Temperature-programmed Retention Indices Research Articles

Modeling the effect of temperature on solid-phase microextraction of volatile organic compounds from air by polydimethylsiloxane coating using finite element analysis

A development of analytical methods based on solid-phase microextraction (SPME) is a very time- and labor-consuming task. The finite element methods have found a wide application in SPME modeling for faster and more accurate optimization of analytical methods. In this work, a computational model for predicting the effect of temperature on extraction of VOCs from air onto SPME coating based on polydimethylsiloxane (PDMS) has been developed using COMSOL Multiphysics® (CMP) software. Most suitable methods and models for estimating the diffusion coefficients of analytes in air and coating, and coating-air distribution constants of the analytes at different extraction temperatures were chosen. The Fuller method was chosen for calculating diffusion coefficients of analytes in air due to its simplicity and reliability. Coating-air distribution constants at different temperatures were estimated using van't Hoff equation. A combination of inverse gas chromatography on a capillary column with a similar stationary phase for estimating diffusion coefficients and linear temperature programmed retention indices (LTPRI) for estimating coating-air distribution constants at initial temperature were chosen for modeling purposes because in most cases it provided lowest values of root-mean-square difference from experimental extraction profiles from 125 mL bulb at 25 and 40 °C. The developed model can be recommended for faster and simpler optimization of the methods of air sampling using PDMS SPME fiber. It can also be used for obtaining extraction profiles at fluctuating temperatures.

Valorization of coffee silverskin industrial waste by pyrolysis: From optimization of bio-oil production to chemical characterization by GC × GC/qMS

Coffee bean tegument (silverskin) is a by-product derived from the coffee roasting process. This residue has been discarded, which constitutes a serious environmental problem. The depletion of oil reserves and environmental issues promoted by the combustion of fossil fuels has generated interest in the use of biomass to obtain fuel and chemicals products. The liquid product obtained by biomass pyrolysis is commonly called bio-oil. Bio-oil is a complex mixture of compounds, and its detailed chemical characterization is necessary to prospect its potential uses. Therefore, this study optimized the final temperature of pyrolysis and N2 flow rate parameters for the pyrolysis of silverskin in a fixed bed reactor, by using a central composite design and response surface. The bio-oil chemical composition was evaluated by comprehensive two-dimensional gas chromatography coupled to rapid-scanning quadrupole mass spectrometry (GC × GC/qMS), combined with the use of standards and linear temperature-programmed retention indices (LTPRI). The optimal values ​​calculated were 560 °C for the final pyrolysis temperature and 49 mL min−1 for the N2 flow rate. The organic phase yield was 15.2% under these conditions. At the optimal conditions, 228 compounds were identified (90.1% of the sample chromatographic volume) in the organic phase. The major chemical class, in terms of volume percentage, were the phenols (26.70%), followed by nitrogen compounds (18.51%). In addition, it is worth mentioning the high representability of the saturated hydrocarbons (8.28%), unsaturated aliphatic (6.69%), and aromatics (7.77%), which together account for 22.74% of the sample chromatographic volume. These results showed that the silverskin bio-oil may have the potential use as a source of chemical inputs.

Gas chromatographic-mass spectrometric determination of glycosides without prior hydrolysis

The article presents for the first time the linear temperature programmed retention indices on a column with stationary phases of 5% phenylpolydimethyl silicone and the mass spectra of trimethylsilyl (TMS) derivatives of 71 glycosides (both commercial preparations and compounds extracted from plant tissues) which were not characterized earlier by these parameters. Converted to their TMS derivatives, the glycosides were thermally stable: they exhibited single peaks on their chromatograms without products of thermal decomposition. Therefore this work demonstrates the suitability of high resolution–high temperature gas chromatography (HR-HT/GC) to analyse different groups of glycosides including compounds with disaccharide moieties without the necessity of their hydrolyses. Since a limited number of commercial and plant-isolated glycosides were available, an attempt was made to assess their retention indices using the known “structure–retention relationships” approach. It was demonstrated that the retention indices of silanised glycosides and their aglycones were characterized by a linear dependence.

