Overview of sample introduction techniques prior to GC for the analysis of volatiles in solid materials

Sample preparation and introduction techniques are very critical steps in gas chromatography analysis and particularly in the analysis of volatiles in solid samples. In these cases, they can be divided into two main categories: direct and indirect approaches, based on how the solid sample is treated, i.e. with and without dissolution (or extraction) of analytes from the solid sample. To enable routine application, coupling with sample preparation techniques (especially solid or solvent‐based microextractions) is needed to achieve automation. Here, an overview of the most common sample introduction techniques for gas chromatography with their advantages and drawbacks is presented and discussed, including references to relevant examples. So, this review can serve as guidance for new users.     

An international round-robin study for the analysis of particulate semi-volatile organics by thermal desorption gas chromatography mass spectrometry

Thermal desorption gas chromatography mass spectrometry (TD-GC/MS) is becoming more commonly used for the quantification and identification of organic compounds in particulate matter (PM), including ambient and source PM such as diesel particulate matter (DPM). It has been proven as an alternative to the traditional solvent extraction (SE) method and liquid injection gas chromatograph mass spectrometry (LI-GC/MS). However, little information is available on how different types of TD-GC/MS systems compare to each other for analysis of real-world PM samples or to direct LI-GC/MS for analysis of PM components in a test solution. To address this, CanmetENERGY Characterization Laboratory initiated a round robin with the participation of 10 laboratories worldwide. Three sample types were analysed: (i) a test solution with a suite of pure compounds commonly found in PM, analysed by TD-GC/MS and LI-GC/MS; (ii) a DPM sample, analysed by TD-GC/MS and SE; and (iii) an ambient PM sample, analysed by TD-GC/MS. The first part of the study showed good overall performance and comparability between the different TD-GC/MS systems and LI-GC/MS method for the analysis of PM components in a test solution, with some variability of results due to system types and parameters used, concentration of calibration standards, and whether or not an internal standards was used. The analysis of the DPM sample showed greater variability between laboratories and methods as many PM components were present near the detection limit and matrix effects particularly affected the TD-GC/MS analysis of heavier n-alkanes. In the last part of the study, for the analysis of an ambient PM sample by TD-GC/MS, the analysis of variance showed good comparison between labs for polycyclic aromatic hydrocarbons (94% non-significant), but slightly lower for n-alkanes (68%) and biomarkers (57%).     

Sample introduction in high speed capillary gas chromatography; input band width and detection limits

The speed of analysis in capillary gas chromatography can be substantially increased by reduction of the column inner diameter. However, special demands are then posed upon instrumental design. In particular, the sampling system is highly critical because it has to be capable of delivering extremely small injection band widths which must be compatible with the column inside diameter. This study focuses on the evaluation of two potentially suitable sample introduction systems with respect to input band width and detection limits and their compatibility with small bore (≦ 100 μm) columns in capillary gas chromatography. One of them allows liquid on-column injection, based on liquid splitting, of only a few nl onto small bore (≦ 100 ?m) fused silica columns. For gases, input band widths as low as 1 ms are obtained with this system. The other one is part of a miniaturized gas chromatograph with extremely low dead volume interfaces and detector volumes. It allows input band widths for gases of a few ms. Without any preconcentration ppm concentrations are measured in gaseous samples with a 80 ?m thick film capillary column. It will be shown that a further reduction of the minimum detectable amount and analysis time is possible with this equipment.     

Sensitive determination of polycyclic aromatic hydrocarbons in water samples using monolithic capillary solid-phase extraction and on-line thermal desorption prior to gas chromatography-mass spectrometry

A methacrylate-based monolithic capillary column has been evaluated for the preconcentration of polycyclic aromatic hydrocarbons (PAHs) from environmental water samples. For this purpose, the monolyte was in situ synthesized in a 6cm×0.32mm id fused-silica capillary. The microextraction unit was fitted to a micro-HPLC pump to pass 10mL of sample. The isolated pollutants were eluted by means of 10μL of methanol, the organic phase being directly collected in a specific interface that can be fitted to the injection port of the gas chromatograph without modification. The interface allows the on-line thermal desorption of the PAHs, avoiding the dilution and providing enough sensitivity to reach the legal limits established for these pollutants in the matrices selected. The limits of detection achieved for 10mL of water ranged between 2.8ng/L (indeno(1,2,3-cd)pyrene) and 11.5ng/L (acenaphthene) with acceptable precision (between 4.5 and 18.2% RSD). The method was applied to the determination of the selected PAHs in tap, river waters and sewage, being fluoranthene and pyrene detected in all of them at concentrations lower than the legal limits established for these compounds in the matrices assayed.     

