Fast capillary columns, sample capacity

Fast chromatography involves the use of narrow-bore columns (typically 0.1-mm i.d.) that will require higher inlet pressures compared with the conventional wide-bore capillary columns. These columns require detectors and computing systems capable of fast data acquisition. The main disadvantage is a much-reduced sample loading capacity. Advances in GC column technology, along with many of the GC-related techniques discussed below, were recently reviewed by Eiceman et... 

Ultra-high flow on capillary columns (0.180 mm i.d.) versus narrow bore (1 mm i.d.) permits to reduce the sample-handling time and to improve column capacity and robustness [14], Moreover, these columns are able to work with sub-2 pm particles, which offer very fast methods to determine the chemical-physical properties of NCE. 

Instead of packed columns, monolithic (continuous bed), analytical, or capillary columns in the form of a rod with flow-through pores offer high porosity and improved permeability. Silica-based monolithic columns are generally prepared by gelation of a silica sol to a continuous sol-gel network, onto which a Cjg or another stationary phase is subsequently chemically bonded. Such columns provide comparable efficiency and sample capacity as conventional columns packed with 5-pm particle materials, but have three to five times lower flow resistance, thereby allowing higher flow rates and fast HPLC analyses. Rigid polyacrylamide, polyacrylate, polymethacrylate, or polystyrene monolithic columns are prepared by in sim polymerization. 

Whilst the advantages of smaller-diameter columns are apparent, typically there has been limited implementation of these columns for fast-GC. This can be attributed to a number of factors, such as the need for high column head pressure, which were historically seen as distinct limitations however, most of these can be overcome by the use of modern instrumentation. Narrow bore capillary columns also have a low sample loading capacity, so small injection sizes (of the order of a few nanograms) are required. 

An important improvement in the separation capacity of GC can be obtained using comprehensive GC X GC. This involves the use of two capillary columns with different separation mechanisms by which the second column separates unresolved compounds that elute from the first. The instrument design uses a modulator interface that couples the two columns. The first column is generally a conventional nonpolar GC column, the second one being a short polar column that allows very fast separation. This technique has proved to be a promising tool for the unambiguous separation of several contaminants such as PAHs, PCBs, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), and pesticides in environmental samples. 

Internal column diameters for fused silica range from 100 to 530 micrometers (0.10-0.53 mm). Some glass capillaries have even larger internal diameters. One-hundred micrometer columns, row one of Table 6.2, have limited sample capacity, and are not well suited for trace analysis. Ease of operation is also limited because of the very limited sample capacity. These small i.d. columns have very good efficiency and produce fast analyses (see Fig. 6.6), but special sampling techniques and high-speed data systems are required to realize their full potential. 

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