Separation of C5-C7 hydrocarbon components on Al2O3 capillary column and its application

The separation and qualitative analysis of 54 common C5-C7 hydrocarbon components on three Al2O3 capillary columns, including S type, KCl type and M type, were investigated in detail. The results showed that the separation of the 54 C5-C7 hydrocarbon components were partly different on these three Al2O3 capillary columns, and the most of C5-C7 hydrocarbon components could be well separated, except that some of them couldn't achieve a baseline separation or co-eluted. Linear temperature programmed retention indices of the 54 C5-C7 hydrocarbon components, including 15 C5 hydrocarbons, 25 C6 hydrocarbons and 14 C7 hydrocarbons, were determined on these three Al2O3 capillary columns. The determination of linear temperature programmed retention indices for these C5-C7 hydrocarbon components on Al2O3 capillary columns provided a basis for the qualitative analysis of them. At the same time, a real pyrolysis gas sample from one of the petrochemical plants was qualitatively analyzed according to the linear temperature programmed retention indices, and the contents of C5-C7 hydrocarbon components were determined. The application field of Al2O3 capillary column was expanded and it is helpful for the analysis of light hydrocarbons in petrochemical enterprises.

Analysis of essential oil ofNardostachys chinensisBatal by GC-MS combined with chemometric techniques

The essential oil extracted from Nardostachys chinensis Batal (NCB) was analyzed by gas chromatography-mass spectrometry (GC-MS) combined with two chemometric resolution methods (CRM), heuristic evolving latent projections (HELP), and selective ion analysis (SIA). Qualitative analysis was performed by comparing the obtained pure mass spectra with those in National Institute of Standards and Technology (NIST) mass spectra database. Identification of some compounds was assisted by comparison of programmed temperature retention indices (PTRIs). The quantitative results were obtained by overall volume integration (OVI). A total of 69 compounds in the essential oil of N. chinensis Batal were identified, accounting for 93.98% of the total content. The major compounds were (−)-spathulenol, epiglobulol, trans-longipinocarveol, and patchouli alcohol which contribute to the antimicrobial and antioxidant activity. The results showed that the efficiency and reliability were greatly improved by use of chemometric tech...

Chemical fingerprinting of Su-He-Xiang-Wan and attribution of major characteristic peaks for its quality control by GC-MS

A simple and facile gas chromatography-mass spectrometer (GC-MS) fingerprint of Su-He-Xiang-Wan (SHXW) was developed, the similarity analysis was conducted, and attribution of the major characteristic peaks was identified for SHXW quality control. GC-MS analysis was performed on a QP2010 instrument (Shimadzu, Japan) equipped with a capillary column of RTX-5MS. The column temperature was initiated at 50 °C, held for 5 min, increased at the rate of 3 °C/min to 120 °C, held for 2 min, and then increased at the rate of 4 °C/min to 220 °C, held for 10 min. Helium carrier gas was used at a constant flow rate of 1.3 mL/min. Mass conditions were ionization voltage, 70 eV; injector temperature, 250 °C; ion source temperature, 250 °C; splitting ratio, 30:1; full scan mode in the 40–500 Da mass ranges with rate of 0.2 s per scan. Attribution of the major characteristic peaks was identified for SHXW by comparing the chemical standards, references of Chinese herbal medicines and the negative controls of prescription samples (NC) of SHXW. With the help of the temperature-programmed retention indices (PTRIs) used together with mass spectra and chemical standards, 25 major characteristic peaks have been identified. Nine volatile medicinal materials were identified in the prescription of SHXW by attributing to the 27 major characteristic peaks. The results demonstrate that the proposed method is a powerful approach to quality control of complex herbal medicines.

