Splitter Design Features

Splitting should be reproducible; reproducibility is, however, affected by many factors.

The sample must encounter inert surfaces only - the injector usually contains a glass liner. Splitting should be linear and nondiscriminatory, i.e. sample should be vaporized rapidly and pass the column inlet as a concentrated, homogeneous plug. Because of the limited heat available, the sample is unlikely to be vaporized instantaneously. Droplets can be carried into the column irreproducibly and evaporation of droplets in the injector leads to fractional distillation. Such behaviour can be overcome by using a mixing device, e.g. packing the liner with glass wool or glass beads. This can cause dilution of the sample (loss of sensitivity) and discrimination owing to adsorption, condensation and decomposition. Adsorption can be reduced by silylation in situ (because packing an injector with silylated glass wool results in fracture of the glass fibres and exposure of new active sites); condensation might be eliminated by increasing the temperature.

Packed injector liners were introduced as thermal reservoirs to encourage rapid vaporization. Calculations show, however, that except for volumes of ca. 0.5 |L, the amount of heat available from carrier gas and liner packing is well below that required for vaporization. Heat is available from the injector wall, but when solvent droplets touch this they are instantly repelled by vaporization of the small amount of solvent in (instantaneous) contact with the wall. It is now accepted that the packing provides a surface that retains the entire sample during slow, controlled evaporation.

The carrier gas is heated so that at high split ratios the gas flow does not cool the injector, leading to variation in the split ratio or incomplete volatilization. If the vent flow is turned off after injection (to conserve carrier gas), ensure: (1) this does not affect column head pressure, hence carrier gas flow rate (back-pressure regulation is claimed to achieve this); (2) there is a small continuous flow from the injector to eliminate back diffusion of partially vented material; and (3) the injector septum is gently purged to atmosphere to eliminate ghost peaks from septum bleed.

It has been suggested that a buffer volume beyond the split point reduces pressure fluctuations that might lead to variations in carrier flow and split ratio; although such buffer volumes do indeed help dispel pressure waves, their benefits have been disputed.

Solar Panel Basics

Solar Panel Basics

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