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.     

Optimizing the relationship between chromatographic efficiency and retention times in temperature‐programmed gas chromatography

A methodology that can maximise the chromatographic efficiency that can be achieved within a defined time frame in temperature‐programmed gas chromatography is described. The efficiency can be defined as the inverse of peak widths measured in retention index units. This parameter can be described by a model similar to the van Deemter equation, which is expanded to account for the effect of the temperature rate in addition to the effect of carrier gas velocity. The model of efficiency is found by response surface methodology, where the temperature rates and the carrier gas velocities are systematically varied in the experiments. A second model that accurately explains the retention time of the last eluting compound can be found from the same experiments, and optimal conditions are found by combining the two models. The methodology has been evaluated with four capillary columns and three carrier gases, using fatty acid methyl esters as analytes. All experiments showed that there is a fairly linear decrease in efficiency with increasing temperature rates. At any temperature rate, optimal velocity is only marginally higher than the velocity that maximises chromatographic efficiency, since the carrier gas velocity has a limited effect on the retention times.     

Glass capillary gas chromatography of chlorinated methyl acetates,propanoates and butanoates on Carbowax 20M and SE-30 columns

Summary The gas chromatography of all chlorinated methyl acetates, methyl propanoates and methyl mono- and dichlorobutanoates has been studied on Carbowax 20M and SE-30 glass capillary columns under various running conditions. The order of elution on a non-polar column was largely determined by the boiling point of esters, whereas on a polar column it was much influenced by the structure of compounds. Complete separation of the combined mixture of all 27 compounds could not be achieved, however, methyl 3,3-dichlorobutanoate was the only ester overlapped on both columns in spite of the various column temperatures used. The best separation of the mixture was on Carbowax 20M with a temperature program from 50°C at 8°C/min, isothermal running conditions leading either to poor separation of volatile components or long analysis time and broad peaks of higher chlorinated esters. The relative retention times for compounds at the various column temperatures are given and the retention order on a polar and on a non-polar column discussed.     

Chromatographic efficiency of polar capillary columns applied for the analysis of fatty acid methyl esters by gas chromatography

The chromatographic efficiency that could be achieved in temperature‐programmed gas chromatography was compared for four capillary columns that are typically applied for analysis of fatty acid methyl esters (FAME). Three different carrier gases, hydrogen, helium and nitrogen, were applied. For each experiment, the carrier gas velocities and the temperature rates were varied with a full 9 × 3 design, with nine levels on the carrier gas velocity and temperature rates of 1, 2 or 3°C/min. Response surface methodology was used to create models of chromatographic efficiency as a function of temperature rate and carrier gas velocity. The chromatographic efficiency was defined as the inverse of peak widths measured in retention index units. The final results were standardized so that the efficiencies that could be achieved within a certain time frame, defined by the retention time of the last compound in the chromatogram, could be compared. The results show that there were clear differences in the efficiencies that could be achieved with the different columns and that the efficiency decreased with increasing polarity of the stationary phase. The differences can be explained by higher resistance to mass transfer in the stationary phase in the most polar columns.     

Identification of FAMEs using ECL values and a three-dimensional Kováts retention index system

n-Alkanes and fatty acid methyl esters have been chromatographed on four capillary columns of different polarity (LM-1 (100% polydimethylsiloxane), CB-225 (25% cyanopropyl, 25% phenyl polysiloxane), LM-100 (immobilized polyethylene glycol), and BPX-70 (70% cyanopropyl polysiloxane)) with the goal of identifying the major fatty acid methyl esters obtained by derivatization of vegetable oils. Chromatographic retention data were employed for the construction of a three-dimensional Kováts retention index system and for the elaboration of a table of equivalent chain length values.     

