Phases, stationary

Any existing stationary phases that can be used in GC can also be used in GCxGC. Table 1 lists some stationary phases that could be, and in most instances have been, used in GCxGC. A variety of stationary phases can be selected according to the intended analyte-stationary phase interaction. Although the number of commercially available columns with very narrow bore dimensions 

The use of conventional columns in the first dimension and a carrier gas-flow rate similar to that used in ID GC enables all injection techniques to be used (e.g., split, splitless, large volume, PTV and SPME) [7]. That further makes any method based on a conventional GC separation amenable to a GCxGC approach without reconsideration or reoptimization of the existing sample introduction technique. 

As we saw earlier, the key point of GCxGC is the interface between the two columns, the modulator. This device ensures high sampling rates and the transfer of the sample from to while respecting Giddings s conservation rules. To guarantee conservation of the first-dimension separation achieved, the fraction eluted from the modulator should be no wider than about one-quarter of the peak width [13]. Recommended is the ensurance of at least three to four cuts per peak of the first dimension by the modulator. Thus, a Pm of 2-8 s is generally chosen. 

The several different modulators that are commercially available can be classified in two main categories thermal modulators and valve-based modulators. Thermal modulators are the most frequently used and in turn can be broken down into two categories those whose principle involves a temperature increase and, inversely, cryogenic modulators. 

Conventional stationary phases separate according to their polarity and to their mutual interactions to the analyte. In contrast, LCPs have the additional technical feature that they separate by the molecular shape of the analyte. The majority of investigations focuses on gas chromatography (GC). 

Low molecular liquid crystal materials may suffer from high volatility. For this reason, separations that need high temperatures are achieved more successfully by LCPs. Liquid crystal materials including LCPs can be characterized by inverse gas chromatography  

Commonly, polymers with a poly(siloxane) backbone and pendant side chain liquid crystalline groups are used. The side chain liquid crystalline groups are organic complexes with zinc, nickel, or crown ethers. This type of polymers is addressed as mesomorphic poly(siloxane). Side chains based on 4-biphenyl-4-allyloxybenzoate exhibit a special separation performance for racemic compounds.  

Certain liquid crystalline side chain polymers based on acrylate have been used as stationary phases for both liquid chromatography (LC) and 

Gc 6 7,68 yjjg preparation of LCPs suitable for the separation of polychlorinated dibenzodioxins, dibenzofurans, and other polychlorinated aromatics have been described. In addition, an extensive compilation of stationary phases for GC composed from LCPs with examples of applications has been presented in the literature.  

It is assumed that the reader is familiar with such common chromatographic concepts as efficiency, selectivity, capacity factors, and theoretical plates, and how these parameters affect and effect chromatographic resolution. Excellent descriptions of these general chromatographic principles have been published. Other reviews on various aspects of carbohydrate separations will be cited in the appropriate Sections. 

Bonded-phase silica and ion-exchange resins in plastic cartridges and mini-columns are very useful for off-line prepurification of samples, especially those for preparative chromatography, when appropriate pre- or guard columns may not be available for on-line clean-up of a sample. 

These have been effectively used to remove lignin and hydrophobic metabolites from plant-derived samples. Bonded-phase mini-columns are also ideal for the prechromatographic purification of a perbenzoylated sugars, glycopeptides (and derived oligosaccharides), and peralkylated oligosaccharides.  

The myriad types of l.c. fittings produced by a great number of vendors have led to confusion among chromatographers about the proper choice for each application. Fortunately, fittings that are more universal in their applications are now available, and one of the most useful of these has a knurled flange on the nut, and a replaceable, polymeric ferrule. These fittings may be sealed by hand and re-used many times without failure. 

The use of in-line filters between injectors and colunm can prevent the accumulation of particulate material on the inlet frit of an analytical column, and can avoid back-pressure problems. A second, and often overlooked, site for filter installation is between the column and the detector. Cartridge-type filtration-units that contain readily changed, 0.2-/i,m filters are commercially available, and they contribute insignificantly to peak broadening. These filters are essential for the prevention of clogged detectors when laboratory-packed columns are used. 

