Open tubular columns minimum

In the previous two chapters, equations were developed to provide the optimum column dimensions and operating conditions to achieve a particular separation in the minimum time for both packed columns and open tubular columns. In practice, the vast majority of LC separations are carried out on packed columns, whereas in GC, the greater part of all analyses are performed with open tubular columns. As a consequence, in this chapter the equations for packed LC columns will first be examined and the factors that have the major impact of each optimized parameter discussed. Subsequently open tubular GC columns will be considered in a similar manner. 

Equation (13) gives the minimum analysis time that can be obtained from an open tubular column, when separating a mixture of defined difficulty, under given chromatographic conditions. It is seen that, in a similar manner to the packed column, the analysis time is inversely proportional to the fourth power of the function (a-1) and inversely proportional to the inlet pressure. The contribution of the function of (k1), to the analysis time is not clear and can be best seen by calculation. It is also seen (perhaps a little surprisingly) that the analysis time is completely independent of the diffusivity of the solute in the mobile phase but is directly proportional to the viscosity of the mobile phase. 

Plate height is reduced in an open tubular column because band spreading by multiple flow paths (Figure 23-19) cannot occur. In the van Deemter curve for the packed column in Figure 23-15. the A term accounts for half of the plate height at the most efficient flow rate (minimum H) near 30 mL/min. If A were deleted, the number of plates on the column would be doubled. To obtain high performance from an open tubular column, the radius of the column must be small and the stationary phase must be as thin as possible to ensure rapid exchange of solute between mobile and stationary phases. 

A generalized plot of the Van Deempter Equation is illustrated in Fig. 11.3. Each of the terms is plotted individually (A = 0 here, as this figure illustrates a plot for an open tubular column) and their sum is shown as the dashed line with a distorted U-shape. The important feature of the plot is that each chromatographic system will have a minimum value for 7/ as a... 

For practical reasons it is desirable to have only the minimum number of columns that will solve one s most frequent separation problems. Open tubular columns are so efficient that fewer of them are usually needed, but it is common to have two to four different phases and several different film thicknesses and lengths. More specific information is given in each of the respective chapters on packed and capillary columns. 

For low k veilues resolution increases very rapidly with increasing k. At large k values the term C 1 and a further k increase will not improve resolution. Thus, the optimum veilues of k lie in the range 1 < k < 10. The minimum analysis time for open tubular columns is achieved under the conditions where k is about 1-2. As a rule, capacity is not a critical parameter in ALOT columns. 

Theoretical performance in gas chromatography. As the inside radius of an open tubular gas chromatography column is decreased, the maximum possible column efficiency increases and sample capacity decreases. For a thin stationary phase that equilibrates rapidly with analyte, the minimum theoretical plate height is given by... 

Figure 3 Progress of frontal zone under application of pulsed electric field. A pulsed electric field was applied in 2-s cycles (electric field was applied for half of the period, and was stopped for the other half). Frontal zone positions were measured from the digital picture on a CRT. The perpendicular line shows the direction of movement of the frontal zone, and 1 mm in length is equal to 167 pixels. The horizontal line shows the time, and the minimum time scale observed is equal to one-fifteenth. A round open-tubular capillary column 75 pm in diameter and 28 mm in length was used. Applied voltage, 1 50 V. Colored sample methanol solution of 1 mM Rhodamine 6G.

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