28 capillaryopen tubular open tubular or capillary

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28 Capillary/Open Tubular Open tubular or capillary columns are finding broad applications. These are mainly of two types: • Wall-coated open tubular (WCOT) <1 m thick liquid coating on inside of silica tube • Support-coated open tubular (SCOT) 30 m thick coating of liquid coated support on inside of silica tube These are used for fast and efficient separations but are good only for small samples. The most frequently used capillary column, nowadays, is the fused silica open tubular column (FSOT), which is a WCOT column.
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29 The external surface of the fused silica columns is coated with a polyimide film to increase their strength. The most frequently used internal diameters occur in the range from 260-320 micrometer. However, other larger diameters are known where a 530 micrometer fused silica open tubular column was recently made and is called a megapore column, to distinguish it from other capillary columns. Megapore columns tolerate a larger sample size .
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35 It should be noted that since capillary columns are not packed with any solid support, but rather a very thin film of stationary phase which adheres to the internal surface of the tubing, the A term in the van Deemter equation which stands for multiple path effects is zero and the equation for capillary columns becomes H = B/ V + C V
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36 Capillary columns advantages compared to packed columns 1. higher resolution 2. shorter analysis times 3. greater sensitivity Capillary columns disadvantage compared to packed columns 1. smaller sample capacity 2. Need better experience
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37 Solid Support Materials The solid support should ideally have the following properties: 1. Large surface area (at least 1 m 2 /g) 2. Has a good mechanical stability 3. Thermally stable 4. Inert surface in order to simplify retention behavior and prevent solute adsorption 5. Has a particle size in the range from 100- 400 m
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38 Selection of Stationary Phases General properties of a good liquid stationary phase are easy to guess where inertness towards solutes is essential. Very low volatility liquids that have good absolute and differential solubilities for analytes are required for successful separations. An additional factor that influences the performance of a stationary phase is its thermal stability where a stationary phase should be thermally stable in order to obtain reproducible results. Nonvolatile liquids assure minimum bleeding of the stationary phase
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39 Weight of liquid stationary phase * 100% %Loading = Increasing percent loading would allow for increased sample capacity and cover any active sites on the solid support. These two advantages are very important, however increasing the thickness of stationary phase will affect the C term in the van Deemter equation by increasing H S , and therefore H t .
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