GC Injection Systems

Many specialized injection systems are used with the GC in the modern laboratory to deliver the complete sample to the column without alteration. The conventional injection method is simply to inject a small volume (about 1 /Л) of a dilution of the sample using a syringe with a fine needle, which pierces a silicon rubber septum and delivers the sample into a heated chamber where it is vaporized and carried on to the column in the stream of carrier gas. Some of the carrier gas containing vaporized sample may be split from the main column flow and vented to reduce the amount of sample delivered to the column. This is known as a ‘split injection’ and is suitable for almost all liquid or soluble materials, even for samples that may contain trace amounts of non-volatile contaminants, as these are deposited in the vaporization chamber of the injector and do not reach the column.

The high temperature of the injector, typically 250 °С, means that this method is not suitable for analytes that are subject to thermal degradation. For these materials, an ‘on-column’ method is preferable, in which the solution of sample is injected directly into the narrow capillary column with a fine needle. On-column injection techniques are also more suitable for extremely dilute samples, as more sample is delivered to the column, but are less suitable for dirty samples contain­ing non-volatile contaminants, which accumulate on the column.

One of the most versatile injection systems is the programmable temperature vaporizer (PTV), a design that incorporates a vaporization chamber with a rapid and accurately controlled heating element, such that the temperature can be programmed to rise following the injection; thus, analytes are swept onto the column without being subjected to temperatures above their boiling point. By electronically controlling the action of the split vent valve, the system can also be used to vent the volatile solvent from dilute samples at a low temperature and then to deliver the analytes to the column by increasing the temperature. This allows much larger volumes of dilute solutions to be injected, thus increasing the absolute amount of analyte on the column without flooding it with solvent.

The use of a headspace injection technique may be preferable if the analytes are contained within a non-volatile or corrosive matrix that cannot be injected directly (perfume in a washing powder, for example). The basic principle of headspace injection is the delivery of a volume of vapour from the space above the sample material to the GC column. This can be achieved in several ways:

—A gas-tight syringe.

—A sample loop and a system of valves to fill the loop with vapour and then direct it onto the column.

—An adsorbent on which the volatile materials are trapped, and subsequently desorbed with a solvent or by heating the adsorbent.

Only materials volatile at the sampling temperature are transferred to the GC. Therefore, most headspace injection systems include some means of either gently heating the vial containing the sample or bubbling a gas through the non-volatile liquid to purge the volatiles, which can then be trapped on an adsorbent. The purge-and-trap method of headspace injection is widely used for analysing very low
levels of volatiles in water, whether they are environmental samples or samples of water containing perfume from washing machines or dish­washers. The trapping of natural volatiles on absorbent cartridges is the basis for headspace analysis of flowers and other natural materials (TerHeide, 1985).

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