Chemical Analysis. Francis Rouessac
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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.
Figure 2.11 Gas analyses. Left, one of the earliest ever chromatograms, obtained point by point and representing a mixture of air, ethylene and acetylene separated on silica gel (E. Cremer and F. Prior, Z. Elektrochem. 1951, 55, 66). Right, an analysis of gas on a modern PLOT column (reproduced courtesy of Supelco).
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 MAIN DETECTORS
Downstream from the column, the chromatograph has one last device: the detector, which provides a signal with the passage of each analyte. All of the signals will form the chromatogram. This detector uses variations in physical amplitude related to the presence of a solute, in addition to that of the carrier gas. Some detectors are universal; that is they are sensitive to practically every solute, while others are selective detectors that are sensitive only to specific compounds (Figure 2.14). The most effective rely on the adaptation of spectral methods, or on mass spectrometry, which not only helps generate the conventional chromatogram but also helps identify each solute (see the Chapter 16 on Mass Spectrometry). Nevertheless, this is expensive, and we always find a number of conventional detectors in laboratories.
2.7.1 Universal or Near‐Universal Detectors
Flame ionization detector (FID)
Considered as almost universal for organic compounds, this is the detector par excellence for GC. The gas flow issuing from the column passes through the flame of a small burner fed by a mixture of hydrogen and air. When a compound is eluted, its combustion results in the release of ions and charged particles responsible for the passage of a very weak current (10−12 μA) between two electrodes (p.d. of 100–300 V). The signal is transformed by an electrometer into a measurable voltage (Figure 2.12). One end of the burner, held at ground potential, acts as a polarization (ground) electrode, while the ring‐shaped second electrode surrounds the flame.
Figure 2.12 FID detector (a) and NPD detector (b). The make‐up gas (or filling gas) is useful when the flow rate of the capillary column is too small. The electrometers used with detectors enable the measurement of intensities that would too small for a galvanometer. Many variants exist depending on the manufacturer.
(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.
Figure 2.13 Thermal conductivity detector. Left, layout demonstrating the dual circulation of the carrier gas. Right, a katharometer unit with the principle of its electrical connections in a Wheatstone bridge type assembly.
Thermal conductivity detector (TCD)
This universal detector, developed in the early days of GC for packed columns, is still in use now. Easy to build, it exists in a number of variants (Figure 2.13), including miniaturized forms (μ‐TCD) for capillary columns.
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)
For several years now, the rise in mass spectrometry has favoured the use of the coupled GC/MS technique, which consists in connecting the chromatograph to a mass detector (a low‐resolution mass spectrometer, see Chapter 16 for more details). Its use as a universal detector for capillary GC has the advantage of leading to a fragmentation spectrum of each eluted compound and, therefore, to their identification when using the spectral libraries. There are several types of interfaces between the chromatograph and the mass spectrometer. The most developed ionization mode is electron impact (EI). From the total ion current (TIC), a chromatogram can be plotted which represents all of the compounds eluted. By choosing a particular ion (selective ion monitoring, or SIM), a selective chromatogram can be produced. This method has become essential, notably in the context of environmental studies. Nevertheless,