Retention Index Values Research Articles

Enantioseparation of chiral amino acids as the N(O,S)-ethoxycarbonylated diastereomeric esters by achiral dual-capillary column gas chromatography.

The enantioseparation of 30 racemic amino acids in a single analysis is described for the determination of their absolute configurations. Two-phase extractive ethoxycarbonyl (EOC) reaction with ethyl chloroformate present in the dichloromethane phase was performed to recover amino acids from alkaline aqueous solutions. The resulting N(O,S)-EOC amino acids extracted into an organic solvent after acidification were reacted with a chiral alcohol such as (S)-(+)-3-methylbutan-2-ol, (S)-(+)-butan-2-ol and (S)-(+)-octan-2-ol for gas chromatographic analysis on achiral dual-capillary DB-5 and DB-17 columns of different polarities. Among the chiral reagents examined, (S)-(+)-3-methylbutan-2-ol provided the best diastereomeric structures in resolving all the racemic amino acids into their enantiomeric pairs with high resolution factors (1.2-8.0). Moreover, the temperature-programmed retention index (I) values measured on the two columns were characteristic of each enantiomer. Hence simple I matching with the reference values was useful in cross-checking for chemical identification and also chiral discrimination. When the present method was applied to a fermented dairy product (Yakult), D-alanine, D-aspartic acid, D-glutamic acid and D-proline were positively detected along with their respective L-forms in addition to glycine.

Characterization of aroma‐impact compounds in cold‐pressed grapefruit oil using time–intensity GC–olfactometry and GC–MS

AbstractThirty‐eight aroma‐active compounds were detected in commercial cold‐pressed grapefruit oil using a time–intensity GC–olfactometry procedure. Identification of the aroma‐active components was based on the combination of sensory descriptors, linear retention index values from DB‐Wax and DB‐1 columns and GC–MS data. Thirteen flavour components, which had not been previously reported in grapefruit oil, were identified. Compounds with the greatest aroma intensities include: 1,8‐cineole, octanal, dodecanal, trans‐4,5‐epoxy‐(E)‐2‐decenal, β‐sinensal and nootkatone. The aroma activities of sulphur compounds such as 4‐mercapto‐4‐methyl‐2‐pentanol and methional were surprisingly low and neither of the two putative grapefruit aroma impact compounds, 4‐mercapto‐4‐methyl‐2‐pentanone and 1‐p‐menthene‐8‐thiol, was detected. Over 200 volatiles have been reported in grapefruit oil, but only 38 have significant aroma activity and 22 of these can be considered major aroma impact compounds. Copyright © 2001 John Wiley & Sons, Ltd.

Molar Refractivity and Connectivity Index Correlations for Henry's Law Virial Coefficients of Odorous Sulfur Compounds on Carbon and for Gas-Chromatographic Retention Indices

Gas–solid chromatography was used to obtain values of the second gas–solid virial coefficient, B2s, in the temperature range from 392 to 511 K for 10 volatile, malodorous organic sulfur compounds; ethanethiol, 1-propanethiol, methyl sulfide, 2-propanethiol, 1-methyl-1-propanethiol, 2-methyl-1-propanethiol, 2-methyl-2-propanethiol, ethyl sulfide, ethyl methyl sulfide, and tert-butyl methyl sulfide. Carbopack C (Supelco Inc.), a graphitized carbon black powder, was used as the adsorbent. Beginning with a theoretical equation for the second gas–solid virial coefficient, it was shown how a quantitative structure–retention relation (QSRR) could be developed to correlate ln B2s and hence chromatographic retention times with calculated molar refractivity and connectivity index values for the thiols and thioethers. It was found that both the gas–solid interaction energies and the ln B2s values could be correlated with calculated adsorbate molar refractivity (r2=0.951) and (r2=0.961), respectively. Connectivity index and molar refractivity together provide a r2=0.989 correlation of the ten ln B2s values at 403 K. A set of 373 organic compounds with retention indices taken from the literature was used to further test our approach. The 373 molecules were divided into 10 structural subgroups and molar refractivities and connectivity indices were calculated and used to correlate their retention index values. An overall correlation of r2=0.982 for the retention indices of the 373 molecules was found.

