Column, capillary dispersion

The peaks eluted from OTC columns are of very low volume and this can create technical difficulties as the volumes of the flow cells in conventional detectors (5-10 jal) are much greater than the eluted peak volumes. If such detectors were used with OTC columns band dispersion could result, thereby negating the inherent advantages of these columns. Therefore, the flow cell volume should be in the range 0.01-1 /il. Similarly, there is a requirement in capillary systems for both a minimal dead volume and a reduction in the injection volume necessitating the development of speciahsed pre-concentration and sampling techniques. One further restriction of OTC columns is their small internal diameter which can result in sample overloading only quantities of less than 10 ng should be used. 

Giddings [2] estimated that, for a well-packed column, (y) takes a value of about 0.6. Equation (11) accurately describes longitudinal dispersion in GC capillary columns and equation (12) accurately describes longitudinal dispersion in GC and LC packed columns. Experimental support for these equations will be given in a later chapter. 

In summary, equation (13) accurately describes longitudinal dispersion in the stationary phase of capillary columns, but it will only be significant compared with other dispersion mechanisms in LC capillary columns, should they ever become generally practical and available. Dispersion due to longitudinal diffusion in the stationary phase in packed columns is not significant due to the discontinuous nature of the stationary phase and, compared to other dispersion processes, can be ignored in practice. 

Figure 5 shows that using average velocity data the extracted value for the multi-path term is negative, which is physically impossible, and, furthermore, for a capillary column should be zero or close to zero. In contrast, the extracted values for the different dispersion processes obtained from data involving the exit velocity give small positive, but realistic values for the multi-path term. 

Equations (2) and (4) allow the permissible extra-column dispersion to be calculated for a range of capillary and packed columns. To allow comparison, data was included for a GC column, in addition to LC columns. The results are shown in Table 1. 

The standard deviation of the extra-column dispersion is given as opposed to the variance because, as it represents one-quarter of the peak width, it is easier to visualize from a practical point of view. It is seen the values vary widely with the type of column that is used, (ag) values for GC capillary columns range from about 12 pi for a relatively short, wide, macrobore column to 1.1 pi for a long, narrow, high efficiency column. 

The packed GC column has a value for (og) of about 55 pi, whereas the high efficiency microbore LC column only 0.23 pi. It is clear that problems of extracolumn dispersion with packed GC columns are not very severe. However, shorter GC capillary columns with small diameters will have a very poor tolerance to extracolumn dispersion. In the same way, short microbore LC columns packed with small... 

Extracolumn dispersion is a major problem for the packed fused silica capillary columns with internal diameters less than 0.35 mm. Peak standeunl deviations will be in the submicroliter range and extensive equipment modification is required for operation under optimum conditions. A reasonable compromise is to esploy injection voluMs of a few hundred nanoliters or less with detector volumes of a similar or preferably smaller size. This demands considerable ingenuity on behalf of the analyst since, as... 

Fused silica capillary columns of various internal bores and of lengths in the range 25 to 50 m are mainly employed for analytical separations. A variety of polar and non-polar column types are available including those open tubular types with simple wall coatings (WCOT), those with coatings dispersed on porous solid-supports to increase adsorbent surface area (SCOT) and porous layer open tubular (PLOT) columns. Important stationary phases include polyethylene glycol, dimethylpolysiloxane and different siloxane copolymers. Various sample introduction procedures are employed including ... 

By replacing conventional 3.5 or 5 jtm columns with sub-2-micron columns, gradient time can be reduced dramatically. The flow rate must be increased for optimal conditions as well but solvent consumption will be less than the amount used by the original method. To use the full power of these columns, an LC instrument must be thoroughly optimized toward lowest extra-column dispersion. The smaller the column (small ID and short length), the more sensitive the performance is to dispersion. With smaller internal diameter columns, the injection volumes and internal diameters of the capillaries should be reduced. 

In order to achieve a good separation between two compounds with close mobilities, it is important to have each compound migrating in narrow bands (zones) through the capillary (column). These narrow bands in the separation column are reflected as narrow peaks in the final electropherogram. The broadness of bands in the electropherogram of a CE separation is determined by the dispersion of the migrating solute zones in the capillary (in the background buffer electrolyte). CE is a dynamic process therefore, dispersion effects are bound to occur. 

Samples of both fulvic and humic acids were suspended in methanol and methylated with diazomethane. Both H and spectra of the free acids were obtained, at 299.94 MHz and 75.42 MHz respectively, on a Varian XL-300 spectrometer having a Nicolet TT-100 PET accessory. Spectra were obtained in D2O, in a 12-mm tube, with deuterated TSP (sodium 3-(trimethylsilyl)propionate-, , 3,3- 4) added as internal reference. GC/MS of methylated acids was conducted on a Hewlett-Packard Model No 5995 GC/MS/DA system equipped with a fused silica capillary column (12 m x. 020 mm ID, Hewlett Packard) internally coated with crosslinked methylene silicone. Infrared spectra were obtained with solid samples dispersed in KBr pellets, by using a Beckman IR-33 spectrophotometer. The various absorption peaks in IR and NMR were interpreted conventionally (9-10). 

The epoxidation of cyclohexene with H202 was carried out at 70 °C in a magnetically stirred three-necked flask equipped with a condenser. In practice, 0.05 g of catalyst was dispersed in the solution containing 0.02 mol of cyclohexene and 20 ml of acetonitrile (solvent). The mixture was then heated to 70 °C under stirring and 0.01 mol of H202 (35 wt.% aqueous solution) was introduced in one lot. The sample was periodically collected and analyzed by gas chromatography (HP 5890 Series II) equipped with FID and HP-1 capillary column. 

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