Programmed-temperature gas chromatographic retention index

Leucas aspera (Willd.) Link Essential Oil from India: β-Caryophyllene and 1-Octen-3-ol Chemotypes

Leucas aspera (Willd.) Link (Lamiaceae) is an annual, branched herb used in traditional medicine as an antipyretic and insecticide. The hydro-distilled essential oil was obtained from the aerial parts of L. aspera growing wild in North West Karnataka region of India and analyzed by gas chromatography equipped with flame ionization detector and gas chromatography coupled with mass spectrometry. Forty-three compounds were identified, representing 98.1% of the total oil. The main constituents were identified as β-caryophyllene (34.2%), 1-octen-3-ol (14.8%), α-humulene (6.3%), α-pinene (5.8%), epi-α-bisabolol (4.6%) and limonene (4.5%). The oil was found to be rich in sesquiterpene hydrocarbons (47.7%), followed by others (long chain hydrocarbons (LCH), oxygenated LCH and phenyl derivative constituents) (20.2%), monoterpene hydrocarbons (14.8%), oxygenated sesquiterpenes (14.8%) and oxygenated monoterpenes (0.6%) type compounds.

What experimental factors influence the accuracy of retention projections in gas chromatography-mass spectrometry?

Programmed-temperature gas chromatographic (GC) retention information is difficult to share because it depends on so many experimental factors that vary among laboratories. Though linear retention indexing cannot properly account for experimental differences, retention times can be accurately calculated, or "projected", from shared isothermal retention vs. temperature (T) relationships, but only if the temperature program and hold-up time vs. T profile produced by a GC is known with great precision. The effort required to measure these profiles were previously impractical, but we recently showed that they can be easily back-calculated from the programmed-temperature retention times of a set of 25 n-alkanes using open-source software at www.retentionprediction.org/gc. In a multi-lab study, the approach was shown to account for both intentional and unintentional differences in the temperature programs, flow rates, and inlet pressures produced by the GCs. Here, we tested 16 other experimental factors and found that only 5 could reduce accuracy in retention projections: injection history, exposure to very high levels of oxygen at high temperature, a very low transfer line temperature, an overloaded column, and a very short column (≤15m). We find that the retention projection methodology acts as a hybrid of conventional retention projection and retention indexing, drawing on the advantages of both; it properly accounts for a wide range of experimental conditions while accommodating the effects of experimental factors not properly taken into account in the calculations. Finally, we developed a four-step protocol to efficiently troubleshoot a GC system after it is found to be yielding inaccurate retention projections.

A practical methodology to measure unbiased gas chromatographic retention factor vs. temperature relationships

Compound identification continues to be a major challenge. Gas chromatography-mass spectrometry (GC-MS) is a primary tool used for this purpose, but the GC retention information it provides is underutilized because existing retention databases are experimentally restrictive and unreliable. A methodology called "retention projection" has the potential to overcome these limitations, but it requires the retention factor (k) vs. T relationship of a compound to calculate its retention time. Direct methods of measuring k vs. T relationships from a series of isothermal runs are tedious and time-consuming. Instead, a series of temperature programs can be used to quickly measure the k vs. T relationships, but they are generally not as accurate when measured this way because they are strongly biased by non-ideal behavior of the GC system in each of the runs. In this work, we overcome that problem by using the retention times of 25 n-alkanes to back-calculate the effective temperature profile and hold-up time vs. T profiles produced in each of the six temperature programs. When the profiles were measured this way and taken into account, the k vs. T relationships measured from each of two different GC-MS instruments were nearly as accurate as the ones measured isothermally, showing less than two-fold more error. Furthermore, temperature-programmed retention times calculated in five other laboratories from the new k vs. T relationships had the same distribution of error as when they were calculated from k vs. T relationships measured isothermally. Free software was developed to make the methodology easy to use. The new methodology potentially provides a relatively fast and easy way to measure unbiased k vs. T relationships.

