Exit velocity,mobile phase

Elution Chromatography The components of the mobile phase supphed to the cohimn ter feed introduction have less affinity for the stationary phase than any of the feed solutes. Under trace conditions, the feed solutes travel through the cohimn as bands or zones at different velocities that depend only on the composition of the mobile phase and the operating temperature and that exit from the cohimn at different times. 

Po) is the pressure at the column exit, and (uq) is the linear velocity of the mobile phase at the column exit. 

Figure 4. HETP Curves for the Same Column and Solute Using the Average Mobile Phase Velocity and the Exit Velocity...

Liquids have relatively low compressibility compared with gases and, thus, the mobile phase velocity is sensibly constant throughout the column. As a consequence, elution volumes measured at the column exit can be used to obtain retention volume data and, unless extreme accuracy is required for special applications, there is no need for the retention volume to be corrected for pressure effects. 

Fig. 6. Schematic presentation of a continuous annular chromatographic reactor. The sample (big arrow) and the mobile phase (small arrows) are continuously introduced from the top of the column, which rotates with the constant angular velocity co. Passing through the column, compound A is converted into the compounds B and C. Due to their different retention they split in three streams and exit at different positions from the column. (Reprinted with permission from [144])... 

This equation is valid everywhere in the column, and particularly at the coliurm exit, hence it can be used to describe the elution chromatogram. A perturbation Aqs of the stationary phase concentration of the additive takes place everywhere a perturbation of the mobile phase concentration of a sample component takes place. These perturbations migrate along the column at the constant component velocity Uz,i = h/(1 -I- Eq. 7.3). When a sample component perturbation... 

The multipath term (A) This term applies to columns packed with support particles. It becomes zero for open tubular columns when the mobile phase velocity is slow enough for the flow to be laminar (i.e., without mrbulent eddies). In a packed column, the paths of individual analyte molecules will differ as they take different routes through the spaces between the particles. Thus they will travel varying distances before they exit the column, and the difference between these distances contributes to band broadening. The relative magnitude of the multipath term depends on the particle and column dimensions. If Fig. 11.3 depicted a packed column, A would be a constant value for all values of u, and would appear as a horizontal line raising the curve by a constant amount. The multipath process is illustrated in Fig. 11.4. 

---