Direct Gas Chromatographic Analysis Research Articles

A gas chromatography-flame ionization detection method for direct and rapid determination of small molecule volatile organic compounds in aqueous phase.

It is still difficult to directly detect low content of volatile organic compounds (VOCs) in water samples by gas chromatography (GC) because when water is the only solvent, it would result in the instability and poor repeatability of peak retention time and peak shape. The adverse effects of water on direct GC analysis of VOCs cannot be significantly reduced or eliminated by simply changing the detection condition of GC. However, it was found that the addition of methanol in samples to a certain final proportion, such as 50 or 75% (v/v), could greatly reduce or eliminate the adverse effects of water. By using 75% (v/v) methanol as a solvent, the standard curves of ethanol, acetic acid, acetone, and isopropanol with correlation coefficient (R 2) over 0.99 were successfully plotted by gas chromatography-flame ionization detection (GC-FID) in a certain concentration range, respectively. The results showed that the retention time and peak shape stability of ethanol, acetic acid, acetone, and isopropanol in aqueous solution were greatly improved by the addition of methanol to final concertation of 75% (v/v). To verify the practical application potential of this method, the method was applied to the detection of components in isopropanol fermentation wastewater. The results showed that the method has well applicability and reliability. The key points in the application of this method were also summarized. This GC analysis method would have a wider and better application prospect in the detection of water-soluble organic matters.

In Situ Catalysis and Extraction Approach for Fast Evaluation of Heterogeneous Catalytic Efficiency.

In situ monitoring of products generated by important heterogeneous catalytic reactions is of great significance for chemical industry, particularly when the products or intermediates are not sufficiently stable or occur at trace-level concentrations. It is therefore highly desirable to develop an integrated in situ catalysis and extraction method, which can simultaneously catalyze the reaction and enrich products while maintaining compatibility with analytical instrumentation. Herein, we propose an approach by depositing different types of metal nanocrystals, including gold, platinum, and palladium nanoparticles, onto fibrous silica microspheres coated fibers for integrated in situ catalysis and extraction. As a proof-of-concept, several typical chemical reactions, including the reduction of p-nitrophenol, epoxidation of styrene, oxidation of benzyl alcohol, and dechlorination of p-chlorophenol, were investigated to validate the feasibility of this method. Our results show that these coatings not only function as catalysts to accelerate the selected reactions but also serve as adsorbents to extract the reactants, intermediates, and products for direct gas chromatographic analysis, suggesting the viability of this approach for the in situ evaluation of catalytic processes. By this approach, the yield, selectivity, and kinetics of a reaction can be readily assessed. This approach can also be extended to investigate the catalytic performance of the same metal nanocrystals with different morphology, surface facet, structure, or surface functionalization. This approach will find broad generality for assessing the catalytic efficiency and selectivity of new catalysts or new chemical reactions and dynamic processes in these reactions.

Gas chromatographic determination of some carboxylic acids: comparison between solution and headspace analysis

Direct and headspace gas chromatographic analysis is compared for the determination of lactic, oxalic, succinic, malic and citric acids. For better chromatographic behaviour, prior to GC analysis carboxylic acids were derivatized using BSTFA as a silylation reagent. Three solvents – acetone, diethyl ether and dimethylformamide – were tested as a derivatization medium and DMF was considered as the best. Derivatization conditions were optimized and analytical characteristics of the direct gas chromatographic determination of analytes in solutions were determined. The method was applied for the determination of citric acid in food. For the analytes dissolved in DMF, the headspace gas chromatographic determination was not sensitive enough. Concentrations of the derivates in the headspace were very low, thus the limits of detection were high and the method was not of practical use. On the other hand, headspace gas chromatography can be applied for identifying of carboxylic acids in solid samples.

