Column packings carbon loading

The most widely used support substance for the manufacture of packing materials in analytical HPLC columns is silica. Silica can be treated with organochlorosilanes or similar reagents to produce siloxane linkages of any derived polarity similar to what is done for GC columns (stationary phases). The most popular materials are octadecyl silane (ODS), which contains a carbon loading of CIS groups and octyl, which contains C8 groups materials such C2, C6, and C22 are also available. 

If significant amounts of interferences still remain in the sample extract after performing the above cleanup procedures, the extract may be subjected to a further cleanup step. Prepare a mixture of active carbon AX-21 and Celite 545, containing 8% and 92%, respectively. This is heated at 130°C for several hours and packed into a chromatographic column. This carbon column is preeluted with toluene and hexane, respectively. The sample extract is now loaded onto the column and the analytes are subsequently eluted with toluene. The eluant is concentrated for analysis. 

The dominant stationary phase in HPLC is modified silica and, to be more specific, octadecyl silica (ODS). It should be pointed out that there could be great differences between various types of ODS materials or even between different batches of the same material. Carbon load, free silanol content, endcapping, type of silica, and coupling chemistry to the Cig moiety, not to mention the several physical characteristics of the packing material all involve the behavior of an ODS column. However, a rather safe generalization is that, in such material, hydrophobic interactions are a dominant mechanism of separation [3-7]. 

Chemically bonded reversed-phase silicas Cig and to a lesser extent Cg sUica cartridges are packed with the same stationary phases as in LC columns, but with a larger particle size." " Although Cig sorbent with high carbon loading and some residual silanol are best suited, they are not able to extract polar compounds from large sample volumes. Therefore Cig silica is rarely used for multrresidue environmental analyses. 

Mozenski and Kucharski [2] examined the pressure-drop, overload limit, and flooding limit of a column (0.5 m diameter) packed with Pall rings (35 mm diameter) and Bialecki rings (35 mm and 50 mm diameter) sprayed with propylene carbonate up to 15 bar. Some specific correlations have been proposed and compared with literature data for atmospheric pressure, particularly with the use of the Sherwood diagram for loading-and flooding capacities. 

In order to obtain a commercial loading of the near-critical extractant, the extraction is sometimes carried out at enhanced pressures in the droplet regime. In such cases the liquid phase does not flow downwards as a film adhering to the packings of a column as is usually assumed, rather it falls down as a swarm of droplets. On the basis of the separation of a mixture of partial glycerides the behavior of packed columns in the droplet regime (instable flowing films) the efficiency of different column installations are compared. A mixture of 55 wt.% propane and 45 wt.% carbon dioxide is used as an extractant. 

Chromosorb Wisa diatomaceous e tii calcined with the addition of sodium carbonate. It is less dense than Chromosorb P, with a packed density of approximately 0.3 g/cm, and is softer with a greater tendency to produce fines. The surface M-ea is about 1 mVg. Celite 545 is simile to Chromosorb W. Chromosorb G is a calcined diatomaceous earth witii a density similar to that of Chromosorb P but tiie surface area is much less, about 0.5 mVg, and it is the least reactive of tiie Chromosorbs. The high resistance to mechanical damage together with the density and inertness m e it very suitable for low-loaded columns. The amount of stationary phase... 

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