Capillary gas chromatography of esters correlations between structure and retention index

Summary The retention index of twenty different homologous series of esters have been determined experimentally on polar (Carbowax 1540) and non-polar (squalane) columns. General equations to calculate retention index have been established by statistical methods. The influence of the length and inductive effect of acid and alcohol chains and the relative position of the carboxylic group are discussed.     

Dicholorbenzyl alkyl ether homologs as retention index markers and internal standards for the analysis of environmental samples using capillary gas chromatography

A homologous series of 2,4-dichlorobenzyl alkyl ethers (DCBEs) have been synthesized and purified for use as retention index calibrants and internal standards. They are stable, sensitive to ECD and FID and ideal for GC-MS, with base peaks at m/z 159/161. The retention index calibration against the n-alkanes for a linear temperature program (LTP) series is given and recommendations made for the most suitable members of the DCBE series for use as internal standards in organochlorine (OC) residue analysis.     

Retention factor and retention index of homologous series compounds in microemulsion electrokinetic chromatography employing suppressed electroosmosis

The retention factor (k) and retention index (I) of homologous series compounds such as alkylbenzenes (BZ), alkylaryl ketones, alkylbenzoates, and alkylparabens in microemulsion electrokinetic chromatography (MEEKC) with suppressed electroosmosis were investigated in a wide range of SDS concentrations ([SDS]), temperatures, and concentrations of organic cosolvents (phi). Using BZ as standards, the retention indices of other homologous series compounds were determined and they were found to be independent of [SDS] and temperature, while are dependent on the types and concentrations of organic cosolvents. The retention factor linearly increases with increasing [SDS], while linearly decreases with increasing temperature. The value of log k linearly decreases with increasing phi for methanol, ethanol, or ACN, while decreases by a second-degree polynomial with increasing phi for 2-propanol. Excellent agreement was found between the observed and predicted values of log k of analytes in MEEKC at given [SDS] and phi, where the predicted values were obtained from modified equations of the linear relationship of log k as functions of [SDS], the number of carbons, and phi. Therefore, both k and I can be used for peak identification of homologous series compounds.     

Factors affecting the retention of polycyclic aromatic hydrocarbons in gas chromatography

Summary The retention index of a planar polynuclear aromatic hydrocarbon on the GC phases OV-101, SE-52, and OV-17 is strictly related only to the boiling point, and less closely to the relative molecular mass. The very approximates relation with connectivity index ins only a consequence of the latter. On a nematic lqiuid crystal phase, a variation of activity coefficient, expressed in terms of a shape factor, also influences the retention of PAH.     

The temperature dependence of retention indices in gas chromatography

Summary A linear dependence of (T–T₁)/[1(T)–1(T₁)] on temperature (considering the retention index 1(T₁) at temperature T₁ as a standard value) is derived. Both ther retention index at an assigned temperature and the temperature dependence of the retention index can be calculated from retention data measured at two temperature-programing rates.     

A new gas chromatographic retention index for pesticides and related compounds

A new gas chromatographic (GC) retention index based on a homologous series of tri-n-alkylamines is proposed for use in the detection of pesticides and related compounds because the standard n-paraffin hydrocarbons used for the Kovats index do not show up well on the nitrogen-phosphorus detectors commonly used in pesticide analysis. Using fused silica bonded phase capillary columns (DB-1 or DB-5), the trialkylamine indices of 106 selected pesticides and related compounds were measured and their relationship to the Kovats index determined.     

Calculation of retention indices in temperature-programmed gas chromatography

Summary A new method is presented for the calculation of the retention indices under linear temperature programming with or without an initial isothermal period. The data calculated by the method are in good agreement with the isothermal retention indices.     

Correlations between Van der Waals' volume and retention index general equation applicable to different homologous series

Summary Correlations between retention index (R_I) and Van der Waals' volume (V_W) in homologous series of n-alcohols, n-aldehydes and esters are studied on the stationary phases Carbowax 1540 and squalane. Influence of functional groups is quantified and a method for obtaining one general equation, valid for an indeterminate number of homologous series, is proposed.     

Advantage of the universal equation over the linear equation for the calculation of retention parameters of homologous series in capillary chromatography

A procedure is suggested for preparation of reproducible and stable galss capillary Apiezon LH+KF columns for the determination of organic compounds containing different functional groups. The advantage of the universal equation over the linear equation for calculation of retention indices for any homologs, including the first members of the series, is shown using the following five homologous series as examples: fatty acid methyl esters, methyl ketones, secondary and tertiary aliphatic amines, and N-alkyl piperidines. The accuracy of the calculation enables the universal equation to be used in computer-assisted identification.     

Comparison of mathemical methods for the calculation of retention indices at high temperature in gas chromatography

The retention indices of three homologous series (2-alkanones, 1-alkanols, cycloalksanones) have been determined at high temperature by the application of two new adaptation methods: A multiparametric least-squares regressions iterative method based on the dertermination of the adjusted retention times and a cubic interpolation directly using the uncorrected retention times without dead time correction. The two methods were applied to two types of columns. The first group includes eight packed columns (seven OV polymethylphenylsiloxane and Apolane-87 stationary phases), while the second includes five glass capillary columns (four methyl-silicons with different film thicknesses and Apolane-87 stationary phases). The retention indices obtained with a multiparametric and a cubic interpolation methods were compared with each other and with those calculated by Grobler's, Guardino's, Kaiser's and Kovàts' methods. The influence of coating, film thickness, and temperature on them was investigated.     

