Monolithic particulate column packings

Advances in column technology have improved the selectivity, stability, and reproducibility of LC analytical columns. For example, analytical columns are packed with a variety of stationary phases, providing enormous versatility in the separation process. This section describes (1) column dimensions, (2) particulate column packings, (3) monolithic column packings, and (4) the use of guard columns. 

It is interesting to compare the flow characteristics of both types of silica columns with respect to the column pressure drop and column permeability. The column pressure drop / flow rate curves of both types indicate a much lower slope of the monolithic columns (not shown) than the particulate columns. In other words, the column pressure drop of the monolithic columns corresponds to that of a column packed with 15 pm silica particles [6]. The column permeability values are listed in Table 3. 

The use of monolithic columns in LC has advanced rapidly since their first introduction in the 1990s [18-21]. In contrast to capillary columns packed with particulate stationary phases, monolithic columns consist of a single continuous support. Monolithic stationary phases can be subdivided in two classes, i.e., polymer-and silica-based materials. 

CEC columns are generally made of fused-silica tubing, usually packed with the appropriate stationary phase. Today, the most commonly used CEC columns have i.d. of 100 p,m or less, with 50 and 75 p,m i.d. being the most popular. The stationary phase is retained in the column by two frits. Column designs can be categorized into two major types OT columns and packed structures, which include packed columns, monolithic columns, and microfabricated stractures (open or continuous beds). Packed capillary columns are most commonly used, as has been demonstrated in numerous papers [9-11]. They can be subdivided into three different categories columns packed with particles, columns with continuous beds fabricated in situ creating a rod-like monolithic structure, and columns with immobilized or entrapped particulate materials. 

The present edition is considerably updated and expanded, covering all aspects of ion chromatography and all developments in this field that have been introduced over the past 10 years. This includes the application of hydrophilic interaction and mixed-mode liquid chromatography for the determination of ionic and ionizable compounds, the introduction of novel detection methods and hyphenated techniques, the design of new separation media for capillary and monolithic columns, columns packed with particulate ion exchangers having smaller particle diameters, and mixed-mode stationary phases. 

Figure 3-22. Cumulative pore volume distribution of different HPLC columns indicating monomodal pore size distribution for polymer monolith and bimodal distributions for both packed particulate silica and silica monolith columns. (Reprinted from reference 90, with permission.)...

The cohuim pressure drop as a function of the flow rate was measured on each cohimn packed with the particulate (Gl-G - G5-G) and monolithic silicas (PKlll - PK118) with n-heptane/ethylacetate (80/20, v/v). The flow rate was increased by 0.5 ml min intervals from 0.5 to 4.0 ml min and the corresponding pressure recorded. The same measurement was performed in the reversed order. The measured pressure was corrected by the pressure drop caused by the cotmecting tubings. This measurement was done without the column. The column permeability Kf was calculated according to equation (1) ... 

The packing in a CEC column plays the key role of providing sites for the sorptive interactions (as in HPLC) as well as supporting EOF. The development of both particulate packings having properties tuned for CEC, and alternative column technologies such as monoliths, is emerging rapidly. 

---