How to count strong adsorption sites by gas chromatography?

Summary A simple gas chromatographic method is presented for the determination of the quantity of the strongest adsorption sites on an adsorbent's surface. The method consists of the blockage of the sites with quasi-irreversibly adsorbed, known amount of a strongly interacting compound and subsequent measuring of the retention of a hydrocarbon during the presence of the blocking compound in the column. Heterogeneity of chromatographic grade silicas is investigated with this method.     

气相色谱法探究聚醚醚酮的热降解及热氧化降解（英文）

Poly(ether ether ketone)(PEEK)was synthesized via polycondensation of hydroquinone with 4,4′-difluorobenzophenone at 320℃for 5 h.Thermal and thermo-oxidative degradation of PEEK was studied over a wide range of temperatures.In an inert medium,decomposition of the polymer occurred in one stage,with the formation of a coke residue accounting for approximately 50%of the original polymer mass.In air,the mass loss curve exhibited two distinct stages.The first stage involved breakdown of the main polymer chain,the speed of which indicated a radical chain failure mechanism.In the second stage,the rate of mass loss clearly decreased,indicating a transition from the radical chain failure mechanism to simple combustion reactions(wherein the polymer combusted completely).To further investigate the nature of the processes occurring during the pyrolysis of PEEK,the investigations were carried out using gas chromatograph under isothermal conditions.It was concluded that during thermal degradation,the decomposition of the polymer starts with the rupture of ketone and ether bonds and proceeds to destruction of the benzene ring at higher temperatures,which is accompanied by the formation of H2O and CH4.Above 500℃,the polymer degradation further involved thermohydrolysis.The thermo-oxidation of PEEK,which was accompanied mainly by the formation ofH2,was noticeable beginning at 325℃.The total yield of the latter indicated oxidation of fragments of the benzene ring.     

Development of a new pyrolyzer for thermal desorption and/or pyrolysis gas chromatography of polymeric materials

A new vertical microfurnace-type pyrolyzer for thermal desorption and/or pyrolysis-gas chromatography has been developed. The pyrolyzer consists of two independent temperature-controlled ovens. Initially, in the desorption process, a sample cup containing the polymeric sample of interest is inserted into an oven at 300°C; the sample is then re-positioned at the upper part of the pyrolyzer where the temperature is maintained at room temperature. The resulting vaporized components such as residual solvents and additives give a desorption chromatogram. The relative peak intensities of desorbed plasticizers in acrylonitrile butadiene-rubber gave a relative standard deviation (RSD) of less than 2%. Subsequently, pyrolysis of the remaining polymer is conducted by dropping the sample cup into the second, pyrolyzing, oven at 55°C; at this stage the pyrogram is recorded. The resulting two chromatograms of desorbed components and pyrolysis products make it easier to characterize the polymer formulation than the complicated pyrogram obtained by an ordinary, single-step pyrolysis.     

Characterization of polymers by multi-step thermal desorption/programmed pyrolysis gas chromatography using a high temperature PTV injector

Thermal treatment hyphenated with gas chromatography is a versatile and powerful tool in the study of polymer characterization. An inexpensive system where thermal treatment at different temperatures occurs inside a Programmable Temperature Vaporization injector (PTV) is described. The samples investigated, commercial plastics, are complex mixtures that contain several polymers and additives. These plastics as well as their pure constituents are subjected to multi-step thermal treatment. The individual chromatograms of the various constituents of the polymeric sample are correlated with those of the final material in order to identify additives (thermal desorption) and degradation products (pyrolysis). Results obtained with the new method indicate the interesting potentials of the technique for the characterization of polymer compositions. Reproducibility of absolute and relative peak areas has been considered and found to be acceptable. The absence of a heated transfer line and switching valves, which are always present in conventional set-ups, eliminates the risk of losses of high molecular weight components. Further advantages of the technique proposed are simplicity, versatility, and its inexpensive nature.     