Correlation of thallium-201 uptake with proliferating cell nuclear antigen in brain tumours.

The aim of this study was to assess the correlation between thallium-201 (201Tl) uptake and proliferative activity measured using the proliferating cell nuclear antigen labelling index (PCNA-LI) in brain tumours. Twenty-nine patients with brain tumours were included in this study. Of these, seven were diagnosed with meningioma, 13 had high grade glioma (HGG) and nine had low grade glioma (LGG). A 210Tl single photon emission computed tomography study was performed on all patients before operation, and 201Tl uptake index (UI) and retention index (RI) values were calculated. Cell proliferation was determined by PCNA. While all of the HGGs and meningiomas showed intense 201Tl accumulation on visual interpretation, eight of the nine LGGs did not show accumulation of 210Tl at the tumour site. 201Tl UI values were 3.23+/-0.89 and 2.67+/-0.66 in HGG, 1.27+/-0.18 and 1.23+/-0.09 in LGG, and 4.35+/-1.60 and 2.52+/-0.78 in meningioma on early and delay images, respectively. There was a statistical difference between the 201Tl UI in HGG and LGG. PCNA-LI values were 16.72+/-6.15%, 1.63+/-0.81% and 2.00+/-1.88% in HGG, LGG and meningioma, respectively. The PCNA-LI in HGG was significantly higher than in LGG and meningioma. While the correlation coefficient between the 201Tl UI and the PCNA-LI was 0.94 in gliomas (n=22), there was no correlation in meningiomas. No statistically significant correlation was found between the RI and the PCNA-LI in gliomas. The presence of a strong positive correlation between 201Tl uptake and PCNA-LI indicates that 201Tl uptake can predict the proliferative activity of the glioma.

Determination of vanillin in orange, grapefruit, tangerine, lemon, and lime juices using GC-olfactometry and GC-MS/MS.

The presence of vanillin in orange, tangerine, lemon, lime, and grapefruit juices has been identified and confirmed using high-resolution GC retention index values, mass spectra, and aroma quality. The impact of vanillin on the flavor score for grapefruit juice is discussed and reported to be minimal. Vanillin concentrations are determined to be in the low parts-per-million range for the various citrus juices. The calculated concentrations in the orange, tangerine, lemon, lime, and grapefruit juices are 0.20, 0.35, 0.41, 0.35, and 0.60 ppm, respectively. Pasteurization produced an average 15% increase in the concentration of vanillin in grapefruit juices. Vanillin did not correlate well with the overall flavor score despite a rather intense signal using OSME gas chromatography-olfactometry software.

Mono-, di- and trimethyl-branched alkanes in cultures of the filamentous cyanobacterium Calothrix scopulorum

The extractable lipids of the cyanobacterium Calothrix scopulorum contain a complex suite of mono-, di- and trimethylheptadecanes. We identified 21 isomers; most of them have not been reported previously. The major compounds are 4,13-, 5,13-, and 4,5-dimethyl-, and 4,5,13- and 4,5,14-trimethylheptadecane. Methyl substitution occurs preferentially, but not exclusively, at positions 4 and 5 (or ω-4 and ω-5) at either end of the carbon chain. In mass spectra, the location of methyl groups at opposite ends of the carbon chain can be deduced from the presence of a characteristic secondary CnH2n−1+ fragment ion. The interpretation of mass spectra was confirmed by calculated retention index values which closely match the measured data. Our data provide evidence that branched alkanes with 17 to 20 carbon atoms can be used as biomarkers for cyanobacterial contributions to sedimentary organic matter.

