Efficiency column diameter reduction

The efficiency obtained from an open tubular column can be increased by reducing the column radius, which, in turn will allow the column length to be decreased and, thus, a shorter analysis time can be realized. However, the smaller diameter column will require more pressure to achieve the optimum velocity and thus the reduction of column diameter can only be continued until the maximum available inlet pressure is needed to achieve the optimum mobile phase velocity. 

Conventionally, analytical SEC columns have been produced with an internal diameter of 7.5 mm and column lengths of 300 and 600 mm. In recent years environmental and safety issues have led to concerns over the reduction of organic solvent consumption, which has resulted in the development of columns for organic SEC that are more solvent efficient (13). By reducing the internal diameter of the column, the volumetric flow rate must be reduced in order to maintain the same linear velocity through the column. This reduction is carried out in the ratio of the cross sectional areas (or internal diameters) of the two columns. Eor example, if a 7.5-mm i.d. column operates at 1.0 ml/min, then in order to maintain the same linear velocity through a 4.6-mm i.d. column the flow rate would be... 

Increased column diameter may cause a reduction in the homogeneity of the packing, resulting in flowrate distortion of the component zones across the column cross-section and higher h (HETP) values. Diminution of column efficiency also can be related to (1) the nonuniformity of liquid coating which in turn affects mass transfer, (2) the radial temperature gradient that... 

Entrainment. If entrainment is excessive, column diameter or tray spacing are usually increased. It has been recommended (2,67) that entrainment from the tray should not exceed about 0.10 lb liquid entrained per pound of liquid flow. At higher values, significant efficiency reduction occurs (34). Depending on the service, a lower or higher value can be set (4). For instance, if the column overhead stream is compressed and no knock-out drum is present, the entrainment that can be tolerated is smaller. Also, for trays operating at a high liquid-to-vapor ratio, 0.1 lb of liquid entrained per pound of liquid is an excessive quantify of entrained liquid, and a lower limit is set. 

The following general trends in the development of new stationary phases for ion chromatography can be identified new stationary phases matrices and bonding chemistries improvement of column efficiency, trend toward reduction in the diameter of separation column, and new chemistry of the bonded functional groups/ layers.ti i ... 

The reduction of axial mixing by use of packings depends on the size of the packing. For good efficiency small packings have to be used. The dependence of axial mixing on the column diameter is the same as in empty bubble columns. 

One advantage of small bore columns is that they are operated at much lower mobile phase flow rates than the 4.6 mm columns, so there is a large reduction in solvent consumption and hence operating costs of the chromatograph. The efficiency of hplc columns does not depend on their diameter but it does depend on the velocity of the mobile phase in the column, so microbore columns are operated at velocities corresponding to the flow rates used with larger columns. If / (cm3 min-1) is the flow rate in a column with diameter d cm, and the mobile phase velocity is v cm min-1, / and v are related by ... 

As in interactive modes of chromatography, reduction in particle diameter reduces mass transfer effects and improves column efficiency in SEC. Column packings with particle diameters of 10 to 12 pm are available for less demanding applications, whereas SEC packings with particle diameters of 4 to 5 pm can be used for applications requiring higher resolution. 

Although HPLC column technology is considered to be a mature field now, improvements and new developments are being made continuously in the stationary phases. One of the improvements has been the reduction in particle sizes. Smaller particles help to improve mass transfer and provide better efficiency. Manufacturers are producing particles down to 1.5 J,m in diameter, although 3- and 5- J,m particles are still the most popular. Because of the smaller particle sizes, the backpressure increases proportionally to the inverse of the square of the particle size. Most commercially available HPLC systems cannot accommodate the pressures required to operate these columns at optimum flow rates. This has led to the introduction of systems that run at high pressures. 

One disadvantage of all silica-based stationary phases is their instability against hydrolysis. At neutral pH and room temperature the saturation concentration of silicate in water amounts to lOOppm. Solubility increases with surface area, decreasing particle diameter drastically with pH above 7.5. This leads also to a reduction of the carbon content. Hydrolysis can be recognized during the use of columns by a loss in efficiency and/or loss of retention. Bulky silanes [32], polymer coating [33], or polymeric encapsulation [34] have been used in the preparation of bonded phases to reduce hydrolytic instability, but most of the RPs in use are prepared in the classical way, by surface silanization. Figure 2.3 schematically shows these different types of stationary phases. 

To meet the high demands of organic trace analysis,21 GC columns have been subject to continuous refinement. This refers not only to the reduction in diameter of the nowadays almost exclusively used capillary columns (separation efficiency increases with decreasing capillary diameter), but also reflects the development in stationary phase technology In order to reduce column bleed (which is essential for mass spectrometric detection), highly cross-linked stationary phases are used to... 

Merely, the choice of column dimension demonstrates the awareness of cost reduction for organic solvents as well as analytical run time and thus of sample throughput and economic efficiency. Only 2 % of published reports make use of a column with an inner diameter (I.D.) as small as 1 mm [12] corresponding to a flow of 50 ul/min. Columns with an I.D. of 2.0-2.1 mm are used in 48 % of reports determining a flow between 200 and 500 ul/min and half of all applications is performed with a column I.D. between 3.0 and 4.6 mm and a flow ranging from 500 to 1,000 ml/min (Tables 5-8). 

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