Length column

Column length affects both the efficiency and the speed of the separation. Longer columns result in longer analysis times. However, the column efficiency tends to increase with length. In general, short columns are used for simple separations. Analytical columns can range from 30 to 300 mm in length. 

Actually, the decrease in column length allows to reach log Poet values up to 7.8 using for example an ODP-50 carhidge with a reasonable analysis time [38]. Moreover the use of acetonitrile as organic modifier and a Discovery RP-amide-Cis 

20x4 mm column has recently shown promising results for the determination of log Pod of very lipophilic compounds [36], 

The analytical column should be as short as possible. The most common lengths for standard columns are 30 or 60 m. Fast GC separations go with 10 or 15 m lengths. Greater lengths are usually not necessary in general residue analysis with GC-MS systems, and are reserved for special well-documented separation purposes as known for instance for FAMEs, or the complex PCB analyses. 

Shorter columns would be desirable for simpler separations, but they are with the same diameter at the limit of the maximum flow for the mass spectrometer 

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Solving the problem (8)-(10), we obtain after some transformations the formula for the dependence of top s column length on time t ... 

Recovery factor Reduced column length Reduced plate height Reduced velocity Relative retention ratio Retardation factor d Retention time Retention volume Selectivity coefficient Separation factor... 

Pressure, column outlet Po Reduced column length A... 

The average linear velocity u of the mobile phase in terms of the column length L and the average linear velocity of eluent (which is measured by the transit time of a nonretained solute) is... 

Under constant pattern conditions the LUB is independent of column length although, of course, it depends on other process variables. The procedure is therefore to determine the LUB in a small laboratory or pilot-scale column packed with the same adsorbent and operated under the same flow conditions. The length of column needed can then be found simply by adding the LUB to the length calculated from equiUbrium considerations, assuming a shock concentration front. 

FIG. 16 36 Dimensionless time-distance plot for the displacement chromatography of a binary mixture. The darker lines indicate self-sharpening boundaries and the thinner lines diffuse boundaries. Circled numerals indicate the root number. Concentration profiles are shown at intermediate dimensionless column lengths = 0.43 and = 0.765. The profiles remain unchanged for longer column lengths. 

The value of Np required to achieve a desired resolution is determined by Eq. (16-168) or (16-171). Since N = L/HTU 2Np = 2L/HETP, Fig. 16-13 or Eq. (16-183) can be used to determine the range of the dimensionless velocity ReSc that maximizes Np for a given particle diameter and column length. 

The allowable pressure drop influences the choice of the particle size and helps determine the column length. Equations for estimating the pressure drop in packed beds are given in Section 6. 

Now, the velocity of a solute band along the column (Z) is obtained by dividing the column length (L) by the retention time, (tr)j consequently. 

Dividing the total variance by the column length (1), the multi-path contribution (Hm) to the overall height of the theoretical plate (H) is obtained. 

Thus, for a packed column length (L), radius (r), with a mobile phase volume equivalent to 60% of the column volume,... 

Column Length 15 cm, Column Diameter 1 mm Particle Diameter 5 micron, k of First Eluted Peak, 1... 

It should be pointed out that equations (14) and (15) do not give an expression for the minimum column lengths, as the optimum particle diameter has yet to be identified. 

The column length, as well as providing the required efficiency, is also defined by the D Arcy equation. The D Arcy equation describes the flow of a liquid through a packed bed in terms of the particle diameter, the pressure applied across the bed, the viscosity of the fluid and the linear velocity of the fluid. The D Arcy equation for an incompressible fluid is given as follows. 

It follows that knowing the optimum particle diameter, the optimum column length can also be identified. It must be emphasized that this optimizing procedure... 

Thus, the minimum column length (L) will be given by... 

Lmin) is the minimum column length, and (e) is the fraction of the column occupied by the mobile phase. 

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