Pyrolysis–gas chromatography/mass spectrometry of peroxynitrite-treated melanins

Peroxynitrite—a strong oxidizing and nitrating agent—is thought to be a mediator of the cytotoxic action of nitric oxide in biological systems. There is some evidence that peroxynitrite-dependent processes may be involved in the pathomechanism of Parkinson's disease. Recently, it has been suggested that neuromelanin, present in the cytoplasm of nigrostriatal neurons, can act as natural scavenger of peroxynitrite. In this study the effect of peroxynitrite on the structure of synthetic models of human neuromelanin was investigated. Model neuromelanins, obtained from dopamine (DA) and/or 5-S-cysteinyldopamine (5-S-CysDA), were pyrolyzed at 770 °C before and after treatment with peroxynitrite and the thermal degradation products separated by gas chromatography were identified by mass spectrometry. After exposure to peroxynitrite, reduced content of the typical pyrolysis products accompanied by elevated levels of low molecular weight gases in the pyrolysates were observed. The changes in pyrolytic patterns were dependent on melanin type and peroxynitrite concentration. It was concluded that interaction with peroxynitrite causes extensive oxidative degradation of the analyzed pigments. This may be of great biological importance with regard to depigmentation of nigrostriatal neurons demonstrated in Parkinson's disease.     

A combined application of thermal desorber and gas chromatography to the analysis of gaseous carbonyls with the aid of two internal standards

In this study, a series of GC calibration experiments were conducted to examine the feasibility of the thermal desorption approach for the quantification of five carbonyl compounds (acetaldehyde, propionaldehyde, butyraldehyde, isovaleraldehyde, and valeraldehyde) in conjunction with two internal standard compounds. The gaseous working standards of carbonyls were calibrated with the aid of thermal desorption as a function of standard concentration and of loading volume. The detection properties were then compared against two types of external calibration data sets derived by fixed standard volume and fixed standard concentration approach. According to this comparison, the fixed standard volume-based calibration of carbonyls should be more sensitive and reliable than its fixed standard concentration counterpart. Moreover, the use of internal standard can improve the analytical reliability of aromatics and some carbonyls to a considerable extent. Our preliminary test on real samples, however, indicates that the performance of internal calibration, when tested using samples of varying dilution ranges, can be moderately different from that derivable from standard gases. It thus suggests that the reliability of calibration approaches should be examined carefully with the considerations on the interactive relationships between the compound-specific properties and the operation conditions of the instrumental setups.     

Advances in the determination of volatile organic solvents and other organic pollutants by gas chromatography with thermal desorption sampling and injection

Summary The problem of the separation of 34 volatile organic chlorinated compounds is solved by using three different GC columns selected according to the needs of the particular separation required. The effect of water vapor contained as moisture in the trapped air on the retention of some characteristic compounds is studied. The influence of dead volumes on trap injection is also studied.Dedicated to Prof. Dr. A. Liberti on the occasion of his 70th birthday.     

Pervaporation prior to gas chromatography for the determination of enological parameters

Summary The use of pervaporation for the separation of volatile species from wine prior to their individual separation by gas chromatography-flame ionization detection (FID) is proposed for the first time. For this purpose, the upper chamber of the pervaporation unit is located in the loop of an HPLC injection valve. By changing the valve to the injection position the evaporated and diffused analytes are led by an helium stream to the chromatographic column for individual separation and subsequent quantitation of the target analytes by FID. The method thus developed was applied to the determination of acetaldehyde, ethyl acetate, methanol and ethanol in wines. Agreement with the recommened methods was obtained in all instances.     

Thermal degradation mechanisms of liquid crystalline aromatic polyesters studied by pyrolysis–gas chromatography/mass spectrometry

The thermal degradation mechanisms of liquid crystalline aromatic polyesters (LCPs) prepared from p-hydroxybenzoic acid (PHB), biphenol (BP), and terephthalic acid (TA) were studied by pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS). The LCP containing deuterated terephthalate units and the LCPs that have different comonomer ratios were examined. On the basis of the pyrolysis products determined, the origin of the main pyrolysis products (benzene, phenol, biphenyl, phenyl benzoate, etc.) from the corresponding comonomer units were estimated and their thermal degradation mechanisms were eventually discussed in detail.     

Indirect hydrogen analysis by gas chromatography coupled to mass spectrometry (GC–MS)

Gas chromatography (GC) is an analytical tool very useful to investigate the composition of gaseous mixtures. The different gases are separated by specific columns but, if hydrogen (H₂) is present in the sample, its detection can be performed by a thermal conductivity detector or a helium ionization detector. Indeed, coupled to GC, no other detector can perform this detection except the expensive atomic emission detector. Based on the detection and analysis of H₂ isotopes by low‐pressure chemical ionization mass spectrometry (MS), a new method for H₂ detection by GC coupled to MS with an electron ionization ion source and a quadrupole analyser is presented. The presence of H₂ in a gaseous mixture could easily be put in evidence by the monitoring of the molecular ion of the protonated carrier gas. Copyright © 2013 John Wiley & Sons, Ltd.     

