Separation impedance

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. 

The separator resistance is usually characterized by cutting small pieces of separators from the finished material and then placing them between two blocking electrodes. The separators are completely saturated with the electrolyte. The resistance (Q) of the separator is measured at a certain frequency by ac impedance techniques. The frequency is chosen so that the separator impedance is equal to the separator resistance. To reduce the measurement error, it is best to do multiple measurements by adding extra layers. The average resistance of single layer is determined from multiple measurements. The specific resistivity, ps cm), of the separator saturated... 

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 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],... 

For mechanically rigid and physically-chemically robust particles, the relationship between column performance, in terms of the separation impedance or column resistance factor (CRF), such that pressure drop and the flow rate is linear over a wide range of flow rates and inversely proportional to square of the particle size, and can be approximated by the relationships... 

Fourth dimensionless parameter, dimensionless separation impedance, E (also known as efficiency ), which embraces retention time, pressure drop, plate number, eluent viscosity and retention factor, is taken as a measure of quality ... 

Note-. The separation impedance of this column is poor E = = 6.4 x... 

The dimensionless separation impedance E depends on u (or F) and has a minimum like a van Deemter curve (middle right). With the Kozeny-Carman factor of ffi = 1000 we come to the relationship ... 

Column type Minimum reduced plate height Minimum reduced velocity Flow resistance parameter Separation impedance... 

Since packed columns can be operated at Emin = 2000 with plate numbers from 10,000 to 1 million, alternative column types will compete effectively only if they can be operated at separation impedance values below 2000. Enlarging the column... 

Column plate count Column hold-up time (s) Column length (m) Reduced plate height Reduced velocity Separation impedance... 

Estimation of the Dillusion Coefficient in the Mobile Phase 2.17 Performance Index and Separation Impedance... 

The performance index is a measure of the quality of the separation. Knox (19) was searching for a criterion that is a purer measure of column performance and criticized the performance index for its dependence on the retention factor and the viscosity. Therefore he defined a dimensionless group, the separation impedance ... 

Performance Index and Separation Impedance 23 Thermodynamics of Chromatogra diy... 

Note. The dependence of the performance index and the separation impedance on the porosity of the colunm is not found in the older literature, since the specific permeability was defined via the linear velocity Uq rather than the flow rate (or superficial velocity) F/nr unfortunately, both permeabilities have been used interchangeably, which is incorrect and causes some confusion. We adhere here to the definition of the specific permeability by lUPAC and are consequently forced to include the porosity factor into the preceding equations.]... 

A separation impedance of 20000 is not a bad value. The minimum E level is around 2000 (h = 2, = 500). If is greater than 10, then HPLC can no longer be regarded as a high-performance process. 

---