Column packing tube selection

The instrumentation of HdC, including a pump, an injector, a column (set), a detector, and a recorder or computer, is very similar to size exclusion chromatography SEC). The essence of this technique is the column. There are two types of HdC columns open microcapillary tubes and a nonporous gel-packed column. This chapter emphasizes column technology and selection and the applications of this technique on the molecular weight analysis of macromolecules. 

Fixed-Bed Adsorption The adsorbent can be packed in a cylindrical column (or tube in small scale) and the diluent passed through for the selective adsorption of the solute of interest. It is one of the most common methods for adsorption in both laboratory or industrial scale separations. 

Adsorption column preparation and loading. In order to obtain satisfactory results, the tube must be uniformly packed with the adsorbent uneven distribution may lead to the formation of cracks and channels and to considerable distortion of adsorption band shapes. If there is any doubt concerning the uniformity of particle size of the adsorbent powder it should be sifted before use to remove the larger particles fines are removed from the adsorbent using a sedimentation procedure immediately prior to column packing. In this the alumina or silica gel adsorbent is stirred into between five to ten times its volume of the selected solvent or solvent system, allowed to settle for five minutes and the supernatant liquor decanted off the procedure is repeated until the supernatant liquid is clear. 

The stationary liquid phase is coated on a solid support and packed into a column (packed column GC), or it is coated on the wall of an open tube (WCOT), as has been discussed. The higher efficiency of OT columns has reduced the necessity for many selective liquids, and the number of OT columns necessary to analyze for all types of analytes is smaller than for packed columns. 

A third sample preparation method is purge and trap, which aims to extract as close to all of the analyte as possible from the solid or liquid sample and is a deviation from headspace sampling. It works by bubbling a purge gas such as helium through the heated sample vial. The gas carries analyte up into an adsorption tube packed with selective stationary phase. After all the analyte has been trapped in the tube, the gas flow is reversed through the tube to remove any residual solvents. The tube is then directed to the injector port and, heated to desorb the analytes, which are then cold-trapped onto the head of the GC column. From there, the concentrated sample is heated for GC separation. 

For monitoring of process streams and product quality, determination of total and individual sulfur compounds is required down to ppm levels. The characterization of individual compounds has involved primarily the combination of GC using packed columns and element-selective detectors. For sulfur compounds, Druschel (100) utihzed a Carbowax 20 M column with a microcoulo-metric detector. A combustion tube at 550°C converts the sulfur compounds into SO2 as they elute from the column. The SO2 is then titrated by electrogenerated iodine. A typical application for this analysis is shown in Figure 13.35. It was used to monitor the removal of condensed thiophenes by hydrotreating of... 

The packed column separations that follow provide effective guidance for selection of the proper stationary phase, support material, tubing, and so on, and to simplify the process. The easiest way to obtain a column is to select one from the following pages. Another approach is to seek guidance from the literature or the Web sites of column manufacturers. 

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