Simple and rapid method to analyse squalene in olive oils and extra virgin olive oils

Olive oil is one of the main vegetable sources of squalene, substance with several interesting properties and applications. This work aims to present and validate a rapid method to quantify squalene in olive oil by gas chromatography (GC) coupled with flame ionization detector and to apply the method to extra virgin olive oils (EVOOs) and olive oils (OOs) from large scale distribution retail, and to some EVOOs from little companies productions (Italian monovarietal Leccino and Frantoio EVOOs, and niche blend Italian EVOOs). A transmethylation of the oil sample is exploited to allow the direct GC analysis avoiding the separation from saponifiable fraction. The optimized conditions provide high sensitivity, short time (total GC run time is 5 min and about 10 min are needed for sample preparation) and require instruments and materials usually available in most of the laboratories. The method was validated by determining linearity, intraday and interday repeatability, recovery, limit of detection and limit of quantification. Within EVOOs investigated, the niche blend EVOO contained significantly higher amount (P < 0.05) of squalene (0.81–1.02 g/100 g) as compared to each of the other EVOOs groups investigated (monovarietal Leccino and Frantoio EVOOs and EVOOs from large scale distribution retail) containing a squalene concentration in the range 0.31–0.65 g/100 g; in OOs, the content, 0.17–0.27 g/100 g, was significantly lower (P < 0.05) as compared to EVOOs.

Solid-phase microextraction of phthalate esters by a new coating based on a thermally stable polypyrrole/graphene oxide composite.

A novel polypyrole/graphene oxide coating was made by the electrochemical polymerization of pyrrole in the presence of sodium dodecyl sulfate and graphene oxide on a platinum wire. The prepared fiber has shown a good thermal stability up to 300°C. The fiber was applied to the direct solid-phase microextraction and gas chromatographic analysis of four phthalate esters. The effect of four parameters on gas chromatography peak area including extraction temperature, extraction time, injection temperature, and ionic strength were investigated. Under the optimized conditions, the detection limits were between 0.042 and 0.26μg/L. The intraday and interday relative standard deviations obtained at 55μg/L, using a single fiber, were 8.2-16% and 17.3-25.6%, respectively. The method was successfully applied to the analysis of phthalate esters in two real samples of boiling water in cheap disposable clear plastic drinking cups showing recoveries from 83 to 120%.

The selection of the standard phase (gas vs. liquid) and the related matrix effect on the direct injection gas chromatographic analysis of VOCs at sub–ppm levels

In this study, the matrix effect on the quantitation of volatile organic compounds (VOCs) at sub–ppm levels has been investigated in relation to the selected standard phase by direct injection gas chromatographic (GC) analysis. To this end, a series of calibration experiments were conducted using both liquid and gas phase standards containing identically a total of 13 target VOCs. Calibration datasets between liquid and gas standards were obtained at the four selected injection volumes, i.e., covering a 1 to 5 and 50 to 500μL range, respectively. The results indicate that injection volume is a sensitive parameter, as sensitivity tends to decrease with increasing injection volume, especially with liquid standards. Loss of analytes in liquid standard occurred noticeably from ones eluting earlier than solvent used for standard (i.e., methanol). If the extent of such loss is expressed by the percent differences in the response factor (RF) values between two standard types, the results were low or insignificant for valeraldehyde (0.48%), benzene (7.6%), toluene (3.3%), and styrene (4.8%) but generally high for the others (i.e., between 10–80%). The relative sensitivities of VOCs in gaseous standards, if computed by normalization against benzene, generally complied well with those derivable from the literature on flame ionization detectors. In contrast, in case of liquid standards, the use of a small injection volume (≤1μL) is recommended to maintain the optimal GC performance in light of the matrix effect.

Changes in the specific migration characteristics of packaging–food simulant combinations caused by ionizing radiation: Effect of food simulant

The primary objective of the present study was to evaluate the extent to which the affinity of the surrounding medium for the migrant, as well as the packaging material, affects the specific migration characteristics of the latter. For this purpose, migration tests were conducted with vinylidene chloride copolymer (PVDC/PVC) in contact with the EU specified solvents simulating all food types: namely, distilled water, 3% w/v acetic acid, 10% v/v ethanol and isooctane. Migration testing was carried out at 40°C for 10 days for the aqueous simulants, and at 20°C for 2 days for the fatty food simulant (EC, 1997; EEC, 1993). In addition, food-grade saran film was subjected to ionizing radiation treatment with a [60Co] source at doses equal to 5, 15 and 25kGy. Acetyl tributyl citrate (ATBC) plasticizer levels were monitored as a function of time for untreated, as well as gamma-irradiated packaging material, with a secondary objective to investigate the effect of ionizing radiation on polymer/migrant/surrounding medium interactions. Depending on the food simulant, determination of the analyte was performed by either direct gas chromatographic analysis, or surfactant (Triton X-114) mediated extraction followed by gas chromatographic-flame ionization detection (GC-FID). ATBC concentrations determined in aqueous and fatty food simulants were 0.216−0.497 and 5.0−5.9mg/L, respectively. Therefore, the most efficient extracting medium of plasticizers in vinyl chloride copolymers is the non-polar isooctane. Moreover, an extremely high rate of ATBC migration into isooctane during the early stages of contact was observed. The above observation verifies the aggressiveness of isooctane towards plastic packaging materials. Amongst the aqueous food simulants tested, the 10% ethanol solution demonstrated the highest migration levels. Gamma-irradiation enhanced ATBC migration; specific migration levels increased with increasing contact time and radiation dose. This was expected, since ATBC did not undergo chemical decomposition upon irradiation up to 25kGy. Finally, specific migration decreased proportionally with increasing polarity of the food-simulating solvent.

