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We present a new chromatogram decomposition method for Gas Chromatography (GC) which represents a chromatogram as a sum of template functions inspired by the analytic solution of mass balance equation. The proposed method starts by approximating GC response of a single gas by these template functions. Consequently, it utilizes the temporal translation and dilation of this approximate response to approximate GC responses of other gases of interest. The results are demonstrated on lab data using calibration bottles containing mixtures of C 1 - C 5. Correlation of the amplitudes of the decomposed responses and injected concentrations indicates linear calibration curves are sufficient to estimate C 1 - C 5 concentrations. The performance of the method is demonstrated by a ratio test where a calibration bottle with C 1 concentration 300 times larger than C 2 and C 3 concentrations is injected into GC. • Expressing response of a GC system to a single gas using functions inspired by the analytic solution of a mass balance equation. • Approximating responses of a GC system to other gases as an affine transformed version of response of the single gas. • Calibration and subsequent quantitative analysis with an application for sensitivity analysis. [ABSTRACT FROM AUTHOR]

Highlights • 701 olive oil samples were differentiated depending on their quality. • GC-IMS data were subjected to two chemometric approaches to classify olive oils. • A non-targeted fingerprinting approach and a targeted one based on markers were used. • In general the markers approach is more suitable for implementation in the industry. • Calibration models from 2014/15 required recalibration to predict oils from 2015/16. Abstract The dual separation in gas chromatography-ion mobility spectrometry generates complex multi-dimensional data, whose interpretation is a challenge. In this work, two chemometric approaches for olive oil classification are compared to get the most robust model over time: i) an non-targeted fingerprinting analysis, in which the overall GC-IMS data was processed and ii) a targeted approach based on peak-region features (markers). A total of 701 olive samples from two harvests (2014–2015 and 2015–2016) were analysed and processed by both approaches. The models built with data samples of 2014–2015 showed that both approaches were suitable for samples classification (success >74%). However, when these models were applied for classifying samples from 2015–2016, better values were obtained using markers. The combination of data from the two harvests to build the chemometric models improved the percentages of success (>90%). These results confirm the potential of GC-IMS based approaches for olive oil classification. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Polar VOC trap to selective infield detection of ethylene from apple headspace. • Pt-nanoparticle- based EC sensor & trap would replace the need for GC validation. • Pulsed mode to improve the reproducibility for an electrochemical sensor. • Ethephon chemistry as a replacement to bottled gas providing in-field calibration. Abstract Ethylene, which regulates fruit ripening processes in some species, is a molecule of central interest to postharvest researchers and technologists. There is a need for a portable, inexpensive, accurate, and repeatable sensor for infield or storeroom measurements. To help ensure repeatable results, we developed a field calibration protocol for a portable electrochemical gas sensor that combines ethephon (2-chloroethylphosphonic acid) with a buffering agent to release a controlled amount of ethylene gas (1–4 μL/L). The ethephon reaction provides an economical source of standard gas for device calibration. This work addresses device selectivity issues through the use of an inline aqueous trap to remove polar volatile organics (50% reduction, as reported by an electrochemical sensor), interfering with the ethylene detection. A complementary gas chromatographic (GC) assay provided an independent validation of these results. A post-harvest study of ethylene production of apples as detected by the electrochemical sensor illustrates significant improvements in the accuracy and precision of these measurements. [ABSTRACT FROM AUTHOR]

Gas chromatography (GC) hyphenated with nitrogen chemiluminescence detection (NCD) and quadrupole time-of-flight mass spectrometry (QTOFMS) was applied for the first time to the quantitative analysis of new psychoactive substances (NPS) in urine, based on the N-equimolar response of NCD. A method was developed and validated to estimate the concentrations of three metabolites of the common stimulant NPS α-pyrrolidinovalerophenone (α-PVP) in spiked urine samples, simulating an analysis having no authentic reference standards for the metabolites and using the parent drug instead for quantitative calibration. The metabolites studied were OH-α-PVP (M1), 2″-oxo-α-PVP (M3), and N,N-bis-dealkyl-PVP (2-amino-1-phenylpentan-1-one; M5). Sample preparation involved liquid-liquid extraction with a mixture of ethyl acetate and butyl chloride at a basic pH and subsequent silylation of the sec-hydroxyl and prim-amino groups of M1 and M5, respectively. Simultaneous compound identification was based on the accurate masses of the protonated molecules for each compound by QTOFMS following atmospheric pressure chemical ionization. The accuracy of quantification of the parent-calibrated NCD method was compared with that of the corresponding parent-calibrated QTOFMS method, as well as with a reference QTOFMS method calibrated with the authentic reference standards. The NCD method produced an equally good accuracy to the reference method for α-PVP, M3 and M5, while a higher negative bias (25%) was obtained for M1, best explainable by recovery and stability issues. The performance of the parent-calibrated QTOFMS method was inferior to the reference method with an especially high negative bias (60%) for M1. The NCD method enabled better quantitative precision than the QTOFMS methods To evaluate the novel approach in casework, twenty post- mortem urine samples previously found positive for α-PVP were analyzed by the parent calibrated NCD method and the reference QTOFMS method. The highest difference in the quantitative results between the two methods was only 33%, and the NCD method's precision as the coefficient of variation was better than 13%. The limit of quantification for the NCD method was approximately 0.25μg/mL in urine, which generally allowed the analysis of α-PVP and the main metabolite M1. However, the sensitivity was not sufficient for the low concentrations of M3 and M5. Consequently, while having potential for instant analysis of NPS and metabolites in moderate concentrations without reference standards, the NCD method should be further developed for improved sensitivity to be more generally applicable. [ABSTRACT FROM AUTHOR]

The present paper is focused on the use of a vacuum ultraviolet absorption spectrometer (VUV) for gas chromatography (GC), within the context of flow modulated comprehensive two-dimensional gas chromatography (FM GC × GC). The features of the VUV detector were evaluated through the analysis of petrochemical and fatty acids samples. Besides responding in a predictable fashion via Beer’s law principles, the detector provides additional spectroscopic information for qualitative analysis. Virtually all chemical species absorb and have unique gas phase absorption features in the 120–240 nm wavelength range monitored. The VUV detector can acquire up to 90 full range absorption spectra per second, allowing its coupling with comprehensive two-dimensional gas chromatography. This recent form of detection can address specific limitations related to mass spectrometry ( e.g., identification of isobaric and isomeric species with very similar mass spectra or labile chemical compounds), and it is also able to deconvolute co-eluting peaks. Moreover, it is possible to exploit a pseudo-absolute quantitation of analytes based on pre-recorded absorption cross-sections for target analytes, without the need for traditional calibration. Using this and the other features of the detector, particular attention was devoted to the suitability of the FM GC × GC-VUV system toward qualitative and quantitative analysis of bio-diesel fuel and different kinds of fatty acids. Satisfactory results were obtained in terms of tailing factor (1.1), asymmetry factor (1.1), and similarity (average value 97%), for the FAMEs mixtures analysis. [ABSTRACT FROM AUTHOR]

