Silica-based packings, mass

SO3H) for cations, is the same in HPLC as in classical ion exchange chromatography. A variety of substrates ranging from cross-linked polystyrene, cross-linked polydextrans, cellulose and silica have been utilised. However, due to the problems of swelling, compressibility and mass transfer encountered with macroreticular polymeric supports and the limited pH stability of silica-based packings, the efficiencies achieved in lE-HPLC applications have been moderate. 

A liquid mobile phase is far denser than a gas and, therefore, carries more momentum. Thus, in its progress through the interstices of the packing, violent eddies are formed in the inter-particular spaces which provides rapid solute transfer and, in effect, greatly increases the effective diffusivity. Thus, the resistance to mass transfer in that mobile phase which is situated in the interstices of the column is virtually zero. However, assuming the particles of packing are porous (i.e., silica based) the particles of packing will be filled with the mobile phase and so there will... 

The promise of monolith is the achievement of a higher performance at a lower backpressure than a packed bed. While this is true in principle, current implementations are limited by the fact that the external wall to the structure is made from PEEK. At the time of this writing, the commercially available monoliths can only be used up to a pressure of 20MPa (200 atm, 3000 psi), while packed bed steel columns can be used up to double this pressure and higher. Also, the preparation of the monolith appears to be cumbersome. At the current time, the silica-based monoliths are available only with an internal diameter of 4.6mm. The speed is thus also limited by the flow rate achievable by the HPLC instrument. At the same time, the detector of choice today is the mass spectrometer, which can tolerate only much... 

A monolithic silica-based CIS stationary phase was used under high flow rate condition (2 mL/min) without significant back pressure in IPC analysis of a recently discovered new drug candidate for the treatment of Alzheimer s disease [15]. Nanoscale IPC using a monolithic poly(styrene-divinylbenzene) (PS-DVB) nanocolumn coupled to nanoelectrospray ionization mass spectrometry (nano-ESl-MS) was evaluated to separate and identify isomeric oligonucleotide adducts. Triethylammonium bicarbonate was used as the IPR. Interestingly, the performance of the polymeric monolithic PS-DVB stationary phase significantly surpassed that of columns packed with the microparticulate sorbents CIS or PS-DVB [16]. 

Totally porous and pellicular silica packings are used in adsorption chromatography for the separation of (usually) non-polar and moderately polar compounds [54-56]. The eluotropic strength of common supercritical fluids is not very high which prevents the elution of polar solutes in a reasonable time and/or with an acceptable peak shape. Mixed mobile phases containing a polar solvent such as methanol, extend the polarity range of compounds that can be eluted but often at the expense of selectivity. Consequently, silica-based, chemically bonded sorbents are the most widely used for the separation of polar and high molecular mass compounds. 

The main advantages of monolithic columns are the superior separation performance and low flow resistance. In addition due to their continuous nature, frits are not required to retain the stationary phase. The production process of monolithic columns is more flexible than that of packed columns e.g., photo-polymerization can be applied to prepare monolithic structures or add selectivity locally. Both polymer- and silica-based monolithic capillary columns have been used for highly efficient separations in LC-mass spectrometry (MS) applications for proteomic research [24,25]. 

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. 

Commercial polyethylenes have a broad molecular mass distribution (see Figure 4.2), and a column or columns with a wide range of molecular size separation is required. Polystyrene/divinylbenzene column packings alone have the necessary range and resolution. Silica-based columns may replace these in the future, since they have better thermal stability, but at the moment are limited by the molecular mass range available. 

Cappiello s research group applied micro-HPLC to the screening of water samples for pesticide contamination. With electron ionization mass spectrometric detection and the use of two microcolumns packed with a Cig silica-based stationary phase, they performed the preconcentration of water samples and successfully detected trace levels of pollutants in similar samples. They also showed that ion-interaction micro-HPLC on a Cig stationary phase with hexylamine as the ion-pair reagent and coupling to particle beam mass spectrometry could be successfully used for the analysis of herbicides ranging from acidic species such as... 

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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