Gas chromatographic determination of the fatty-acid content of heat-treated green beans

A gas chromatographic method that employs flame ionization detection, a DB-Wax capillary column with helium as the carrier gas, and a split-splitless (1:15) injector was used to determine the effects of different heat treatments on the fatty-acid content of whole green beans (Phaseolus vulgaris, L.). A one-step-extraction-methylation method was used to obtain fatty acid methyl esters from raw and steamed, boiled, pressure-cooked, and microwave-cooked green beans. The fatty-acid profile changed slightly, but heat treatment produced an increase in the fatty-acid content.     

Analysis of polychlorinated biphenyls (PCB) by glass capillary gas chromatography

Summary The composition of seven technical PCB-mixtures (Aroclor [Monsanto, USA] und Clophen A [Bayer, FRG]) has been investigated by high-resolution thin-film glass capillary gas chromatography with electron-capture detector. Methylpolysiloxane (SE 30) and purified Apiezon L have been used as liquid phases. Identification of the single PCB components has been performed by comparison of their retention indices with those of polychlorinated biphenyls defined by synthesis or with values calculated from retention index increments. For marking the individual PCB compounds a systematic numbering has been used.Herrn Prof. Dr. R. Bock zum 65. Geburtstag gewidmet     

Gas chromatographic data for 187 nitrogen- or phosphorus-containing drugs and metabolites of toxicological interest analysed on methyl silicone capillary columns

Retention data, retention indices and retention times relative to diazepam, associated with 187 nitrogen- or phosphorus-containing drugs and metabolites likely to be found in toxicological analysis, are presented. The work was carried out with cross-linked methyl silicone, siloxane-deactivated fused-silica capillary columns in different gas chromatographs equipped with nitrogen-phosphorus flame ionization detectors and a gas chromatograph-mass spectrometer. Results show the high reproducibility offered by the capillary system, which permits the reduction of identification problems and analysis time.     

Temperature program optimization by computer simulation for the capillary GC analysis of fatty acid methyl esters on biscyanopropyl siloxane phases

The selectivity of capillary columns coated with biscyanopropyl siloxane stationary phases for the separation of fatty acid methyl esters has been optimized by means of computer-assisted column temperature optimization software. Temperature programming rates yielding the highest resolution in the shortest analysis time were selected for split, splitless, and on-column injection operated in the constant pressure and pressure programmed modes.     

Sensitivity of the methylbenzenes and chlorobenzenes retention index to column temperature, stationary phase polarity, and-number and chemical nature of substituents

Retention indices of methylbenzenes and chlorobenzenes on two fused silica capillary columns, HP-5 (diphenylsiloxane 5% diphenyldimethylsiloxane) and ZB-WAX (polyethylene glycol), have been calculated at various isothermal temperatures and compared with literature data. The retention index temperature effect was studied for each solute, finding greater retention index the higher the column temperature. A comparison between the straight line fit and the fit to the recently proposed equation I = A + B/T +C ln T was carried out. The effect of the stationary phase polarity on the retention index was checked. In general, a greater retention index was found for the more polar stationary phase. The retention indices of the chlorobenzenes are greater than the retention indices of the methylbenzenes, irrespective of the stationary phase and the column temperature. In addition, the influence of the methyl/chlorine substitution on the benzene molecule was investigated at each temperature. The retention indices increased as the number of substituents (methyl/chlorine) increased. The retention index increments of methyl and chloro derivatives are also discussed, which permits to compare the effect of both, methyl or chlorine, chemical functions, for a fixed substituent number in the benzene molecule.     