In this study, adsorption isotherms of o-xylene, ethylbenzene and toluene liquid solutions on a commercial 13X zeolite adsorbent were measured using the gas chromatographic headspace technique. According to this technique, the vapor, in equilibrium with the liquid phase and solid adsorbent, is analyzed using a GC column. In the first part of the study, a novel technique was introduced based on liquid calibration curve procedure. In addition, the earlier introduced vapor calibration technique was further extended to measure single component isotherms of non-ideal liquid solutions. In this part of the study, both vapor and liquid calibration techniques were extended to measure liquid adsorption isotherms of the multi-component solutions involving both, ideal solutions e.g., aromatic sorbates, and non-ideal solutions, e.g., aromatic sorbates and n-decane as a solvent. Consequently, Raoult's law and extended Raoult's vapor-liquid equilibrium (VLE) law for calculations of ideal and non-ideal solutions were applied, respectively. Extended Wilson and NRTL models were used to determine the sorbates' activity coefficients in the non-ideal liquid solutions. The multicomponent isotherms involving both ideal and non-ideal liquid solutions were experimentally determined at 30, 60 and 90 °C and also predicted by the extended Sips adsorption model. Based on these results, the selectivity of the NaX zeolite towards different sorbates from the selected mixtures were determined at different temperatures and concentrations. [ABSTRACT FROM AUTHOR]

Gas chromatography coupled with high resolution time of flight mass spectrometry (GC–HR-TOFMS) has gained popularity for the target and suspect analysis of complex samples. However, confident detection of target/suspect analytes in complex samples, such as produced water, remains a challenging task. Here we report on the development and validation of a two stage algorithm for the confident target and suspect analysis of produced water extracts. We performed both target and suspect analysis for 48 standards, which were a mixture of 28 aliphatic hydrocarbons and 20 alkylated phenols, in 3 produced water extracts. The two stage algorithm produces a chemical standard database of spectra, in the first stage, which is used for target and suspect analysis during the second stage. The first stage is carried out through five steps via an algorithm here referred to as unique ion extractor (UIE). During the first step the m / z values in the spectrum of a standard that do not belong to that standard are removed in order to produce a clean spectrum and then during the last step the cleaned spectrum is calibrated. The Dot-product algorithm, during the second stage, uses the cleaned and calibrated spectra of the standards for both target and suspect analysis. We performed the target analysis of 48 standards in all 3 samples via conventional methods, in order to validate the two stage algorithm. The two stage algorithm was demonstrated to be more robust, reliable, and less sensitive to the signal-to-noise ratio (S/N), when compared to the conventional method. The Dot-product algorithm showed lower potential in producing false positives compared to the conventional methods, when dealing with complex samples. We also evaluated the effect of the mass accuracy on the performances of Dot-product algorithm. Our results indicated the crucial importance of HR-MS data and the mass accuracy for confident suspect analysis in complex samples. [ABSTRACT FROM AUTHOR]

A novel method for rapid, quantitative determination of trace permanent gases and carbon dioxide in ultra-high purity ammonia by dual-channel two-dimensional GC-PDHID is presented. An improved matrix back-flush-to-vent approach combining back-flush column switching technique with auxiliary NaHSO 4 ammonia trap is described. The NaHSO 4 trap prevents traces of ammonia from entering the analytical column and is shown not to affect the impurity content of the sample. The approach allows shortening the analysis time and increasing the amount of measurements without extensive maintenance of the GC-system. The performance of the configuration has been evaluated utilizing ammonia- and helium-based calibration standards. The method has been applied for the analysis of 99.9999+% ammonia purified by high-pressure distillation at the production site. [ABSTRACT FROM AUTHOR]

We describe modified gas chromatography electron-impact/triple-quadrupole mass spectrometry (GC–EI/MS/MS) utilizing a newly developed hydrogen-injected self-cleaning ion source and modified 9 mm extractor lens. This instrument, with optimized parameters, achieves quantitative separation of 62 polycyclic aromatic hydrocarbons (PAHs). Existing methods historically limited rigorous identification and quantification to a small subset, such as the 16 PAHs the US EPA has defined as priority pollutants. Without the critical source and extractor lens modifications, the off-the-shelf GC–EI/MS/MS system was unsuitable for complex PAH analysis. Separations were enhanced by increased gas flow, a complex GC temperature profile incorporating multiple isothermal periods, specific ramp rates, and a PAH-optimized column. Typical determinations with our refined GC–EI/MS/MS have a large linear range of 1–10,000 pg μl −1 and detection limits of <2 pg μl −1 . Included in the 62 PAHs, multiple-reaction-monitoring (MRM) mode enabled GC-EI/MS/MS identification and quantitation of several constituents of the MW 302 PAH isomers. Using calibration standards, values determined were within 5% of true values over many months. Standard curve r 2 values were typically >0.998, exceptional for compounds which are archetypally difficult. With this method benzo[a]fluorene, benzo[b]fluorene, benzo[c]fluorene were fully separated as was benzo[b]fluoranthene, benzo[k]fluoranthene, and benzo[j]fluoranthene. Chrysene and triphenylene, were sufficiently separated to allow accurate quantitation. Mean limits of detection (LODs) across all PAHs were 1.02 ± 0.84 pg μl −1 with indeno[1,2,3-c,d] pyrene having the lowest LOD at 0.26 pg μl −1 and only two analytes above 2.0 pg μl −1 ; acenaphthalene (2.33 pg μl −1 ) and dibenzo[a,e]pyrene (6.44 pg μl −1 ). [ABSTRACT FROM AUTHOR]

Satureja hortensis L. and Oliveria decumbens Vent. are known for their diverse effects in drug therapy and traditional medicine. One of the most interesting properties of their essential oils is good antioxidant activity. In this paper, essential oils of aerial parts of S . hortensis L. and O. decumbens Vent. from different regions were obtained by hydrodistillation and were analyzed by gas chromatography–mass spectrometry (GC–MS). Essential oils were tested for their free radical scavenging activity using 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay to identify the peaks potentially responsible for the antioxidant activity from chromatographic fingerprints by numerous linear multivariate calibration techniques. Because of its simplicity and high repeatability, orthogonal projection to latent structures (OPLS) model had the best performance in indicating the potential antioxidant compounds in S . hortensis L. and O . decumbens Vent. essential oils. In this study, P -cymene, carvacrol and β-bisabolene for S . hortensis L. and P -cymene, Ç -terpinen, thymol, carvacrol, and 1,3-benzodioxole, 4-methoxy-6-(2-propenyl) for O . decumbens Vent. are suggested as the potentially antioxidant compounds. [ABSTRACT FROM AUTHOR]

