Retention time defined

In Equation la, W is the weight of the stationary phase and tN is the net retention time defined as... 

Mean retention time defined by VjQ Zero size population density Nucleation rate Concentration of a solution Rate constant from Eq. (4.28)... 

The rate of elution of a component of the mixture is determined by the relative affinities of the component for the stationary and mobile phases. In gas chromatography, the elution rate is most often expressed as a retention time, defined as the time between the placement of the sample on the column and the maximum elution of the component in question. The basic principle responsible for the great utility of all chromatographic methods is the same Even compounds with very similar structural features frequently are eluted from the column at sufficiently different rates that an analytical or preparative separation is possible. If the efficiency of a gas chromatographic column is expressed in theoretical plates for comparison with distillation (Sec. 2.4.1), it is found that the gas chromatographic column provides 50 times as many theoretical plates as an efficient distillation column (e.g., a spinning-band column) of the same length. 

GLC determines directly the adjusted retention time, defined by eqn (2.30). It depends, obviously, on K of the solute-solvent system in question. Eqns (2.33), (2.35) and (2.37) transform t n into the specific retention volume, F, [16] which is also a fimction of JT. Taking account of eqns (2.37) and (2.42) the following equation relating K and F, results ... 

The identities of the solutes are defined such that solute A always has the smaller retention time. Accordingly, the selectivity factor is equal to 1 when the solutes elute with identical retention times, and is greater than 1 when is greater than fr A-... 

Now that we have defined capacity factor, selectivity, and column efficiency we consider their relationship to chromatographic resolution. Since we are only interested in the resolution between solutes eluting with similar retention times, it is safe to assume that the peak widths for the two solutes are approximately the same. Equation 12.1, therefore, is written as... 

An eluted solute was originally identified from its corrected retention volume which was calculated from its corrected retention time. It follows that the accuracy of the measurement depended on the measurement and constancy of the mobile phase flow rate. To eliminate the errors involved in flow rate measurement, particularly for mobile phases that were compressible, the capacity ratio of a solute (k ) was introduced. The capacity ratio of a solute is defined as the ratio of its distribution coefficient to the phase ratio (a) of the column, where... 

When the volume of the vessel is divided by the flow rate, retention time and dilution rate are defined in the following equation ... 

The basis of chromatography is in the differential migration of chemicals injected into a column. The carrier fluid takes the solutes through the bed used for elution (mobile phase). The bed is the stationary phase. Based on mobility, the retention-time detectors identify the fast and slow-moving molecules. Based on internal or external standards with defined concentration, all unknown molecules are calculated in a developed method by software. GC columns are installed in an oven which operates at a specified temperature. A diagram of an oven with GC column is shown in Figure 7.16. 

From a quantitative perspective, each peak is defined by two parameters, i.e. the position of its baseline and the retention time boundaries, with those derived by the computer system being shown in Figure 3.27. It is not the intention of this present author to discuss how these have been determined but simply to point out that their positions may have a significant effect on the accuracy and precision of any quantitative measurements, especially, as in Figure 3.27, when the baseline is not horizontal and the signals from each of the components are not fully resolved. It is usual for the software to allow the analyst to override the parameters chosen by the computer to provide what they consider to be more appropriate peak limits and/or baseline positions. 

Each of the three approaches will be applied in this section to the transformed retention times of the 23 chalcones with eight chromatographic elution methods in Table 31.2. The transformation is defined by the successive operations of logarithms, double-centering and global normalization which is typical for the method of spectral map analysis (SMA) ... 

Multiway and particularly three-way analysis of data has become an important subject in chemometrics. This is the result of the development of hyphenated detection methods (such as in combined chromatography-spectrometry) and yields three-way data structures the ways of which are defined by samples, retention times and wavelengths. In multivariate process analysis, three-way data are obtained from various batches, quality measures and times of observation [55]. In image analysis, the three modes are formed by the horizontal and vertical coordinates of the pixels within a frame and the successive frames that have been recorded. In this rapidly developing field one already finds an extensive body of literature and only a brief outline can be given here. For a more comprehensive reading and a discussion of practical applications we refer to the reviews by Geladi [56], Smilde [57] and Henrion [58]. 

Any data matrix can be considered in two spaces the column or variable space (here, wavelength space) in which a row (here, spectrum) is a vector in the multidimensional space defined by the column variables (here, wavelengths), and the row space (here, retention time space) in which a column (here, chromatogram) is a vector in the multidimensional space defined by the row variables (here, elution times). This duality of the multivariate spaces has been discussed in more detail in Chapter 29. Depending on the chosen space, the PCs of the data matrix... 

As explained before, the scores of the spectra can be plotted in the space defined by the two principal components of the data matrix. The appearance of the scores plot depends on the way the rows (spectra) and the columns have been normalized. If the spectra are not normalized, all spectra are situated in a plane (see Fig. 34.5). From the origin two straight lines depart, which are connected by a curved line. We have already explained that the straight line segments correspond with the pure spectra which are located in the wings of the elution bands (selective retention time... 

A well-developed records management program that defines rules by which records and documents are handled from creation to retirement is necessary for a successful electronic data archiving process. In a GLP-compliant system, records are classified by type, such as study-specific raw data, reports, personnel records, etc. Each record type has a defined retention time. 

The partition coefficient of a substance between several Immiscible solvent pairs can be combined with retention time data to confirm the identity of a substance when a pure standard is available [706]. Devised by Bowman and Beroza, the substance specific partition coefficient ("p-value") was defined as the fractional amount of substance partitioning into the less polar phase of an equal-volume, two-phase system. Only nanogram quantities of sample are required for the measurement and p-values are often sufficiently characteristic to distinguish between closely related substances. 

Therefore, a 4a separation (R = 1), in which peak retention times differ by four times the width at half-height, corresponds to a 2% area overlap between peaks.1 The maximum number of peaks that could be separated in a given time period assuming a given value of R, is defined as the peak capacity.1 The peak capacity must be greater — usually much greater — than the number of components in the mixture for a separation to succeed. The resolution of two compounds can also be written in terms of the number of plates of a column, N, the selectivity, a, and the capacity factors, k, and k j, as12... 

A peak on a chromatogram is usually identified by some measure of retention, for example retention time or chart length where the length or time is measured from the injection point to the peak apex. A useful quantity to locate or identify a peak is the capacity factor (k ) which is defined as follows ... 

---