Recent developments in the gas chromatographic retention index scheme

Identification of Additives in Polypropylene and Their Degradation under Solar Exposure Studied by Gas Chromatography-Mass Spectrometry

Additives are absolutely essential in the development of commercial polymeric materials. Accordingly, an exhaustive control of composition and evolution in these additives over time is necessary to validate their performance and safety during their shelf life and, consequently, their ultimate applications. Gas chromatography coupled with mass spectrometry, GC-MS, is described in the present work to identify and analyze the content of a wide variety of additives, commonly used in industrial polymeric materials. First, the identification under the present experimental protocol of additives with a relatively high molecular weight (Irganox 1330 and Irganox 1010) has been successfully attained. Second, the evolution under solar exposure over time has been analyzed by GC-MS for 11 additives and derived substances, which have been identified in a commercial polypropylene sample, estimating the corresponding depletion times. In addition, the resultant increase of carbonyl groups in the polymeric macrochains along the photo-oxidation has been also determined by infrared spectroscopy. Therefore, GC-MS is found to be a reliable tool for the analysis of the evolution of commonly used polymer additives under specific degradation conditions, which can be very useful in the formulation of improved future additivations.

Estimation of Kováts retention indices using group contributions

We have constructed a group contribution method for estimating Kováts retention indices by using observed data from a set of diverse organic compounds. Our database contains observed retention indices for over 35,000 different molecules. These were measured on capillary or packed columns with polar and nonpolar (or slightly polar) stationary phases under isothermal or nonisothermal conditions. We neglected any dependence of index values on these factors by averaging observations. Using 84 groups, we determined two sets of increment values, one for nonpolar and the other for polar column data. For nonpolar column data, the median absolute prediction error was 46 (3.2%). For data on polar columns, the median absolute error was 65 (3.9%). While accuracy is insufficient for identification based solely on retention, it is suitable for the rejection of certain classes of false identifications made by gas chromatography/mass spectrometry.

Alkylphenol retention indices

A novel type of retention indices for alkylphenols and related compounds are proposed. The alkylphenol retention indices (APRI) use para-substituted n-alkylphenols as reference series. APRI for para-n-alkylphenols are per definition equal to the number of carbon atoms in the alkyl substituent; the value for phenol is zero. Application of the APRI system with different types of derivatisation of the phenolic hydroxy group showed that the derivatisation has limited influence on these indices. Especially para-substituted alkylphenols gave APRI values that could be transferred with high accuracy from one type of derivative to another. By comparing results obtained with different gradients in temperature-programmed GC, it was also shown that APRI is less affected by chromatographic conditions than retention indices based on n-alkanes.

Retention Indexes for Temperature-Programmed Gas Chromatography of Polychlorinated Biphenyls

A noninteger retention index was defined based on a series of PCB internal standards, namely congeners 8 (2,4'-dichlorobiphenyl), 31 (2,4',5-trichlorobiphenyl), 44 (2,2',3,5'-tetrachlorobiphenyl), 101 (2,2',4,5,5'-pentachlorobiphenyl), 138 (2,2',3,4,4',5'-hexachlorobiphenyl), 180 (2,2',3,4,4',5,5'-heptachlorobiphenyl), and 194 (2,2',3,3',4,4',5,5'-octachlorobiphenyl). These retention index markers are common congeners present in technical mixtures and most environmental samples, and they show a linear dependence of retention time on the number of chlorine atoms, in the temperature-programmed analysis. The index values are calculated with a single regression equation instead of the Van den Dool and Kratz equation. The retention indexes of all 209 PCBs on two commonly used columns (DB-XLB and DB-5), as well as on a supplementary column of DB-17 in capillary gas chromatography, were determined using this system. The reliability of the retention index is quite good, with the average 95% confidence limits for three measurements on each PCB being +/-0.1 index unit under the same chromatographic conditions and +/-0.4 index unit under different column head pressures. The effect of heating rate of the programmed runs on the retention index was also investigated. The inversion of the elution order of some congener pairs on the DB-XLB column for different temperature heating rates was observed. Our index values were compared with those of Castello and Testini.

Standardless screening of chemical warfare agents based on gas chromatographic data

A method for calculation of the retention time of a compound in a temperature-programmed mode of GC analyses is proposed. The method is based on a detailed consideration of thermodynamics of chromatographic process and account of the actual state of a capillary column. The suggested approach permits estimation of probability of a poisonous agent in tests by means of information stored in the database, eliminating a need for reference standard of such an agent.