SPME is extremely sensitive to experimental parameters affecting liquid–gas and gas–solid distribution coefficients. Our aims were to measure the weights of these factors and to design a multivariate strategy based on the addition of a pool of internal standards, to minimize matrix effects. Synthetic but real-like wines containing selected analytes and variable amounts of ethanol, non-volatile constituents and major volatile compounds were prepared following a factorial design. The ANOVA study revealed that even using a strong matrix dilution, matrix effects are important and additive with non-significant interaction effects and that it is the presence of major volatile constituents the most dominant factor. A single internal standard provided a robust calibration for 15 out of 47 analytes. Then, two different multivariate calibration strategies based on Partial Least Square Regression were run in order to build calibration functions based on 13 different internal standards able to cope with matrix effects. The first one is based in the calculation of Multivariate Internal Standards (MIS), linear combinations of the normalized signals of the 13 internal standards, which provide the expected area of a given unit of analyte present in each sample. The second strategy is a direct calibration relating concentration to the 13 relative areas measured in each sample for each analyte. Overall, 47 different compounds can be reliably quantified in a single fully automated method with overall uncertainties better than 15%. [ABSTRACT FROM AUTHOR]

This work demonstrates a novel method for rapid determination of total solid content in viscous liquid (polymer-enriched) samples. The method is based multiple headspace extraction gas chromatography (MHE-GC) on a headspace vial at a temperature above boiling point of water. Thus, the trend of water loss from the tested liquid due to evaporation can be followed. With the limited MHE-GC testing (e.g., 5 extractions) and a one-point calibration procedure (i.e., recording the weight difference before and after analysis), the total amount of water in the sample can be determined, from which the total solid contents in the liquid can be calculated. A number of black liquors were analyzed by the new method which yielded results that closely matched those of the reference method; i.e., the results of these two methods differed by no more than 2.3%. Compared with the reference method, the MHE-GC method is much simpler and more practical. Therefore, it is suitable for the rapid determination of the solid content in many polymer-containing liquid samples. [ABSTRACT FROM AUTHOR]

Highlights: [•] A GC method was developed to detect the viscosity of concentrated black liquors. [•] Methanol is used as the tracer and its release profile is measured by MHE-GC. [•] Two correlation approaches were established for the calibration method. [•] The presented method is simple and practical for black liquor viscosity testing. [Copyright &y& Elsevier]

Comprehensive two-dimensional gas chromatography coupled with a flame photometric detector (GC×GC-FPD) was developed for analysis of 16 organophosphorus pesticides (OPs) in food matrices. The column set, modulation period and modulator temperature were investigated in this study, and suitable conditions chosen. Three pairs of co-eluting OPs in 1D GC were baseline separated in GC×GC with a non-polar/polar column set (DB-5×DB-1701), a 4s modulation period, and modulator temperature of 50°C. Compared to GC-FPD, the identification of OPs is more accurate in GC×GC due to the additional identification parameter, 2 t R. Second dimension t R reproducibility is in the range 0–0.85% RSD in GC×GC-FPD. Excellent calibration graph linearities were obtained for 16 OPs over the concentration range from 5 to 1000μgL−1. The limits of detection were 1.5–5.6μgL−1 indicating significant improvement compared to those obtained in 1D GC. Six OPs were found at low concentrations (0.79–5.12μgkg−1) in 5 samples analysed in six different food matrices. The good reproducibility of parallel results showed that GC×GC-FPD for OP analysis in food matrices is a satisfactory and valid technology. [ABSTRACT FROM AUTHOR]

Abstract: High-purity carbon monoxide (CO) gas as a certified reference material (CRM) was analyzed using a micro gas chromatograph (micro-GC) with a micro thermal conductivity detector. The main reason to select the micro-GC was its compact size to provide a safe experimental environment. Thus, both the CO gas cylinder and the micro-GC were placed inside a draft shield with a limited small space. Peaks of several impurities were found in chromatograms of the micro-GC. The main impurities in the CO gas were helium, hydrogen, oxygen, nitrogen and carbon dioxide. It was found that helium was the impurity of the highest concentration. High-accuracy calibration gas mixtures were used to construct calibration lines for the micro-GC. The mixtures were precisely prepared by the gravimetric blending method. Calibration lines had enough linearity and accuracy for quantitative analyses of the impurities. The values of detection limit of the impurities were 0.7–4μmol/mol. The purity of the high-purity CO gas was around 99.996%, which was the value estimated from the sum of the concentrations of the impurities. [Copyright &y& Elsevier]

Abstract: A simple method for establishing calibration curves with sorbent-filled thermal desorption tubes has been demonstrated for nitroaromatic and nitramine vapor samples using a thermal desorption system with a cooled inlet system (TDS–CIS), which was coupled to a gas chromatograph (GC) with an electron capture detector (ECD). The method relies upon the direct liquid deposition of standard solutions onto the glass frit at the head of sorbent-filled thermal desorption tubes. Linear calibration results and ideal system conditions for the TDS–CIS–GC–ECD were established for mixtures containing both cyclotrimethylenetrinitramine, a.k.a. RDX, and 2,4,6-trinitrotoluene (TNT). Because of the chemical characteristics of RDX, a higher TDS–CIS flow rate relative to the optimized approach for TNT was required for efficient RDX desorption. Simultaneous quantitation of TNT and RDX using the direct liquid deposition method with optimized instrumentation parameters for RDX were compared to results from a standard split/splitless GC inlet and a CIS. [Copyright &y& Elsevier]

Abstract: In this study, two important sensorial parameters of beer quality – bitterness and grain taste – were correlated with data obtained after headspace solid phase microextraction – gas chromatography with mass spectrometric detection (HS-SPME–GC–MS) analysis. Sensorial descriptors of 32 samples of Pilsner beers from different brands were previously estimated by conventional quantitative descriptive analyses (QDA). Areas of 54 compounds systematically found in the HS-SPME–GC–MS chromatograms were used as input data. Multivariate calibration models were established between the chromatographic areas and the sensorial parameters. The peaks (compounds) relevant to build each multivariate calibration model were determined by genetic algorithm (GA) and ordered predictors selection (OPS), tools for variable selection. GA selected 11 and 15 chromatographic peak areas, for bitterness and grain taste, respectively; while OPS selected 17 and 16 compounds for the same parameters. It could be noticed that seven variables were commonly pointed out by both variable selection methods to bitterness parameter and 10 variables were commonly selected to grain taste attribute. The peak areas most significant to the evaluation of the parameters found by both variable selection methods fed to the PLS algorithm to find the proper models. The obtained models estimated the sensorial descriptors with good accuracy and precision, showing that the utilised approaches were efficient in finding the evaluated correlations. Certainly, the combination of proper chemometric methodologies and instrumental data can be used as a potential tool for sensorial evaluation of foods and beverages, allowing for fast and secure replication of parameters usually measured by trained panellists. [Copyright &y& Elsevier]

