Plunger-in-barrel (5-1000 |L) and plunger-in-needle (0.5 or 1 |L) syringes are widely available. The latter are used to dispense quantities up to 1 |L, although the 5 |L syringe can be used to dispense 0.3 |L with reasonable repeatability, especially if used on an autosampler, although the amount injected is not necessarily that indicated on the graduated scale. With the plunger-in-barrel syringe, needle volume is relevant if the injector temperature is above the solvent boiling point. When the plunger-in-needle syringe is used for vaporizing injection the needle temperature starts to increase when the needle penetrates the septum; the sample should, therefore, be partly withdrawn into the needle, otherwise part will be discharged into the septum.

It is difficult to determine the volume of sample remaining in the needle after vaporizing injection. Much will be forced from the syringe by explosive vaporization; this will cool the needle (passage of cold solution; absorption of latent heat of vaporization) and some sample will inevitably evaporate from inside the needle, depositing involatile residue. Such problems decrease in significance with increasing injection volume (but are immensely significant in capillary GC). Passage of sample through a hot steel needle can lead to decomposition of unstable sample components.

These considerations have led to the development of several different syringe-handling techniques.

Filled needle The plunger is moved quickly up and down to eject air and the syringe is removed from the sample with the plunger fully depressed. The sample is injected by pushing the syringe through the septum without moving the plunger. The needle volume only is injected, some sample is inevitably 'injected' into the septum, and much of the sample evaporates from the inner surface of the needle.

Cold needle The sample is withdrawn into the barrel and the syringe is inserted through the septum and the plunger depressed immediately. It is hoped that most of the sample passes through the needle in the liquid state with the needle still cold. Sample remaining in the needle, however, evaporates as the needle warms.

Hot needle Performed as above but the needle is left to warm in the injector for 3-4 s before rapid depression of the plunger. Much of the sample is rapidly ejected from the needle as a result of rapid explosive vaporization in the first stages of injection. The amount of sample evaporating from the inside of the needle is probably less than for cold needle injection.

Solvent flush A small volume (c. 1 |L) of solvent is withdrawn into the barrel, then the sample (possibly with an air barrier between the two to prevent mixing). Injection is performed by hot or cold needle injection. The sample can be sandwiched between two portions of solvent.

Air flush As above, but with air in place of solvent. However, the continuous introduction of oxygen into the column is not recommended for high temperature work because of the deleterious effect on the stationary phase.

Air and solvent As above, but with both air and solvent.

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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