Optimization of Standard Gas Chromatographic Methodology for the Determination of Trans Fat in Unlabeled Bakery Products

Analytical parameters and quality control measures were optimized for standard direct gas chromatographic (GC) analysis of trans fat in unlabeled bakery products, and differences in concentrations measured in samples assayed with and without the modifications were evaluated. Total lipid was extracted with chloroform/methanol from homogenized cakes, cookies, doughnuts, pastries, muffins, and commercially available reference materials (NIST SRM 2387 Peanut Butter, LGC7103 Sweet Digestive Biscuit). Total lipid was saponified and fatty acids were derivatized to methyl esters (FAME) and analyzed by GC using a 100% nonbonded bis-cyanopropyl polysiloxane column (100 m × 0.25 mm, 0.2 µm film). Total FAME entering GC was optimized to separate C18:1-12t and C18:1-13t isomers that occur at a significant level but often remain unresolved from the C18:1-9c peak in products containing partially hydrogenated vegetable oils. Silver-ion solid-phase extraction was used to validate identification of the major C18:1 cis/trans peaks. For samples assayed by the standard method and with specific sample-to-sample quality control and GC optimization, the former underestimated trans fat by 2 to 5 g/100 g (0.2–0.6 g/serving) in some products. Differences were less for foods containing <1 g/100 g, but nonetheless could have implications for labeling because trans fat levels <0.5 g/serving may be declared zero according to U.S. Food and Drug Administration regulations. The practical modifications and controls described can be implemented in routine standard GC analysis to increase the consistency and validity of trans fat measurement across a range of samples with unknown fat content and fatty acid composition.

A rapid gas chromatographic method for the determination of histamine in fish and fish products

A gas chromatographic (GC) method, which reduced the time for determination of histamine in fish and fish products to less than 20 min, was demonstrated. Contrary to traditional GC method, histamine in sample was initially extracted with alkaline methanol and injected into a GC column (CP-SIL 19CB) for analysis without derivatization. Internal standard used in this protocol was 1,9-nonanediol. Detection limit for histamine by this method was about 5 μg/g. Standard addition test indicated 98–111% (CV: 2.7–7.8%) of recovery for tuna flesh and 99–102% (CV: 2.7–8.9%) for shrimp meat after adding with authentic compound, suggesting that using direct GC analysis for histamine determination was feasible.

Direct capillary gas chromatographic analysis and thermal stability of bile acid esters of glucose

This paper deals for the first time with a direct method for analysis of the α and β anomers of bile acid esters of glucose by capillary gas chromatography (CGC) without the need for a hydrolytic step. The bile acid esters were derivatized to their trimethylsilyl (TMS) ethers, which in turn were chromatographed on a short (7m) metal capillary column chemically coated with a thin (0.15 μm) film of thermostable, non-polar polydimethylsiloxane. Satisfactory CGC separation of the isomeric bile acid esters was achieved on the column; the β anomers eluted before the corresponding α isomers. Particularly noteworthy is that the α anomers are partially isomerized to the corresponding β anomers, and that both anomers are partially decomposed during CGC analysis, demonstrating the chemical specificity and thermal instability of the bile acid esters.

Fast gas chromatography analysis of N-carbamates with cold on-column injection.

Direct gas chromatographic (GC) analysis of thermally labile N-carbamates is studied by using fast GC with cold on-column injection. With the greatly reduced injection temperature, short column lengths, high flow rate, and fast temperature-programming rate, the exposure of carbamates to high temperature is reduced and degradation can be avoided. Nine N-carbamates in EPA Method 531 are eluted without thermal decomposition. The relative standard deviation percentage for peak areas average 1.9% for all of the carbamates analyzed. A conventional GC instrument is employed to simplify the experiments. GC-mass spectrometry is used to monitor the decomposition peaks.