Abstract: The polar organic chemical integrative sampler (POCIS) was relatively recently developed to trap hydrophilic organic micropollutants in aquatic environments. The aim of this article is to compile information from numerous references involving the POCIS in order to discuss the evaluation of time-weighted average concentrations (e.g., calibration methods, sampling rates, performance and reference compounds) and to review critically the different in situ applications (e.g., screening or quantifying micropollutants, and coupling to toxicity tests), application domains (e.g., molecules analyzed, and sampling media) and analytical protocols (e.g., processing, analysis, and exposure duration). [Copyright &y& Elsevier]

Abstract: The aim of our study was to provide an innovative headspace-gas chromatography–mass spectrometry (HS-GC-MS) method applicable for the routine determination of blood CO concentration in forensic toxicology laboratories. The main drawback of the GC/MS methods discussed in literature for CO measurement is the absence of a specific CO internal standard necessary for performing quantification. Even if stable isotope of CO is commercially available in the gaseous state, it is essential to develop a safer method to limit the manipulation of gaseous CO and to precisely control the injected amount of CO for spiking and calibration. To avoid the manipulation of a stable isotope-labeled gas, we have chosen to generate in a vial in situ, an internal labeled standard gas (13CO) formed by the reaction of labeled formic acid formic acid (H13COOH) with sulfuric acid. As sulfuric acid can also be employed to liberate the CO reagent from whole blood, the procedure allows for the liberation of CO simultaneously with the generation of 13CO. This method allows for precise measurement of blood CO concentrations from a small amount of blood (10μL). Finally, this method was applied to measure the CO concentration of intoxicated human blood samples from autopsies. [Copyright &y& Elsevier]

Abstract: A simple and sensitive method for the determination of dimethylsulfide (DMS) and dimethylselenide (DMSe) employing a gas chromatograph–photoionization detector with headspace sampling (HS-GC-PID) was developed. Parameters affecting the HS-GC-PID sensitivity including matrix effect, heating temperature, extraction time and carrier gas flow-rate were optimized. The matrix effect was required for standard addition calibration method for quantification purposes. The method has been validated when rectilinear relationship was between the concentrations of analytes and peak area in the range of 0.2–50ngmL−1 for DMSe and 0.5–100ngmL−1 for DMS, the correlation coefficients were from 0.995–0.998, and the limits of detection for DMS and DMSe were 25pgmL−1 and 48pgmL−1, respectively. The proposed method was further applied to quantification of DMS and DMSe in urine samples. [Copyright &y& Elsevier]

Abstract: Digital image analysis was applied to the products of simple colour presumptive tests for opiates. Adobe Photoshop software was used for colour analysis to obtain analytical data in the form of a Red Green Blue (RGB) value. Calibration curves were developed for morphine, codeine, and diamorphine hydrochloride and the developed tests successfully applied to seized heroin samples to demonstrate the application of the technique in a forensic case context. Good agreement with gas chromatographic quantification results was obtained for the illicit samples analysed and a wide linear range and low detection limit for all drugs under test facilitated the application to illicit samples. The results show great potential for use as a semi-quantitative field test for illicit drug compounds. [Copyright &y& Elsevier]

Abstract: Solid-phase microextraction (SPME) has undergone a surge in popularity within the field of analytical chemistry in the past two decades since its introduction. Owing to its nature of extraction, SPME has become widely known as a quick and cost-effective sample preparation technique. Although SPME has demonstrated extraordinary versatility in sampling capabilities, the technique continues to experience a tremendous growth in innovation. Presently, increasing efforts have been directed towards the engineering of novel sorbent material in order to expand the applicability of SPME for a wider range of analytes and matrices. This review highlights the application of ionic liquids (ILs) and polymeric ionic liquids (PILs) as innovative sorbent materials for SPME. Characterized by their unique physico-chemical properties, these compounds can be structurally-designed to selectively extract target analytes based on unique molecular interactions. To examine the advantages of IL and PIL-based sorbent coatings in SPME, the field is reviewed by gathering available experimental data and exploring the sensitivity, linear calibration range, as well as detection limits for a variety of target analytes in the methods that have been developed. [Copyright &y& Elsevier]

Abstract: Volatile organic compounds (VOCs) play an important role in the chemistry of the atmosphere and in biogeochemistry. They contribute to the oxidative capacity of the atmosphere, particle and air pollutants, as well as to the production of greenhouse gasses (for instance ozone). Among analytical techniques for their determination in the atmosphere gas chromatography coupled with mass spectrometry (GC–MS) offers several advantages. However, for an accurate quantification calibration with standard substances is necessary. A quantitative structure–property relationship (QSPR) model for the prediction of MS response factors was developed on basis of our experimental measurements for the quantification of ozone precursors present in the atmosphere. A linear correlation between chemical structures and response factors was established by using a 7-parameter MLR model. The average error in the prediction of response factors was calculated by cross-validation procedure and was below 20%, which is sufficient for the determination of VOCs in the air. The proposed procedure is time consuming so it is more suited for the quantification of tentatively identified organic compounds during the reprocessing of MS chromatograms in cases when the original sample is no longer available. [Copyright &y& Elsevier]

Abstract: A new approach for target quantitative analysis for comprehensive two-dimensional gas chromatography (GC×GC), interval Multi-way Partial Least Square (iNPLS) is presented and evaluated in this paper. In iNPLS, the two-dimensional chromatogram is split in small sections; each of these pieces is treated as an independent new chromatogram. Separated conventional NPLS calibration models for the concentration of the target analyte are built for each of the pieces of the whole chromatogram, and the best model is selected for quantitative analysis. An algorithm for iNPLS running on MatLab platform was written, preliminarily evaluated with using solutions of model compounds with different chemical properties and subsequently applied to quantify some allergens in perfume samples. The results were found to be adequate, and good precision and accuracy was obtained even for poorly resolved peaks. [ABSTRACT FROM AUTHOR]