Polyimide polymer glass-free capillary columns for gas chromatography

Polymeric polyimide capillary tubing, both uncoated and coated with stationary phases of two polarities, is explored for use as capillary columns for gas chromatography (GC). These glass-free polyimide columns are flexible and their small winding diameter of less than a cm around a solid support makes them compatible for potential use in portable GC instruments. Polyimide columns with dimensions of 0.32 mm i.d. × 3 m are cleaned, annealed at 300°C, and coated using the static method with phenylmethylsilicone (PMS). Separations of volatile organics are investigated isothermally on duplicate sets of polyimide columns by GC with a flame ionization detector using split injection. Unlike the uncoated ones, the coated polyimide columns successfully separate Grob test mix classes of alkanes, amines, and fatty acid methyl esters. The relative standard deviations for retention time and peak area are 0.5 and 2.5 , respectively. With the 3 m PMS-coated column connected to a retention gap to permit operation at its optimum flow rate of 30 cm/s, a plate count of 3200 or plate height of 1 mm is possible. Lack of retention and tailing peaks are evident for the polyimide polymer capillary columns as compared to that of a 3 m commercial cross-linked PMS fused silica capillary. However, headspace analyses of an aromatic hydrocarbon mix and a Clearcoat automotive paint sample are viable applications on the PMS polyimide polymer column.     

Temperature‐programmed capillary high‐performance liquid chromatography with atmospheric pressure chemical ionization mass spectrometry for analysis of fatty acid methyl esters

A new capillary high‐performance liquid chromatography method with atmospheric pressure chemical ionization mass spectrometry was developed for the analysis of fatty acid methyl esters and long‐chain alcohols. The chromatographic separation was achieved using a Zorbax SB‐C18 HPLC column (0.3 × 150 mm, 3.5 μm) with a mobile phase composed of acetonitrile and formic acid and delivered isocratically at a flow rate of 10 μL/min. The column temperature was programmed simply, using a common column oven. Good reproducibility of the temperature profile and retention times were achieved. The temperature programming during the isocratic high‐performance liquid chromatography run had a similar effect as a solvent gradient; it reduced retention times of later eluting analytes and improved their detection limits. Two atmospheric pressure chemical ionization sources of the mass spectrometry detector were compared: an enclosed conventional ion source and an in‐house made ion source with a glass microchip nebulizer. The enclosed source provided better detectability of saturated fatty acid methyl esters and made it possible to determine the double bond positions using acetonitrile‐related adducts, while the open chip‐based source provided better analytical figures of merit for unsaturated fatty acid methyl esters. Temperature‐programmed capillary high‐performance liquid chromatography is a promising method for analyzing neutral lipids in lipidomics and other applications.     

Contribution of adsorption to rentension data in capillary gas chromatography part II. Non-polar stationary phases

Summary The influence of adsorption on the dependence of retention data upon film thickness of the non-polar stationary phases OV-101 and Apiezon L coated on glass (series of capillary columns with non-deactivated and Carbowax 20 M deactivated surface) and fused silica has been studied. Limiting values of Kovats indices, I, of solutes which exhibit tendency to hydrogen bonding are calculated for each series of columns.Presented at the 14th International Symposium on Chromatography London, September, 1982     

The effect of the trifluoropropyl group in polysiloxane stationary phases used for capillary gas chromatography

Four poly(methyl 3,3,3-trifluoropropyl siloxanes) with trifluoro-propyl group content (group substitution) between 8 and 35 percent have been synthesized and characterized as stationary phases for gas chromatography in borosilicate glass capillary columns. Results are compared with those from two commercial stationary phases–a polydimethylsiloxane and a poly(methyl 3,3,3-trifluoropropyl siloxane) with a fifty percent trifluoropropyl group content (group substitution). Retention index values, McReynolds constants, polarity (as defined by McReynolds) and retention polarity (as defined by Takács) increase regularly with the trifluoropropyl group content of the stationary phase. The temperature coefficient of the retention indices of the McReynolds probes, and that of the polarities, have been determined at temperatures between 60 and 180 °C. Specific retention volumes do not follow the linear dependence on trifluoropropyl group content observed for retention indices or polarities. Substances with electron-donor groups show maximum retention for a trifluoropropyl group content of ca 30%, whereas the retention of hydrocarbons, halogenated compounds, and alcohols decreases as the degree of trifluoropropyl group substitution increases from 0 to 50%. It is felt that a polysiloxane with a trifluoropropyl group content of ca 30 to 35% would be the best choice for the separation of ketones, nitro compounds or amines.     

