berger [26] has compared GC, pSFC, cSFC, LC and CE on the basis... 

Column impedance E has been introduced by Bristow and Knox (7) ... 

Flow controllers Many detectors are flow sensitive and, thus, a flow controller may be essential. The controllers are simple pneumatic devices that will maintain a constant mass flow rate independent of the column temperature or column impedance. 

Before looking at the overall control scheme, let us discuss what is meant by the column impedance. 

In a packed column, however, the situation is quite different and more complicated. Only point contact is made between particles and, consequently, the film of stationary phase is largely discontinuous. It follows that, as solute transfer between particles can only take place at the points of contact, diffusion will be severely impeded. In practice the throttling effect of the limited contact area between particles renders the dispersion due to diffusion in the stationary phase insignificant. This is true even in packed LC columns where the solute diffusivity in both phases are of the same order of magnitude. The negligible effect of dispersion due to diffusion in the stationary phase is also supported by experimental evidence which will be included later in the chapter. 

Column Type Minimum Reduced Plate Height Minimum Reduced Velocity flow Resistance Parameter Separation Impedance... 

The possibility of obtaining significant improvements in performance by using semi-packed and open tubular columns is clearly illustrated by the values for the separation impedance in Table 1.17. Variation of the reduced plate height with the reduced velocity for an open tubular column is given by equation (1.82), assuming that the resistance to mass transfer in the stationary phase can be neglected... 

The separation impedance represents the difficulty of achieving a certain performance and should be minimized for optimum performance. The highest performance is achieved by a column which combines low resistance to flow and mlnlnua dispersion of chromatographic solute bands. 

CUORICINO crystals are grouped in elementary modules of four elements (see Fig. 15.7) held between two copper frames joined by copper columns. PIPE pieces are inserted between the copper and TeOz, as a heat impedance and to clamp the crystals. There is a 6 mm gap between crystals with no material in between. The four detectors are mechanically coupled some of the PTFE blocks and springs act simultaneously on two crystals. 

We consider a point source axisymmetric example as illustrated in Figure 10.20. We shall examine the rise of the plume, zv, as a function of time. As the hot gases rise due to the source Q, initiated at time t = 0, the gases at the front or cap encounter cooler ambient air. The hot gases in the cap, impeded by the air, form a recirculating zone, as illustrated in Figure 10.20. Entrainment of air occurs over the vertical plume column and the cap. The warmed, entrained air forms the gases in the plume. 

Figure 1 The column tube end is drawn to a fine tip. To prevent fouling the ion source with packing material a small amount of coarser HPLC material is placed inside the tip. By coating with high-temperature epoxy glue (EPO-TEK 353ND) the tip is mechanically protected. This design gives the best chromatographic results. A 30-cm packed column with 3 ym Spherisorb ODS particles gives almost 5 x 10 theoretical plates (3). The separation impedance (unretained solute time per plate multiplied by the pressure drop per plate and divided by the viscosity of the eluent) has been calculated as 750.

In pHPLC, there are numerous types of columns used. The comparison and characterization of these columns are often discussed in terms of thermodynamic properties and kinetic characteristics. The retention factor, k, selectivity, a, and the peak asymmetry are believed to be representative parameters for the thermodynamic properties, while the kinetic characteristics are often expressed in dimensionless magnitudes of reduced plate height, h, separation impedance, E, and flow resistance factor, ( ). 3... 

In the course of this work two phenomena were noted for which no explanation was sought. When ammonium bicarbonate was added to sea water to prepare a regenerant solution, a white precipitate was formed which was settled out and discarded with no attempt made at chemical analysis. Toward the end of some of the regeneration cycles and at the start of some of the feed cycles, gas was evolved in the ion exchange columns, which eventually disappeared. This gas, which was not identified, caused serious flow impedance. 

Many reports (78-84) investigated the differences in packed and capillary supercritical fluid chromatography. Unfortunately, the rift between packed and capillary column users of SFC impeded the development of the science. This rift is a likely cause of the current low interest in SFC. Ideally, the unique features of the mobile phase is the area of scientific exploration that should be exploited. Choice of column size or type should be dependent upon the analytical problem to be solved. 

The resistance of the electrolyte, Ra, represents the ohmic resistance of the complete column of electrolyte between both electrodes. Note that in this way of presentation, the Faradaic impedance cannot necessarily be correlated with an occurring process, therefore it is represented by a general symbol ZF. It can be seen in this case that the total impedance consists of contributions from the working and the counter/reference electrodes, and knowledge of the behaviour of only the working electrode needs manipulation. A simple possibility is to use a CE/RE electrode with much larger surface than the WE. However, in the work presented in this book, this manipulation is not done, because the system used here will be used in applications as a set of two identical electrodes. In addition, one of the parameters that we are interested in is the electrolyte resistance, a parameter that is suppressed when using electrodes of different surface areas. 

Silica gel G (30), and Uni si 1 (31,32). All of these LC procedures are hampered by the absence of an adequate detection system. Although the lack of on-line detection has impeded the adaptation of the LC procedures to HPLC, preparative HPLC of glycolipids has been performed on silica SI 60 with post-column, off-line TLC detection (30) and with a moving wire detector (31). The procedure described below for per-O-benzoylation of glycolipids with benzoic anhydride in pyridine and DMAP as catalyst avoids N-benzoylation problem and provides a convenient method for the detection and preparative isolation of glycolipids. The application of this procedure for the isolation of 15 mg of glycolipids in a single HPLC run is described. 

In pressure-driven operation, considerable band broadening was observed at high linear velocity, although the separation impedance was much lower than that of a particle-packed column owing to the much lower flow resistance. The separation impedance (E = AP to / r N2 = (AP / N) (to / N) (l/r )) expresses the total column performance in terms of the reciprocal number of theoretical plates per unit time and pressure drop. Because the contributions of the B- and C-terms are expected to be similar for a pressure-driven mode and an electro-driven mode, the difference in performance can be attributed to the greater contribution of the A-terms in Eqn. 5.2 in the pressure-driven mode. The contribution of the A-term is known to be less in CEC than in HPLC [6],... 

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