Abstract: A novel ionic liquid (IL) bonded fused-sil-ica fiber for headspace solid-phase microextraction (HS-SPME)/gas chromatography-flame ionization detection (GC-FID) of methyl tert-butyl ether (MTBE) in a gasoline sample was prepared and used. The new proposed chemically bonded fiber has better thermal stability and durability than its corresponding physically coated fiber. Another advantage is that no spacer was used for the purpose of bonding the IL to the surface of the fused-silica. The latter advantage makes the preparation of these fibers easier with lower cost than those prepared using sol–gel method. The ionic liquid 1-methyl-3-(3-trimethoxysilyl propyl) imidazolium bis(trifluoromethylsulfonyl) imide was synthesized and cross linked to the surface of the fused-silica fiber. Then, the chemically IL-modified fibers were applied to the headspace extraction of MTBE. The chemically IL-modified fibers showed improved thermal stability at temperatures up to 220°C relative to the physically IL-modified fibers (180°C). The chemically bonded IL film on the surface of the fused-silica fiber was durable over 16 headspace extractions without any significant loss of the IL film. The calibration graph was linear in a concentration range of 2–240μgL−1 (R 2 =0.996) with the detection limit of 0.1μgL−1 level. The reproducibility (RSD %, n =6) of the new IL bonded fused-silica fiber (8.9%) was better than the physically coated fiber (12%) suggesting that the proposed chemically IL-modified fiber is more robust than the physically IL-modified fiber. The optimum extraction conditions were the followings: 40°C extraction temperature, 12min extraction time, 30s desorption time and sample agitation at 200rpm. [ABSTRACT FROM AUTHOR]

Abstract: A novel solid-phase microextraction fiber based on a stainless steel wire coated with Au nanoparticles was prepared and has been applied, coupled with gas chromatography, to the extraction of aromatic hydrophobic organic chemical pollutants in rainwater and soil extract. The solid-phase microextraction fiber exhibited excellent extraction efficiency and selectivity. Effects of extraction time, extraction temperature, ionic strength, stirring rate and desorption conditions were investigated and optimized. Single fiber repeatability and fiber-to-fiber reproducibility were less than 7.90% and 26.40%, respectively. The calibration curves were linear in a wide range for all analytes. Correlation coefficients ranged from 0.9941 to 0.9993. The as-established SPME-GC method was used successfully to two real natural samples. Recovery of analytes spiked at 10μgL−1 and 100μgL−1 ranged from 78.4% to 119.9% and the relative standard deviations were less than 11.3%. [ABSTRACT FROM AUTHOR]

Abstract: The development of a two phase hollow fiber liquid-phase microextraction technique, followed by gas-chromatography-flame ionization detection (GC-FID) for the profiling of the fatty acids (FAs) (lauric, myristic, palmitic, stearic, palmitoleic, oleic, linoleic, linolenic and arachidic) in vegetable oils is described. Heptadecanoic acid methyl ester was used as the internal standard. The FAs were transesterified to their corresponding methyl esters prior to the extraction. Extraction parameters such as type of extracting solvent, temperature, extraction time, stirring speed and salt addition were studied and optimized. Recommended conditions were extraction solvent, n-tridecane; extraction time, 35min; extraction temperature, ambient; without addition of salt. Enrichment factors varying from 37 to 115 were achieved. Calibration curves for the nine FAs were well correlated (r 2 >0.994) within the range of 10–5000μgL−1. The limit of detection (signal:noise, 3) was 4.73–13.21ngL−1. The method was successfully applied to the profiling of the FAs in palm oils (crude, olein, kernel, and carotino cooking oil) and other vegetable oils (soybean, olive, coconut, rice bran and pumpkin). The encouraging enrichments achieved offer an interesting option for the profiling of the minor and major FAs in palm and other vegetable oils. [ABSTRACT FROM AUTHOR]

Abstract: In order to determine flavours (diacetyl, pentanedione, acetoin and acetaldehyde) in beer, gas chromatography coupled with headspace sampling technique was developed in our laboratory. The calibrations were accomplished by six level standard addition methods, and relative coefficients were ⩾0.999. Repeatability experiments were accomplished by detecting 20 successive same standard samples, which showed that detection precision was <2% relative standard deviation (RSD) for three flavours (diacetyl, pentanedione and acetaldehyde), and detection precision of acetoin was <2.5% (RSD). Four flavours gradually increased and then decreased during the process of fermentation, and they leveled off when the fermentation was over. The ratio (diacetyl/pentanedione) reflected the degree of contamination: normal beer had a ratio of approximate 1; the ratio (diacetyl/pentanedione) was higher than 1, displaying that beer was contaminated by microbes. Our data suggested that gas chromatography coupled with headspace sampling technique could be used successfully to determine flavours (diacetyl, pentanedione, acetoin and acetaldehyde) in beer. [Copyright &y& Elsevier]

Abstract: A novel analytical method is reported that combines continuous solid-phase extraction and gas chromatography for the determination of 22 carboxylic acids in water. The highly polar and hydrophilic analytes were preferentially sorbed on a mixture of LiChrolut EN–Supelclean ENVI-18 (1:1) sorbent column and eluted with methanol; this extraction process did not require derivatisation. The extract was analysed by gas chromatography coupled to a flame ionisation detector as well as a mass spectrometer with electron impact (EI) or positive chemical ionisation modes. The highest sensitivity was achieved when using MS-EI, with good linearity in calibration curves and low detection limits (2–40ngL−1) for 50mL of sample. The entire procedure from raw aqueous sample to a ready-to-inject methanol solution of the acids requires less than 15min. Another benefit of this method is the good accuracy (recoveries between 93 and 102%) and precision (relative standard deviation, 3.4–6.2%), which allows the determination of carboxylic acids in environmental water and in real chlorinated and ozonated drinking water. [ABSTRACT FROM AUTHOR]

Abstract: Dispersive liquid–liquid microextraction (DLLME) coupled with gas chromatography–mass spectrometry-selective ion monitoring (GC–MS-SIM) was applied to the determination of methyl tert-butyl ether (MTBE) in water samples. The effect of main parameters affecting the extraction efficiency was studied simultaneously. From selected parameters, volume of extraction solvent, volume of dispersive solvent, and salt concentration were optimized by means of experimental design. The statistical parameters of the derived model were R 2 =0.9987 and F =17.83. The optimal conditions were 42.0μL for extraction solvent, 0.30mL for disperser solvent and 5% (w/v) for sodium chloride. The calibration linear range was 0.001–370μgL−1. The improved detection limit with the aid of chemometrics was 0.3ngL−1. The relative standard deviation (RSD) with n =9 for 0.1mgL−1 MTBE in water with and without internal standard was 2.7% and 3.1%, respectively. Under the optimal conditions, the relative recoveries of spiked MTBE in different water samples were in the range of 100–105%. [ABSTRACT FROM AUTHOR]

