Alkane Retention Research Articles

A multi-location peak parking approach for calculation of second dimensional retention indices for improved volatile compound identification with cryogen-free comprehensive heart-cut two-dimensional gas chromatography.

Comprehensive heart-cut multidimensional gas chromatography (CH/C MDGC) without a cryogenic trapping device was developed with an established approach for calculation of first and second dimensional retention indices (1I and 2I) for improved compound identification. A first dimensional (1D) DB-1MS column (60 m) and a second dimensional (2D) DB-WAX column (60 m) were applied with a Deans switch (DS) using a constant H/C window of 0.2 min and a periodic multiple heartcut strategy comprising 225H/C throughout the CH/C. 1I was calculated based on comparison of the middle of the heartcut time with the alkane retention times on the 1D column. A multi-location peak parking approach using sixteen sets of automated injections of alkane references was also established with the least square curve fitting method for construction of the alkane isovolatility curves which were applied for 2I calculation. The untargeted compound analysis of a perfume sample was then performed according to comparison with the libraries of mass spectra, 1I and 2I. The CH/C MDGC system with a 25 h analysis time showed a peak capacity (nc) of 9198 and 128 separated peaks with 71 compounds successfully identified according to MS, 1I and 2I library match under the established error approximation criteria. Furthermore, relationship between the analysis time and number of separated peaks was proposed based on the set of 84 identifiable compounds. With the compensation of lower separation performance and greater I errors, the analysis time could be reduced by applying a 2.5 min H/C window with a total analysis time of 2 h and nc of 1134.

Effects of Coated Capillary Column, Derivatization, and Temperature Programming on the Identification of Carica papaya Seed Extract Composition Using GC/MS Analysis

Some of the common practices to assess the composition of plant extract, including Carica papaya seed extract (CPSE) are direct injection of the extract, compound separation using polyethylene glycol capillary column (DB-WAX), and non-linear-temperature programming (NLTP) of GC/MS analysis. This study specifically compared the coating of capillary column, sample derivatization, and temperature programming of GC/MS to determine the composition of CPSE. The retention indices (RI) of the detected compounds were determined and compared to the reference RI. In particular, 5% phenyl–95% methylpolysiloxane (HP-5MS)-, DB-WAX-, and biscyanopropyl polysiloxane (HP-88)-coated capillary columns were used to identify the composition of CPSE. For this study, HP-5MS column, which separated the highest number of compounds (26 compounds) from CPSE, was deemed as the most suitable column. The GC/MS analysis of derivatized CPSE identified 21 compound groups, where fatty acids and fatty acid methyl esters served as the major compounds (80.23%), followed by these compounds in decreasing order: amides > nitriles > sterols > fatty aldehydes > organic acids. A stronger correlation determination between the carbon number and alkane retention time of linear-temperature programming (LTP) (R2 = 0.9859) was found, as compared to its correlation determination with NLTP (R2 = 0.9175), which exhibited an almost equal RI of LTP to the reference RI. Conclusively, GC/MS analysis for the derivatized CPSE using HP-5MS column separation and LTP is highly recommended. • This study (1) analyzed the composition of Carica papaya seed extract (CPSE) by comparing sample derivatization, type of column, and temperature programming of the GC/MS and (2) developed retention indices (RI) of volatiles and TMS-formed compounds of CPSE using linear (LTP) and non-linear (NLTP) temperature programming. • HP-5MS column separated the highest number of compounds (26 compounds), followed by DB-WAX (13 compounds) and HP-88 (11 compounds) at 90% similarity match towards NIST 11 library. • Twenty-one groups of compounds were identified in derivatized and non-derivatized CPSE using GC/MS, where fatty acids and fatty acid methyl esters were the dominant compounds (80.23%). • The RI for LTP was almost equal to the reference RI as reported from previous literature, while RI for NLTP showed different values from the reference RI due to the stronger correlation determination between carbon number against retention time of alkane of LTP (R2 > 0.9859) as compared to NLTP (R2 > 0.9175). • This study recommended a GC/MS analysis of derivatized CPSE with HP-5MS column separation and LTP.

