Retention indices structures

The distribution of tritium in compounds 137-150 can be determined by tritium NMR spectroscopy without chemical manipulations115. The structure retention index relationship (SR IR)116 has been used for identification of unknown radioactive peaks and to differentiate by-products from radioimpurities from extraneous sources. 

A set of n = 209 polycyclic aromatic compounds (PAC) was used in this example. The chemical structures have been drawn manually by a structure editor software approximate 3D-structures including all H-atoms have been made by software Corina (Corina 2004), and software Dragon, version 5.3 (Dragon 2004), has been applied to compute 1630 molecular descriptors. These descriptors cover a great diversity of chemical structures and therefore many descriptors are irrelevant for a selected class of compounds as the PACs in this example. By a simple variable selection, descriptors which are constant or almost constant (all but a maximum of five values constant), and descriptors with a correlation coefficient >0.95 to another descriptor have been eliminated. The resulting m = 467 descriptors have been used as x-variables. The y-variable to be modeled is the Lee retention index (Lee et al. 1979) which is based on the reference values 200, 300, 400, and 500 for the compounds naphthalene, phenanthrene, chrysene, and picene, respectively. 

Liu, S., Yin, C., Cai, S., Li, Z. Chemom. Intell. Lab. Syst. 61, 2002, 3-15. Molecular structural vector description and retention index of polycyclic aromatic hydrocarbons. 

The concentration data obtained from each sample analysis were expressed as fractional parts and normalized to sum to 100. The normalized data were statistically analyzed, and three principal components (A=3, Equation [1]) were calculated. The PCB constituents (varlbles) are numbered sequentially and correspond to peak 1, peak 2,. .. to peak 105. The structure and retention index of each constituent in the mixture were reported by Schwartz et al. (9). The tabular listing of the data is availedile from the present authors. 

Figure 11.5 shows the structures of some of the major components in peppermint oil. The use of the retention index system is illustrated in Figures 11.6 and 11.7 for peppermint oil run in comparison with n-alkane standards on both a weakly polar OV-5-type column and a polar carbowax column. 

Wehrli and Kovats (1) introduced the concept of the retention index to help confirm the structure of organic molecules. This method utilizes a series of normal alkanes as a reference base instead of one compound as in the relative retention method. Identification can be assisted with the use of the retention... 

Estimation of pv for PCBs Burkhard et al. [8] compared the predictive capability of 11 different methods to estimate pvap for PCBs at 25°C. The comparison includes solely structure-based methods and methods that require the input of Tm, Th, and the entropy of fusion, ASfUS, or a gas-liquid chromatographic retention index. 

The retention index (RI) was determined on a 30 m x 0.32 mm fused silica column (Supelcowax 10 Supelchem Germany). Structure not established by MS. 

Zarei, K. and Atabati, M. (2005). Predictions of GC retention indexes for insect-produced methyl-substituted alkanes using an artificial neural network and simple structural descriptors. J. Anal. Chem., 60, 732-737. 

The international tests have shown that a GC/ FTIR instrument is not a necessity for successful testing because two other techniques (GC/EI/MS, GC/CI/MS, NMR, or even GC retention index monitoring) can be used as the required different techniques. The major advantages of GC/FTIR are that it gives information on different characteristics of a chemical than, for example, MS and NMR. Therefore, it can make the overall spectral elucidation easier and faster. Also, the first guesses , that is, candidate structures for synthesis are more accurate when FUR is used (see ref. (25) for examples). The latter advantage is emphasized when analyzing phosphorus chemicals. 

A dimensionless molecular retention index Me parameter can be defined as the sum of Mr (relative molecular weight) and a structural increment W. Contained in W are all the additive contributions of the functional groups (see Eq. 4-52) which differ from a hypothetical n-alkane with the same Mr value. According to definition, the W values of the n-alkanes are always equal to zero. In this manner it is possible to estimate the partition coefficients of any given organic compound between a gas and any given liquid or polymer with help of additive structural increments. 

Donnelly JR, Munslow WD, Mitchum RK, Sovocool GW (1987), J. Chromat. 392 51-63. Cor-relation of structure with retention index for chlorinated dibenzo-p-dioxins ... 

Donnelly JR, Sovocool GW (1992), Chemosphere 25 1299-1304. Gas chromatographic elution order and elution shift modelling for isomer specific analysis of halogenated dioxins" Donnelly JR, Munslow WD, Grange AH Pettit TL, Simmons RD, Sovocool GW (1991a), J. Chromat. 540 293-310. Correlation of structure with linear retention index for bromo- and bromochlorodibenzo-p-dioxins and bromodibenzofurans"... 

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]. 

It is often the case that a peak in a gas chromatogram can be identified from its relative retention index (or time). This provisional identification can be confirmed by spiking the unknown sample with an authentic sample which should co-chromatograph with the unknown. However, unambiguous identification of an unknown compound demands structural analysis, which can either be done on-line (e.g., using coupled GC-MS equipment) or off-line. 

In addition to GC properties, Kurz [4] has investigated HPLC retention of 106 PCDE congeners. Retention indexes (RI) were measured on three reversed phase columns having phenyl, cyanopropyl, and octadecyl phases. Like HRGC retention, HPLC retention of PCDEs depends on the structure. LC data of PCDEs have been used to determine water solubility and octanol-water partition coefficients [66]. 

Kier, L.B. and Hall, L.H. (1979). Molecular Connectivity Analyses of Structure Influencing Chromatographic Retention Indexes. J.Pharm.ScL, 68,120-122. 

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. 

Acevedo-Martmez, J., Escalona-Arranz, J.C., Villar-Rojas, A., Tellez-Palmero, F., Perez-Roses, R., Gonzalez, L. and Carrasco-Velar, R. (2006) Quantitative study of the structure-retention index relationship in the imine family. J. Chromat., 1102, 238-244. 

Ren, B. (2003a) Atom-type-based AI topological descriptors for quantitative structure-retention index correlations of aldehydes and ketones. Chemom. Intell. Lab. Syst., 66, 29-39. 

None of the detectors previously described yield any information as to the nature of the compound eluted. At most they are selective. Compounds identification proceeds with the use of an internal calibration based on retention times or requires the knowledge of retention indexes (cf. paragraph 2.10). When the chromatogram is very complex, a confusion of identity could occur. To counteract this, several complementary detectors could be associated (Figure 2.17), or a detector able to convey structural information based on spectroscopic data, or elemental composition of the analytes. The retention time and specific characteristics for each compound could then be known. These detectors lead to stand-alone analysis techniques for which the results depend only on the ability of the column to separate properly the constituents of the sample mixture. 

Elizalde-Gonzalez MP, HutflieB M, Hedden K (1996) Retention index system, adsorption characteristics and structure correlations of polycyclic aromatic hydrocarbons in fuels. JHigh Res Chrom 19, 345-352. 

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 is useful also for identifying a compound from its retention time relative to those of similar compounds in a homologous series (those that differ in the number of carbon atoms in a similar structure, as in alkane chains). The index I is defined as... 

We would like to critically examine the interpretation of the regression equation (2) that it supports a notion that the Chromatographic retention index for benzenoids is shape dependent. As one can see from equation (1), most of the correlation between the structure and the Chromatographic retention index as the property is described by the connectivity index that is bond additive and size dependent. The conclusion therefore ought to be that the Chromatographic retention values are mostly bond additive and size dependent. Because of this pronounced bond-additivity, clearly we can understand why indices that are global are inferior in this particular application. 

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