Fast Gas Chromatography Analysis of N-Carbamates with Cold On-Column Injection

Direct gas chromatographic (GC) analysis of thermally labile N-carbamates is studied by using fast GC with cold on-column injection. With the greatly reduced injection temperature, short column lengths, high flow rate, and fast temperature-programming rate, the exposure of carbamates to high temperature is reduced and degradation can be avoided. Nine N-carbamates in EPA Method 531 are eluted without thermal decomposition. The relative standard deviation percentage for peak areas average 1.9% for all of the carbamates analyzed. A conventional GC instrument is employed to simplify the experiments. GC-mass spectrometry is used to monitor the decomposition peaks.

Determination of diethyl ether and methyl iodide in dimethylcadmium using headspace analysis

The possibility of direct gas-chromatographic analysis of dimethylcadmium was demonstrated. A procedure for determining impurities of diethyl ether and methyl iodide using headspace analysis was developed. The limits of detection for diethyl ether and methyl iodide were 3 x 10-4 and 2 x 10-3 mol %, respectively.

Anaerobic degradation of 1,2-dichloropropane in batch and continuous culture

Microbial degradation of 1,2-dichloropropane (DCP) was studied in a laboratory scale continuous flow fluidized bed reactor using polyurethane foam cubes as a carrier for the mixed culture. The anaerobic enrichment culture derived from Saale River sediment and was immobilized prior to utilization in the reactor. The DCP degradation performance was monitored by direct gas chromatographic analysis. A variety of different co-substrates were investigated for their ability to support DCP dechlorination during reactor operation. Continuous DCP removal efficiencies over 90% were achieved with a model water at substrate loading rates of up to 700 μmol/(L · d) with methanol and sodium acetate as co-substrates at 24 h hydraulic retention time in the reactor. In batch experiments the degradation potential of the culture for other chlorinated organics, like 1, 2, 3-trichloropropane and dichlorodiisopropyl ether was investigated.

Quantitative analysis of long-chain trans-monoenes originating from hydrogenated marine oil.

Gas chromatography (GC) is used for the analysis of trans-fatty acids in partially hydrogenated vegetable oils. Although trans-isomers of C18 carbon length predominate in partially hydrogenated vegetable oils, trans-isomers of C20 and C22 carbon length occur in partially hydrogenated fish oil. We report a simple silver ion chromatographic combined with capillary GC technique for quantitative analysis of trans-monoenes derived from partially hydrogenated fish oil. Silver nitrate thin-layer chromatographic (TLC) plates are developed in toluene/hexane (50:50, vol/vol). Fatty acid methyl esters are separated into saturates (Rf 0.79), trans-monoenes (Rf 0.49), cis-monoenes (Rf 0.27), dienes (Rf 0.10), and polyunsaturated fatty acids with three or more double bonds remaining at the origin. The isolated trans-monoenes are quantitatively analyzed by capillary GC. The technique of argentation TLC with GC analysis of isolated methyl esters is highly reproducible with 4.8% variation (i.e., coefficient of variation, CV%) in R. values and 4.3 and 6.9% CV% in quantification within batch and between batch, respectively. Furthermore, the combined technique revealed that direct GC analysis underestimated the trans-content of margarines by at least 30%. In this study, C20 and C22 trans-monoenes were found in relatively large quantities; 13.9% (range 10.3-19.6%) and 7.5% (range 5.3-11.5%), respectively, in margarine purchased in 1995, but these C20 and C22 trans-monoenes were much reduced (0.1%) in a fresh selection of margarine purchased in 1998. Compositional data from labels underestimated the trans-content of margarines, especially those derived from hydrogenated marine oil. Low levels of C20 transmonoenes (range 0.1-0.3%) and C22 trans-monoenes (range 0.0-0.1%) were identified in adipose tissue obtained from healthy volunteers in 1995, presumably indicating consumption of partially hydrogenated fish oil.

Interactions of flavor compounds with pectic substances.