Retention in high-performance liquid chromatography at −196 °C

Recently, we have developed ultralow-temperature high-performance liquid chromatography (HPLC) at −196°C using liquid nitrogen-based mobile phases. In this study, a retention model for ultralow-temperature HPLC, in which adsorption exchange and “pseudo partition” modes are combined, is proposed to describe the effect of the mobile phase composition on the retention of analytes. The experimental results agreed well with the equation of the proposed model. It was revealed that, in the low and high additive concentration regions, adsorption exchange and pseudo partition retention, respectively, dominate the analyte retention. The difference in the retention behavior between bare-silica and octadecylsilyl-modified silica (ODS) columns was also studied. Retention of alkanes in the ODS column was greater than that in the bare-silica one. Addition of both ethane and ethylene to the mobile phase in the ODS column afforded the elution of propylene, which was not eluted with the bare-silica column at −196°C.

High Performance Liquid Chromatography at −196 °C

Ultralow temperature high-performance liquid chromatography (HPLC) was developed using a liquefied gas as the mobile phase. HPLC separation of low molecular weight alkanes at -196 °C with liquid nitrogen mobile phase was successfully achieved, whereas their GC separation at -196 °C using helium gas mobile phase failed to elute the analytes due to strong adsorption. Prior to the further study of HPLC at -196 °C, the effect of column temperature on the chromatographic behavior was investigated, and it was found that the retention of analytes drastically increased when the column temperature was over the boiling point of the mobile phase. As the study of retention control in HPLC at -196 °C, the mobile phases of nitrogen and methane mixtures were investigated. The addition of methane to the nitrogen mobile phase suppressed the retention of the analytes (tetra-deuterated methane, ethane, and propane), that is, the retention on HPLC at ultralow temperature could be controlled by the mobile phase composition, akin to the typical retention in HPLC. The selectivity toward the n- and iso-alkane in HPLC at -196 °C was altered compared with that in GC separation at room temperature. A significant enhancement of retention of alkanes compared with alkanes were observed in HPLC at -196 °C.

IMatch: A retention index tool for analysis of gas chromatography–mass spectrometry data

A method was developed to employ National Institute of Standards and Technology (NIST) 2008 retention index database information for molecular retention matching via constructing a set of empirical distribution functions (DFs) of the absolute retention index deviation to its mean value. The effects of different experimental parameters on the molecules’ retention indices were first assessed. The column class, the column type, and the data type have significant effects on the retention index values acquired on capillary columns. However, the normal alkane retention index ( I norm) with the ramp condition is similar to the linear retention index ( I T ), while the I norm with the isothermal condition is similar to the Kováts retention index ( I). As for the I norm with the complex condition, these data should be treated as an additional group, because the mean I norm value of the polar column is significantly different from the I T . Based on this analysis, nine DFs were generated from the grouped retention index data. The DF information was further implemented into a software program called iMatch. The performance of iMatch was evaluated using experimental data of a mixture of standards and metabolite extract of rat plasma with spiked-in standards. About 19% of the molecules identified by ChromaTOF were filtered out by iMatch from the identification list of electron ionization (EI) mass spectral matching, while all of the spiked-in standards were preserved. The analysis results demonstrate that using the retention index values, via constructing a set of DFs, can improve the spectral matching-based identifications by reducing a significant portion of false-positives.

Dual-Injection System with Multiple Injections for Determining Bidimensional Retention Indexes in Comprehensive Two-Dimensional Gas Chromatography

A new instrumental approach for collection of retention index data in the first (1D) and second (2D) dimensions of a comprehensive two-dimensional (2D) gas chromatography (GCxGC) experiment has been developed. First-dimension indexes were determined under conventional linear programmed temperature conditions (Van den Dool indexes). To remove the effect that the short secondary column imposes on derived 1D indexes, as well as to avoid handling of pulsed GCxGC peaks, the proposed approach uses a flow splitter to divert part of the primary column flow to a supplementary detector to simultaneously generate a conventional 1D chromatogram, along with the GCxGC chromatogram. The critical 2D indexes (KovAts indexes) are based upon isovolatility curves of normal alkanes in 2D space, providing a reference scale against which to correlate each individual target peak throughout the entire GCxGC run. This requires the alkanes to bracket the analytes in order to allow retention interpolation. Exponential curves produced in the 2D separation space require a novel approach for delivery of alkane standards into the 2D column by using careful solvent-free solid-phase microextraction (SPME) sampling. Sequential introduction of alkane mixtures during GCxGC runs was performed by thermal desorption in a second injector which was directly coupled through a short transfer line to the entrance of the secondary column, just prior to the modulator so that they do not have to travel through the 1D column. Thus, each alkane mixture injection was quantitatively focused by the cryogenic trap, then launched at predetermined times onto the 2D column. The system permitted construction of an alkane retention map upon which bidimensional indexes of a 25-perfume ingredient mixture could be derived. Comparison of results with indexes determined in temperature-variable one-dimensional (1D) GC showed good correlation. Plotting of the separation power in the second dimension was possible by mapping Trennzahl values throughout the 2D space. The methodology was applied to the separation of a standard mixture composed of 25 analytes (very diverse in polarity and structure) suspected to be allergens in perfume samples. The method will allow straightforward determination of temperature-variable retention indexes of target analytes.

