Читать книгу Chemical Analysis - Francis Rouessac - Страница 102

1 As long as settings remain unchanged, GC chromatographs are able to reproduce the retention time of a compound to the nearest second, in the case of several successive injections. This can only be obtained by perfect control of all parameters: temperatures, flow rates, pressures, and carrier gas purity.

2 The range of use of GC depends on the volatility of compounds. The upper limit of this range is reached if molecular weight exceeds the 500 Da boundary, or if hydrogen bonds or dipole–dipole interactions are created between compounds.

3 At equal volatility, compounds elute from the column by following the order of their distribution coefficients in the stationary phase. The carrier gas does not participate in concentration equilibria. The two main factors determining the behaviour of an analyte are its volatility and analyte‐stationary phase interactions.

4 The carrier gas must be free from oxygen in order not to alter the analytes, which are weakened when they are brought to a high temperature in the instrument’s oven. In general, hydrogen is chosen as the carrier gas. It enables faster analyses than nitrogen or helium, without altering the efficiency (N) of the separation.

5 An abundance of capillary columns for GC is on the market, either for general use or for specific separations. They are classified according to their retention index and their polarity, as defined by their McReynolds constants. For the GC‐MS technique, we choose grafted, cross‐linked, and low‐bleed columns, which are able to interact with the molecules that require separation.

6 A new column is accompanied by a document specifying its efficiency (N), with the conditions of acquisition of that value, as well as its retention indexes for five test compounds, with different, universally used chemical properties.

7 Detectors are either general, such as FID, which is by far the most common, owing to its sensitivity and its linearity, or they are adapted to a category of compounds or are even specific to a single compound, thus simplifying the chromatograms when the matrix is complex and enabling a better quantification of the analytes in question.

8 The Kovats index of a compound is calculated from the retention times of adjacent n‐alkanes. It is of interest because it depends only on the stationary phase and not on other characteristics of the column or apparatus. Index tables help to identify compounds by comparison of their retention indexes, without worrying about retention times, which are variable.