Cold OnColumn Injection

This term denotes direct transfer of liquid sample into the column inlet, i.e. the sample is introduced to the column as a liquid rather than being evaporated from the syringe. Because the technique exploits solvent trapping, the column inlet must be kept cool, i.e. 10°C below the solvent boiling point (cf. 20°C below for splitless injection). In this way explosive evaporation of sample does not lead to its being flushed back into the cold injector where it might be lost completely or from where it might slowly drift back on to the column, leading to band broadening. The point of injection must, however, be positioned within the temperature control of the oven, usually 5-10 mm from the oven wall (a greater distance increases the chance of the needle being heated). The original design had the injector body only cooled. Later versions had the addition of secondary cooling so that during injection the section of column in the oven, and into which injection took place, could be cooled, thus increasing the range of temperatures over which the technique could be used. The injector body should be isolated from the hot insulating material of the oven top.

Injection is performed with a special syringe with a needle of o.d. 0.05 mm less than the column i.d. (so it does not plug the column and restrict carrier gas flow. The syringe is inserted into the column via a seal that prevents escape of carrier gas (column head pressure should be maintained). The carrier gas line to the injector should be through wide-bore tubing so that any slight leakage through the needle-seal does not affect the column head pressure. For borosilicate columns needle entry into the column is aided by chamfering the column inlet. This is not possible with fused silica because the column wall is too thin.

Injection speed depends on the injector. Better quantitative results are obtained by rapid injection, sending the sample well into the column to reduce the possibility of sample adhering to the needle and being removed when the syringe is withdrawn. Reproducible injection of 0.5-8 |L is possible. Smaller quantities are more difficult because they cannot be measured accurately with a 5 or 10 |L syringe and the speed at which the sample is ejected from a 0.5 or 1 |L syringe is insufficient to carry the sample well away from the syringe needle. If fast injection of these sample volumes is performed at oven temperatures near the solvent boiling point there are no problems with expanding clouds of solvent vapour being forced back past the syringe needle (with consequent sample loss). At higher temperatures it is essential that the injector is fitted with secondary cooling so that sample is ejected into a cool section of the column. If secondary cooling is not available, it might be necessary to inject slowly at or about the solvent boiling point so that the vapour formed on evaporation from the column wall (not from the syringe needle) is carried away as it is formed. Some loss on the needle usually accompanies this technique, but losses are less than if the sample were to be forced explosively back into the injector body. Losses might be reduced by increasing the carrier flow and/or leaving the syringe in the injector for a few seconds after injection, to promote evaporation of the more volatile materials. However, this technique will lead to discrimination of another type, especially if the syringe needle becomes warm. Typical speeds for slow injection of different volumes of pentane are: 0.125 |L, 0.5 s; 0.5 |L, 1 s; 2 |L, 5 s; 8 |L, 20 s. If injections are much slower than this the solvent vapour pressure might be insufficient for condensation and no solvent trapping will occur. For volumes less than 0.2 |L rapid injection can be performed without secondary cooling, as the amount of vapour produced is insufficient to cause the sample to be expelled backwards.

In addition to cold trapping, which with on-col-umn injection is effective for injections as small as 0.1 |L (cf. 0.5-1 |L for splitless injection), cold-trapping can also be exploited. Because cold trapping is less effective than solvent trapping the difference between elution and injection temperatures should be 80-120°C if effects are to be comparable.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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