Gas Chromatographic and Mass Spectrometric Characterization of Trimethylsilyl Derivatives of Some Terpene Alcohol Phenylpropenoids

This paper reports mass spectra and linear temperature programmed retention indices (I(T)) for 31 esters of mono- and sesquiterpene alcohols with hydroxycinnamic acids. In this study, 14 phenylpropenoids were synthesized by esterification of terpenols with p-coumaric, ferulic, caffeic and sinapinic acids. Other phenylpropenoids were semi-quantitatively isolated by column chromatography from exudates covering the buds of two birch species (Betula pubescens and B. litwinowii). Main diagnostic ions in phenylpropenoids mass spectra were determined by GC/MS analysis. The possibility of predicting I(T) values was demonstrated with standard error of prediction between 3 and 11 of retention index units.

Detection of extraction artifacts in the analysis of honey volatiles using comprehensive two-dimensional gas chromatography

Extraction using headspace solid phase microextraction (HS-SPME) coupled to comprehensive two-dimensional gas chromatography with flame ionisation detection (GC×GC–FID) was employed to evaluate the effect of SPME fractionation conditions (heating time and temperature) on the generation of artifacts. The occurrence of artifacts was more pronounced at higher fractionation temperatures and times which caused significant changes in the chromatographic profiles. The identification of the volatile fraction of the honey blend was performed through a two-dimensional gas chromatograph coupled to a mass spectrometer with time of flight analyser (GC×GC–ToFMS) by comparing the first dimension linear temperature programmed retention index (1D-LTPRI) with the peak’s identities provided by the mass spectral similarity search. Several artifacts were found and identified – such as hydroxymethylfurfural, methyl–furone and furfural – and some of them were not previously detected as such in honey samples. These compounds were either the result of hydrolysis or thermal decomposition of components already present in the honey samples. This occurrence was attributed to the increased detectability provided by GC×GC compared to conventional GC. The possible emergence of previously unknown extraction artifacts as a general tendency related use of GC×GC instead of conventional GC is discussed as a result of these observations.

Botanical and Geographical Origin Characterization of Polish Honeys by Headspace SPME-GC× GC-TOFMS

Volatile organic compounds (VOCs) composition of Polish honeys obtained from various geographical regions was studied. Headspace solid phase microextraction (HS-SPME) using Divinylbenzene/Carboxen/Polydimethylsiloxane-coated fibers was used in combination with comprehensive two-dimensional gas chromatography–time–of–flight mass spectrometry (GC× GC–TOFMS) because of the complexity of the samples. Acacia, linden, rapeseed, buckwheat and honeydew honeys were studied in this project.A total of 329 compounds were identified during the investigations. Positive identification of 82 compounds was achieved using analytical standards, while tentative identification of 247 compounds was based on comparison of deconvoluted mass spectra and linear temperature programmed retention indices (LTPRI) with NIST 2005 library and literature values, respectively. Many VOCs identified in Polish honey samples were commonly present in honeys from other geographical regions of Europe (e.g. Italy, Corsica). They included certain acyclic and cyclic alkanes, acyclic and cyclic alkenes, aromatic hydrocarbons, oxygenated aromatic compounds, alcohols, aldehydes, ketones, esters, ethers, nitriles, organic sulphides, phenolic compounds and terpenes. GC× GC–TOFMS profiles for the same types of honey samples were remarkably similar. Eight botanic discriminants were selected among all compounds. Their identity was confirmed by comparison with authentic standards. Furthermore, possible geographical discriminants for Polish honeys were identified, including 19 for acacia, 3 for linden, 3 for rapeseed and buckwheat and 6 for honeydew honeys. Keywords: Discriminants, honey, HS–SPME/GC× GC–TOFMS, origin traceability, volatile organic compounds.

