Nanogram-scale preparation and NMR analysis for mass-limited small volatile compounds

[Progress in the application of preparative gas chromatography in separating volatile compounds]

Gas chromatography (GC) yields superior separations of low boiling point volatile compounds. Therefore, preparative gas chromatography (Prep GC) was established by combining analytical GC with a sample collection system at the end of the column, enabling the efficient isolation of the volatile components from complex matrices and their subsequent collection after GC. As Prep GC is based on an analytical gas chromatograph, its injection, separation, detection, and fraction collection systems are continuously optimized and upgraded to improve the recoveries and purities of the target compounds. Prep GC, in combination with modern spectroscopic techniques (such as UV-Vis absorption spectroscopy, infrared absorption spectroscopy, Raman spectroscopy, mass spectrometry, X-ray diffraction, and nuclear magnetic resonance spectroscopy), enables accurate structural elucidation of a target compound. Reports of the separations of various volatile components from complex matrices using Prep GC have recently increased annually, revealing promising application prospects. However, Prep GC also displays several disadvantages, such as the failure to separate thermolabile compounds, high separation costs, and the likely introduction of exogenous contamination. Based on the recent related research, this review summarizes the evolution of the structure of Prep GC and its application in isolating essential oil monomers, insect pheromones, volatile food and plant components, geological biomarkers, and persistent environmental pollutants. Finally, this review also summarizes and prospects the use of Prep GC in separating volatile components to provide a reference for the expansion of its applications.

Combining High-Resolution Gas Chromatographic Continuous Fraction Collection with Nuclear Magnetic Resonance Spectroscopy: Possibilities of Analyzing a Whole GC Chromatogram

This study presents, for the first time, the successful application of analyzing a whole gas chromatography (GC) chromatogram by nuclear magnetic resonance (NMR) spectroscopy using a continuous repeatable and stable (n = 280) high-resolution (HR) GC fractionation platform with a 96-well plate. Typically with GC– or liquid chromatography–mass spectrometry analysis, (isomer) standards and/or additional NMR analysis are needed to confirm the identification and/or structure of the analyte of interest. In the case of complex substances (e.g., UVCBs), isomer standards are often unavailable and NMR spectra too complex to achieve this. This proof of concept study shows that a HR GC fractionation collection platform was successfully applied to separate, purify, and enrich isomers in complex substances from a whole GC chromatogram, which would facilitate NMR analysis. As a model substance, a chlorinated paraffin (CP) mixture (>8,000 isomers) was chosen. NMR spectra were obtained from all 96 collected fractions, which provides important information for unravelling their full structure. As a proof of concept, a spectral interpretation of a few NMR spectra was made to assign sub-structures. More research is ongoing for the full characterization of CP isomers using multivariate statistical analysis. For the first time, up to only a few CP isomers per fraction were isolated from a highly complex mixture. These may be further purified and certified as standards, which are urgently needed, and can also be used for persistency, bioaccumulation, or toxicity studies.

Tools in the Investigation of Volatile Semiochemicals on Insects: From Sampling to Statistical Analysis

The recognition of volatile organic compounds (VOCs) involved in insect interactions with plants or other organisms is essential for constructing a holistic comprehension of their role in ecology, from which the implementation of new strategies for pest and disease vector control as well as the systematic exploitation of pollinators and natural enemies can be developed. In the present paper, some of the general methods employed in this field are examined, focusing on their available technologies. An important part of the investigations conducted in this context begin with VOC collection directly from host organisms, using classical extraction methods, by the employment of adsorption materials used in solid-phase micro extraction (SPME) and direct-contact sorptive extraction (DCSE) and, subsequently, analysis through instrumental analysis techniques such as gas chromatography (GC), nuclear magnetic resonance (NMR) and mass spectrometry (MS), which provide crucial information for determining the chemical identity of volatile metabolites. Behavioral experiments, electroantennography (EAG), and biosensors are then carried out to define the semiochemicals with the best potential for performing relevant functions in ecological relationships. Chemical synthesis of biologically-active VOCs is alternatively performed to scale up the amount to be used in different purposes such as laboratory or field evaluations. Finally, the application of statistical analysis provides tools for drawing conclusions about the type of correlations existing between the diverse experimental variables and data matrices, thus generating models that simplify the interpretation of the biological roles of VOCs.

Qualitative and quantitative analysis of chemicals emitted from the pheromone gland of individual Heliothis subflexa females

The chemicals emitted from the sex pheromone gland of individual Heliothis subflexa females were sampled using a short section of thick-film megabore fused silica capillary column, and the pheromone glands of the same females were extracted after the effluvia collection. Both samples were treated with a silylation reagent, and then subjected to gas chromatography-chemical ionization-mass spectrometry for quantitative and qualitative analysis of all components. The total amount of all 11 components emitted from the glands of calling females was 153 ng/female/hr, which was substantially higher than previously reported. The ratios of the pheromone components in the volatile emissions and pheromone gland extracts were generally similar to previous studies, but with notable differences. The collections of volatiles and gland extractions contained, respectively: Z9-14:Ald (1.57%, 1.35%), 14:Ald (3.78%, 1.51%), Z7 + Z9-16:Ald (9.60%, 3.59%), Z11-16:Ald (76.14%, 18.94%), 16:Ald (2.95%, 2.17%), Z9-16:OH (0.07%, 7.21%), Z11-16:OH (1.11%, 49.04%), Z7-16:OAc (0.48%, 1.73%), Z9-16:OAc (1.32%, 4.02%), and Z11-16:OAc (2.98%, 10.43%). The thick-film megabore column is an efficient approach for sampling the headspace for semiochemicals.

