Column packings pellicular

To protect the column from compounds that irreversibly adhere to or partition into the column packing, a precolumn may be used. A pellicular packing coated with C g material (Cjg Corasil II, Waters Associates) has proven beneficial, yet has a minimally detrimental effect on compound resolution. 

Arsenic species were preconcentrated on Zipax, a pellicular anion exchange material and separated on a column packed with high performance liquid chromatography grade strong anion exchange resin, then continuously reduced with sodium tetrahydroborate and detected by atomic absorption spectrometiy. Detection limits were 2ng for arsenite, arsenate and monomethylarsinate and lng for dimethylarsonate. 

Silica gel columns. The following columns have been tested in this experiment two 2 mm ID x 61 cm Porasil A columns (35-75 /u,m) (or hand-packed columns) two 3.9 mm ID x 15 cm /uPorasil columns (10 /u,m) two 3.9 mm ID x 15 cm Resolve Silica columns (5 or 10 /am). Other silica gel columns should work in this experiment however, they have not been tested. A pellicular column (packed or purchased) may also be used (e.g., 2-2 mm ID x 61 cm Corasil II columns). 

The only way to increase the rate of diffusion of solutes is to raise the temperature substantially, but this would lead to problems in thermally unstable analytes. The alternative is to reduce the distance through which the molecules diffuse. Efficient separation then requires the use of smaller particles for column packings. This apparently simple expedient has evolved into a new practice of LC that is competitive with GLC in speed and resolution of complex mixtures and applicable to many more materials than GLC. Packing materials comprised of particles as small as 5 pm are currently available. Smaller particles are extremely difficult to handle and give an almost impermeable column. To solve this problem, solid glass beads of 30-50 pm in diameter can be coated with a layer of porous material. These are called pellicular beads. The porous layer may serve as a solid stationary phase or be coated with a very thin layer of liquid stationary phase with an extremely large surface area. 

The HPLC of large biomolecules such as proteins and DNA often requires specialized columns packed with wide-pore polymer or silica-based bonded phase with extra-low silanol activity.1215 Alternate approaches are pellicular materials or very small nonporous particles. Some of these columns are packed in PEEK or titanium hardware to allow the use of high-salt mobile phase and to prevent possible protein denaturing by metallic leachates. Further details on bio-separations and application examples are discussed in Chapter 7. 

Figure 19-32. Diagrams of the general types of column packings Lett rally porous, Middle pellicular, Right microporous.

Pellicular particles technical term for a synthetic HPLC column packing material consisting of microglass beads, 10-50 pm in diameter with a 2-3 pm surface film of active stationary phase material. These packings have only about 10% of the capacity of microporous materials but produce columns with greater separation efficiencies due to the very rapid equilibrium processes that occur with such regular particles and thin stationary phase films. 

Fig. 11.5.4. HPLC separation of leukotriene standards. Chromatographic conditions column, 5 im ODS (250x4.6 mm I.D.) coupled to a guard column packed with Corasil (ODS) pellicular packing mobile phase, methanol-water-acetic acid-ammonium hydroxide (67 33 0.08 0.04), pH 6.2 flow rate, 1 ml/min detection, UV at 280 nm. The peaks shown include two non-enzymatically formed d -/rans-leukotriene B4 (diHETE) isomers of leukotriene B4. Reproduced from Metz et al. (1982), with permission.

The preceding analysis applies spedfically to porous particles. However, several workers have suggest that pellicular, small-particle columns might be especially suited for these separations [p. 243 or Ref. (79)]. Some of the inherent di vantagesof very small partides could be avoid, while taking advantage of the smaller C term [Eq. (33)] of pellicular particles. Finally, columns packed with very snoall nonporous pa des hold similar promise, and l- rm-particle columns of this type have now been reported (7/7). 

Columns packed with pellicular stationary phases of small particle size have low permeability and, therefore, cannot be operated at relatively high flow rates due to the pressure limitation of commercial HPLC instruments. However, due to the non-porous structure, micropellicular particles are generally more stable at higher temperature than conventional porous materials. Consequently, in the... 

For the analysis of nucleotides, the optimum conditions using a 1 mm X 3 m stainless steel column packed with a pellicular anion exchange resin are ... 

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