Analysis of volatile components in herbal pair Semen Persicae‐Flos Carthami by GC‐MS and chemometric resolution

Analysis of volatile components in herbal pair (HP) Semen Persicae-Flos Carthami (SP-FC) was performed by GC-MS coupled with chemometric resolution method (CRM). Furthermore, temperature-programmed retention indices were used together with mass spectra for identification of the volatile components. With the help of CRM, the two-dimensional data obtained from GC-MS instruments were resolved into a pure chromatogram and a mass spectrum of each chemical compound. By use of these methods upon two-dimensional data, 26, 49, and 59 volatile chemical components in essential oils of single herb Semen Persicae, Flos Carthami, and HP SP-FC were determined qualitatively and quantitatively, accounting for 78.42, 81.08, and 82.48% total contents of essential oil of single herb Semen Persicae, Flos Carthami, and HP SP-FC, respectively. It is shown that the accuracy of qualitative and quantitative analysis can be enhanced greatly by means of CRM. It is further demonstrated that the numbers of volatile chemical components of HP SP-FC are almost the addition of those of two single herbs, but the main volatile chemical components of the former are completely different from those of single herb Semen Persicae or Flos Carthami because of chemical reactions and physical changes occurring in the process of decocting two single herbs. This means that chemical components especially pharmacologically active compounds in the recipe might be different from those of single herbs.

Analysis of Essential Oils and Fatty Acids from Platycodi Radix using Chemometric Methods and Retention Indices

The chemical composition of the essential oils and fatty acids among nine groups of Platycodi Radix in China was analyzed by gas chromatography-mass spectrometry. Complicated components were resolved using chemometric methods. Simultaneously, the various features among heuristic evolving latent projections, selective ion analysis and alternative moving window factor analysis were compared by using some experimental data. Temperature-programmed retention indices were applied in further identification of the chemical composition of the essential oils. The equivalent chain length, fraction chain length, and an established special retention indices library integrated with mass spectrometry were also applied to further identify the composition of fatty acids, including total fatty acids, esterified fatty acids and free fatty acids. A total of 121 different compounds accounting for 95.12-98.74% were identified among the essential oils. Chemical polymorphisms and variation existed in the essential oils of Platycodi Radix. Sixteen components were identified in fatty acids, and linoleic acid (18:2n-6c) and other unsaturated acid possess a characteristic majority.

Determination of flavor components of rice bran by GC-MS and chemometrics

The flavor composition of rice bran was investigated using solid-phase microextraction (SPME), which was selected from four extraction methods. Additionally, static headspace extraction (SHS), accelerated solvent extraction (ASE) and simultaneous distillation extraction (SDE), followed by GC-MS analysis with the help of heuristic evolving latent projections (HELP) were also used. The effects of the most important factors, including fiber coating, extraction time, and temperature of SPME, on the flavor components of rice bran were studied. Qualitative analysis of the flavor components was obtained by a mass spectra similarity search using pure mass spectra resolved by HELP with the aid of automated mass spectral deconvolution and identification system (AMDIS) software and temperature-programmed retention indices (PTRIs), while quantitative analysis was conducted using the overall volume integration (OVI) technique. A total of 43 out of 76 compounds were tentatively identified, accounting for 82.76% of the total flavor compounds. The flavor compounds were mainly esters, alkanes, alcohols, ketones, aldehydes, and fatty acids, with a composition of 22.24%, 22.16%, 17.75%, 9.06%, 5.72%, and 4.18%, respectively. Together, these results indicate that analyzing the rice bran flavor profile may be more reasonable than solely monitoring free fatty acids for understanding, evaluating and controlling the instability of rice bran.

Modeling of Programmed-Temperature Retention Indices of a Diverse Set of Natural Compounds by Subspace Orthogonal Projection

Modeling of Programmed-Temperature Retention Indices of a Diverse Set of Natural Compounds by Subspace Orthogonal Projection

Simultaneous sampling and analysis of indoor air infested with Cimex lectularius L. (Hemiptera: Cimicidae) by solid phase microextraction, thin film microextraction and needle trap device