Nowadays, non-invasive tomographic methods, such as NMR spectroscopy, are used to reveal hydrodynamics [19] and temperature profiles [20] in CEC capillaries. The EOF in CEC is in the microliter or nanoliter per second range and can be measured by weighing the mass of eluent transferred, by determining the zeta potential or the current under different EOF conditions and evaluating the residence time of neutral markers (e.g. alcohols). Otherwise, miniaturized flow sensors comprising Prandtl tubes, piezoelectric elements, etc., can be used (21). 

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Gas phase chromatography is a separation method in which the molecules are split between a stationary phase, a heavy solvent, and a mobile gas phase called the carrier gas. The separation takes place in a column containing the heavy solvent which can have the following forms ... 

The column is swept continuously by a carrier gas such as helium, hydrogen, nitrogen or argon. The sample is injected into the head of the column where it is vaporized and picked up by the carrier gas. In packed columns, the injected volume is on the order of a microliter, whereas in a capillary column a flow divider (split) is installed at the head of the column and only a tiny fraction of the volume injected, about one per cent, is carried into the column. The different components migrate through the length of the column by a continuous succession of equilibria between the stationary and mobile phases. The components are held up by their attraction for the stationary phase and their vaporization temperatures. 

The large number of stationary phases can be classified in two groups ... 

Apolar stationary phases having no dipolar moments, that is their center of gravities of their positive and negative electric charges coincide. With this type of compound, the components elute as a function of their increasing boiiing points. The time difference between the moment of injection and the moment the component leaves the column is called the retention time. 

Poiar stationary phases which have a polar moment. These phases interact with the dipoiar moments of poiar components themselves and those components capable of induced polarization such as aromatics. 

The adsorbent, the stationary phase, fills a column of a few decimeters in length and 5 to 10 mm in diameter. The column is swept continually by a solvent or mixture of solvents (the liquid phase). 

The mixture to be studied is injected by syringe into the head of the column and the molecules comprising the mixture are adsorbed in varying degrees by the stationary phase and desorbed by the liquid phase. At the end of this succession of equilibria, the components of the mixture, more or less separated from each other, leave the column with the solvent. 

The resolution of capillary columns enables the separation of all principal components of a straight-run gasoline. The most frequently used stationary phases are silicone-based, giving an order of hydrocarbon elution times close to the order of increasing boiling point. 

The probes are assumed to be of contact type but are otherwise quite arbitrary. To model the probe the traction beneath it is prescribed and the resulting boundary value problem is first solved exactly by way of a double Fourier transform. To get managable expressions a far field approximation is then performed using the stationary phase method. As to not be too restrictive the probe is if necessary divided into elements which are each treated separately. Keeping the elements small enough the far field restriction becomes very week so that it is in fact enough if the separation between the probe and defect is one or two wavelengths. As each element can be controlled separately it is possible to have phased arrays and also point or line focussed probes. 

The last modification in the Rayleigh integral concerns the delay of propagation between the two points which is simply the time taken by the energy to propagate along the path of stationary phase. It is denoted T and given by,... 

Twenty-eight chiral compounds were separated from their enantiomers by HPLC on a teicoplanin chiral stationary phase. Figure 8-12 shows some of the structures contained in the data set. This is a very complex stationary phase and modeling of the possible interactions with the analytes is impracticable. In such a situation, learning from known examples seemed more appropriate, and the chirality code looked quite appealing for representing such data. 

Figure 8-12. Examples of las eluted enantiomers In a chromatographic separation on chiral HPLC with teicoplanin stationary-phase.

Chromatography (Section 13 22) A method for separation and analysis of mixtures based on the different rates at which different compounds are removed from a stationary phase by a moving phase... 

Table 11.13 McReynolds Constants for Stationary Phases in Gas Chromatography...

Similar stationary phases Temp., °C McReynolds constants USP code... 

In these expressions, dp is the particle diameter of the stationary phase that constitutes one plate height. D is the diffusion coefficient of the solute in the mobile phase. 

Ion-Exchange Equilibrium. Retention differences among cations with an anion exchanger, or among anions with a cation exchanger, are governed by the physical properties of the solvated ions. The stationary phase will show these preferences ... 

To accomplish any separation of two cations (or two anions) of the same net charge, the stationary phase must show a preference for one more than the other. No variation in the eluant concentration will improve the separation. However, if the exchange involves ions of different net charges, the separation factor does depend on the eluant concentration. The more dilute the counterion concentration in the eluant, the more selective the exchange becomes for polyvalent ions. 

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