Ion Chromatography - Column Efficiency
Today, pellicular packings are no longer in common use. High efficiency columns are best obtained in ion chromatography using silica based materials as the stationary phase either with the ion exchange moiety chemically bound to the silica surface or adsorbed from solution in the mobile phase onto its surface.
The dispersive bonded phase, thus, carries the ion exchanger absorbed on its surface However, care must be taken when employing silica based ion exchange media as most silicas are soluble in mobile phases outside the pH range of 4.0 to 8.0 and even at a pH of 8.0 dissolution of the silica can occur with the release of the bonded material. As a consequence, clever modification of the distribution system may be necessary if high resolution and high speed is required in a separation using silica based ion exchange material. Unfortunately, fast and efficient separations in ion chromatography can be difficult to achieve using other types of material as the stationary phase.
A book on ion chromatography is not the place for a detailed discussion on chromatography theory. However, some understanding of the processes that cause a solute band to spread during its passage through a column is necessary, not only to help diagnose column problems, but also to help in operating the ion chromatograph in the most efficient manner. Consequently, the individual processes of band dispersion in a packed column will be briefly considered.
The effect of each dispersion process on the column efficiency will be illustrated using the curves shown in figure 7. There are three major dispersion processes that take place in a column each producing its own contribution to the total dispersion. As the spreading effect is measured as the peak variance resulting from each process and each process is random in nature, the overall variance of the eluted peak can be obtained by summing all the individual variances resulting from each dispersion process.
The three dispersion processes that will be considered will be Multi-path Dispersion, Dispersion from Longitudinal Diffusion and Dispersion From Resistance to Mass Transfer. .It will be seen that certain dispersion processes will dominate in different types of chromatography column. For example there is no packing in a capillary column so there can be no multi-path dispersion and it will be seen that different mass transfer components will also dominate in different column types.
Figure 7. The HETP Curve (A graph relating the variance per unit length of a solute passing through the column against the linear velocity of the mobile phase)