Stationary-phase film, chemically bonding

It may be difficult to imagine a liquid mobile phase used with a liquid stationary phase. What experimental setup allows one liquid to move through another liquid (immiscible in the first) and how can one expect partitioning of the mixture components to occur The stationary phase actually consists of a thin liquid film chemically bonded to the surface of finely divided solid particles, as shown in Figure 11.8. It is often referred to as bonded phase chromatography (BPC). Such a stationary phase cannot be removed from the solid substrate by heat, reaction, or dissolving in the mobile phase. 

FIGURE 11.8 An illustration of partition chromatography. A thin liquid film chemically bonded to the surface of finely divided solid particles is the stationary phase. 

Choose the appropriate column length and determine the optimum film thickness. The use of chemically bonded columns has many advantages. Rearrangement of the stationary phase film is impossible because of the chemical bonding therefore, the efficiency of the columns is prolonged. Contaminated columns may be washed with solvents to regain their original performance. 

Every column (including chemically bonded columns) will have some column bleed. The amount of column bleed will increase with increasing column temperature, film thickness, column diameter, and column length. The base line starts to rise approximately 25-50° below the upper temperature limit of the stationary phase. After a column is installed in a GC/MS system, a background spectrum should be obtained for future reference. 

To maximise separation efficiency requires low H and high N values. In general terms this requires that the process of repeated partitioning and equilibration of the migrating solute is accomplished rapidly. The mobile and stationary phases must be mutually well-dispersed. This is achieved by packing the column with fine, porous particles providing a large surface area between the phases (0.5-4 m2/g in GC, 200-800 m2/g in LC). Liquid stationary phases are either coated as a very thin film (0.05-1 pm) on the surface of a porous solid support (GC) or chemically bonded to the support surface as a mono-molecular layer (LC). 

The chromatographic column used was a wall-coated, open tubular column (WCOT) (J W Scientific) with a DB-1 Durabond chemically bonded stationary phase that had a nominal film thickness of 0.25 pm. The column was 60 m long X 0.32 mm i.d. The DB-1 stationary phase has chromatographic properties similar to SE-30. 

The area available for the stationary phase will directly affect the phase ratio. If a solid material is used as the stationary phase in a packed column, if a liquid phase is deposited on a solid adsorbent with a constant film thickness, or if chemically bonded phases are employed, the phase ratio (through VJ will be directly proportional to the available surface area. The surface area of an adsorbent is usually given per unit weight (i.e. the specific surface area in m2/g). However, it should be noted that the relevant quantity is the surface area per unit volume (m2/ml) in the packed column. 

Retention in supercritical chromatography is affected by the nature of both the mobile and the stationary phase. A variety of stationary phases, including high boiling liquids, polymer films, solid supports and chemically bonded monolayers, has been used. 

It is not yet clear what type of stationary phase will be most useful for SFC. Liquid stationary phases will almost inevitably be of insufficient stability. Polymeric films of various thickness and varying degree of cross-linking have been used. Preferably, such polymeric phases should be covalently bonded to the column wall (open columns) or a solid support (packed columns). Alternatively, solid adsorbents or chemically bonded monolayers may be applied. 

The thin film of stationary phase, if untreated, may be rinsed away by mobile phase under the high pressures used. One method of dealing with this is chemically bonding the liquid phase to the solid support. Porous silica beads are esterified with various... 

Chemical bonding of stationary phases has been shown to increase the stability of the stationary phase compared with conventionally coated films. A chemically bonded phase may be regarded as one that is not extractable by solvents that do not attack the phase. 

In GLC, separation occurs based on differences in partitioning of the sample components between the carrier gas and the liquid phase. A wide selection of liquid phases makes GLC a versatile separation technique. Further, the liquid phase can be a polymer or a chemically bonded phase. In all cases, the liquid phase is film coated or chemically bonded onto a solid support surface or a column wall. Liquid-bonded phases overcome the problem of leakage of the stationary phase material into the carrier. They are used commonly in LC also, and the process to fabricate... 

Although not offered in every combination, today s state-of-the-art columns are available in a variety of lengths, and with internal diameters ranging from 0.05 mm to 0.75 mm, and with cross-linked chemically bonded films of stationary phase that range from 0.1 to 8 um in thickness. 

In this case the stationary phase is a substance that is covalently bonded as a thin film to a solid support. This solid support can be the internal capillary walls of fused silica capillary columns, or small silica particles that have been chemically treated to facilitate bonding of the stationary phase and are packed into the column itself. The stationary phase can be a liquid in the pure state at the operating temperature range of the column (e.g., Cg hydrocarbon) but nonliquids are also used (e-g-, Cjg). Chromatographic separation results from the differing ratios of solubilities (expressed as their partition coefficients K ) of the various analytes in the mobile and stationary phases. 

For the general separation of volatile heilogenated hydrocarbons a column should be used with a high retention. Since the halogenated hydrocarbons do not have a bonded group capable of hydrogen-bond formation, they can be analyzed on virtually siny type of stationary phase giving symmetrical peeiks. The most favored phases are the chemically bonded methylphenylsiloxanes with a film thickness of 1 -3 urn. 

Initially, non-polar stationary phases only (of the methyl silicone type) were used in high-temperature GC, and cross-linking and chemical-bonding improved the properties of the columns appreciably. More polar bonded phases, consisting of phenylmethyl silicones, later came into use and are available commercially. At present, these have a temperature limit of about 360 C, and while this will no doubt be improved, the ultimate limit may depend on the pyrolysis temperature of triacylglycerols. The optimum thickness of the liquid film for high-temperature GC is about 0.1 to 0.12 o.m. 

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