Capillary column separations packed

Figure 14.12 Schematic diagram of the Refomiulyser system Inj, split injector Cl, polar capillary column C2, packed column to retain the alcohols C3, packed Porapak column for the separation of the oxygenates C4, non-polar capillary column C5, packed 13X column A/E cap, Tenax trap to retain the ar omatics Olf. trap, cap to retain the olefins Pt, olefins hydrogenatOT A cap, cap to retain the -alkanes FID, flame-ionization detector.

There are a variety of liquid phases available for both packed and capillary columns. In packed columns, these are coated onto a solid support in capillary columns they are coated onto the capillary column wall. At the height of packed column use, there were over 200 liquid phases in common use. With the much higher separating power of capillary columns, this number has dropped to five most common liquid phases with perhaps two dozen specialty materials. A summary of common liquid phases and their application is shown in Table 14.5. The capillary column and instrument vendors also have extensive information on how to properly use packed and capillary columns and on applications available in their literature and on their WWW-sites. ... 

There are two types of separation column used in gas chromatography— capillary columns and packed columns. 

Solvent modifiers and additives can be used to adjust the retention and selectivity of separation in packed-column SFC. Similar effects have been reported with open-tubular capillary SFC. The advantage of capillary column over packed column arises from the differences in permeability. Pressure ramps are much easier to use in capillary columns to modify the solvent strength (via density modification) as compared to packed columns. Therefore it should be entirely feasible, with capillary SFC, to combine the benefit of solvent density (pressure) programming with simultaneous modification of the solvent strength. ... 

Gas-solid chromatography (GSC) and gas-liquid partition chromatography (GLC) are variations of GC. In GSC separations occur primarily by differences in absorption at the solid phase surface. In GLC a nonvolatile liquid is coated or chemically bonded onto particles of column packing or directly onto the wall of a capillary column. Separation occurs primarily by differences in solute partitioning between the gaseous mobile phase and the hquid stationary phase. 

A good comparison of packed and glass capillary column separations was demonstrated by Grob (8). The two chromatograms shown in Figure 6 are of the same lake water extract, run on the same stationary phase (OV-1). Packed and wall coated open tubular (WCOT) columns and methods are compared in Table I. 

Figure 6. Comparison of packed and capillary column separations of a river water extract. Dotted lines indicate corresponding sample com-...

The separation impedance is a measure of column performance and allows one to compare, for example, capillary columns to packed beds. The performance index is a measure of the quality of the separation and contains a strong dependence on analysis time. 1 would prefer a criterion that preserves the... 

Volatile and aromatic components Separation of volatile components is achieved on either fused silica capillary columns or packed columns. Individual volatile components are detected with a FID and identified by the use of reference standards. Methods using specific detectors, such as the NPD, sulfur-specific flame photometric detector, and mass-selective detector (MSD) have also been used. The MSD has the additional advantage of providing structural identification of the individual components. 

At present, GSC is widely used in analytical practice, and the capillary variant of GSC is used most effectively and most often. Considering the advantages of capillary columns over packed columns, only the higher efficiency of the former is often mentioned. At the same time, capillary columns are also characterized by faster separations, by more reproducible temperature regimes, by the small sizes of the apparatus needed, and by lower consumption of carrier gases and sorbents they also extend applications to the lower temperatures. 

It has been found that mixing several different liquid phases in one column will produce a selectivity directly proportional to the sum of the parts mixed together [8-12]. Usually it does not matter if the phases are kept separate in the column or are mixed together. A few useful ones are listed in Table 5.5. Some are commercially available, for example, those used in EPA methods for wastewater analysis. Since this flexibility cannot easily be attained with capillary columns, mixed packings represent one of the unique advantages of packed columns. 

The effects of the type of carrier gas and its flow rate upon a capillary column performance can be significant. The flow rate has to be accurately set, measured, and reproduced to fully experience the high efficiencies available with capillary columns. Unlike packed columns, small variations in the carrier gas flow rate can have significant effects on the separations obtained with capillary columns. Even the type of carrier gas can affect separation quality [2]. 

Separators. As long as packed columns with high gas-flow rates were used, it was necessary to separate the carrier gas from the analytes. This was accomplished by a variety of devices called separators examples include the Biemann-Wat-son separator and the jet separator. The latter was subsequently used in LC/MS interfaces. Due to the replacement of packed GC columns by capillary columns, separators are no longer used they are described in detail in [16]. 

This simple illustration shows how the basic choice of capillary versus packed columns determines the nature of the resulting separation and how optimization of flow and temperamre can produce further improvements. Now we will leave packed columns behind and consider in more detail the various avenues available for optimization of capillary-column separations. 

Fig. 4. Examples of gas chromatographic separations on capillary and packed columns, (a) Packed column separation of mixed hydrocarbons (b) capillary column separation of some flavor compounds. Reproduced from D.W. Grant, Capillary Qas Chromatography, 1996. John Wiley Sons Ltd. Reproduced with permission.

Figure 2.123 Simulation of a packed column separation performed on a 30 m capillary column. Carrier gas has been changed from nitrogen to helium and compared with a 3% OV17 packed column, the 5% phenyl-methylpolysiloxane capillary column separates four previously unresolved peak pairs in a similar run time. Column 30 m TRACE...

In GC, we distinguish between separation on packed columns and on capillary columns. Briefly, packed columns have a large inner diameter (2-6 mm) and are filled with an inert support coated with the liquid stationary phase, while capillary columns are open tubes with small inner diameter, the liquid stationary phase coating the inner wall. 

Used in virtually all organic chemistry analytical laboratories, gas chromatography has a powerful separation capacity. Using distillation as an analogy, the number of theoretical plates would vary from 100 for packed columns to 10 for 100-meter capillary columns as shown in Figure 2.1. 

In gas chromatography (GC) the sample, which may be a gas or liquid, is injected into a stream of an inert gaseous mobile phase (often called the carrier gas). The sample is carried through a packed or capillary column where the sample s components separate based on their ability to distribute themselves between the mobile and stationary phases. A schematic diagram of a typical gas chromatograph is shown in Figure 12.16. 

A chromatographic column provides a location for physically retaining the stationary phase. The column s construction also influences the amount of sample that can be handled, the efficiency of the separation, the number of analytes that can be easily separated, and the amount of time required for the separation. Both packed and capillary columns are used in gas chromatography. 

Bentone-34 has commonly been used in packed columns (138—139). The retention indices of many benzene homologues on squalane have been determined (140). Gas chromatography of C —aromatic compounds using a Ucon B550X-coated capillary column is discussed in Reference 141. A variety of other separation media have also been used, including phthaUc acids (142), Hquid crystals (143), and Werner complexes (144). Gel permeation chromatography of alkylbenzenes and the separation of the Cg aromatics treated with zeofltes ate described in References 145—148. 

The assay of ethyleneamines is usually done by gas chromatography. Compared to packed columns, in which severe tailing is often encountered due to the high polarity of the ethyleneamines, capillary columns provide better component separation and quantification. Typically, amines can be analyzed using fused siUca capillary columns with dimethyl silicones, substituted dimethyl silicones or PEG Compound 20 M as the stationary phase (150). 

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