Alcohol Contributes to Attraction of Heliothis (= Chloridea) virescens Males to Females

Female-emitted volatile sex pheromones in most moths are composed of biosynthetically related blends of fatty acid derivatives, such as aldehydes, acetate esters and alcohols. In many moths, as in the noctuid Heliothis (Chloridea) virescens, the pheromone gland contains alcohols (e.g., (Z)-11-hexadecen-1-ol, hereafter Z11-16:OH) that may serve dual functions as pheromone components as well as precursors of other pheromone components. The relative importance of Z11-16:OH to male attraction in H. virescens has been controversial. It occurs in the pheromone gland in relatively large amounts, but several studies could neither detect Z11-16:OH in gland emissions nor attribute any conspecific behavioral function to it in flight- tunnel assays. Trapping assays in the field, however, have more consistently documented that the addition of Z11-16:OH increased trap catch. Using a short section of thick film megabore column, in combination with derivatization and GC-CI-SIM-MS, we determined that Z11-16:OH is emitted from the sex pheromone gland during calling. Field trapping studies demonstrated that trap catch increased when Z11-16:OH was added to a 2-component minimal blend and to a 6-component blend. Behavioral observations in the field confirmed that more males responded to a pheromone blend that contained a low blend ratio of Z11-16:OH, but ≥5% Z11-16:OH depressed both male behavior and trap catch. We conclude that Z11-16:OH should be considered a component of the sex pheromone of H. virescens females.

Improving the productivity of a multidimensional chromatographic preparative system by collecting pure chemicals after each of three chromatographic dimensions

The enhanced sample collection capability of a heart-cutting three-dimensional GC-prep system is reported. In its original configuration, a highly pure component can be usually collected after the last (3D) column outlet by means of a dedicated preparative station. The latter is located after the last chromatographic column, and this poses the requirement for multiple heart cuts even for those components showing satisfactory degree of purity after the first (or second) separation dimension. The feasibility to collect pure components after each chromatographic dimension is here described, employing a three-dimension MDGC system equipped with high-temperature valves, located inside the first and second GC ovens, with the aim to improve the productivity of the collection procedure. In addition to a commercial preparative collector located at the 3D outlet, two laboratory-made collection systems were applied in the first and second dimension, reached by the effluent to be collected trough a high-temperature valve switching the heart-cut fraction between either the detector (FID), or the collector. Highly pure sesquiterpene components were collected, namely: patchouli alcohol after the first column [poly(5% diphenyl/95% dimethylsiloxane)], α-bulnesene after a second column coated with high molecular weight polyethylene glycol, and α-guaiene after an ionic-liquid based column (SLB-IL60), used as the third dimension. Purity levels ranging from 85 to 95% were achieved with an average collection recovery of 90% (n=5). The following average amounts were collected per run: 160μg for α-guaiene, 295μg for α-bulnesene, and 496μg for patchouli alcohol.

Pollination by sexual deception-it takes chemistry to work

Semiochemicals are of paramount importance in sexually deceptive plants. These plants sexually lure specific male insects as pollinators by chemical and physical mimicry of the female of the pollinator. The strategy has evolved repeatedly in orchids, with a wide diversity of insect groups exploited. Chemical communication systems confirmed by field bioassays include: alkenes and alkanes in bee pollinated Ophrys species, keto-acid and hydroxy-acids in scoliid wasp pollinated O. speculum, and cyclohexanediones and pyrazines in thynnine wasp pollinated Chiloglottis and Drakaea orchids, respectively. In Ophrys, stearoyl-acyl carrier protein desaturase (SAD) enzymes have been confirmed to control species level variation in alkene double bond position. The production of cyclohexanediones in Chiloglottis unexpectedly depends on UVB light, a phenomenon unknown for other plant specialised metabolites. Potential biosynthetic pathways for other systems are explored, and alternative approaches to further accelerate chemical discovery in sexually deceptive plants are proposed.

