The injector, which is the sample’s entrance to the chromatograph, has different functions. Besides its role as an inlet for the sample, it must vaporize, mix with the carrier gas and bring about the sample at the head of the column. The characteristics of the injectors, as well as the modes of injection, differ according to column type. The use of an automatic injection system can significantly enhance measurement precision.
Direct vaporization injector
Direct vaporization injector
For packed and megabore columns (see paragraph 2.5), which typically use a flow rate of about 10 mL/min, direct vaporization is a simple way to introduce the sample. Any model of this type comprises a metal tube with a glass sleeve (called the insert). It is heated to the average boiling temperature of the compounds being chromatographed. The needle of the micro-syringe containing the sample pierces the septum, made of silicone rubber, which closes the end of the injector. The other end, also heated, is connected directly to the column (Figure 2.4). Once all of the liquid plug has been introduced with a syringe it is immediately volatilized and enters the column entirely within a few seconds, 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 micro-syringe 01 L, can saturate the column. Special injectors are used which can operate in two modes, with or without flow splitting (also called split or splitless).
In the split mode a high flow rate of carrier gas arrives in the vaporization chamber where it mixes with the vapours of injected sample (Figure 2.5). A vent
valve, regulated at between 50–100 mL/min, 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 1 : 20 and 1 : 500. Only the smallest fraction, containing an amount of sample equal to the ratio of division, will penetrate into the column.
Split ratio = (split outlet flow rate +column outlet flow rate)
column outlet flow rate
Split ratio = (split outlet flow rate +column outlet flow rate)
column outlet flow rate
When working with capillary columns, this type of injector is also used for
very dilutes samples in the splitless mode. In this mode a smaller volume of solution is injected very slowly from the micro-syringe during which bleeding valve 2 (Figure 2.5) is maintained in a closed position for 0.5 to 1 minute in order that the vaporized mixture of compounds and carrier solvent are concentrated in the first decimetre of the column. The proper use of this mode of injection, which demands some experience, requires a program that starts with a colder temperature in order that the solvent precedes the compounds onto the column. The re-opening of valve 2 provides an outlet for an excess of solvent-diluted sample. Some less volatile compounds are eliminated and that can interfere with the results of the analyses.
very dilutes samples in the splitless mode. In this mode a smaller volume of solution is injected very slowly from the micro-syringe during which bleeding valve 2 (Figure 2.5) is maintained in a closed position for 0.5 to 1 minute in order that the vaporized mixture of compounds and carrier solvent are concentrated in the first decimetre of the column. The proper use of this mode of injection, which demands some experience, requires a program that starts with a colder temperature in order that the solvent precedes the compounds onto the column. The re-opening of valve 2 provides an outlet for an excess of solvent-diluted sample. Some less volatile compounds are eliminated and that can interfere with the results of the analyses.
In quantitative analysis, the use of the split/splitless injector can result in concentration errors owing to a strong discrimination between compounds that are of different volatility: the composition of the fraction entering the column is different to that which is eliminated from the injector. This mode of operation should be avoided when
Cold on-column injection
This is not a vaporization technique. The sample is deposited directly into the capillary column (‘cold on-column’orCOC). A special micro-syringe, whose needle (steel or silica) is of 0.15 mm diameter, is necessary for penetrating the column which is cooled to 40
C before being allowed to return to its normal operating temperature. This procedure, useful for thermally labile compounds or high boiling compounds, is difficult to master without the aid of an autosampler. It is known not to discriminate between compounds of different volatilities (Figure 2.5).
This is not a vaporization technique. The sample is deposited directly into the capillary column (‘cold on-column’orCOC). A special micro-syringe, whose needle (steel or silica) is of 0.15 mm diameter, is necessary for penetrating the column which is cooled to 40
C before being allowed to return to its normal operating temperature. This procedure, useful for thermally labile compounds or high boiling compounds, is difficult to master without the aid of an autosampler. It is known not to discriminate between compounds of different volatilities (Figure 2.5).
Programmed temperature vaporization injector
This injector, named PTV (programmed temperature vaporizer), is conceptually similar to the split/splitless model. The temperature of the injection chamber can be programmed to effect a gradient, e.g. from 20 up to 300
C, in a few tens of seconds (Figure 2.6). So, the advantages of the split/splitless injection are combined with those of the cold injection onto the column.
It becomes possible to inject greater volumes with standard syringies avoiding needle-induced discrimination. Furthermore, compounds having low boiling points (particularly solvents) can be eliminated.
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 vaporization chamber and immediately the vent valve is opened and the injector is heated. As the sample is not instantaneously vaporized, the solvent and the different compounds penetrate onto the column in the order of their boiling points. In this way the column is never overcharged.
This injector, named PTV (programmed temperature vaporizer), is conceptually similar to the split/splitless model. The temperature of the injection chamber can be programmed to effect a gradient, e.g. from 20 up to 300
C, in a few tens of seconds (Figure 2.6). So, the advantages of the split/splitless injection are combined with those of the cold injection onto the column.
It becomes possible to inject greater volumes with standard syringies avoiding needle-induced discrimination. Furthermore, compounds having low boiling points (particularly solvents) can be eliminated.
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 vaporization chamber and immediately the vent valve is opened and the injector is heated. As the sample is not instantaneously vaporized, the solvent and the different compounds penetrate onto the column in the order of their boiling points. In this way the column is never overcharged.
• Splitless cold injection: this mode is employed for trace analysis. The vent valve is closed during the injection. The injection chamber is then heated in order to transfer the sample to 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. Vent flow rate is very high and can attain 1000 mL/min to eliminate all of the solvent. The injector is then heated to permit transfer of the less volatiles compounds onto the column, the bleed valve now being closed (splitless mode). In this way it is possible to inject up to 50 L in a single injection or up to 500 L of sample solution, over several injections. This method eliminates the step of the preliminary concentration of sample prior to injection.
• Injection with elimination of solvent: the sample is introduced into the cold injector after which the vent valve is opened. Vent flow rate is very high and can attain 1000 mL/min to eliminate all of the solvent. The injector is then heated to permit transfer of the less volatiles compounds onto the column, the bleed valve now being closed (splitless mode). In this way it is possible to inject up to 50 L in a single injection or up to 500 L of sample solution, over several injections. This method eliminates the step of the preliminary concentration of sample prior to injection.