Index retention data

A retention index (RI) was used to allow comparison of compound retention data between columns of different equipment, e.g. GC and GC-MS. Straight chain hydrocarbons (alkanes) were assigned an index of 100 times the number of carbon atoms in the molecule. The RI s of all compounds within all the extracts were calculated by using the difference between the retention indices of the alkane eluting before and after that compound ... 

GLC-retention index (ly) data (137,138). Thermal decomposition data (139,151). 

The suitability of a stationary phase for a specific separation depends upon the selectivity of the phase. This is a measure of the degree to which polar compounds are retarded relative to their elution on a nonpolar phase. A systematic method for expressing the retention data is based on retention indices. For this sytem, the retention indices of the n-paraffins are by definition equal to 100 times the number of carbon atoms in the molecule. For example, the retention index for n-hexane is 600 and for n-octane 800. These values are defined and apply regardless of the column used and regardless of the temperature. 

Many factors can influence the Kovats index which can make it unreliable at times for the characterization of gas chromatographic behavior, although it generally varies less than the relative retention with temperature, flow, and column variation. However, for many it is the preferred method of reporting retention data. 

Many scales, either empirical or measured, have been proposed for the hydrophobicity of amino acid residues in proteins (Nakai and Li-Chan, 1988). The most extensive study on tlje hydrophobicity index of amino acids was published by Wilce et al. (1995). The authors derived four new scales of coefficients from the reversed-phase high-performance liquid chromatographic retention data of 1738 peptides and compared them with 12 previously published scales. 

The usefulness of retention data from gas chromatography can be enhanced by reporting standardized times or retention indices (RI), which involves expressing retention in terms of a ratio of the retention time (RT) of an analyte to the RT of a standard. Retention scaling based on the Kovats (1965) method requires the chromatographic separation of a homologous series of normal paraffins, esters, and others, producing an index that is the ratio of the RT of an analyte minus the RT of a less retentive standard to the RT difference between... 

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... 

A most important contribution to the above means of identification is the Kovats retention index system [28]. The Kovats retention index of a compound is 100 times the number of carbon atoms in a hypothetical n-alkane that would display in the given system the same retention as the compound in question. Hence, the retention index system essentially is also based on the regularities between the retention data and number of carbon atoms in homologous compounds. The concept of the Kovats retention index system is illustrated by the model in Fig. 3.7, which shows a plot of log A) values for homologous compounds of the type CH3(CH2) X and for n-alkanes against carbon number. It is apparent that the retention index of, e.g., C2H5X is 560, i.e., 7(C2HSX) = 200 +... 

From the Th-FFF retention data, it is possible to obtain a molar mass distribution after a suitable calibration for the determination of the Mark-Houwink constants (straight-line plot of log(D/DT) vs. log M [15]). Another possibility is to couple an absolute molar mass detector like MALLS (see Sect. 4.3.2) or a suitable detector combination such as an on-line viscometer coupled with a refractive index detector. This possibility does not require prior knowledge of DT... 

TTie structural features are represented by molecular descriptors, which are numeric quantities related directly to the molecular structure rather than physicochemical properties. Examples of such descriptors include molecular weight, molecular connectivity indexes, molecular complexity (degree of substitution), atom counts and valencies, charge, molecular polarizability, moments of inertia, and surface area and volume. Once a set of descriptors has been developed and tested to remove interdependent/collinear variables, a linear regression equation is developed to correlate these variables with the retention parameter of interest, e.g., retention index, retention volume, or partition coefficient The final equation includes only those descriptors that ate statistically significant and provide the best fit to the data. For more details on QSRR and the development and use of molecular descriptors, the reader is referred to the literature [188,195,198,200-202 and references therein]. 

Identify any peaks which appear on the chromatograms by reference to the standard chromatogram and 10 the retention data listed in Table 9 the index of Gas Chromatographic Data in Part 3 may also be consulted. Corroborative evidence must be obtained from both systems. For quantification, prepare standard solutions of the identified drug containing 0., 10, 20, 50. and 100 Lig/ml in plasma. Examine at least three standards by the same method as used for the sample. Duplicate analyses of samples and standards are essential. 

Identify any peaks which appear on the chromatograms by reference to the standard chromatogram and to the retention data listed in Table 11 the indexes of Gas Chromatographic Data in Part 3... 

Applications The most extensive sources of information on applications of gas chromatography are Gas Chromatography Literature— Abstracts and Index and Gas Chromatography Abstracts. Compilations of retention data are available from several sources two are Gas Chromatographic Retention Data, by Mc-Reynolds and The Handbook of Chromatography. ... 