Retention correlation maps in comprehensive two-dimensional gas chromatography

A method of generating a retention index base for comprehensive two-dimensional gas chromatography (GC×GC) has been developed for the first and second axes of the two dimensional analysis space. The approach is described for an alkane base, and also with 2-methyl ketone and fatty acid methyl ester standards. The method involves repetitive, timed, sequential injections of the reference compound standard mixtures, and from the resultant data developing a retention correlation map. The first dimension generates linear temperature programmed indices, whilst the second dimension may be interpreted on the basis of ‘pseudo isothermal’ retention indices. From this map, retention information should be obtainable for any solute eluting within the allowed analysis region. It is proposed that this procedure will allow prediction of comprehensive two-dimensional gas chromatography (GC×GC) retention data or positions for compounds in the 2D separation space, andwill also support development and understanding of rational column choices for GC×GC. The second dimension cannot be easily interpreted based on an alkane retention series, and so a more polar retention index base will be required.

Gas chromatographic characterization of the adsorption properties of the natural adsorbent CACMM2

The adsorption behavior of n-pentane, n-hexane, n-heptane, benzene and cyclohexane on the natural adsorbent CACMM2 at 60°C was studied by use of gas chromatography. The variation of the differential enthalpy of adsorption and the entropic factor were determined and are similar than those obtained on such non-specific adsorbents as dehydroxylated silica. Retention of alkanes depends mainly on the boiling point. The separation of the mixtures pentane–benzene, pentane–cyclohexane, hexane–benzene, hexane–cyclohexane, heptane–benzene and heptane–cyclohexane is achieved in 1 min.

Effect of high temperature in determination of the mathematical dead time and the constants of the n‐alkane retention time curve in gas chromatography

AbstractIn gas liquid chromatography, the column dead time and the constants of the n‐alkane retention time curve are calculated by the multiparametric least‐squares regression iterative method at high temperature between 180 and 270°C. The method was applied to two types of columns. The first group includes eight packed columns (seven OV polymethylphenylsiloxane and Apolane‐87), while the second includes five glass capillary columns (four methylsilicone with different film thicknesses and Apolane‐87). The calculated tM and b values were compared with those obtained by Guardino's, Grobler's, and Kaiser's methods. The influences of coating thickness and temperature thereon were investigated.

Retention of cyanoalkanes and cyanoalkylbenzenes by bonded stationary phases in supercritical fluid chromatography

The retention of cyanoalkanes and cyano-alkylbenzenes during SFC was investigated on alkyl and cyanoalkyl-bonded stationary phases and compared to alkane retention. The particular behaviour of short chain homologues was attributed to a silanophilic interaction with surface-OH groups on the silica and inhibition of specific interactions between solutes and C10CN bonded groups.

Surface thermodynamic analysis of finishes on Kevlar 29* fabrics by inverse gas chromatography

The surface thermodynamic properties of fibers from a variety of Kevlar 29 fabrics have been measured using inverse gas chromatography (IGC). The enthalpy (ΔH° A), entropy (ΔS° A), and free energy (ΔG° A) of adsorption, and the dispersive component of the surface free energy (γd s) have been calculated from alkane retention volumes on fibers with various surface finishes, in addition to commercially scoured and laboratory cleaned samples. All of the samples, with the exception of the laboratory cleaned sample, have low surface energies consistent with the presence of surface finishes or, in the case of the scoured samples, significant surface contamination. A majority of these samples also exhibit anomalous alkane retention volumes around room temperature. This phenomenon, absent in the laboratory cleaned sample, is consistent with a glass transition in the surface film. Infrared analysis of material extracted from these samples supports this conclusion. The detection of glass transitions in surface films...