Abstract: Genetic algorithm based multivariate calibration models were generated for infrared spectroscopic determination of aluminum rolling oil additives and contaminants such as gear and hydraulic oils. Two different additives and six different suspected contaminants were investigated in the base oil lubricant. Routine analysis samples from 9 different aluminum rolling systems were collected in a period of 2 months in an aluminum rolling plant and gas chromatography (GC) is used as the reference method. Infrared absorbance spectra of the samples were then collected and the reference values obtained with GC were used together with these spectra for model building. Inverse least squares method was optimized with a genetic algorithm by selecting the most contributing regions of the infrared spectra for each component. The R 2 values between GC and multivariate spectroscopic determinations were around 0.99 indicating a good correlation between the two methods. Performance of genetic algorithm based multivariate calibration models were also compared with partial least squares (PLS) method. The study showed that infrared spectroscopy coupled with multivariate calibration can be used for continuous monitoring of additives and contaminants in aluminum rolling oil. By this way, analysis time is significantly reduced and simultaneous determination of all the components can be accomplished. [ABSTRACT FROM AUTHOR]

Abstract: A simple solvent microextraction method termed vortex-assisted liquid–liquid microextraction (VALLME) coupled with gas chromatography micro electron-capture detector (GC-μECD) has been developed and used for the pesticide residue analysis in water samples. In the VALLME method, aliquots of 30μL toluene used as extraction solvent were directly injected into a 25mL volumetric flask containing the water sample. The extraction solvent was dispersed into the water phase under vigorously shaking with the vortex. The parameters affecting the extraction efficiency of the proposed VALLME such as extraction solvent, vortex time, volumes of extraction solvent and salt addition were investigated. Under the optimum condition, enrichment factors (EFs) in a range of 835–1115 and limits of detection below 0.010μgL−1 were obtained for the determination of target pesticides in water. The calculated calibration curves provide high levels of linearity yielding correlation coefficients (r 2) greater than 0.9958 with the concentration level ranged from 0.05 to 2.5μgL−1. Finally, the proposed method has been successfully applied to the determination of pesticides from real water samples and acceptable recoveries over the range of 72–106.3% were obtained. [ABSTRACT FROM AUTHOR]

Abstract: HS-SPME analysis of terpenes does usually have inherent quantification problems when working with complex samples, especially due to the matrix effect of the substrate or the calibration solution. Three different terpene carrier matrices were compared: methanol, synthetic oil and milk fat obtained by centrifugation from milk cream. Considerable differences in calibration sensitivity parameters were observed depending on the matrix used and on the type of terpene standard analysed. For milk sample quantification purposes internal standard method was preferred using milk fat as calibration matrix. Linearity range, repeatability, recovery and limits of detection and quantification were determined. Validation parameters were different depending on the concentration and molecular structure of each terpene analysed, particularly between mono- and sesquiterpenes. The method was useful to determine in an accurate manner the terpene content in milk samples from pasture fed animals, and it will help to establish objective terpene levels to differentiate milks from specific production systems. [Copyright &y& Elsevier]

Abstract: A solid phase extraction cleanup step substantially improved analytical efficiency and data quality for measurements of non-polar and moderately polar organic molecular marker concentrations in airborne particulate matter. Rapid gas chromatography column deterioration was evident after very few samples in the absence of a cleanup step, resulting in the need for frequent recalibration. High molecular weight polycyclic aromatic hydrocarbons, were among the species most strongly impacted by the deterioration, exhibiting deviations as high as 30–40% from expected calibration verification standard values after only a few injections. Column deterioration and calibration verification failure were eliminated by introducing a solid phase extraction step prior to analysis and a total of 58 samples were analyzed with no unacceptable deviation of calibration verification standards from target values [Copyright &y& Elsevier]

Abstract: Carbon dioxide (CO2) is a greenhouse gas that makes by far the largest contribution to the global warming of the Earth''s atmosphere. For the measurements of atmospheric CO2 a non-dispersive infrared analyzer (NDIR) and gas chromatography are conventionally being used. We explored whether and to what degree argon content can influence the determination of atmospheric CO2 using the comparison of CO2 concentrations between the sample gas mixtures with varying Ar amounts at 0 and 18.6mmolmol−1 and the calibration gas mixtures with Ar at 8.4, 9.1, and 9.3mmolmol−1. We newly discovered that variation of Ar content in calibration gas mixtures could undermine accuracy for precise and accurate determination of atmospheric CO2 in background air. The differences in CO2 concentration due to the variation of Ar content in the calibration gas mixtures were negligible (<±0.03μmolmol−1) for NDIR systems whereas they noticeably increased (<±1.09μmolmol−1) especially for the modified GC systems to enhance instrumental sensitivity. We found that the thermal mass flow controller is the main source of the differences although such differences appeared only in the presence of a flow restrictor in GC systems. For reliable monitoring of real atmospheric CO2 samples, one should use calibration gas mixtures that contain Ar content close to the level (9.332mmolmol−1) in the ambient air as possible. Practical guidelines were highlighted relating to selection of appropriate analytical approaches for the accurate and precise measurements of atmospheric CO2. In addition, theoretical implications from the findings were addressed. [Copyright &y& Elsevier]

Abstract: In this study, the reliability of thermal desorbing technique was investigated using the gaseous standards of five reduced sulfur compounds (RSCs: hydrogen sulfide, methane thiol, dimethyl sulfide, carbon disulfide, and dimethyl disulfide). A series of calibration experiments for RSCs were performed using gas chromatography (GC) with pulsed flame photometric detector (PFPD) that is interfaced with a thermal desorber (TD) unit. These calibration data were evaluated by means of two contrasting concepts: fixed standard concentration method (FSC: variable volumetric injection of standard gases prepared at a given concentration) and fixed standard volume method (FSV: injection of multiple standards with varying concentrations at a given volume). When the results of both methods were compared, RSCs generally showed enhanced sensitivity with increasing concentration (FSC) and sample loading volume (FSV). This study highlights that TD-based calibration properties are practically undistinguishable between different sample transfer approaches (e.g., FSV and FSC). As a result, the calibration properties of RSCs derived by thermal desorption technique are greatly distinguished from those of direct injection into GC. [Copyright &y& Elsevier]

Abstract: In this study, the fundamental aspects of gas chromatography with a pulsed flame photometric detector were investigated through the calibration of gaseous reduced sulfur compounds based on the direct injection method. Gaseous standards of five reduced sulfur compounds (hydrogen sulfide, methane thiol, dimethyl sulfide, carbon disulfide, and dimethyl disulfide) were calibrated as a function of injection volume and concentration level. The results were evaluated by means of two contrasting calibration approaches: fixed standard concentration method (variable volumetric injection of standard gases prepared at a given concentration) and fixed standard volume method (injection of multiple standards with varying concentrations at a given volume). The optimum detection limit values of reduced sulfur compounds, when estimated at 100μL of injection volume, ranged from 2.37pg (carbon disulfide) to 4.89pg (dimethyl sulfide). Although these detection limit values improved gradually with decreasing injection volume, the minimum detectable concentration (e.g., in nmolmol−1 scale) remained constant due to a balance by the sample volume reduction. The linearity property of pulsed flame photometric detector also appeared to vary dynamically with changes in its sensitivity. According to this study, the performance of pulsed flame photometric detector, when tested by direct injection method, is highly reliable to precisely describe the behavior of reduced sulfur compounds above ∼20nmolmol−1. [Copyright &y& Elsevier]