GC Behavior of disaccharide trimethylsilyl oximes

Twenty-five disaccharides are converted to trimethylsilyl oximes and injected on two capillary columns of different polarities (methyl and phenyl-methyl silicone). Their Kovats retention indices are determined at three temperatures. Multivariate data analysis is used in order to obtain relations between retention and some structural features. Phenylmethyl silicone have better chromatographic characteristics for the assayed compounds. The two columns can provide useful chromatographic information when a very complex disaccharide mixture has to be analyzed.     

Prediction of programmed temperature retention times on linked capillary columns of different polarity

Systems formed by serial connection of capillary columns of different polarity were studied with methods previously used to predict the behavior of linked capillary columns under isothermal conditions and to obtain programmed temperature gas chromatography (PTGC) retention times of the individual columns starting from isothermal data. The two calculation methods were simultaneously applied in order to predict PTGC retention times of the series system starting from isothermal data obtained on the two individual columns. Experimental retention values measured using different temperature programs on the individual columns and on the series systems were found to agree with those calculated.     

Behavior of n-alkanes on poly(oxyethylene) capillary columns. Evaluation of interfacial effects

The solvation behavior of n-alkanes on poly(oxyethylene) was studied employing capillary gas chromatography. Interfacial effects were discriminated and evaluated through the analysis of retention data from six commercial fused-silica capillary columns, having film thicknesses of 0.15-5 microm. Expressions for the mixed retention mechanism in capillary columns were deduced from assumptions of a general character. Partition coefficients were determined for the n-alkanes up to 28 carbon atoms, at temperatures ranging from 40 to 240 degrees C. In agreement with other authors, it was observed that interfacial phenomena contribute poorly to the chromatographic retention, being negligible over 140 degrees C for homologues with less than 16 carbons.     

Gas chromatography of tricyclic spiro esters

Summary The separation of some tricyclic spiro esters on fused-silica capillary columns coated with dimethylsiloxane, cyanopropyl methyl siloxane and poly(ethylene glycol) stationary phases was investigated. Retention indices were determined at two temperatures to interpret their chromatographic behaviour. The repeatability of the measurements was 0.2, 0.3 and 0.5 index units. The influence of the polarity of the stationary phases on the chromatographic retention of these tricyclic spiro esters is discussed.     

Distribution coefficients of n-alkanes measured on wall-coated capillary columns

Distribution coefficients K of n-alkanes were determined in wide ranges of temperature and carbon numbers from gas chromatographic retention data measured on wall-coated poly(dimethylsiloxane) commercial capillary columns. A discussion is centered on how to mitigate the difficulties for an accurate determination of K when using weakly retentive columns, as those bearing very high phase ratios or short lengths. Particularly, the errors associated with the estimation of the gas hold-up and the phase ratio of the column are considered. The chromatographic importance for determining K of n-alkanes relies on the fact that these are the most commonly applied references for reporting relative thermodynamic parameters such as the Kovats Index and the relative retention. A great amount of information has been compiled in this form. If K of the reference is known, absolute values of distribution coefficients for a myriad of substances are readily obtainable. The knowledge of K(T) functions of solutes in wide ranges of temperature is a primary necessity in temperature-programmed gas chromatography. This knowledge is needed for the prediction of absolute retention times and for computing separation optimizations of mixtures containing several critical pairs of analytes.     

Identification of fatty acids in gas chromatography by application of different temperature and pressure programs on a single capillary column

A method for the identification of fatty acid methyl esters (FAME) based on the analysis of shifts in equivalent chain lengths (ECL) is described. The method is based on two-dimensional retention data achieved on one capillary column. Various temperature and pressure programs are applied on the same cyanopropyl column and the shifts in the ECL values are analysed by multivariate methods. The chain length, number of double bonds, and the double bond positions can be determined with high accuracy. The same procedure is suitable for determination of the number of trans and cis double bonds in trans fatty acids, and for detection of artefacts in fatty acid methyl ester chromatograms.