The introduction of GC as an analytical technique has had a profound impact on both qualitative and quantitative analysis of organic compounds. Identification of compounds by GC can be accomplished by their retention times on the column as compared to known reference standards, by inference from sample treatment prior to chromatography, " or by the concept of retention index. " The latter method and tables of retention indices " with associated conditions have been reported. " Although qualitative data and analytical techniques for identification of compounds are well-established " and relative retention data for over 600 substances also have been published, " the main utility of GC undoubtedly lies in its powerful combination of separation and quantitative capabilities. Use in quantitative analysis involves the implementation of two techniques being performed concurrently, i.e., separation of components and subsequent quantitative measurement. 

Instrumental methods in chemistry have dramatically increased the availability of measurable properties. Any molecule can be characterized by many different kinds of data. Examples are provided by Physical measures, e.g. melting point, boiling point, dipole moment, refractive index structural data, e.g. bond lengths, bond angles, van der Waals radii thermodynamic data, e.g. heat of formation, heat of vaporization, ioniziation energy, standard entropy chemical properties, e.g. pK, lipophilicity (log P), proton affinity, relative rate measurements chromatographic data, e.g. retention in HPLC, GLC, TLC spectroscopic data, e.g. UV, IR, NMR, ESCA. 

Structural identifications have been made431,432 of polycyclic methylpolysiloxanes of the general formula [(CH iOj 5)]x - [(CH3)2SiO]j, x and y being between 2 and 5 based on retention index additivity. Retention data for a series of siloxanes were determined on squalane, Apiezon L, QF-1 or Polysorbate 60. The value of the retention index is related to the structural characteristics of the molecule, which are regarded as additive and to a stationary phase factor. 

The Kovats retention index [137], using n-alkanes as a series, is most commonly used for both internal and external data comparisons. With certain precautions in mind, this retention system is usable for temperature-programmed runs. Within the field of biological investigations, fatty acid esters [133] and steroidal hydrocarbons [134] were also used to standardize the retention data within a compound class. However, the use of the so-called methylene units [135] is basically identical with the Kov its system. 

Under the same column conditions, authentic samples of methyl esters and alcohols gave the following retention data in Kovats retention index units ... 

Not only variations in the pressure at constant temperature influence column-to-column retention data the role of the column hold-up volume as well as the mass of stationary phase present in the column is also important. The net retention volume caleulated from the adjusted retention volume corrects for the column hold-up volume (see Table 1.2). The specific retention volume corrects for the different amount of stationary phase present in individual colunms by referencing the net retention volume to unit mass of stationary phase. Further correction to a standard temperature of 0°C is discouraged [16-19]. Such calculations to a standard temperature significantly distort the actual relationship between the retention volumes measured at different temperatures. Specific retention volumes exhibit less variability between laboratories than other absolute measures of retention. They are not sufficiently accurate for solute identification purposes, however, owing to the accumulation of multiple experimental errors in their determination. Relative retention measurements, such as the retention index scale (section 2.4.4) are generally used for this purpose. The specific retention volume is commonly used in the determination of physicochemical properties by gas chromatography (see section 1.4.2). 

There has been a great deal of interest in the relationship between solute structure and retention for several reasons [248,249,251,271]. If predictive relationships could be developed then a method would exist to test the reliability of retention data, to predict the retention index of an analyte that was unavailable for study, or to predict the value of certain physical properties of an analyte that could be correlated through its retention index. 

Normally, a graphical procedure is not required to determine retention indexes. Instead, adjusted retention data are calculated by interpolation from a chromatogram of a mixture of the solute of interest and two or more alkane standards. 

For the comparison of analytical data, e.g. molecular spectra and/or chromatographic retention data, a similarity index in the form of a P-value, as used in hypothesis testing, has been developed. This similarity index requires that unknown and reference compounds be characterised by a set of continuous feature quantities qi... qj... q. In the case of mass spectra these may be the peak intensities at a certain number of selected masses, whereas for C-NMR spectra the chemical shifts could be used. 

Retention index data can be employed to recognize molecular structures of unknown monofunctional compounds. This was principally demonstrated by Huber and Reich C113, 2373. Retention data of 199 compounds on ten different stationary phases were utilized for cluster analysis, KNN--classifications and the computation of classifiers with the learning machine. The minimum number of stationary phases was only two for the classification of aromatic compounds, four for alcohols and 13 for aldehydes and ketones. A two-step classification procedure was developed. 

Isoindoles of o-phthalaldehyde (OPA)/N acetyl-L-cysteine (NAC) are commonly used for detection of amino acids (RiCH(COOH)NH2) RPLC. A particular hydrophobicity scale was established with the retention data of these derivatives in mobile phases of SDS at pH 3, using the glycine derivative as a reference [20], Linear relationships were obtained between the ratios of log k of each amino acid derivative to log k of the glycine derivative (which was called quantitation of hydrophobicity index, QH), and log Pq for the Ri substituents (tTri). 

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