Performance evaluation of a versatile multidimensional chromatographic preparative system based on three-dimensional gas chromatography and liquid chromatography-two-dimensional gas chromatography for the collection of volatile constituents

The present research deals with the multi-collection of the most important sesquiterpene alcohols belonging to sandalwood essential oil, as reported by the international regulations: (Z)-α-santalol, (Z)-α-trans bergamotol, (Z)-β-santalol, epi-(Z)-β-santalol, α-bisabolol, (Z)-lanceol, and (Z)-nuciferol. A versatile multidimensional preparative system, based on the hyphenation of liquid and gas chromatography techniques, was operated in the LC-GC-GC-prep or GC-GC-GC-prep configuration, depending on the concentration to be collected from the sample, without any hardware or software modification. The system was equipped with a silica LC column in combination with polyethylene glycol-poly(5% diphenyl/95% dimethylsiloxane)-medium polarity ionic liquid or β-cyclodextrin based GC stationary phases. The GC-GC-GC-prep configuration was exploited for the collection of four components, by using a conventional split/splitless injector, while the LC-GC-GC-prep approach was applied for three low abundant components (<5%), in order to increase the quantity collected within a single run, by the LC injection of a high sample amount. All target compounds, whose determination is hampered by the unavailability of commercial standards, were collected at milligram levels and with a high degree of purity (>87%).

Rapid isolation of high solute amounts using an online four-dimensional preparative system: normal phase-liquid chromatography coupled to methyl siloxane-ionic liquid-wax phase gas chromatography

This study reports the recent evolution of a multidimensional GC-GC-GC preparative system, now combined with an online LC preseparation step, operated under normal phase conditions. It is herein shown that the four-dimensional instrument can collect sample components with a concentration lower than 10%, in a short time period, while maintaining a high level of analyte purity. The LC dimension allows (I) the injection of higher sample amounts, compared to "direct" GC injection; (II) a polarity-based preseparation, leading to the GC injection of simplified subsamples, and thus reducing the possibility of coelutions; (III) to eliminate the essential-oil "matrix", replacing it with the LC mobile phase (the GC system is more protected from potential contamination); (IV) the LC mobile phase is of much lower viscosity with respect to a pure, or highly concentrated essential oil, avoiding difficulties in the syringe sample withdrawal process, prior to GC injection. System optimization was performed by using standard solutions; in addition, a very complex sample, namely, vetiver essential oil, was subjected to the preparative process, with the scope of isolating two low-amount constituents (namely, α-amorphene and β-vetivone). The latter two sesquiterpenoids, which accounted for 1.7 and 4.0% of the sample (considering the volatiles), respectively, were successfully collected at the milligram level, in a one-day work period, with a purity degree in excess of 90%.

Simple preparative gas chromatographic method for isolation of menthol and menthone from peppermint oil, with quantitative GC-MS and (1) H NMR assay

The quantitative performance of a simple home-built preparative gas chromatography (prep-GC) arrangement was tested, incorporating a micro-fluidic Deans switch, with collection of the target compound in a deactivated uncoated capillary tube. Repeat injections of a standard solution and peppermint sample were made into the prep-GC instrument. Individual compounds were eluted from the trapping capillary, and made up to constant volume. Chloronaphthalene internal standard was added in some cases. Recovered samples were quantitatively assayed by using GC-MS. Calibration linearity of GC-MS for menthol standard area response against number of injections (2-20 repeat injections) was excellent, giving R(2) of 0.996. For peppermint, menthol correlation over 2-20 repeated injections was 0.998 for menthol area ratio (versus IS) data. Menthone calibration for peppermint gave an R(2) of 0.972. (1) H NMR spectroscopy was conducted on both menthol and menthone. Good correspondence with reference spectra was obtained. About 80 μg of isolated menthol and menthone solute was collected over a sequence of 80 repeat injections from the peppermint sample, as assayed by 600 MHz (1) H NMR analysis (∼100% recovery for menthol from peppermint). A procedure is proposed for prediction of number of injections required to acquire sufficient material for NMR detection.

Unusual macrocyclic lactone sex pheromone of Parcoblatta lata, a primary food source of the endangered red-cockaded woodpecker

Wood cockroaches in the genus Parcoblatta, comprising 12 species endemic to North America, are highly abundant in southeastern pine forests and represent an important prey of the endangered red-cockaded woodpecker, Picoides borealis. The broad wood cockroach, Parcoblatta lata, is among the largest and most abundant of the wood cockroaches, constituting >50% of the biomass of the woodpecker's diet. Because reproduction in red-cockaded woodpeckers is affected dramatically by seasonal and spatial changes in arthropod availability, monitoring P. lata populations could serve as a useful index of habitat suitability for woodpecker conservation and forest management efforts. Female P. lata emit a volatile, long-distance sex pheromone, which, once identified and synthesized, could be deployed for monitoring cockroach populations. We describe here the identification, synthesis, and confirmation of the chemical structure of this pheromone as (4Z,11Z)-oxacyclotrideca-4,11-dien-2-one [= (3Z,10Z)-dodecadienolide; herein referred to as "parcoblattalactone"]. This macrocyclic lactone is a previously unidentified natural product and a previously unknown pheromonal structure for cockroaches, highlighting the great chemical diversity that characterizes olfactory communication in cockroaches: Each long-range sex pheromone identified to date from different genera belongs to a different chemical class. Parcoblattalactone was biologically active in electrophysiological assays and attracted not only P. lata but also several other Parcoblatta species in pine forests, underscoring its utility in monitoring several endemic wood cockroach species in red-cockaded woodpecker habitats.