Retention time locking procedure for comprehensive two-dimensional gas chromatography

In gas chromatography (GC) reproducible retention times are in many cases highly favorable or in some cases even required. In one-dimensional GC, retention time shifts can be eliminated or minimized using a procedure called retention time locking (RTL). This procedure is based on adjusting the (constant) column head pressure. Unfortunately, this RTL procedure cannot be used in comprehensive two-dimensional gas chromatography (GC × GC) given the fact that peaks will shift in both dimensions. Adjusting the column head pressure in GC × GC will only minimize or eliminate the primary retention time shifts. In this paper, a fast and easy to perform, two-step retention time locking procedure for two-dimensional gas chromatography (2D-RTL) is proposed and its feasibility is demonstrated. This 2D-RTL procedure involves adjustment of the column head pressure or constant column flow, followed by the adjustment of the so-called effective secondary column length. The secondary column length is increased or decreased, simply by moving it stepwise through the modulator. It is demonstrated that retention time shifts in both the primary- and secondary-dimension, which may occur after e.g. replacing the column set, can be minimized to less than half peak base width. The proposed 2D-RTL procedure is used successfully for approximately 1 year in our laboratory.     

Capillary gas chromatography of hydrogenation products of 1,5,9-cyclododecatriene stereoisomers

Summary The separation of the products obtained by the hydrogenation of 1,5,9-cyclododecatriene stereoisomers on fused-silica capillary columns coated with dimethylsilicone and cyanopropylmethylsilicone stationary phases was investigated. Retention indices of cyclododecatrienes, cyclododecadienes, cyclododecenes and cyclododecane were determined at three temperatures. The respective standard deviations were 0.3 and 0.6 i.u. The influence of the polarity of the stationary phases on the chromatographic retention of these cyclic hydrocarbones is discussed.     

Extrapolation and interpolation of gas chromatographic retention times

Summary An empirical method of extrapolating and interpolating gas chromatographic retention times obtained at three equally spaced isothermal temperatures is described. The accuracy of the method was evaluated from retention time data obtained using packed glass columns. A procedure for constructing retention time tables for homologous series and the derivation of an equation for calculating retention times as a function of temperature is also presented.     

Kovats and lee retention indices determined by gas chromatography/mass spectrometry for organic compounds of environmental interest

Retention indices of standard organic compounds of environmental interest were determined by gas chromatography/mass spectrometry, using a DB-5 fused-silica capillary column. Retention indices are useful references for tentative compound identification by gas chromatography, or confirmation by gas chromatography/mass spectrometry. They provide elution order for isomers that might be indistinguishable based on mass spectra. Modified Kovats and Lee retention indices are given for polycyclic aromatic hydrocarbons; sulfur heterocycles; nitrogen heterocycles; aromatic amines; oxygen heterocycles; phenols; alcohols; ketones; alkanes; nitriles; and methylesters of fatty, dicarboxylic, and aromatic acids for comparison and reference. Retention index values for heterocycles by gas chromatography/mass spectrometry are comparable with gas chromatography values previously reported.     

Comparing columns for gas chromatography with the two-parameter model for retention prediction

The retention times of selected compounds in temperature programmed gas chromatography were predicted using a two-parameter model, on the basis of thermodynamic data obtained from isothermal runs on seven capillary columns, primarily substituted with 5% diphenylsiloxane. The scope for using thermodynamic data obtained from isothermal runs on one column to optimize separation on a different column or a different instrument setup was investigated. Additionally, the predictive utility of thermodynamic data obtained using a DB-5 column that had been in use for three years was compared to that of a new column of the same model. It was found that satisfactory separation could be achieved on one capillary column or instrument setup on the basis of thermodynamic data obtained using a different column or instrument set-up.     

A thermodynamic criterion for the identification of the functional group of aliphatic compounds by gas chromatography

Summary A thermodynamic criterion, Q, is suggested which permits to estimate the difference in the functional group energy of interactions for two different stationary phases. The linear dependence of Q on the homolog number m of any series R_m X can be used as the thermodynamic criterion for the identification of substances by GC analysis.Enlarged text of the paper presented at the Fourth Danube Symposium on Chromatography, Bratislava, Czechoslovakia, September 1983.     

Revisiting the problem of correcting retention parameters for pressure and temperature in gas chromatography

Summary It has been shown [1, 2] that the compressibility correction factor equals the ratio of gas pressure at the column outlet to the average pressure in the column,j=p _o /p _c , and, therefore, by multiplying by this factor, all experimentally measured retention volumes and flow rates are converted from ambient pressure to the average pressure in the column. This makes retention volumes corrected in this way independent of pressure. In contrast, correcting retention times for gas compressibility has no physical meaning and terms such as “corrected retention time” and “net retention time” should not be used. Similarly, recalculating corrected retention volumes to a standard temperature of 273 K appears to provide a thermodynamically sound basis for comparison of data obtained at different temperatures. In reality, it distorts actual relationships and should not be used. Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997.