Air in a room infested by Cimex lectularius L. (Hemiptera: Cimicidae) was sampled simultaneously by three different sampling devices including solid phase microextraction (SPME) fiber coatings, thin film microextraction (TFME) devices, and needle trap devices (NTDs) and then analyzed by gas chromatography–mass spectrometry (GC–MS). The main focus of this study was to fully characterize indoor air by identifying compounds extracted by three different microextraction formats and, therefore, perform both the device comparison and more complete characterization of C. lectularius pheromone. The NTD technique was capable of extracting both (E)-2-hexenal and (E)-2-octenal, which were previously identified as alarm pheromones of bedbugs, and superior NTD recoveries for these two components allowed reliable identification based on mass spectral library searching and linear temperature programmed retention index (LTPRI) technique. While the use of DVB/CAR/PDMS SPME fiber coatings provided complementary sample fingerprinting and profiling results, TFME sampling devices provided discriminative extraction coverage toward highly volatile analytes. In addition to two alarm pheromones, relative abundances of all other analytes were recorded for all three devices and aligned across all examined samples, namely, highly infested area, less infested area, and control samples which were characterized by different bedbug populations. The results presented in the current study illustrate comprehensive characterization of infested indoor air samples through the use of three different non-invasive SPME formats and identification of novel components comprising C. lectularius pheromone, therefore, promising future alternatives for use of potential synthetic pheromones for detection of infestations.

HS-SPME/GC–MS analysis of volatile and semi-volatile organic compounds emitted from municipal sewage sludge

The aim of the research involved identification and semi-quantitative determination of unknown volatile and semi-volatile organic compounds emitted to air by sewage sludge formed in the process of municipal wastewater treatment in a sewage treatment plant. Samples taken directly after completion of the technological process as well as the sludge stored on the premise of the sewage treatment plant were analyzed. A simple method using off-line headspace solid-phase microextraction combined with gas chromatography-mass spectrometry has been proposed for extraction and detection of organic pollutants. For reliable identification of compounds, combination of two independent parameters: mass spectra and linear temperature programmed retention indices were employed. Over 170 compounds of different structure were identified including aliphatic and aromatic hydrocarbons, alcohols, esters, carbonyls, as well as sulfur, nitrogen, and chlorine containing compounds. The prevailing substances included: ethyl ether, n-hexane, p-xylene, o-xylene, mesitylene, m-ethylbenzene, limonene, n-decane, n-undecane, and n-dodecane. A few compounds such as methanetiol, dimethyl polisulfide, octaatomic sulfur, phthalic anhydride, and indoles were identified in the sludge for the first time.

Reproducibility of Programmed-Temperature Retention Indices under Average Linear Velocity Carrier Gas Control of GC and GC–MS

Programmed-temperature retention indices (PTRIs) are useful for the identification and quantification of chemicals by means of GC and GC/MS. To obtain reproducible PTRIs, we studied the influence of average linear velocity and column length under the constant average linear velocity control (CVC) using 55 pesticides and 33 fatty acid methyl esters. The PTRIs decreased with increasing average linear velocity, and the degree of decrease differs with the compounds. The PTRIs, however, were almost constant as long as the same average linear velocity was used even if the column length was changed. In addition, the correlation between the variation ratio in PTRI and the increase in average linear velocity was linear. Using the linear relationship between PTRIs and average linear velocities under CVC, highly reproducible PTRIs were obtained even if the capillary column length changes. Furthermore, the PTRIs determined by GC with FID, ECD, and FPD under atmospheric pressure were applied to GC/MS under vacuum or vice versa. From these results, it was confirmed that because CVC provides reproducible retention indices, CVC is advantageous for qualitative and quantitative analysis by GC and GC/MS.

