Chemical Analysis. Francis Rouessac
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differ depending on the nature of samples and types of analyses. Historically, the first injectors, called direct vaporization injectors, were intended for packed columns or macrobores, in which the carrier gas flow rate must be at least 6–7 ml/min. Any model of this type comprises a metal tube with a glass sleeve (called the insert). It is swept by the carrier gas and heated to a sufficient temperature so that the less volatile solutes of the sample vaporize. The sample is introduced with a microsyringe, piercing the septum placed at the entrance of the injector (Figure 2.4). The other end is connected directly to the column. Once all of the sample has been introduced with a syringe, it is immediately volatilized and enters the column, swept along by the carrier gas.
Split/splitless injector
For capillary columns able to handle only a small capacity of sample, even the smallest volume that it is possible to inject with a microsyringe (0.1 μl) can saturate the column. Special injectors are used that can operate in two modes, with or without flow splitting (also called split or splitless). This helps to adjust the sample fraction sent into the column as compared to the total quantity introduced in the chromatogram.
In split mode, a flow of carrier gas (e.g. 100 ml/min) arrives in the vaporization chamber where it mixes with the vapours of the injected sample (Figure 2.5). A vent valve separates this flow into two fractions, of which the largest portion is vented from the injector, taking with it the majority of the sample introduced. The split ratio typically varies between 20:1 and 500:1. Only the smallest fraction, containing an amount of sample equal to the split ratio, will penetrate into the column.
Figure 2.5 Injectors. Above left, injection chamber with splitter (vent 2 regulates the split). Below, a typical chromatogram obtained in splitless mode. For solvent peaks that are superimposed upon those of the compounds, a selective detector which does not “see” the solvent is recommended.
Splitless mode is used only for very dilute samples. The contents of the microsyringe are injected very slowly from the microsyringe, during which bleeding valve 2 (Figure 2.5) is maintained in a closed position for 0.5–1 minute in order for the vaporized mixture of compounds and the carrier solvent to be concentrated in the first few decimetres of the column, where the temperature is lower than the boiling point of the solvent. The opening of valve 2 provides an outlet for an excess of sample. The solvent precedes the compounds in the column and on the chromatogram. For either one of these injection techniques, chromatographs with software‐controlled parameters (flow rates and pressures) have a gas saver, which reduces the split ratio after the injection phase.
The split injector may lead to a poor evaluation of mixture composition, following a discrimination between compounds with varying volatilities: the composition of the fraction entering the column is not the same as the composition of the eliminated fraction. Therefore, it is a problem in quantitative analysis, where external standards are not advised.
Cold on‐column injection (COC)
The sample is injected directly into the capillary column (cold on‐column or COC). It is vaporized just after introduction. A special microsyringe, whose needle (steel or silica) is just 0.15 mm in diameter, is necessary for penetrating the column, which is cooled to 40°C before being allowed to return to its normal operating temperature. This process, useful for fragile compounds, is difficult to master in order to correctly eliminate the solvent(s). It is known not to discriminate between compounds of different volatilities (Figure 2.6). The injector has a special duckbill septum. While it is useful for thermally labile compounds, there is a risk of column pollution if nonvolatile compounds are present.
Programmed temperature vaporization (PTV) injector
This PTV injector is considered to be the most universal. It can be used successively as a cold on‐column injector and then as a split/splitless injector. The temperature of the injection chamber can be programmed to create a gradient, e.g. from 20 up to 300°C, in a few tens of seconds (Figure 2.6). So, the advantages of split/splitless injection are combined with those of cold injection onto the column. The advantages are:
Absence of discrimination due to the needle.
Use of classic syringes.Figure 2.6 PTV injector, with programmable temperature and cold on‐column injector. To make fast temperature gradients, the injection chamber is surrounded by a heating element or cooled by the circulation of a cold gas.
Elimination of the solvent or low‐boiling point compounds.
Larger injection volume.
The three principal modes of operation are named split cold injection, splitless cold injection and injection with elimination of solvent.
Split cold injection: the sample is introduced into the cold vaporization chamber. The vent valve is then immediately opened and the injector is heated. As the sample is not instantaneously vaporized, the solvent and the different compounds penetrate the column in the order of their boiling points. In this way, the column is never overloaded.
Splitless cold injection: this mode is employed for trace analysis. The vent valve is closed during injection. The injection chamber is then heated in order to transfer the sample into the column, which is maintained cold.
Injection with elimination of solvent: the sample is introduced into the cold injector, after which the vent valve is opened. The vent flow rate is very high and can reach 1,000 ml/min in order to eliminate all of the solvent. The injector is then heated to permit transfer of the less volatile compounds onto the column, with the vent valve now being closed (splitless mode). In this way, it is possible to inject up to 50 μl in a single injection or up to 500 μl of sample solution in a volatile solvent, over several injections. This method eliminates the preliminary concentration step of the sample prior to injection.
Injection after pyrolysis. Solid samples that are not soluble in solvents may nevertheless be the subject of a GC analysis by placing a device upstream from the injector. This device brings the sample to 600°C to decompose it into smaller molecules. The composition of these molecules will be used to identify the starting sample. The related injector is generally of the PTV type. This method is used for polymers, paints, rubber, additives or textiles.
2.4 TEMPERATURE‐CONTROLLED OVEN
The gas chromatograph comprises a temperature‐controlled enclosure, or ventilated oven, in which the column is placed. This column can be heated uniformly to more than 400°C, if necessary. The enclosure must have low thermal inertia, enabling a rapid but controlled temperature rise when a temperature gradient is included in the analysis (gradient can reach 130–140°C/min. with excellent stabilization at 0.05°C). By installation of a cryogenic valve fed with nitrogen or carbon dioxide in the liquid state, the oven can be very quickly regulated at low temperature. This valve is essential when we work with an autosampler and a temperature gradient, as temperature cycles must