Column, capillary limitations

Interfaces for Gas Chromatography Two types of gas chromatographic column, capillary and packed, are in routine use (see page 178). However, the width of the chromatographic peak places limitations on the performance of the mass spectrometer, and the difference in flow rates affects the way in which each type of column must be interfaced to the mass spectrometer. 

Gas chromatography is the most widely used analytical technique for studying organic chemicals that are hazardous to the environment and that may potentiaUy have adverse human health effects. The conventional packed column systems used by the majority of analysts are valuable for certain analyses. However, along with other problems, packed columns have limited separation ca )abilities, therefore they do not meet the demands of todays s analytical requirements. The very recent developments in the manufacture of open tubular, large bore fused silica columns (0.53 mm ID) and more conventional capillary columns ( 0.35 mm ID) are significant and provide the analyst with superior... 

The load capacity of a capillary column is limited, usually less than 50 ng per peak or component, whereas it can be up to 10 g per peak in packed columns [ 5 ]. In most cases the sample volume to be injected has to be much smaller than in conventional systems. 

Linear dynamic range of the 30 ii TCD is good when used with a packed column. When the 30)ii TCD is coupled with WCOT columns, the limited sample capacity of the columns severely limits its analytical utility. While the dynamic range can be somewhat adjusted by judicious selection of the split ratio of a capillary Injector, major and trace components can not be both simultaneously determined. The upper end of linear range is always limited by column capacity. 

While the developments in capillary GC were slow in coming in the late 1960 s, many researchers then considered the support-coated open tubular and micropacked columns to be viable alternatives to the conventional capillaries. Although some interesting results were reported about 10 years ago [94,95] on the performance of such columns, they were largely overshadowed by the rapid advances in technology of wall-coated columns. The limited column permeability of micropacked columns and an excessive surface activity of support-coated open tubular columns are the major drawbacks of these column types. However, they may still offer a suitable compromise between sample capacity and column efficiency in certain special instances. 

Whereas oligonucleotide libraries were traditionally partitioned by filtration or capture on beads or on affinity columns, capillary electrophoresis has been validated as a promising alternative. Aptamers displaying a high affinity were obtained in a very limited number of selection rounds. Nonequilibrium capillary electrophoresis of equilibrium mixtures led to the identification of DNA aptamers... 

During the last years glass capillary columns have replaced packed columns in many laboratories. The packed column has limited resolution and therefore less specifity. Many investigators routinely now work with commercially available capillary columns. 

The number of theoretical plates available in fractional distillation [www h> columns is limited by column holdup (see Techniques 2,3, and website discussion of distillation theory). Thus, distillations of less than 500 (xL are generally not practical. Gas-chromatographic columns, on the other hand, operate most efficiently at the microscale or submicroscale levels, where 500 (xL would be an order of magnitude (even 3-8 orders of magnitude in the case of capillary columns) too large. 

Open tubular or capillary columns have open unrestricted path for the gas within the column. These columns are about 15-30 meters in length with an inside diameter of about 0.25 mm. The inner wall of these columns is coated with the liquid stationary phase to about 1 m in thickness. The open tubular columns are of two kinds. One is known as the wall coated open tubular column [WCOT) in which the liquid phase is coated on the column wall. These columns have limited sample capacity and are unsuitable for large-scale separations. The second type is known as support coated open tubular columns (SCOT). In these columns a porous layer is formed on the inside wall of the tubing. The porous layer can either be formed by chemical treatment of the inner wall or is deposited on the inner wall. The support is coated in such a way that the inherent property of the capillary columns, i.e., the unrestricted gas flow is retained. The inert porous layer is then impregnated with the liquid stationary phase. These columns have a higher sample capacity. 

An important problem with all liquid stationary phases is their tendency to bleed from the column. The temperature limits listed in Table 12.2 are those that minimize the loss of stationary phase. When operated above these limits, a column s useful lifetime is significantly shortened. Capillary columns with bonded or... 

Despite their importance, gas chromatography and liquid chromatography cannot be used to separate and analyze all types of samples. Gas chromatography, particularly when using capillary columns, provides for rapid separations with excellent resolution. Its application, however, is limited to volatile analytes or those analytes that can be made volatile by a suitable derivatization. Liquid chromatography can be used to separate a wider array of solutes however, the most commonly used detectors (UV, fluorescence, and electrochemical) do not respond as universally as the flame ionization detector commonly used in gas chromatography. 

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