Читать книгу Chemical Analysis - Francis Rouessac - Страница 69

The most common injection method is where a microsyringe is used (Figure 2.3) to inject a very small quantity of sample in solution (1 μl or less) through a rubber septum sealing the injection chamber. Some very elaborate septa have been developed (Figure 2.4) for microsyringes, which are an integral part of the automatic injectors found in most current instruments (Figure 2.1).

Figure 2.3 Microsyringe for GC and principle of an injection loop installed in a continuous process. The model chosen for this illustration has a cone shape, adapted to several septa or automatic injectors. In this model, the piston enters the needle to deliver the entire sample and prevent any dead volume. At the bottom, there is an example of an injection loop for gas or liquids (also see Figure 3.5).

Figure 2.4 Direct vaporization injector used for packed columns. The typical septum is an elastomer disc but there are also more sophisticated versions, including the Merlin Microseal, which can be used thousands of times.

(Source: Courtesy of Agilent Technologies.)

Automatic injectors with a sample holder carousel are called autosamplers. With some of them, we can select the injection method (liquid, headspace, SPME (see Chapter 21)) and, if necessary, conduct a short preparation of each sample beforehand (dilution, addition of an internal standard, derivatization). These devices complete the chromatogram, improve results, and shorten analysis times. They have become essential for multi‐analyte methods in environmental analysis (search for PAHs, mycotoxins, pesticides).

When there is too little analyte to be detected (detectability limit) or to be quantified (quantification limit), we must make a preconcentration. To do so, we have such techniques as SPME (solid phase micro‐extraction) or SBSE (stir bar sorptive extraction). These techniques use the possibility of adsorbing an analyte on a solid phase, and inversely to desorb in the injector when subjected to heat (see Chapter 21).