Narrow-bore

Note. For most compounds, the circular bore in C should have a diameter of not less than i cm. otherwise the final ejection of the pellet may prove very difficult. Some powdered substances do not bind well in the press however, and for these compounds a cylinder having a narrower bore (3 -5 mm.) is desirable. 

A narrow bored column that usually does not contain a particulate packing material. 

The narrow bore of the capillary column and the relative thickness of the capillary s walls are important. When an electric field is applied to a capillary containing a conductive medium, such as a buffer solution, current flows through the capillary. This current leads to Joule heating, the extent of which is proportional to the capillary s radius and the magnitude of the electric field. Joule heating is a problem because it changes the buffer solution s viscosity, with the solution at the center of the... 

Removing a restriction plate that limits the flow into a vessel and that has been taken into account when sizing the vessel s relief valve. A length of narrow bore pipe is safer than a restriction plate, as it is less easily removed. 

Rodding out narrow bore lines is sometimes necessary. But before doing so, a ball valve or cock should be fitted on the end (Figure 17-lb). It is then possible to isolate the flow when the choke has been cleared, even if the original valve will not close. 

Generally, size exclusion chromatography is carried out using columns with an internal diameter of 7.8 mm. However, some SEC applications require the use of expensive solvents. For this purpose, size exclusion columns with a smaller internal diameter (4.6 mm) have been developed. Of course one should use proportionally lower flow rates with narrow-bore columns. If the standard column size uses a flow rate of 1 ml/min, then the smaller 4.6-mm columns should be used at a flow rate of 0.35 ml/min. This provides the same linear velocity as 1 ml/min on 7.8-mm columns. The decreased flow rate reduces solvent consumption and solvent disposal cost. The performance of the smaller diameter columns is not compromised if properly optimized instrumentation is used. 

Smaller diameter columns are especially useful when expensive solvents are used. Figure 11.3 shows the analysis of poly (1,4-butylene terephthalate) using a Waters Alliance narrow-bore GPC system, quantitated against narrow polymethylmethacrylate standards. In this case, the solvent used is hexaflu-oro-2-isopropanol with 0.05 M sodium trifluoroacetic acid at a flow rate of... 

FIGURE I 1.3 Analysis of poly(l, 4-butylene terephthalate) usinga Waters Alliance narrow-bore GPC system. Columns 4.6 X 300 mm Styragel HR 2, HR 3, and HR 4. Mobile phase hexafluoroisopropanol. 0.35 ml/min at 30 C. (Chromatogram courtesy of Peter Alden, Waters Corp.)... 

Narrow-bore columns are most useful for the analysis of polymers that are difficult to analyze in inexpensive solvents. However, if the appropriate equipment is available, good results can be obtained for a broad range of standard analyses. A comparison of an analysis of standards between an equivalent bank of conventional 7.8-mm and solvent efficient 4.6-mm columns is shown in Fig. 11.4. The columns used were Styragel HR 0.5, 1, 2, and 3 columns at 35°C with tetrahydrofuran (THF) as the solvent. The flow rate was 1 ml/min for the conventional columns (Fig. 11.4A) and 0.35 ml/min for the solvent-efficient 4.6-mm columns (Fig. 11.4B). If the correct equipment is available, the reduced solvent consumption of these solvent-efficient Styragel columns is of value to the environmentally conscious user. 

Several issues need to be considered when operating narrow-bore SEC columns. 

The system dead volume must be reduced to an absolute minimum, particularly when using very efficient narrow-bore SEC columns. Extra column dispersion becomes a greater consideration as the column volume is reduced, and dead volume should be minimized in all parts of the system, including injection valves, connecting tubing, and detectors, if the column performance is to be realized. 

Sample loading must be reduced in accordance with the column inside diameter. Polymers exhibit high solution viscosity, and in order to avoid band broadening due to viscous streaming the sample concentration must be reduced for narrow-bore columns. Overloading effects become noticeable at much lower concentrations using 4.6-mm columns compared to 7.5-mm columns because of the effective sample concentration in a smaller volume column. 

Meehan, E., Oakley, S. A., Warner, F. P., (1992). Narrow Bore Columns for Size Exclusion Chromatography. Presented at Pittcon 92, New Orleans. 

Such effects principally cannot be observed in multi band detectors such as a UV diode array detector or a Fourier transform infrared (FTIR) detector because all wavelengths are measured under the same geometry. For all other types of detectors, in principle, it is not possible to totally remove these effects of the laminar flow. Experiments and theoretical calculations show (8) that these disturbances can only be diminished by lowering the concentration gradient per volume unit in the effluent, which means that larger column diameters are essential for multiple detection or that narrow-bore columns are unsuitable for detector combinations. Disregarding these limitations can lead to serious misinterpretations of GPC results of multiple detector measurements. Such effects are a justification for thick columns of 8-10 mm diameter. 

Capillary column A narrow bore tube (0.25-1 mm ID) typically 30-100 m long (usually of deactivated fused silica), whose walls are coated with a liquid stationary phase to produce high-efficiency separations (N > 100,000). 

Several mercury electrodes combine the features of the DME and HMDE. In particular, one employs a narrow-bore capillary that produces DMEs with drop lives of 50-70 s (14). Another involves a controlled-growth mercury drop (15). For this purpose, a fast-response valve offers a wide range of drop sizes and a slowly (step-by-step) growing drop. 

If the platinum wire is to be used as an electrode it may be preferable not to flatten and shape it as in the Housekeeper method, but to use a mercury cup on the outside of the apparatus (Figure 42, III). The fine wire is fused into a constricted part of a narrow bore tube and the... 

S. Porter and N. Johnston, The application of narrow-bore HPLC columns to the analysis of veterinary drugs, in Proceedings of the EuroResidue II Conference, pp. 538-542 (1993). 

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... 

Where Q, is the minimum detectable amount, R the detector noise level and s the detector sensitivity [135,146,151,152]. For a concentration sensitive detector the minimum detectable concentration is the product of Q, and the volumetric gas flow rate through the detector. The minimum detectable amount or concentration is proportional to the retention time, and therefore, directly proportional to the column radius for large values of n. it follows, then, that very small quantities can be detected on narrow-bore columns. 

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