Isothermal Kováts retention indices of sulfur compounds on a poly(5% diphenyl-95% dimethylsiloxane) stationary phase

Isothermal Kováts retention indices of 21 sulfur compounds relevant to the fuel gas and food industries are reported on a poly(5% diphenyl-95% dimethylsiloxane) capillary column stationary phase. Measurements were performed at four temperatures and the temperature dependence of the values modeled with Antoine-type equations. Indices were calculated using a non-linear technique, and the predicted values were found to agree with values obtained using traditional logarithmic predictions. We demonstrate that there is sufficient separation between retention indices to predict the identity of a compound by its retention index.

Retention index calculation without n-alkanes--the virtual carbon number

For the fast gas chromatographic identification of separated components the retention index is still one of the most often used tools, although mass spectrometry is available in almost all analytical laboratories. For the calculation of the retention indices it is not necessary to use n-alkanes or any other homologous series. If the analyte contains some compounds, not necessarily belonging to a homologous series, with well-known retention indices those compounds can be used as index references and the index of the other compounds can be calculated as is done using n-alkanes. The only difference is that instead of the carbon number of the n-alkanes, virtual carbon numbers of the reference compounds should be used. The method of calculation, and the effect of this method of calculation on the reproducibility are discussed in this paper, and analyses of some halogenated compounds and marjoram oil are used as experimental examples.

Prediction of retention indices. V. Influence of electronic effects and column polarity on retention index

The retention index increment for addition of a methylene group to an analyte molecule is shown for 1-halo-n-alkanes to be different from 100 i.u., a value that is customarily assigned according to the current convention in retention index prediction. In temperature-programmed gas chromatography using linearly interpolated retention index I, a linear regression equation, I=AZ+(GRF), with the number of atoms (Z) in the molecule as variable can describe the retention of 16 homologous series of organic compounds on non-polar and polar columns with characteristic A (linear regression coefficient) and (GRF) (group retention factor) values. A molecular model of retention on the basis of electron density and electron density distribution relative to that of n-alkane is proposed. This model brings out the inter- and intramolecular electronic effects in the analyte molecule and its dipole-dipole interaction with the stationary liquid phases, as variations in the A value. The (GRF) value varies with the connectivity ability of a functional group for extended conjugation, substitution, etc., but is most influenced by hydrogen bonding (H-bonding) with the stationary liquid phase. One can estimate the sequence of elution of a mixture of organic compounds from any two of the three parameters on the right-hand side of the above equation or retrieve the retention indexes of an entire homologous series from its A and (GRF) values. The fact that each analyte molecule has its own A value on different columns makes column difference (deltaI) compound-specific rather than column-specific, a departure from previous assumptions.

Retrieval of structure information from retention index

The chromatographic identity of a compound can be determined by four parameters, namely, I, A, Z and (GRF). These are interrelated in a linear regression equation, given in the paper as Eq. 8. The retrieval of structural information from retention data requires the introduction of a new meaning to the Kováts retention index, the use of column difference (delta I) to characterize functional groups, the redefinition of the role of electronegative oxygen and nitrogen atoms, and the division of retention index (I) into contributions from atoms and from functional groups. The separation of retention index (I) into molecular and interaction contributions is a necessary condition for retention index prediction from structure and also for structure information retrieval from retention data. According to Eq. 8 the retention index is uniquely determined by three parameters, namely A, Z and (GRF). For prediction of retention index, the A value is assigned a value of 100 index units (i.u.), the Z value is obtained directly from the compound, and the (GRF) value is pre-calibrated. In Eq. 10, the m and n values represent the pre-calibrated terms for a quantitative structure-retention index relationship. These terms account for the positive and negative retention contributions from polar and polarizable atom groups. All atom groups that are different from methylene and methyl groups will interact with the stationary phase and contribute to retention. The m and n values for various functional, polar and polarizable atom groups and their column differences (delta I values) are the results of interactions between the solute and the stationary phase and are structure dependent. The interaction increases with increasing polarities of the solute and the stationary phase. The column difference not only reflects the strength of the interaction, but is also characteristic of the functional and polarizable groups. The retrieval of structural information from retention data is equivalent to obtaining Z and (GRF) values from known I and delta I values, which is straightforward for monofunctional compounds. For multi-functional compounds, additional data will be needed for retrieval of structural information. These can be obtained from derivatization of the unknown compound, from its chemical reactions with other reagents, from GC-MS analysis and from structure match using internal or external standards. The additional data required will depend upon the complexity of the unknown structure. This approach demonstrates that a system can be devised to utilize GC retention characteristics uniquely for structure elucidation.