Capacity capillary column performance

The effect of capillary inner diameter on column efficiency is quite predictable the column efficiency increases as the diameter decreases. However, this increased performance is at the expense of sample capacity. Capillary columns that are most commonly used today have inner diameters between 0.2 and 0.3 mm. While the sample capacity corresponding to such column dimensions is adequate for the combined GC/MS, wide-bore capillary columns are required for most remaining peak identification techniques. The wide-bore (0.5-0.7 mm, i.d.) columns may tolerate up to microgram amounts. The column technologies for the wide-bore and conventional capillary columns frequently differ, as an extensive geometrical modification of the column inner surface is needed for the former column type. 

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. 

Most of the N-nitrosamines analyses by GC-MS have been performed using capillary columns. The use of these columns avoids the necessity of intensive clean-up of the extracts prior to the analysis. The clean-up is needed for removing most of the many potentially interfering substances contained in environmental samples. On the other hand, the packed columns present the advantage of their greater sample capacity. The electron impact (El) ionization mode is the most frequently... 

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. 

Chromatographic performance in terms of speed of analysis and resolving power has been found to be poor relative to conventional small particle packed columns. Although packed capillary columns have larger capacities their permeability is reduced. 

The standard procedure in gas chromatography is to perform 1 D GC on a 30-to 60-m-long capillary column, under temperature-programming conditions, thereby achieving a peak capacity of some 100-150. If more resolution is desired, the higher potential of 2D GC is invoked (Figure 1). 

The selection of a capillary column depends on the complexity of the sample to be analyzed. The column length, the internal diameter, the stationary phase, and its fdm thickness determine the separation power (resolution), the sample capacity. the speed of analysis, and the detectability or sensitivity. Theoretical considerations [12], [13] indicate that for capillary columns with thin films (< 1 pm), the Wniin value is roughly equal to the column diameter. This is illustrated in Figure 4. which shows experimental H-u curves for columns varying in internal diameter. H was calculated for dodecane at I00°C with hydrogen as carrier gas. measured experimentally is indeed very close to deduced theoretically. By knowing this, the maximum plate number that a capillary column can provide may be calculated without performing any analysis ... 

Coupled systems include multidimensional and multimodal systems. Multidimensional chromatography involves two columns in series preferably two capillary columns, with different selectivity or sample capacity, to optimize the selectivity of some compounds of interest in complex profiles or to provide an enrichment of relevant fractions. In multimodal systems, two chromatographic methods or eventually a sample preparation unit and a chromatographic method are coupled in series. Coupled systems that received much interest in recent years are multidimensional CGC (MDCGC), the combination of high-performance liquid chromatography with CGC (HPLC-CGC) and the on- or off-line combination of supercritical fluid extraction with CGC (SFE-CGC). Multidimensional and multimodal techniques in chromatography arc described in detail in [65],... 

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