Abstract: The precision and long-term stability of pyrolysis probe set-point temperature calibration of a commercially available coiled-filament pyrolyzer were assessed for a variety of polymers, including Kraton® D1107, high-density polyethylene (HDPE), and low-density polyethylene (LDPE). While plots of peak area ratios for Kraton® and HDPE versus pyrolysis set-point temperatures produced statistically significant linear curves at the 95% confidence level, poor precision was observed at each of the set-point temperatures. Plots of peak area ratios for LDPE, in particular for n-C16 alkyldiene/n-C16 alkene peak area ratios, also exhibited good linearity but showed significant improvements in precision at each set-point temperature. In addition, replicate analysis over a 10-month period of peak area ratios for polymers pyrolyzed at a set-point temperature of 900°C confirmed the improved method precision obtained from pyrolysis of LDPE and analysis of the n-C16 alkyldiene/n-C16 alkene ratio when compared to the precision obtained from pyrolysis of Kraton® D1107 or high-density polyethylene. [Copyright &y& Elsevier]

Abstract: This paper presents a detailed study on the calibration of a thermal desorption–gas chromatography–mass spectrometry (TD–GC–MS)-based methodology for quantification of volatile organic compounds (VOCs) in gaseous and liquid samples. For the first time, it is documented to what extent three widely encountered problems affect precise and accurate quantification, and solutions to improve calibration are proposed. The first issue deals with the limited precision in MS quantification, as exemplified by high relative standard deviations (up to 40%, n =5) on response factors of a set of 69 selected VOCs in a volatility range from 16Pa to 85kPa at 298K. The addition of [2H8]toluene as an internal standard, in gaseous or liquid phase, improves this imprecision by a factor of 5. Second, the matrix in which the standard is dissolved is shown to be highly important towards calibration. Quantification of gaseous VOCs loaded on a sorbent tube using response factors obtained with liquid standards results in systematic deviations of 40–80%. Relative response factors determined by the analysis of sorbent tubes loaded with both analytes and [2H8]toluene from liquid phase are shown to offer a reliable alternative for quantification of airborne VOCs, without need for expensive and often hardly available gaseous standards. Third, a strategy is proposed involving the determination of a relative response factor being representative for a group of analytes with similar functionalities and electron impact fragmentation patterns. This group method approach indicates to be useful (RSD≈10%) for quantifying analytes belonging to that class but having no standards available. [Copyright &y& Elsevier]

Abstract: The sample matrix can cause an enhancement in the observed chromatographic response for pesticide residues in a matrix extract compared with the same concentration in a matrix-free solution. The matrix increases the transfer of pesticides from hot vaporizing injectors by reducing the thermal stress for labile compounds and by masking active sites in the injector responsible for the adsorption or decomposition of polar pesticides. The use of different injector types and matrix simplification procedures can reduce matrix-induced enhancement but do not eliminate it. The most effective strategy is to use matrix-matched calibration standards or analyte protectants which equalize the response enhancement for calibration standards and sample extracts. From a practical point of view it is important that the method used to correct for matrix-induced enhancement is compatible with low system maintenance. The different approaches for correcting matrix-induced enhancement for calibration in pesticide residue analysis are discussed and compared in this review. [Copyright &y& Elsevier]

Abstract: The effect of the amount of matrix compounds in comprehensive two-dimensional gas chromatographic (GC×GC) separation of polycyclic aromatic hydrocarbons (PAHs) was investigated. The second dimension column was overloaded by progressively increasing the matrix amount and the separation was evaluated by inspecting peak widths, asymmetries, resolution, retention times, peak areas and volumes of the target compounds. In quantitative analysis, two different calibration methods were tested for both raw sediment extracts and cleaned extracts, i.e. for extracts in which the matrix was removed by a liquid chromatographic clean-up step. The quality of the separation was not significantly disturbed in terms of peak width, asymmetry and resolution when the amount of matrix was increased. However, the depth of the valley between PAH and preceding matrix peak increased with an increase in matrix amount and the repeatabilities of retention times, peak areas and volumes decreased. In quantitative analysis, calibration by using areas had a tendency to underestimate the trace amounts of PAH, especially for the non-cleaned samples. In case of volume calibration, the most accurate results were obtained by external calibration for both cleaned and non-cleaned samples. [Copyright &y& Elsevier]

Abstract: A recently introduced two-dimensional fatty acid retention index system (2D-FARI) was used as basis for prediction of equivalent chain lengths (ECL) of fatty acid methyl esters (FAME) on a BPX-70 stationary phase. Models for the relationship between 2D-FARI data and ECL values of a calibration sample with 30 common fatty acids were established by a simple multivariate regression. The models were thereafter applied on 2D-FARI data for other FAMEs and used to predict the ECLs for these compounds. The 2D-FARI values for the fatty acids in the calibration sample are given by definition. Thus, the only information necessary to calculate the ECL value for a compound run under identical conditions as the calibration sample is the 2D-FARI values for the compound, which can be acquired from literature data. The method was validated with test sets analysed with different temperature and flow programs. ECLs of various marine FAME and trans isomers of Eicosapentaenoic and Docosahexaenoic acid were predicted with root mean squared error of prediction from 0.002 to 0.012 ECL units. [Copyright &y& Elsevier]

Abstract: In order to evaluate the extent of analytical biases involved in the GC calibration, we conducted a series of experiments to examine the calibration methods of trace gas components. For the purpose of this comparative study, gaseous standards of reduced sulfur compounds (RSC) including hydrogen sulfide (H2S), methanethiol (CH3SH), dimethyl sulfide (DMS), carbon disulfide (CS2), and dimethyl disulfide (DMDS) were calibrated by the combination of a GC/PFPD technique and a loop-injection method. In the course of this study, two different types of calibration methods were tested and compared: incremental-injection of a given standard with the fixed standard concentration (FSC) versus injection of multiple standards (with different concentrations) at the fixed standard volume (FSV). In the case of the FSV calibration, a notable increase in the GC sensitivity is apparent with decreasing loop size (or injection volume). For instance, the calibration slope for RSC obtained using a 10μl loop system was approximately three times higher than that for a 250μl one. However, the results obtained by the FSC method exhibit much poorer sensitivity than its counterpart with slight differences in their sensitivities across different standard concentrations (due to such factors as the matrix effect from varying injection volumes). Thus, the overall results of this study confirm that the detailed characterization of the selected calibration method (e.g., the use of FSV approach relative to FSC) is of primary significance to perform an accurate quantification of trace gases. [Copyright &y& Elsevier]

