Packed and Capillary Columns

In addition to providing a separation where peaks have narrower bandwidths compared to those of a packed column counterpart, a properly prepared fused-silica capillary column, which has an inert surface (less potential for adverse adsorptive effects toward polar species), yields better peak shapes (i.e., bands are sharper with less peak tailing), which facilitates trace analysis as well as providing more 

TABLE 3.2 Comparison of Wall-Coated Capillary Columns with Packed Columns 

At higher column oven temperatures with increased linear velocity of carrier gas, capillary separations can be achieved that mimic those on a packed column but with a shortened time of analysis. The reduced amount of stationary phase in a 

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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. 

The combination of chromatography and mass spectrometry (MS) is a subject that has attracted much interest over the last forty years or so. The combination of gas chromatography (GC) with mass spectrometry (GC-MS) was first reported in 1958 and made available commercially in 1967. Since then, it has become increasingly utilized and is probably the most widely used hyphenated or tandem technique, as such combinations are often known. The acceptance of GC-MS as a routine technique has in no small part been due to the fact that interfaces have been available for both packed and capillary columns which allow the vast majority of compounds amenable to separation by gas chromatography to be transferred efficiently to the mass spectrometer. Compounds amenable to analysis by GC need to be both volatile, at the temperatures used to achieve separation, and thermally stable, i.e. the same requirements needed to produce mass spectra from an analyte using either electron (El) or chemical ionization (Cl) (see Chapter 3). In simple terms, therefore, virtually all compounds that pass through a GC column can be ionized and the full analytical capabilities of the mass spectrometer utilized. 

Molecular diffusion (the B term) applies to both packed and capillary columns and derives from the fact that all molecules in the gas phase will diffuse into any available space. It is minimized by using an increased flow rate (see the carrier gas velocity in the denominator) and by using a high molecular weight carrier gas. 

Mass transfer (the C term), which involves collisions and interactions between molecules, applies differently to both packed and capillary columns. Packed columns are mostly filled with stationary phase so liquid phase diffusion dominates. The mass transfer is minimized by using a small mass of low-viscosity liquid phase. Capillary columns are mostly filled with mobile phase, so mass transfer is important in both the gas and liquid phases. A small mass of low-viscosity liquid phase combined with a low-molecular weight carrier gas will minimize this term. 

Important column dimension parameters for packed and capillary columns... 

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. ... 

The thermal conductivity detector (TCD) is a classical detector for both packed and capillary columns. A schematic representation of a modern... 

Many reports (78-84) investigated the differences in packed and capillary supercritical fluid chromatography. Unfortunately, the rift between packed and capillary column users of SFC impeded the development of the science. This rift is a likely cause of the current low interest in SFC. Ideally, the unique features of the mobile phase is the area of scientific exploration that should be exploited. Choice of column size or type should be dependent upon the analytical problem to be solved. 

Several packed and capillary columns have been reported in the U.S. EPA methods, research literature, and manufacturers product catalogs. Some common columns are described below. 

Chlorinated pesticides mixture can be separated on both packed and capillary columns. Some of the columns and conditions are as follows ... 

There are several types of chiral derivatizing reagents commonly used depending on the functional group involved. For amines, the formation of an amide from reaction with an acyl halide [147,148], chloroformate reaction to form a carbamate [149], and reaction with isocyanate to form the corresponding urea are common reactions [150]. Carboxyl groups can be effectively esterified with chiral alcohols [151-153]. Isocynates have been used as reagents for enantiomer separation of amino acids, iV-methylamino acids, and 3-hydroxy acids [154]. In addition to the above-mentioned reactions, many others have been used in the formation of derivatives for use on a variety of packed and capillary columns. For a more comprehensive list, refer to References 155-159. 