Comparison of Retention Indices of Some Explosives and Related Compounds Calculated by Different Mathematical Methods in Reversed-Phase Liquid Chromatography

In reversed phase liquid chromatography, the retention indices of some neutral and acidic explosives and related compounds (nitramines, nitroaromatics, aminoaromatics, and nitrophenols) based on the alkan-2-one, alkyl aryl ketone, and 1-nitroalkane retention index standards have been determined by the application of a new mathematical adaptation method, viz. a multiparametric least-squares regression iterative method. This method was applied to two types of columns. The first group includes six octadecyl-C18 columns with different packing materials and obtained from different manufacturers, while the second group comprises one octyl-C8 column. The retention indices have been extensively studied using either methanol-water or methanol-phosphate buffer mobile phase systems. The calculated multiparametric retention indices values were compared with those obtained by Guardino's, Grobler's, and Kovats' methods. The influences of the concentration of the organic modifier in the mobile phase, aqueous mobile phase pH, and the column packing material on the retention indices of the explosives were also investigated. Good agreement was observed between the retention indices calculated by the use of the four mathematical methods for both neutral and acidic explosives.

Independent of local properties mathematical models for the calculation of retention indices in programmed temperature gas chromatography

Retention indices of some phthalates separated in temperature programming on SE-30 packed column were calculated by smoothing calibration data with Bezier curves, and from 2 to 6 order B-splines. The values thus obtained were compared to the corresponding ones calculated in a classical way. Whatever the standard n-alkane mixtures used (homologous series, alternate members with even or odd carbon atoms, any mixture with consecutive members not exceeding 4 carbon atoms between each two) the B-splines interpolations lead to retention indices values in better agreement with these ones, although Bezier curve smoothing still leads to values more consistent with the scheme of retention indices. Referring the phthalates to n-alkane stan - dard mixtures, with consecutive members not exceeding 3 or 4 carbon atoms between each 2, connecting the calibration data by B-splines, with orders from 2 to 5, and selecting the set of retention indices corresponding to the smallest value of the sum of squared second divided differences one may recover, with a good accuracy, classical programmed retention indices.

Gas chromatography–atomic emission detection for quantification of polycyclic aromatic sulfur heterocycles

Gas chromatography–microwave induced plasma atomic emission detection (GC–MIP–AED) was used for analysis for 34 tri-, tetra-, penta- and hexacyclic aromatic sulfur heterocycles (PASHs). Responses on the AED sulfur trace were investigated with regard to concentration linearity and to dependence on type of compound. Retention indices were estimated using a retention index system for polycyclic aromatic compounds. Linear response to sulfur was found from 100 pg to 90–160 ng of analyte for all PASHs. Compound-independent calibration with a randomly selected PASH reference substance yielded deviations in response between 4% and 12%. Retention index values were found to depend on the gas chromatographic settings applied.

Applications of an HPLC-DAD drug-screening system based on retention indices and UV spectra.

An analytical database, using high-performance liquid chromatography (HPLC) with diode array ultraviolet (UV) detection, of over 250 toxicologically relevant drugs has been extensively applied to both clinical and forensic toxicology. This general drug screening system, based on a mixed-phase (octadecyl/cyanopropyl) column and gradient reversed-phase HPLC, can identify a wide range of basic, acidic, and neutral drugs and metabolites, some of which are not amenable to gas chromatography and thin-layer chromatography. Compounds are identified using both retention index (RI) value (calculated by interpolation between a series of reference drug markers) and UV spectral data. It has been previously shown that this chromatographic system provides long-term reproducibility and has potentially useful selectivity differences compared with those based on octadecylsilane columns. Development has been undertaken to improve the speed and practicality of the system for emergency toxicology screening by increasing the rate of the elution gradient, while maintaining the applicability of the RI database. The database has also been found to be an important tool in determining optimum strategies for the adaptation of the system for quantitative analyses of drugs and metabolites under isocratic conditions.

Gas chromatographic retention index system for polychlorinated biphenyls: possibilities and limitations

The identification of individual polychlorinated biphenyls (PCBs) congeners would be easier if a proper reference series were available to calculate retention indices when using electron capture detectors, which are insensitive to the n-alkanes. In a previous work, a mixture of PCBs showing a linear dependence of retention on the number of chlorine atoms in both isothermal and programmed temperature analysis was suggested for reference purposes. Retention index values referred to this series, I PCB, can be calculated and used for identification. In this work a wide collection of columns was tested in order to check the linearity of the proposed reference series. The effect of the initial temperature and heating rate of the programmed runs on the reproducibility of the results was investigated. Retention data of the congeners belonging to the reference series are given together with retention indices of toxic PCB congeners calculated for various columns. The inversion of the elution order or the shift of retention values on columns of different polarities are shown.

