Packing material chemical stability

Another approach to increase HPLC speed is the use of higher temperatures. The viscosity of a typical mobile phase used in reversed-phase separation decreases as the column temperature is increased. This allows an HPLC system to operate at a higher flow rate without suffering too much from increased back pressure. Zirconia-based packing materials provide excellent physical and chemical stability. They have been used successfully for high-throughput bioanalysis at elevated temperatures.9... 

Chemical modification of the adsorbent surface significantly alters practically all properties of the base material. Dense coverage of the adsorbent surface with inert ligands usually expands chemical stability of the packing material. 

The expectation that graphitized carbon-packing material would show high chemical stability over the entire pH range (pH=0-14), and ultimate hydrophobic properties, led to a number of investigations in this area. There... 

The stationary phases used in reversed-phase chromatography, when it was first introduced, comprised of a non-polar substance (e.g. squalene) coated on to a silica-based support. These are now seldom used. The stability of such systems is low, because the forces holding, say, squalene to even a silylated silica are so weak that the stationary phase is easily washed from the column. A compromise reversed-phase packing material was developed, which had a polymeric hydrocarbon stationary phase on the support, but although quite successful it has now been superseded by a chemically bonded stationary phase of which some examples are discussed below. 

Several materials have been proposed as alternatives to quartz gravel pack materials and have been tested for stability under steam injection conditions. Phenolic resin-coated gravel has been used in thermal wells (79). The coated grains agglomerate with temperature to form a consolidated pack that does not require a screen. Alumina-based materials have been reported to be stable in the high pH environment to over 300 °C (78), but others (80) found that dissolution of these materials can occur. Weaver and Knox (80) proposed an alternate material, SRG, whose chemical formulation is not revealed. A steel wool filter has been used as an alternative to gravel packs in thermal wells (58, 59, 81, 82). 

The attributes of a SEC column packing material are listed in Table 2. As indicated, the support must be optimized with respect to specific resolution, efficiency, column pressure, and mechanical, chemical, and thermal stability. Recovery of mass and activity is particularly important in the analysis and purification of biopolymers. It also plays a role in the analysis of nonbiochemical synthetic polymers on silica-based SEC columns. In addition to recovery losses by adsorption, the recovery for both groups of polymers can also be reduced by polymer degradation as a result of, for instance, mechanical shear. 

NPLC stationary phases include metal oxides and moderately or strongly polar chemically bonded phases. Unmodified silica gel and silica-based bonded phases are most frequently used nowadays. Considerable effort in the development of new HPLC column packing materials in the past years has resulted in significant improvement of the column efficiency, reproducibility, and increased stability at elevated temperatures and at higher pH, enabling better compatibility with HPLC/mass spectrometry techniques and rapid analyses. Even though the new column technologies were primarily focused on RPLC separations, normal-phase HPLC also benefits from the improved properties of the support materials with uniform small particles and well-defined pore size. 

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