Airborne antituberculosis activity of Eucalyptus citriodora essential oil

The rapid emergence of multi- and extensively drug-resistant tuberculosis (MDR/XDR-TB) has created a pressing public health problem, which mostly affects regions with HIV/AIDS prevalence and represents a new constraint in the already challenging disease management of tuberculosis (TB). The present work responds to the need to reduce the number of contagious MDR/XRD-TB patients, protect their immediate environment, and interrupt the rapid spread by laying the groundwork for an inhalation therapy based on anti-TB-active constituents of the essential oil (EO) of Eucalyptus citriodora. In order to address the metabolomic complexity of EO constituents and active principles in botanicals, this study applied biochemometrics, a 3-D analytical approach that involves high-resolution CCC fractionation, GC-MS analysis, bioactivity measurements, and chemometric analysis. Thus, 32 airborne anti-TB-active compounds were identified in E. citriodora EO: the monoterpenes citronellol (1), linalool (3), isopulegol (5), and α-terpineol (7) and the sesquiterpenoids spathulenol (11), β-eudesmol (23), and τ-cadinol (25). The impact of the interaction of multiple components in EOs was studied using various artificial mixtures (AMxs) of the active monoterpenes 1, 2, and 5 and the inactive eucalyptol (33). Both neat 1 and the AMx containing 1, 2, and 33 showed airborne TB inhibition of >90%, while the major E. citriodora EO component, 2, was only weakly active, at 18% inhibition.

"Retention projection" enables reliable use of shared gas chromatographic retention data across laboratories, instruments, and methods

Gas chromatography/mass spectrometry (GC/MS) is a primary tool used to identify compounds in complex samples. Both mass spectra and GC retention times are matched to those of standards; however, it is often impractical to have standards on hand for every compound of interest, so we must rely on shared databases of MS data and GC retention information. Unfortunately, retention databases (e.g., linear retention index libraries) are experimentally restrictive, notoriously unreliable, and strongly instrument dependent, relegating GC retention information to a minor, often negligible role in compound identification despite its potential power. A new methodology called "retention projection" has great potential to overcome the limitations of shared chromatographic databases. In this work, we tested the reliability of the methodology in five independent laboratories. We found that, even when each lab ran nominally the same method, the methodology was 3-fold more accurate than retention indexing because it properly accounted for unintentional differences between the GC/MS systems. When the laboratories used different methods of their own choosing, retention projections were 4- to 165-fold more accurate. More importantly, the distribution of error in the retention projections was predictable across different methods and laboratories, thus enabling automatic calculation of retention time tolerance windows. Tolerance windows at 99% confidence were generally narrower than those widely used even when physical standards are on hand to measure their retention. With its high accuracy and reliability, the new retention projection methodology makes GC retention a reliable, precise tool for compound identification, even when standards are not available to the user.

Easy and accurate calculation of programmed temperature gas chromatographic retention times by back-calculation of temperature and hold-up time profiles

Linear retention indices are commonly used to identify compounds in programmed-temperature gas chromatography (GC), but they are unreliable unless the original experimental conditions used to measure them are stringently reproduced. However, differences in many experimental conditions may be properly taken into account by calculating programmed-temperature retention times of compounds from their measured isothermal retention vs. temperature relationships. We call this approach "retention projection". Until now, retention projection has been impractical because it required very precise, meticulous measurement of the temperature vs. time and hold-up time vs. temperature profiles actually produced by a specific GC instrument to be accurate. Here we present a new, easy-to-use methodology to precisely measure those profiles: we spike a sample with 25 n-alkanes and use their measured, programmed-temperature retention times to precisely back-calculate what the instrument profiles must have been. Then, when we use those back-calculated profiles to project retention times of 63 chemically diverse compounds, we found that the projections are extremely accurate (e.g. to ±0.9 s in a 40 min ramp). They remained accurate with different temperature programs, GC instruments, inlet pressures, flow rates, and with columns taken from different batches of stationary phase while the accuracy of retention indices became worse the more the experimental conditions were changed from the original ones used to measure them. We also developed new, open-source software (http://www.retentionprediction.org/gc) to demonstrate the system.

