Separation packed columns

Figure 6.3 Conparlson of the separation of the octylphenol poly(ethylene glycol) ether, Triton X-16S on a packed column, left, and an open tubular column, right, using UV detection. For the packed column separation al0cmx2mmI.D. column packed with Nucleosil C g, d. 3 micrometers, temperature > 170 C, and mobile phase carbon dioxide (2 ml/min] and methanol (0.15 nl/rnin). pressure programmed from 130 to 375 bar in 12 min were used. For the open tubular column separation a 10 m x 50 micrometers I.O., SB-Biphenyl-30, temperature = 175°C, mobile phase carbon dioxide (0.175 ml/min) and 2-propanol (0.0265 ml/min) pressure programmed, 125 bar for 5 min, then ramped from 125 to 380 bar over 19.5 min, and held at 380 bar for 15 min. were used. (Reproduced with permission from ref. 57. Copyright Preston Publications, Inc.) ...

For packed column separations using Pall rings, the height of a transfer unit (HTU) may be estimated as follows ... 

Packed column separation and analysis is certainly valuable for many routine tests and even very useful and necessary at times. However, it cannot meet the analytical requirements of many of today s investigations. For example, if the sample size is a limiting and critical factor, as in blood testing or other biological samples, or if complex mixtures are of interest, the packed column will in most cases fail to generate satisfactory results. 

Gorak, A. and Vogelpohl, A., Experimental Study of Ternary Distillation in a Packed Column, Separ. Sci. Technol., 20, 33-61 (1985). 

Most contemporary instruments for supercritical fluid chromatography are optimized for either open tubular or packed column separations and may provide a poor compromise for column types other than those they were designed for [146]. The small volumes and low flow rates associated with open tubular columns make greater demands on instrument design than typical packed columns, which have dimensions similar to those commonly used in liquid chromatography. 

Figure 6.5 shows chromatograms of the same sample on a packed and capillary column. On top is a packed-column separation of AROCHLOR 1260 , a commercial blend of polychlorinated biphenyl compounds. A two-meter column of two-millimeter, i.d. glass was used with an electron capture detector. This chromatogram exhibits a plate number of about 1,500, and we observe about 16 peaks with this sample. 

All initial work in gas chromatography was performed on packed columns and the first commercial instruments accepted only packed columns. Later, when open tubular capillary columns were invented, only one manufacturer (Perkin-Elmer) produced them, so most chromatographers continued to use packed columns. As a result, much of the early literature reports only packed column separations. Today, however, it is estimated that over 80% of all analyses are made on capillary columns. 

Even if excellent equipment is used, but the sample is passed over a badly packed" column, separation efficiency is poor. Therfore, the goal is to pack the chromatographic bed as densely and... 

Continuous passage of a suitable eluant (mobile phase) through the packed column separates the components of the sample applied to the column. This process is known as column development. There are two main techniques of elution (ij tsocratic elution, and (ii) gradient elution. 

Examination of the parameters in the van Deemter expression (Equation 2.44), term by term, provides a basis for optimizing a packed column separation. The plate height, h, of a packed column may be represented as the sum of the eddy diffusion, molecular diffusion and mass transfer effects. Thus, to attain maximum column efficiency, each term in the plate height equation should be minimized ... 

Three stages in the evolution of the capillary-column technology are presented in Figure 3.15 a packed-column separation and two separations with a stainless-steel and glass capillary column. Better resolution is evident with the capillary... 

While open tubular columns are comparable with packed columns in terms of efficiency under optimal flowrate conditions, longer open tubular columns provide more theoretical plates and thus have greater resolving power. A typical 30-min-long packed-column separation with a 2-m column will generate a typical peak capacity of 30-100 peaks. A 50-m-long open tubular column can generate a peak capacity of several hundred peaks in the same timeframe. 

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. 

Fig. 4. Examples of gas chromatographic separations on capillary and packed columns, (a) Packed column separation of mixed hydrocarbons (b) capillary column separation of some flavor compounds. Reproduced from D.W. Grant, Capillary Qas Chromatography, 1996. John Wiley Sons Ltd. Reproduced with permission.

Figure 2.123 Simulation of a packed column separation performed on a 30 m capillary column. Carrier gas has been changed from nitrogen to helium and compared with a 3% OV17 packed column, the 5% phenyl-methylpolysiloxane capillary column separates four previously unresolved peak pairs in a similar run time. Column 30 m TRACE...

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