Operational theory and instrumental implementation of the thermal gradient programmed gas chromatography (TGPGC) mode of analysis

Thermal gradient programmed gas chromatography (TGPGC) separations involve the application of three-dimensional thermal field programming. The operational aspects associated with TGPGC are such that rapid GC analyses can be achieved for complex and broad-volatility-range samples using open tubular columns with high thermal compliance, physical compactness, and geometric conformity. Elapsed times for TGPGC analyses of complex samples have ranged from 50 to 300 s. The application of rapidly changing thermal fields (or temperature surfaces) in TGPGC is accomplished through the use of a column sheath assembly. Recent research has centered upon different column sheath assembly designs and instrumental implementation of the TGPGC mode of operation. The instrumental implementation of the TGPGC mode for conducting GC and GC-MS analyses places special demands upon the design and performance of numerous operational aspects of the system, e.g., the response-time characteristics associated with analyte detection and the analytical output signal handling capabilities.     

Study of modified silicas by inverse gas chromatography. Influence of chain length on the conformation of n-alcohols grafted on a pyrogenic silica

Summary A fumed silica surface was systematically modified by the grafting of n-alkyl chains with increasing carbon numbers. The samples were characterized by the dispersive component of the surface energy, their specific interaction potential and enthalpies of adsorption of polar, in particular, alcohol probes. It is shown that the variation of the surface properties and adsorption capacities depend on the chain length of the graft. For instance, minimum values are recorded when the surface coverage by methylene groups corresponds either to one or two CH₂ surface layers. This behaviour is related to the mobility of the grafted alkyl chains, mobility which was examined by solid state NMR.     

Development of multi‐response optimization and quadratic calibration curve for determination of ten pesticides in complex sample matrices using QuEChERS dispersive liquid–liquid microextraction followed by gas chromatography

In this study, QuEChERS combined with dispersive liquid‐liquid microextraction is developed for extraction of ten pesticides in complex sample matrices of water and milk. In this regard, effective factors of proposed extraction technique combined with gas chromatography with flame ionization detector were designed, modeled, and optimized using central composite design, multiple linear regression, and Nelder–Mead simplex optimization. Later, univariate calibration model for ten pesticides was developed in concentration range of 0.5–100 ng/mL. Surprisingly, quadratic calibration behavior was observed for some of the pesticides. In this regard, Mandel's test was used for evaluating linearity and types of calibration equation. Finally, four pesticides followed linear calibration curve with sensitivity (0.23–0.66 mL/ng), analytical sensitivity (0.20–0.32), regression coefficient (0.988–0.995), limit of detection (0.39–1.83 ng/mL), and limit of quantitation (1.30–6.10 ng/mL) and six of them followed quadratic calibration curve with sensitivity (0.18–0.93 mL/ng), analytical sensitivity (0.25–0.86), regression coefficient (0.944–0.999), limit of detection (0.59–1.92 ng/mL), and limit of quantitation (1.96–6.40 ng/mL). The calculated limits of detection were below the maximum residue limits according to European Union pesticides database of European Commission. Finally, the proposed analytical method was used for determination of ten pesticides in water and milk samples.     

Quantitation in capillary gas chromatography with emphasis on the problems of sample introduction

Sampling techniques for practical quantitative capillary GC have to meet certain principal requirements. Both the absolute and the relative peak areas (e.g. column loads) must be reproducible with high precision and at high accuracy; discrimination of certain constituents according to their volatility should not take place on sampling. On the basis of systematic studies, the three most reliable sampling techniques used for GC analyses with the aim of achieving precise and accurate quantitative data proved to be the following: On-column, injection, splitless PTV injection, and an optimized version of split sampling called “cooled needle split” injection. The on-column technique can be optimized by using precolumns with wider internal diameters and without stationary phase coatings to overcome the problems of large liquid sampling volumes and for automation. The PTV technique should only be used in the splitless mode because discrimination cannot be suppressed completely with the split mode. All three of the techniques can be operated automatically, either to avoid “human interference”, i.e. to improve precision or for unattended operation to save man-power.