Capillary sample capacity

Several techniques are available for introducing samples into capillary columns which generally have a much lower sample capacity than packed columns. 

Several approaches towards monolithic GC columns based on open pore foams prepared in large diameter glass tubes were reported in the early 1970s [26,27, 110]. However, these columns had poor efficiencies, and the foams possessed only limited sample capacities in the gas-solid GC mode. Subsequent experiments with polymerized polymer layer open tubular (PLOT) columns where the capillary had completely been filled with the polymer were assumed to be failures since the resulting stationary phase did not allow the gaseous mobile phase to flow [111]. 

An open-tubular column is a capillary bonded with a wall-supported stationary phase that can be a coated polymer, bonded molecular monolayer, or a synthesized porous layer network. The inner diameters of open-tubular CEC columns should be less than 25 pm that is less than the inner diameters of packed columns. The surface area of fused silica tubing is much less than that of porous packing materials. As a result, the phase ratio and, hence, the sample capacity for open-tubular columns are much less than those for packed columns. The small sample capacity makes it difficult to detect trace analytes. 

Packed capillary columns (Figure 8) have a greater sample capacity than open-tubular columns because of the increased surface area and, hence, greater phase ratio. Greater sample capacities result in increased sensitivity and selectivity. More than 95% of the CEC columns... 

For most of the columns listed in Table D 1.2.1, various dimensions are available for each of the column types. Generally, a column with an i.d. from 0.20 to 0.32 mm is regarded as a capillary column. A column with an i.d. of 0.45 or 0.53 mm is called a megabore column. Both capillary and megabore columns are commonly used in FAME analysis. There are advantages and disadvantages for both capillary and megabore columns. Capillary columns have better resolution but less sample capacity,... 

Electrodriven separations, such as capillary electrophoresis (CE) and capillary electrochromatography (CEC), are based on the different electrophoretic mobilities in an electric field of the molecules to be separated. They provide a higher separation efficiency then conventional HPLC since the electrophoretic flow (EOF) has a plug-flow profile. Whereas the mobile phase in CE is driven only by the electro-osmotic flow, it is generated in CEC by a combination of EOF and pressure. CEC has a high sample capacity which favours its hyphenation with NMR. 

The medium-film thickness is about 0.3-0.6 pm and generally offers the best compromise of sample capacity, retentivity, and phase stability. The phase ratio determines the capacity of the column and influences its retentivity of solutes. The phase ratio (j8) can be defined as the ratio of the inner column radius to that of the product of twice the stationary-phase film thickness or 0 = r/2df. We can now also use phase ratios to group film thicknesses and now say that thick-film columns have phase ratios of less than about 80. (In capillary SFC the typical stationary-phase film thicknesses are 0.1-0.3 pm.) The effective phase ratio can change in capillary SFC, depending on the characteristics of the stationary phase and the operating density [57]. The change in phase ratio can be attributable to a swelling of the stationary phase under certain SFC conditions. 

In GC/MS, analytical packed columns are still often used, mainly because of their well-established characteristics, simple injection technique, high sample capacity, and the fact that most instrumentation has been designed for operation with such columns. However, the separation efficiency and transfer to the MS are in most cases inferior to open tubular (capillary) columns. 

Thin-layer chromatography (TLC) is another liquid-liquid partition technique applicable to polysaccharides, but in two dimensions. In TLC, the M cutoff boundaries between separated molecules are sharpened, because diffusion is minimized or eliminated in favor of capillary transport. The sample capacity of a TLC plate is in microliters. Resolution is enhanced further at high solvent pressure (Rombouts and Thibault, 1986). 

In GC we have a real choice between packed columns (dp = 100-200 pm 150-65 mesh) and open columns (dc= 50-500 pm). Capillary columns have the advantage of enhanced speed of analysis (eqn.7.6). In order to exploit this advantage, narrow-bore capillaries (dp< 100 pm) should ideally be used. However, such columns require relatively high inlet pressures (especially for high plate counts) and considerable experimental modifications and have a very low sample capacity [702],... 

Because of all these reasons, so-called wide-bore capillaries (dc 500 pm) have recently gained considerable popularity. These columns, which are usually provided with a thick (about 1 pm) film of stationary phase, behave in a fairly similar way to packed columns. They show low pressure drops (allowing them to provide a much higher efficiency than packed columns), may easily be installed in most instruments and have a high sample capacity. However, they also behave similar to packed columns in terms of separation speed. Therefore, the current capillary columns with diameters between 100 and 300 pm form a reasonable compromise between instrumental limitations and theoretical promises. 

Wide-bore capillary GC columns allow easy operation and have a large sample capacity. However, they give rise to relatively long analysis times. 

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