Chemically inert stationary phase

This problem was remedied by the discovery of methods for chemically bonding the active stationary phase to the inert support. Most chemically bonded stationary phases are produced by covalent modification of the surface silica. Three modification processes are shown in Equations 3.6-3.8. 

Here c and are the concentrations of the substance under test in the stationary and mobile phase respectively, F" and F are the volumes of the stationary and mobile phase in the column, and is the retention time of an inert substance. From measurements of and f the capacity ratio k, and from this the partition coefficient K, can be calculated. In Figure 37 experimentally determined log k values are plotted against the density p and the pressure p of supercritical carbon dioxide at 40 C as a mobile phase for SFC experiments with alkanes having between 10 and 30 carbon atoms a chemically bonded stationary phase (Carbowax 400 on Porasil C) was used and further details are given in the caption of Figure 37. > The curves show that with increasing density the retention time f and the capacity ratio k decrease and consequently the... 

The most common mobile phases for GC are He, Ar, and N2, which have the advantage of being chemically inert toward both the sample and the stationary phase. The choice of which carrier gas to use is often determined by the instrument s detector. With packed columns the mobile-phase velocity is usually within the range of 25-150 mF/min, whereas flow rates for capillary columns are 1-25 mF/min. Actual flow rates are determined with a flow meter placed at the column outlet. 

The main criteria for selecting a stationary phase are that it should be chemically inert, thermally stable, of low volatility, and of an appropriate polarity for the solutes being separated. Although hundreds of stationary phases have been developed, many of which are commercially available, the majority of GLC separations are accomplished with perhaps five to ten common stationary phases. Several of... 

As mentioned before, there are two common types of GC gas-liquid chromatography (GLC) and gas-solid chromatography (GSC), depending on the physical state of the stationary phase. GSC is seldom used. In GLC the analyte is partitioned between the mobile phase (gas) and a liquid phase, which is retained on an inert solid support. The liquid phase should ideally possess a low volatility (so that it does not volatilise with the analyte), be thermally stable and chemically inert, and have favourable solvent characteristics. 

The stationary phases coated on the inert supports are similar in the two classes of columns. Liquids of different polarities are chosen for the separation of non-polar or polar compounds a relatively small number of solvents, chemically inert and heat resistant, are sufficient for most residue analyses. The most commonly used for organophosphorus compounds vary from low-polarity hydrocarbons, methylsilicones, phenylsilicones, phenylmethylsilicones or carboranesiloxanes 165 168 175 178 180 182 184 192,195,197,198,... 

Packed columns contain an inert and stable porous support on which the stationary phase can be impregnated or bound (varying between 3 to 25%). The solid support is made of spheres of approximately 0.2 mm in diameter, obtained from diatomites, silicate fossils (such as kieselguhr, tripoli) whose skeleton is chemically comparable to amorphous silica. These materials, which have a specific surface area ranging from 2 to 8 m2/g, have been commercialised by several companies such as Johns Manville, under the name of Chromosorb , and are used universally. Other synthetic materials have been developed such as Spherosil , made of small silica beads. All of these supports have a chemical reactivity comparable to silica gel because of the presence of silanol groups. 

I Liquid-liquid partition chromatography, where the sample components are partitioned between a moving liquid phase and a stationary liquid phase deposited on an inert solid. The two solvent phases must be immiscible. The stationary phase may he a large molecule chemically bonded lo the surface of a solid (bonded liquid phase) lo prevent loss by solubility in the moving phase. This method can also be subdivided into normal-phase systems, in which Ihe moving phase is less polar than the stationary phase, and reverse-phase systems, in which it is more polar. 

Pure fluids. Carbon dioxide is often the mobile phase of choice for SFC, since it has relatively mild critical parameters, is nontoxic and inexpensive, chemically inert, and is compatible with a wide variety of detectors including the flame ionization detector (FID) used widely in GC and the UV absorbance detector employed frequently in HPLC (7). The usefulness of carbon dioxide as a mobile phase in many instances is somewhat limited, however, because of its nonpolarity (8), and many polar compounds appear to be insoluble in it. For a sample containing polar compounds, pure carbon dioxide may not be the proper mobile phase. The elution of polar compounds is often difficult and the peak shapes for these polar compounds are sometimes poor. This latter difficulty is commonly observed with nonpolar supercritical fluids and may be due to active sites on the stationary phase rather than any inherent deficiency in the fluid itself. 

As the mobile phase (inert gas) carries the chemicals through the column, repeated partitioning of molecules takes place between the mobile and the stationary phases, and molecules of same (or similar) chemical and physical nature are separated into groups. These groups reach the end of the column at different times. This process is called elution, and it can be compared to a marathon race all runners start at the same time, but reach the finish line at different times due to the differences in their athletic ability. 

The most critical properties of a capillary column are resolution, support inertness, retention reproducibility, thermal stability, and column bleed. To provide fast, reliable, and accurate analysis, it is important that the stationary phase, internal diameter (ID) of the column, film thickness, and length of the column be chosen with a view to the particular application. CWC-related chemicals differ greatly from each other in their chemical and physical properties and thus the selection of the stationary phase is in most cases a compromise between resolution and analysis time. The most suitable stationary phases for the separation of chemicals related to the CWC are listed in Table 1, along with their structures and polarities (24). 

There are two other phases indicated in figure 3.8. The first is a so-called pyrocarbon material. Such a stationary phase is formed by pyrolizing an organic layer on a silica substrate. The idea is to combine the mechanical strength of silica with the chemical inertness of carbon. The value of 14 used here can be thought of as typical for carbonaceous materials. These materials do not seem to behave like non-polar phases in the tradition of chemically bonded phases for RPLC, but rather like phases of intermediate polarity. Hence, as for silica, they may be most useful in the reversed phase mode for the separation of very polar molecules using aqueous mobile phases. 

The carrier gas (1) acts as the mobile phase it is typically a chemically inert gas (e.g. N2, H2, Ne) - that is one that will not react with either the stationary phase or the sample being investigated. 

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