Capacity fused-silica capillary columns

Although it is possible to chemically bond polymethyl-siloxane polymers on fused silica capillary columns with film thicknesses up to 5 m (high capacity columns), these capillary columns provide a relatively low resolution. 

A 0.53-mm-i.d. column offers the best of both worlds, because it combines the attributes of a fused-silica capillary column with the high sample capacity and ease of use of a packed column. Analytical methods developed with a packed column can be easily transferred for many applications to a megabore column... 

A 0.53-mm-i.d. column offers the best of both worlds, because it combines the attributes of a fused-silica capillary column with the high sample capacity and ease of use of a packed column. For many applications, analytical methods developed with a packed column can easily be transferred to a megabore column with the appropriate stationary phase. Peaks generated with a megabore column typically are sharper and exhibit less tailing compared to those with a packed column. Redistribution of the stationary phase can occur at the inlet of a packed column with large injections of solvent and leave an exposure of silanol sites on a diatomaceous earth support. With a cross-linked phase in a megabore... 

The use of pyrolysis apparatus with GC-MS systems imposes particular requirements on them. The sample quantity applied must correspond to the capacity of commercial fused silica capillary columns. It is usually in the microgram range... 

Figure 1.3. Thin fused silica capillary tubes similar to (and frequently smaller than) those shown here are often used for the analytical-scale separation of complex mixtures by chromatography and electrophoresis. The inside diameters of these capillaries are only 220 /im (lower two) and 460 fim (upper). The diameters of the smaller tubes are 680 times less than that of the LC column shown in Figure 1.2. The cross-sectional area, roughly proportional to separative capacity, is over 400,000 times less. (Photo by Alexis Kelner.)...

Porous layer open tubular (PLOT) columns were used to separate basic proteins and peptides [15]. The use of these types of columns was prompted by their high permeability and by the relatively high loading capacity due to an increased surface area by the porous layer. The authors showed that under conditions of reversed-phase chromatography at acidic pH, the EOF mobility was over 8-fold higher than that in raw fused-silica capillary, which is an indication of the high surface charge present in the porous layer. As expected, the EOF... 

Column Initial experiments use a fused-silica capillary 30-50 cm long and 50- or 75-p.m I.D. Short columns are appropriate for scouting experiments. The complexity of the sample dictates the required column length. For 2-10 analytes use 35-40 cm 11-50 analytes 50-60 cm 50-80 analytes 70-80 cm and > 80 analytes 90-100 cm. Smaller diameter columns (25 or 50 p,m) provide higher separation efficiency but lower sample loading capacity. 

Usually, the stationary phase including CDs in GC was prepared by depositing the modified CDs dissolved in a dimethylformamide solution onto the support (e.g. Chromosorb W) and then evaporated the solvent by vacuum heat-treatment. Some reports described glass or fused silica capillary GC column coated with peralkylated a-, P- and y-CDs dissolved in polysiloxanes. However, the coverage of CD in the surface is limited and results in limited preparative capacity [26]. In addition, the kinetics of inclusion are relatively slow and result in poor peak shape which also hinders the use of CDs as preparative phases. 

The macroporous monolith approach, introduced by Frechet and Svec, seemed to address many of the problems associated with open-tubular and particle packed columns. First, the adsorptive capacity of capillary monoliths has been found to be 3-5 orders of magnitude larger than that of both open channel and bead-packed columns [32]. Next, since the polymerization takes place within the fused-silica capillary, the tedious process of packing the capillary columns may be avoided. Furthermore, the limitations in chromatographic efficiency caused by irregularities in particle packing and by the nonuniformity of particle sizes are eliminated. 

LC columns are usually packed with silica gel or silica derivatized with amino or cyano functions. Online coupling mostly involves 2 mm i.d. columns, compromising between the capacity for matrix material (e.g., edible oil) and the fraction volumes to be transferred. Furthermore, the commonly used flow rates (200-400 pi min well suit the requirements of transfer into GC, as evaporation rates through an early vapor exit are in the same range. Initially, some applications involved packed fused silica capillary LC columns with fraction volumes in the order of 5-10 pi. However, most applications presuppose higher capacities for the sample matrix. 

From measurements of the internal dieuaeter before and after etching, it was found that, in fact, about 1 silica was removed from the suifece of the capillaries. This indicates that creation of a thick porous layer on fused silica capillaries by etching is not possible [101], However, retention on an etched capillary compared to an untreated one is about twice as high (in LC). Etching of fused silica capillaries with 1M potassium hydroxide creates an activated surface, but the capacity of the silica layer (in LC) is too small. However, we think that this method may be useful for gas chromatography capillary columns. 

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. 

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