Development of a high-performance liquid chromatography retention index scale for toxicological drug screening

An efficient and practical analytical method for correcting HPLC retention data has been produced using an HPLC diode-array UV detector system. The system is based on retention indices (RI) and is to be used primarily for the identification of toxicologically relevant drugs involved in clinical and forensic toxicology. The RI correction method was chosen as it provided a slightly greater degree of reproducibility than using relative retention time (RRT), particularly for acidic and neutral drugs. Development of the system involved the establishment of the optimal chromatographic conditions and extensive studies of the stability of the system. An acetonitrile gradient elution was used with RI values determined by interpolation from a series of specifically chosen basic and acidic/neutral marker drugs which eluted at regular intervals to produce a linear RI scale. It was found that two separate RI scales were required for basic and acidic/neutral drugs. The use of multiple drug markers as primary retention index standards had not been previously applied to HPLC general drug screening and comparison with a recently published database suggests that the system may also provide improved selectivity.

Predictive strategies for determining retention indices of some allylic alcohols and their esters by gas chromatography

Improved structure retention index relationships were used to predict retention index values for some allylic alcohols and unsaturated esters on polar and non-polar stationary phases. Using this method, we were also able to establish group retention factors, for these systems. In these computations, it was found that our predicted retention index values had percentage differences to within ±3.00% of the observed retention index in more than 95% of the experiments performed. This reconfirmed the applicability of the retention index increment methodology. The importance of n–π orbital overlap of lone pairs of electrons and C=C bonds in these molecules was also shown to be a crucial factor in establishing the strength of intra- and intermolecular interactions between solute and stationary phases.

Determination of retention indices of polychlorobiphenyls by using other compounds detectable by electrondashcapture detection or selected polychlorobiphenyls as the reference series

The highest sensitivity to polychlorobiphenyls (PCBs) is offered by electrondashcapture detection (ECD), but the easy identification of individual congeners remains unresolved. Published retention data are of limited usefulness, as the lack of a uniform style and the dependence on the analysis conditions do not permit their general use. Retention index values of PCBs cannot be easily referred to ndashalkanes, due to the very low response of these compounds to ECD, whereas other well detected reference series can be used: ndashalkyl iodides were found useful for the determination of retention indices of the first congeners of the PCB series. Retention indices obtained by using selected DCMA (Dry Colour Manufacturers' Association) PCB reference mixtures can be directly used or converted to standard Kováts indices. A mixture of PCBs that shows a linear dependence of retention on the number of chlorine atoms on both isothermal and programmed-temperature analysis is suggested for reference purposes. Other methods and standard mixtures used for identification and quantitation purposes are reviewed and discussed.

Comparability of RP-HPLC Retention Indices of Drugs in Three Databases

Abstract Three databases of HPLC retention indices in the 1-nitroalkane scale were recently established (IFM = Institute of Forensic Medicine, MCL = Microchemistry Laboratory, MTSS = Merck Tox Screening System). In two of them (IFM and MTSS) virtually identical chromatographic conditions were applied but different RP-columns. In the third system (MCL) a different column and different elution conditions were used. Among a total of 1149 drugs examined in all three labs, 258 drugs were common for IFM and MTSS, 179 were common for MCL and MTSS, 162 were common for IFM and MCL, and 135 drugs were common for all three databases. Good agreement of retention index values in IFM and MTSS was demonstrated (r2 = 0.9778). The correlations between IFM and MCL and MTSS and MCL data were less strong (r2 = 0.9245 and 0.9339, respectively), due to some deviating results. These deviations concerned mainly substances with pKa values between 1–4 and were probably caused by the differences in the pH of the two mobile phases. ...

