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crown ethers, 4, diamides), cyclodextrins are by far the most commonly used. They include three types of sites: A, axial hydroxyl, B, equatorial hydroxyl and C, hydroxymethyl. Their reactivities are sufficiently different to enable selective reactions and thus obtain some 50 phases (e.g. 2, ungrafted cycloSil‐B phase from Chromoptic). Partial chromatograms of natural extracts, demonstrating the separation of optical isomers of carvone.

Schematic illustration of gas analyses.

      Historically, silica gel, a thermostable material that is insensitive to oxygen, was one of the first compounds to serve as a stationary phase for GC columns (Figure 2.11). Today, solid phases have become much more elaborate.

      2.7.1 Universal or Near‐Universal Detectors

      Flame ionization detector (FID)

Schematic illustration of FID detector (a) and NPD detector (b).

      (Source (a): Modified from Cremer, E. and Prior, F. (1951), Anwendung der chromatographischen Methode zur Trennung von Gasen und zur Bestimmung von Adsorptionsenergien. Zeitschrift für Elektrochemie und angewandte physikalische Chemie, 55, 66–70. https://doi.org/10.1002/bbpc.19510550115. (b): Courtesy of Supelco.)

      For organic compounds, the intensity of the signal is sensitive to the mass flow of the sample, except in the presence of heteroelements, such as halogens. The latter may change the response and several simple compounds, such as water, carbon dioxide or ammonia, do not give any response. Thus, the area under the peak reflects the mass m of the compound eluted (dm/dt integrated between the beginning and end of the peak). An FID detector is not affected by variations in flow rate, which can lead to errors with some types of detectors. The sensitivity of this detector is expressed in Coulombs/g of carbon, and the dead volume of the detector is null. The detection limit is in the order of 2 or 3 pg/s, and the linear dynamic range reaches 108; however, concentrated solutions do not lead to the best resolution.

      To evaluate the overall quantity of volatile organic compounds (VOCs) in polluted air, there exist small portable instruments housing a flame ionization detector that allows the measurement of the carbon factor of the atmosphere examined, without prior chromatographic separation.

Schematic illustration of thermal conductivity detector.

      Thermal conductivity detector (TCD)

      Its operating principle is based on the thermal conductivity of gas mixtures as a function of their composition. The main part of this detector is the katharometer, a thermostatted metal unit that is brought to a temperature slightly higher than that of the column and which includes thermistors located in tiny cavities. In the given example, the katharometer includes four thermistors, placed two‐by‐two and fed as indicated either with carrier gas sourced upstream from the injector or with the mobile phase downstream from the column. When a solute elutes, the conductivity of the mixture (carrier gas + compound) decreases with respect to that of the carrier gas alone. The thermal equilibrium is disrupted and this results in a variation in the resistance of one of the filaments, which is proportional to the concentration of the compound in the carrier gas. The dynamic range of this detector extends over some six orders of magnitude, and while its sensitivity is quite average (from ng to mg), it is being used more frequently thanks to the rise of micro‐GC (see Section 2.9.2).

      Mass spectrometry detector (MSD)

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