For packed columns, typical values [701] are h = 3 and v= 3, so that h/ v= 1. For open columns typically h = 1.5 and v= 5, so that /i/ v= 0.3. Consequently, capillary columns will lead to analysis times that are about three times shorter (for dp— dc) for the same separation (N and k constant). Therefore, in principle, capillary columns are superior to packed columns. Unfortunately, capillary columns cannot always be used. This arises from the occurrence of the diffusion coefficient (Dm) in eqn.(7.6). Typically, Dm is 10,000 times larger in gases than it is in liquids. This necessitates the use of very small particles (typically 5-10 pm) in HPLC columns. If we compare packed and capillary columns with dp= which is a reasonable assumption for GC [702], then capillary columns with very small internal diameters need to be considered for LC [703]. Such very narrow columns impose extreme demands on the instrumentation, and at present open tubular columns cannot be used for practical LC separations. 

Unlike mobile phase terms, the stationary phase term in both packed and capillary columns, Csv, varies throughout the column, increasing continuously from inlet to outlet as v increases. Its average value is given by... 

Barry E.F. 2004. Columns Packed and capillary column selection in gas chromatography. In Modem Practice of Gas Chromatography (Barry, E.F., Ed.). Wiley-Interscience, Hoboken, N.J., pp. 65-191. 

It is evident that many workers in GC-MS use columns of inadequate efficiency to handle complex mixtures and attempt to improve this situation by careful analysis of the MS data. This undoubtedly can lead to omissions and inaccurate identifications and it seems likely that in the future capillary columns, once their preparation is routine, will be very widely used. Figure 1.7 shows the chromatographic profiles of a derivat-ised urine steroid extract on packed and capillary columns. The vastly improved resolution in the latter instance is immediately obvious. 

Both packed and capillary columns are used with the SFE-GC tandem. The detector is usually of the mass spectrometry, flame ionization or electron capture type. The most suitable interface for each application will be that allowing the extractant to be removed prior to the column — and hence to the detector. 

In SFC, both packed and capillary columns are used, each with their specific advantages and disadvantages. Packed columns in SFC are very similar to those used in HPLC, with the most often used stationary phases being modified silicas. Column selectivity follows the same rules as it does in HPLC with aromatic hydrocarbons (e.g., more retained on octadecyl silica column than on bare silica) [5]. A great variety of different selective stationary phases... 

In addition to the packed and capillary columns, progress has been made in the development of micro GC columns on silicon chips. These microdevices have great potential for high-speed GC, a miniature GC, and eventually even a pocket GC (see Chapter 10 for a discussion on nanotechnology). 

Operationally, both packed and capillary columns require careful control of the column, injector, and detector temperatures. Control of the column temperature is achieved when the column is placed in an oven or when the column is heated directly by resistive heating.Injector and detector temperatures usually are controlled by electrical resistance... 

Separation in GC is based on the vapour pressures of volatilized compounds and their affinities for the liquid stationary phase, which coats a solid support, as they pass down the column in a carrier gas. The practice of GC can be divided into two broad categories, i.e. packed- and capillary-column based. For... 

In studies of polysaccharides structure, the alditol acetate procedure remains the most widely used GLC procedure. The advent of high-resolution glass capillary columns has allowed very efficient separations. Recent applications of these columns to alditol acetate separations have been described (55-57). The alditol acetate procedure requires reduction of the sugars with sodium borohydride. After removal of boric acid, the sample is acetylated by conventional means. Various polar stationary phases have been used in GLC separation of alditol acetates, in both packed and capillary columns. A low-polarity phase was used in a report ( ) which demonstrated the separation of trimethylsilylated alditols, and the neutral sugars in a hemicellulose sample were resolved. 

Like gas chromatography, supercritical fluid chromatography can be practised both in packed and capillary column techniques. Observations made earlier under gas chromatography in regard to column performance of capillary columns compared with that of packed columns, are also valid for supercritical fluid chromatography. 

An overview of capillary gas chromatography is presented. Selected environmental applications, such as PCB s in water, PAH s in airborne particulate matter, and TCDD s at the part-per-trillion level illustrate the separation and analysis of complex mixtures. The chromatographic performance, characteristics, and trade-offs of packed and capillary columns are described in terms of permeability and efficiency, sample capacity, choice of stationary phase, high temperature capabilities, quantitative accuracy, and the development of GC separation methods. 

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

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