Correlation of Retention Indices with Resolution and Selectivity in Reversed-Phase HPLC and GC

The solvent selectivity triangle concept for characterizing high-performance liquid chromatographic (HPLC) mobile phase liquids consistently fails to group solvents according to selectivity for resolving aromatic compounds in reversed-phase HPLC. Selectivity differences between solvents classified in the same group often exceed those for solvents classified in different groups. Nevertheless, solvents characterized by simple relationships between resolution and 2-keto alkane retention indices correlate accurately with experimental resolution for 16 aromatic compounds with 12 reversed-phase solvents in binary mixtures with water. Also, by adjusting for the relative slopes of the 2-keto alkane retention index standards in HPLC or n-alkane standards in gas chromatography (GC), retention index differences are directly correlated with resolution, whereas nonadjusted retention index differences fail to correlate with resolution. Anomolies in McReynolds constants used in the classification of GC liquid phases are eliminated by adjustment for relative slope differences for the n-alkane standards that occur with the various phases.

Thermodynamic studies into a sorption mechanism within the cross-linked polysiloxane stationary phases

The retention mechanism has been studied for n-alkane analytes within a cross-linked commercial poly(dimethylsiloxane) stationary phase and laboratory-synthesized poly(methyl-n-alkylsiloxane) substrates with different alkyl chain lengths. Model capillary columns were prepared with these phases and the specific retention volumes of the analytes measured within the range of 80-160/sup 0/C. The following thermodynamic calculations indicate that bulk dissolution appears to be the dominant mechanism for retention of alkanes within these stationary phases.

Correlation between reversed-phase retention and solute molecular surface type and area : II. Retention of low polarity non-hydrocarbonaceous surfaces

On octadecylsilica (ODS) systems and pure methanol as eluent, the retention of homonuclear proton-free solutes like P 4, As 4, S 9, Se 8 and sulphur homocycles S n with n > 9 appears to depend only on their respective total molecular surface area (TSA). When plotted versus TSA, their ln k′ values lie on one straight line. The retention of these solutes is distinctly higher than that of n-alkanes having equal TSA. In terms of retention index, I K, the difference is about +500 units. It is proposed that the lower alkane retention is due to an additional interaction with the eluent methanol. The retention increments produced by chalcogen atoms in RS n R, RSe n R and RTe n R chains are also proportional to their respective surface areas, and are influenced to a certain degree by the length of the terminal alkyl groups R. The retention of proton-free heteronuclear solutes like S n Se 8− n , hexachlorobenzene, hexabropmobenzene and carbon disulphide is lower than with homonuclear proton-free solutes, but higher than with alkanes if normalized to equal TSA. This is explained by the existence of stronger bond dipoles and an increased polarizability.

Mathematical dead time determination and calculation of the constants of the n‐alkane retention time curve on OV polymethyl phenylsilicone stationary phases in gas chromatography

AbstractThe column dead time and the constants of the n‐alkane retention time curve in gas‐liquid chromatography are calculated by the Wentworth iterative method on series of OV polymethylphenylsilicones with an increasing percentage of phenyl substituent. The values obtained are in good agreement with those calculated by the methods of Gröbler and Kaiser and with the experimental ones reported in the literature for stationary phases studied at 120°C and 150°C. The scatter of experimental retention times of n‐alkanes and tM and b and c and Kováts retention indices are calculated.

Relationship between retention behaviour and molecular structure of alkanes in reversed phase high performance liquid chromatography

The retention data of 54 alkanes C5-C11 with different degrees of branching were determined in order to study the relationship between retention behaviour and the structure of alkanes by reversed phase high-performance liquid chromatography. The structural parameters influencing the retention of alkanes are described. The connectivity index was employed to correlate the structures of alkanes with their retention data.

The linearity of the log-plot of adjusted retention times versus carbon number

The linearity of the log (adjusted retention time of normal alkanes) versus carbon number is of interest because of the possibility to calculate retention indices by logarithmic interpolation over a wider range of carbon atoms than as defined by Kovats who considered the adjusted retention times of two neighbouring (even) normal alkanes. Although by some authors this logplot is considered to have a certain curvature, at least in a part of the plot it appears from our data that no curvature can be observed at all. One of the reasons that curvature has been found is due to the general assumption in Gas Chromatography that the adjusted retention time of methane equals zero. This induces an error which is responsible for the observed curvature in the initial region of the logplot. There is a way to get around this error viz by calculating the column dead volume point from three “standard” normal alkane retention times. On this base retention indices of other normal alkanes were calculated and appeared to be multiples of 100 within a small non systematic experimental error. An Algol 60 programme as used for the calculation of the retention indices is added. The use of a standard programme is almost imperative in this sort of calculations and will add to a more accurate standardisation of retention index determinations as carried out in different laboratories.