Chemical Composition and Antimicrobial Activity of the Essential Oils from Cleome Spinosa

Five different essential oil extractions of the aerial parts of Cleome spinosa Jacq. were examined. The oils obtained by hydrodistillation of the whole aerial parts, aerial parts without flowers (fruit, leaves and stem), flowers, fruits and leaves have been examined by GC-FID and GC-MS. The chemical profiles of the oils reveal the dominance of oxygenated sesqui- and diterpenes, with the exception of the fruit oil, which contained a high content of fatty acids. The most abundant compounds from the whole aerial parts were ( Z)-phytol (31.3%), integerrimine (5.5%) and incensole (4.0%). The major compounds from the aerial portion without flowers were caryophyllene oxide (10.5%), (-)-spathulenol (7.5%) and Z-phytol (6.9%). In the flower oil, the main components were 7-α-hydroxy manool (23.8%), incensole (9.2%) and sclareol (8.7%). The chief constituents in the fruit oil were tetradecanoic acid (40.6%), ( Z)-phytol (6.58%) and sclareol (4.5%). In the leaf oil, ( Z)-phytol (19.5%), 7-α-hydroxy manool (6.8%) and caryophyllene oxide (4.36%) were the predominant compounds. Antimicrobial activity of the oil obtained from the whole aerial part was evaluated against nine microbial strains using a filter paper disc-diffusion method. The volatile oil showed moderate action against seven of the eight bacteria strains used, with significant inhibitory activity against Streptococcus pyogenes Group A when compared with the standard antibiotics, ampicillin and gentamicin. The fungus, Candida albicans was less sensitive to the essential oil. The oils showed moderate insecticidal activity against Cylas formicarius elegantalus, but possessed no antioxidant activity as indicated by the DPPH method. This represents the first report on the chemical composition of the essential oils from C. spinosa found in Jamaica and the in vitro antioxidant, insecticidal and antimicrobial potential of the oil from the aerial parts.

Conversion of programmed-temperature retention indices from one set of conditions to another

In order to make programmed-temperature retention index (PTRI) data be shared by other chromatographers and laboratories, conversion of PTRI from one set of experimental conditions to another is investigated in detail in this work. It was found that the differences between the PTRIs at different heating rates are structurally dependent, especially the number of ring in molecules. Thus, with the help of molecule constitutional descriptors, equations of PTRI conversion to certain initial temperature, heating rate, and stationary phase were obtained with high correlation coefficients and low standard deviations. Calculation errors of PTRI conversion between different heating rates and between different initial temperatures were from 1.1 to 2.9 retention index units (i.u.), which is in the same order with experiment errors. It is well known that reproducibility of PTRI on a polar column is not as good as that on an apolar column because of the apolarity of the n-alkane homologues. Thus, topological descriptors were used for PTRI conversion between two columns with different polar stationary phases, giving better results than those obtained by constitutional descriptors. This shows that topological descriptors could provide more molecular structural information than constitutional descriptors. However, as constitutional descriptor has the advantages of clear physical meaning and very simple calculation, it is our first selection when the PTRI calculation accuracy is satisfied. The method developed is simple in calculation, easy to be performed with high accuracy.

Identification of degradation products of ibuprofen arising from oxidative and thermal treatments

Ibuprofen is a widely utilised analgesic anti-inflammatory drug. It is sensitive to oxidation and photodegradation. In this work, the oxidative and thermal degradations were investigated. The treatments adopted allowed the detection of 13 degradation products, seven of which have never been reported: hydratropic acid, 4-ethylbenzaldehyde, 4-(1-carboxyethyl)benzoic acid, 1-(4-isobutylphenyl)-1-ethanol, 2-[4-(1-hydroxy-2-methylpropyl)phenyl]propanoic acid, 1-isobutyl-4-vinylbenzene, 4-isobutylphenol. For 1-(4-isobutylphenyl)-1-ethanol, the in vitro toxic effects have already been described in the literature. To detect all degradation products, two RP-HPLC methods and a GC-MS procedure were developed or modified from the official monographs. The identification was conducted by evaluating chromatographic and spectral data and the structural attributions were confirmed by simple and univocal synthesis. Moreover, the actual presence of these molecules in marketed medicinal products was investigated.