Polydimethylsiloxane-based permeation passive air sampler. Part I: Calibration constants and their relation to retention indices of the analytes

A simple and cost effective permeation passive sampler equipped with a polydimethylsiloxane (PDMS) membrane was designed for the determination of time-weighted average (TWA) concentrations of volatile organic compounds (VOCs) in air. Permeation passive samplers have significant advantages over diffusive passive samplers, including insensitivity to moisture and high face velocities of air across the surface of the sampler. Calibration constants of the sampler towards 41 analytes belonging to alkane, aromatic hydrocarbon, chlorinated hydrocarbon, ester and alcohol groups were determined. The calibration constants allowed for the determination of the permeability of PDMS towards the selected analytes. They ranged from 0.026cm2min−1 for 1,1-dichloroethylene to 0.605cm2min−1 for n-octanol. Further, the mechanism of analyte transport across PDMS membranes allowed for the calibration constants of the sampler to be estimated from the linear temperature programmed retention indices (LTPRI) of the analytes, determined using GC columns coated with pure PDMS stationary phases. Statistical analysis using Student's t test indicated that there was no significant difference at the 95% probability level between the experimentally obtained calibration constants and those estimated using LTPRI for most analyte groups studied. This correlation allows the estimation of the calibration constants of compounds not known to be present at the time of sampler deployment, which makes it possible to determine parameters like total petroleum hydrocarbons in the vapor phase.

Comparative analysis of volatile constituents between herbal pair flos lonicerae-caulis lonicerae and its single herbs

Gas chromatography-mass spectrometry (GC-MS) and the chemometric resolution method (alternative moving window factor analysis, AMWFA) were used for comparative analysis of volatile constituents in herbal pair (HP) flos lonicerae-caulis lonicerae (FL-CL) and its single herbs. The temperature-programmed retention index (PTRI) was also employed for the identification of compounds. In total, 44, 39, and 50 volatile chemical components in volatile oil of FL, CL and HP FL-CL were separately determined qualitatively and quantitatively, accounting for 87.22%, 94.54% and 90.08% total contents of volatile oil of FL, CL and HP FL-CL, respectively. The results show that there are 32 common volatile constituents between HP FL-CL and single herb FL, 33 common volatile constituents between HP FL-CL and single herb CL, and 10 new constituents in the volatile oil of HP FL-CL.

Determination of Essential Oil Composition from Osmanthus fragrans Tea by GC-MS Combined with a Chemometric Resolution Method

The essential oils of the Osmanthus fragrans tea (OFT), Osmanthus fragrans (OF) and green tea (GT) had been extracted by steam distillation and analyzed by gas chromatography-mass spectrometry (GC-MS) with the help of heuristic evolving latent projections (HELP), an effective chemometric resolution method (CRM). The overlapping peak clusters were resolved into pure chromatograms and pure mass spectra with HELP. The qualitative analysis was performed by similarity searches in the National Institute of Standards and Technology (NIST) mass spectra database with the obtained pure mass spectrum of each component. Identification of some compounds was also assisted by comparison of temperature-programmed retention indices (PTRI) with authentic standards included in our own laboratory database under construction. The quantitative results were obtained by overall volume integration (OVI) method. A total of 67, 73 and 53 components in essential oils of the OFT, OF and GT were identified, accounting for 90.83%, 93.65% and 89.97% total contents of the essential oil of OFT, OF and GT, respectively.

GC–MS Combined with Chemometrics for Analysis of the Components of the Essential Oils of Sweet Potato Leaves

Essential oils from the leaves of two cultivars of sweet potato have been investigated by gas chromatography–mass spectrometry with the help of two chemometric resolution methods, heuristic evolving latent projections and selective ion analysis, and use of temperature-programmed retention indices. The overall volume integration technique was used for quantitative analysis. Eighty-four and forty-five components, accounting for 89.39 and 98.08% of the oils, were tentatively identified in the two cultivars. Thirty-four components were identical in both cultivars. Major constituents of Shuiguo were germacrene D (21.83%), germacrene B (15.17%), caryophyllene (12.44%), and n-hexadecanoic acid (11.24%). Caryophyllene (28.73%), γ-muurolene (13.07%), and β-caryophyllene epoxide (9.04%) were the major components of Xiangshu-17. These results indicate the method is suitable for analysis of the two-dimensional data obtained from essential oils.