Kovats Retention Indices of Halocarbons on a Hexafluoropropylene Epoxide-Modified Graphitized Carbon Black

Kovats retention indices of 97 halocarbons related to research on alternative refrigerants, propellants, foaming agents, and blowing agents have been measured on a packed column stationary phase consisting of a 5% (mass/mass) coating of a low molecular weight polymer of hexafluoropropylene epoxide on graphitized carbon black. The measurements on each fluid have been made at four temperatures, and the thermal dependence of the indices has been modeled with appropriate equations. The modeled values are suitable for the identification of these compounds by gas chromatography on both laboratory and field instrumentation. The values are also useful for the optimization of more sophisticated analyses needed in specific situations. The results indicate that the stationary phase chosen will provide separation of nearly all the fluids of interest. Moreover, we show that there is sufficient spread in the retention index values to facilitate fluid identification. The measurements also appear to fit a qualitative triangular property diagram that has been useful for classifying alternative refrigerant fluids and related compounds.

Prediction of programmed temperature retention indices on capillary columns of different polarities

Abstract Open tubular capillary columns filled with bonded liquid phases of different polarities were used for the separation of substituted aromatic compounds (chlorobenzenes and chlorophenols). The retention index values obtained during programmed temperature analyses were predicted by starting from retention data obtained under isothermal conditions. The calculation method previously used for the prediction of programmed temperature retention times was modified and simultaneously applied to the sample mixture and to the reference n-alkanes series. Experimental retention index values measured in different temperature programming conditions were found to agree with the calculated ones.

Ethyl glucuronide: an unusual ethanol metabolite in humans. Synthesis, analytical data, and determination in serum and urine.

Ethyl glucuronide (ethyl beta-D-6-glucosiduronic acid), a minor ethanol metabolite in serum or urine, was determined by gas chromatography-mass spectrometry. Prior to this, ethyl glucuronide was synthesized by the reaction of acetobromo-glucosiduronic acid with ethanol. For the determination of ethyl glucuronide, serum samples were precipitated with acetone, and urine specimens were analyzed after evaporation to dryness. The residues were derivatized with acetic anhydride. Capillary gas chromatography was used to find a retention index value of 1920 for the triacetyl derivative. The mass spectrum of the acetylated ethyl glucuronide was recorded. The calibration is linear in the range investigated (0.1-150 mg/L), and the detection limit is 0.1 mg/L. In individual specimens containing between 0.1 and 4 g ethanol per liter serum, ethyl glucuronide could be detected at concentrations between 3 and 14 mg/L and in the corresponding urine specimens at concentrations between 3 and 130 mg/L.

Sorption characteristics ofN-alkylimidazoles under conditions of capillary chromatography

The retention indices forN-n-alkyl (C1-C5) substituted imidazoles with Me, Et, Pr, and Bu groups in different positions of the cycle were determined. Two capillary columns with OV-101/KF and PEG-40M/KF were used. The two nitrogen atoms of the imidazole molecule were shown to have different effects on the contributions of the alkyl groups to the retention indices. The maximum and minimum contributions are observed for the substituents at the 5 and 4 positions of the imidazole cycle, respectively. An increase in a size of the CmH2m+1 substituent attached to the N(1) atom has a notable effect only on the contributions of the alkyl groups at the 2 and 5 positions. The retention indices values for a homologous series with ann-alkyl group attached to the N or C(2) atom were described by a universal type equation. The obtained equations can be used for predicting the retention indices of new homologs and identification ofN-alkylimidazoles in complex mixtures.

Influence of biological matrix on chromatographic behavior and detection of selected acidic, neutral, and basic drugs examined by means of a standardized HPLC-DAD system.

Human serum, autopsy blood, and liver samples were extracted by means of liquid-liquid extraction at acidic and at basic pH and at basic pH following acid hydrolysis. Six selected basic and six selected acidic or neutral drugs were added to the extracts. The spiked and blank samples were examined by means of reversed-phase high-performance liquid chromatography (HPLC) under standardized conditions using diode array detection. The retention index (RI) values of drugs were determined using a 1-nitroalkane scale and secondary standards. The RI values were virtually unaffected by biological matrix. On the other hand, the detection of some substances may be impaired by matrix peaks occurring regularly in biological extracts. The retention index values and UV spectra of three identified (phenethylamine, tryptamine, and indole) and nine unidentified substances were included in an HPLC database.