Chromatography carrier gas properties

Thermal conductivity detector. The most important of the bulk physical property detectors is the thermal conductivity detector (TCD) which is a universal, non-destructive, concentration-sensitive detector. The TCD was one of the earliest routine detectors and thermal conductivity cells or katharometers are still widely used in gas chromatography. These detectors employ a heated metal filament or a thermistor (a semiconductor of fused metal oxides) to sense changes in the thermal conductivity of the carrier gas stream. Helium and hydrogen are the best carrier gases to use in conjunction with this type of detector since their thermal conductivities are much higher than any other gases on safety grounds helium is preferred because of its inertness. 

Figure 9-1 Schematic diagram of a gas-liquid chromatography apparatus. The detector is arranged to measure the difference in some property of the carrier gas alone versus the carrier gas plus effluent sample at the exit. Differences in thermal conductivity are particularly easy to measure and give reasonably high detection sensitivities.

TCD) detector or the flame-ionization (FID) detector, which are the two most common detectors in gas chromatography, respond to all (organic) compounds except the carrier gas. On the contrary, a selective detector responds to a range of compounds with a common physical or chemical property. Representatives of the latter group of detectors are the nitrogen-phosphorus detector (NPD), the electron capture detector (ECD), the mass selective detector (MSD) and - last, but not least - the tandem mass spectrometer (MS/MS). 

There are several reports in the literature that measure binary adsorption equilibria using gas chromatography [4,S,6]. In GC techniques the adsorbent is equilibrated with a continuous flow of carrier gas (gas 1). Then a pulse of gas 2 is injected at the column inlet. A peak of the gas 2 is eluted at the exit of the column after some time. Net retention time (or volume) is calculated from the first moment of the peak after correcting for void volume (by measuring the retention time of a non-adsorbing species). If the carrier gas is inert (i.e. helium) the net retention time is related to the pure component Henry s constant. Typical binary measurements reported so r use a mixture of the two gases as carrier and introduce a small perturbation in composition. The net retention volume is related to the thermodynamic properties by [4]... 

In the experiments on accelerators the required short-lived products of nuclear reactions are converted into volatile compounds and separated by gas-solid chromatography techniques in a continuous regime. Open columns of a meter in length and a few millimeters in diameter are used. The linear velocity of the carrier gas has varied from centimeters to meters per second. To optimize the separation process, it is important to understand how the experimental conditions, the properties of the separated species and other factors affect the shape and position of the resulting adsorption zone. 

Using these methods is similar to reconstructing a puzzle. How the retention and vaporization mechanisms can be quantitatively analyzed, and the predicted retention times improved, based on molecular properties calculated in silica, are fundamental questions in chromatography. In gas chromatography, no solvent is used except in special cases where water vapor and ionic gas are mixed with the carrier gas. The basic retention mechanisms depend on the strength of the molecular interaction with the stationary phase, and the vaporization mechanism depends on the properties of the analytes. 

Gas chromatography (GC) is a chemical analysis technique for separating chemicals in a complex sample. In a gas chromatography setup, the sample is passed through a narrow tube known as the column, through which different chemical constituents of a sample pass in a gas stream (carrier gas, mobile phase) at different rates depending on their various chemical and physical properties and on their interaction with a specific column filling, referred to as the stationary phase. Interaction of the analytes with the stationary phase causes each one to exit the column at a different time (retention time). Separated chemicals are detected and identified at the end of the column. Miniaturization of GC systems can lead to small size and extremely low power consumption. 

The appropriate sample size depends on whether the investigations are carried out in the range of infinite dilution or of finite-concentration . At infinite dilution, the concentrations of the components in the carrier gas may be neglected and thus the gaseous phase may be considered ideal. At finite concentration, a correction for retention calculations should be introduced, which makes them more complicated. Certain limitations of GC methods for physicochemical determinations should also be mentioned. These are mainly restricted to the study of interactions that occur on solids, in liquids, in the mobile gas phase, and at their interfaces. While measurements can be made simultaneously, it is possible that interference from other physicochemical properties of the materials may introduce inaccuracy. Another important limitation is the volatility of the stationary phase in gas-liquid chromatography. 

As a rule, a modifying agent is both a component of the carrier gas and a constituent of the stationary phase (surface layer of adsorbent). The situation is somewhat reminiscent of the retention in liquid chromatography, where the surface properties depend on the concentration of the modifier in the mobile phase. Analyte retention volume depends on the nature and partial pressure of the carrier gas additive as well as on the nature of adsorbent surface. Therefore the use of a phase system composed of a (modifier-carrier gas) mixture as mobile phase offers the possibility of fine tuning of selectivity and retention by adjusting the modifier content of both phases [10]. 

The idea of gradient elution (and pressure programming) for gas chromatography have been proposed and discussed for many years (see, for example, [10, 41, 73-80]). This method would involve the use of condensable vapor of modifiers in the carrier gas. Modifiers would adsorb in or on the stationary phase, altering the properties of the phase and, consequently, the selectivity of the system and the resolution of the analyzed mixture (see, for example, [10, 41, 72-75, 81-84]). 

Another important instrument required in modem LC is a sensitive or selective detector for continuous monitoring of the column effluent. In GC. the differences in physical properties of the mobile phase (carrier gas) and the sample are great enough for universal detectors with good sensitivity to be used (e.g., flame ionization detector, thermal conductivity detector, - Gas Chromatography). The problem in LC is that the physical properties of the mobile phase and the sample are often very similar, which makes the use of a universal detector impossible. Nevertheless, presently available LC detectors are very sensitive, are generally selective, and have a relatively wide range of applications (see Table 1). 

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