Abstract: In-depth evaluation of an analytical method to detect and quantify long chain fatty acids (C8–C16) at trace level concentrations (25–1000μg/l) is presented. The method requires derivatization of the acids with methanolic boron trifluoride, separation, and detection by gas chromatography–mass spectrometry. The calibration experiments passed all the tested performance criteria such as linearity, homoscedasticity, and ruggedness. The detection limits and related quantities were computed by applying the method detection limit, and the calibration line approximation. The values obtained by applying the latter approach were more reliable and consistent with the actual statistical theory of detection decisions and yielded the following concentrations: C8, 87.6μg/l; C10, 45.2μg/l; C11, 39.9μg/l; C12, 37.7μg/l; C14, 41.4μg/l and C16, 40.6μg/l. Two different gas–liquid chromatographic columns were tested and similar results achieved, which shows the ruggedness of the method. [Copyright &y& Elsevier]

Abstract: The aim of this study was to identify and quantify an unknown peak in the chromatogram of a very complex mixture, a straight oil metalworking fluid (MWF). The fraction that permeated through a thin nitrile polymer membrane had less mineral oil background than the original MWF did at the retention time of the unknown peak, thus facilitating identification by total ion current (TIC) gas chromatography–mass spectrometry (GC–MS). The peak proved to be di-n-octyl disulfide (DOD) through retention time and mass spectral comparisons. Quantitation of DOD was by extracted ion chromatogram analysis of the DOD molecular ion (mass-to-charge ratio (m/z) 290), and of the m/z 71 ion for the internal standard, n-triacontane. Linear models of the area ratio (y) of these two ions versus DOD concentration showed a systematic negative bias at low concentrations, a common occurrence in analysis. The linear model of y 0.8 (from Box–Cox power transformation) versus DOD concentration showed negligible bias from the lowest measured standard of 1.51mg/L to the highest concentration tested at 75.5mg/L. The intercept did not differ statistically from zero. The concentration of DOD in the MWF was then calculated to be 0.398±0.034% (w/w) by the internal standard method, and 0.387±0.036% (w/w) by the method of standard additions. These two results were not significantly different at p ≤0.05. The Box–Cox transformation is therefore recommended when the data for standards are non-linear. [Copyright &y& Elsevier]

Abstract: Calibration of SPME by the injection of liquid standard solution is a common method for SPME quantitative analysis by gas chromatography. The feasibility of this calibration method relies on the assumption that sample transfer efficiencies are the same for both the liquid injection and the SPME fiber injection. Sample transfer efficiencies for liquid injection and SPME fiber injection were studied in this paper. The results showed that the sample transfer efficiency for liquid injection was affected by several factors, such as the dimensions of the liner, the presence of the wool, and the temperature of the injector. The sample transfer efficiency for SPME fiber injection was affected by the cross-sectional area of the space between the column and the liner, the carrier gas flow rate, and the length of the column inside the liner. An equation was proposed to estimate the sample loss rate for SPME injection. It was found that the use of a direct injection (DI) liner and program temperature vaporizing provides high sample transfer efficiencies, for both liquid injection and SPME fiber injection. When a common SPME straight liner is used, large outer diameter (o.d.) pre-column will help to achieve high sample transfer efficiency. [Copyright &y& Elsevier]

Abstract: The accurate and reproducible analysis of long-chain polyunsaturated fatty acids (PUFA) is of growing importance. Especially for labeling purposes, clear guidelines are needed in order to achieve optimum accuracy. Since calibration standards cannot be used for method validation due to the instability of PUFAs, there is no direct way to check for the absence of systematic errors. In this study the sources of error that weaken the accuracy were evaluated using theoretical considerations and calibration standards with corrected composition. It was demonstrated that the key role for optimum accuracy lies in the optimization of the split injection system. Even when following the instructions outlined in the official methods of the American Oil Chemist''s Society (AOCS), systematic errors of more than 7% can arise. Clear guidelines regarding system calibration and selection of appropriate internal standards (IS) can improve precision and accuracy significantly. [Copyright &y& Elsevier]

Abstract: In an effort to provide detailed information concerning the analysis of reduced S compounds, we carefully examined the response properties of four different compounds (H2S, CH3SH, DMS, and DMDS) using the GC/PFPD system. Such properties were assessed mainly in terms of both absolute detectability in mass (or concentration) and relative calibration patterns across different S gases. The results of our analysis, when inspected in terms of calibration data over a wide concentration range, indicated that the patterns could be affected greatly by the absolute content of S analyzed with notable differences among different S compounds. The causes of such differences appeared to be associated with different processes (or mechanisms) such as either some type of (adsorptive) loss within the system (H2S) or moderate variabilities in linearity (DMS and DMDS). Relative response factors (RRF) of these S gases, when computed by their calibration slope values through normalization of the least variable results (CH3SH), were found to increase in the order of H2S, CH3SH, DMS, and DMDS. The DL values of all S gases for our analytical system were typically estimated in a few pg or a few ppb range (e.g., at 500μl sample volume) with considerable differences in their linearity responses. [Copyright &y& Elsevier]

Abstract: In the present paper, the authors propose an alternative methodology for the determination of the calibration curves of a gas chromatograph employed in natural gas analysis. Such methodology, coming from a modification to the regression technique known as Generalised Distance Regression (GDR), was applied to the data obtained during the calibration procedure of the gas chromatograph lodged in the “Quality Control” laboratory of the Snam Rete Gas (San Donato Milanese, Milan), accredited with the Calibration Service in Italy (SIT). Binary mixtures, gravimetrically prepared according to existing international standards in the abovementioned laboratory, were used as reference standards for the instrument calibration. In order to obtain the gas chromatograph calibration curves, the GDR technique was first applied, to consider the heteroscedastic uncertainty both in the dependent and independent variables. However, for some chemical components, a polynomial curve of order higher than one were obtained. In general, since the gas chromatograph is affected by time drifts, it would be favourable to have straight lines as calibration curves. In this way, the periodical check (usually made daily) would be extremely simplified. In fact, to determine the drift entity for a line, two checkpoints would be necessary, and if the line would pass through the origin, a single point would be necessary. In order to simplify the management of the time drift check procedure, the authors introduced a modification to the GDR technique, obtaining the Modified Generalised Distance Regression, which allows one to impose a straight line as calibration curve for each component, obtaining an unbiased estimation of the calibration coefficients and of their variance–covariance matrix. [Copyright &y& Elsevier]