Stationary characterization parameter

The Rohrschneider scheme has been modified by McReynolds [213], The modifications include the use of some more convenient test probes (for instance, ethanol often yields very low retention indices, nitromethane often yields poorly shaped peaks), the use of a somewhat higher temperature and a factor of 100 to avoid decimal points in the stationary phase parameters. The McReynolds probes are shown in table 2.7. Considering these modifications, I feel that no justice is done when McReynolds constants are tabulated. It appears to be more appropriate to refer to these constants as Modified Rohrschneider constants , or Rohrschneider constants, modified according to McReynolds . An additional set of five probes may be used according to McReynolds, but the five extra parameters are not very helpful for characterization purposes. 

A table consist of rows, one row for each object (stationary phase), and columns, one column for each variable (stationary phase characterization parameter). 

The stationary-state velocity per unit acceleration is a parameter which characterizes the settling particle and is called the sedimentation coefficient s ... 

For a while till now, our research group has been involved in studies of the properties of limit flames. Most of the results reported in this chapter were obtained for propane flames, under normal atmospheric conditions, in 300 mm long channels, with a square cross-section. The experimental procedure was described previously [25]. A flame propagating through a stationary mixture in a quenching tube or quenching channel can be characterized by the parameters defined in Figure 6.1.1. 

Another fairly important stationary phase in straight phase PLC is aluminum oxide. Comparable with silica gel also in the case of aluminum oxides, hydroxyl groups at the surface of this adsorbent are responsible for the selective retention of sample molecules. The relevant physical parameters for the characterization of aluminum oxides suitable for straight phase PLC are the following ... 

One other parameter characterizing the ion exchange is the concentration- [8] or weight-based [9] distribution coefficient K = CJC, where and represent the concentrations of analytes in the stationary and mobile phases, respectively. 

The behaviour of the correlation functions shown in Fig. 8.5 corresponds to the regime of unstable focus whose phase portrait was earlier plotted in Fig. 8.1. For a given choice of the parameter k = 0.9 the correlation dynamics has a stationary solution. Since a complete set of equations for this model has no stationary solution, the concentration oscillations with increasing amplitude arise in its turn, they create the passive standing waves in the correlation dynamics. These latter are characterized by the monotonous behaviour of the correlations functions of similar and dissimilar particles. Since both the amplitude and oscillation period of concentrations increase in time, the standing waves do not reveal a periodical motion. There are two kinds of particle distributions distinctive for these standing waves. Figure 8.5 at t = 295 demonstrates the structure at the maximal concentration... 

By using external reservoirs, some of these parameters can be kept at values different from those of thermodynamic equilibrium, / /, j = 1, , m < n. As a result, a non-equilibrium state arises, which is characterized by nonvanishing values of some fluxes /,-, i = 1, s < r and of the corresponding forces Xj. Examples of such processes are diffusion and related effects, Peltier effect, etc.45,46. Such a state can either be stationary or time-dependent, stable or unstable. 

Section 2.3 describes various methods for the characterization and classification of mobile and stationary phases for chromatography. In section 2.3.1 the solubility parameter is introduced as a quantitative definition of the word polarity . Section 2.3.2 describes the characterization of GC stationary phases according to Rohrschneider and section 2.3.3 the classification of LC solvents according to Snyder. The applicability of the different methods is summarized in section 2.3.4. 

Rohrschneider [205,210] has developed a scheme for the characterization of stationary phases for gas chromatography. The scheme is based on the retention index (/). The retention index is a dimensionless retention parameter, designed to be independent of flow rate, column dimensions and phase ratio. The retention index of a solute is defined as 100 times the number of carbon atoms in a hypothetical n-alkane, which shows the same net retention time as that solute. This definition is illustrated in figure 2.2. By plotting the logarithm of the net retention time against the number of carbon atoms in n-alkanes, a straight line is obtained. The net retention time for a solute may then be located on the vertical axis, and the retention index found on a horizontal scale, which represents 100 times the scale for na... 

The Rohrschneider scheme has no significance, until the parameters for five different solutes (or five different stationary phases) are known. With fewer parameters, the system is undefined, while more than 25 known parameters create a problem of consistency. Realizing that the characterization scheme is completely empirical, Rohrschneider made a very convenient choice for the characterization of stationary phases. The probe solutes and their parameters are listed in table 2.5. 

The solubility parameter may be used to characterize the overall polarity of compounds (solutes, mobile and stationary phases) in chromatography. It may also be used to predict the polarity (solvent strength) of mixtures (see section 3.2). 

The retention index (/) was introduced in section 2.3.2 as a reproducible means for reporting GC retention data. The retention index was therefore found to be highly useful as the basis for a characterization scheme for stationary phases in GLC. In this chapter, however, the capacity factor and not the retention index has been used to find expressions, which describe the influence of the various relevant parameters on the retention. Besides... 

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