Investigation has been conducted using equilibrium dialysis on the sorption of flavor compounds (FC) in solutions of low- and high-esterified pectinates (LEP and HEP, respectively). The compounds include 2-acetyl pyridine, 2,3-diethyl pyrazine, 2-acetyl thiophene and some normal 2-ketones. A method of direct gas chromatographic analysis has been developed for the determination of the FC in aqueous solutions at concentrations as low as 10(-3)% v/v. Additional information was obtained by the analysis of circular dichroism (CD) and apparent molar heat capacity. The results indicate: (1) In LEP solutions, sorption of normal aliphatic 2-ketones with chain length greater than C6 presumably proceeds via van der Waals interactions between alkyl groups of the 2-ketone and hydrophobic regions of the pectinate. The sorption increases with increased alkyl chain length. The Gibbs energy of the methylene group transferring from the solvent to the LEP solution is found to be 1.8 kJ/mol for 2-ketones at C7-C9. (2) In acidic media, binding of heterocyclic FC with pectinates mostly proceeds via hydrogen bonding involving the hydrogen atoms in undissociated carboxyl groups in the pectinate macromolecules. It can also be affected by the self-association of the pectinate macromolecules, depending on pH and pectinate concentration. (3) Under neutral conditions, the presence of Mg2+, Ca2 and Zn2+ ions or the formation of the calcium-pectinate gel network has little effect on the sorption of 2-ketones with the LEP. However, in acidic media, metal ions inhibit the sorption of FC through hydrogen bonding. The sorption of 2-octanone in acidic media depends extremely on Ca++ concentration by the correlationship with changes in the structure of LEP solutions in terms of apparent molar heat capacity.

Gas chromatographic profiling analysis of urinary organic acids from nonsmokers and smokers

A rapid profiling and screening procedure is described for the comparative analysis of urinary organic acids among the groups of nonsmokers and smokers. The procedure involves solid-phase extraction of organic acids using Chromosorb P in normal-phase partition mode, with subsequent single-step conversion to tert.-butyldimethylsilyl derivatives, followed by direct gas chromatographic (GC) analysis on dual-capillary columns. A total of forty-two organic acids were positively identified by retention index ( I) matching in urine samples (0.25 ml) from eleven nonsmokers and fifteen smokers studied. When the GC profiles were simplified to their corresponding organic acid I spectra in bar graphical form, characteristic patterns were obtained for each individual as well as for each average of nonsmoking and smoking groups. When stepwise discriminant analysis was performed on GC data after omitting hippuric acid, seven acids were selected as the variables most discriminating between smokers and nonsmokers. The star symbol plots drawn based on these discriminants were characteristic of each individual and group average, enabling to distinguish smokers from nonsmokers. And canonical plot produced by canonical discriminant analysis using the same variables as the data vectors displayed two separate clusters representing each group.

Use of Cold On-Column Injection for the Analysis of Putrescine and Cadaverine by Gas Chromatography

Putrescine and cadaverine are biogenic diamines that are produced during fermentation or spoilage of different food products. Therefore, they are used as indicators to determine food quality. Unfortunately, their chemical analysis is not an easy task. The use of cold on-column gas chromatographic (GC) injection together with a new base-deactivated fused silica capillary column is evaluated for the direct GC analysis of putrescine and cadaverine. Multiple injections of standard mixtures containing putrescine, cadaverine, and heptylamine are made in order to determine reproducibility. Standard mixtures at concentrations from 20 to 400 ppm are also injected to determine the linearity of response with both the column and the flame ionization detector. Excellent resolution of the three amines is achieved as well as symmetrical peaks. Multiple injections produce reproducibilities of 1 % for peak areas and 0.03% for retention times. A good linear response is obtained between concentrations of 20 to 400 ppm.

An improved method for the recovery of petroleum hydrocarbons from fish muscle tissue

A method based on fish muscle hydrolysis with NaOH followed by addition of CaCl 2, and steam distillation, with direct analysis of a dichloromethane extract of the distillate by capillary gas chromatography (GC), was evaluated for the recovery of bioaccumulated water-soluble hydrocarbons from high-fat salmon muscle with the objectives of combining simplicity and minimal contamination by other materials. The 17 hydrocarbons tested as model compounds were primarily aromatics in the benzene-methylnaphthalene range, including several substituted benzenes and naphthalenes but as well included some shorter chain aliphatic compounds. The mixture is representative of the major hydrocarbons present in the muscle of salmon which have been exposed to the seawater-soluble fraction of crude oil. Recovery studies were conducted at three concentration levels. The standard solutions were spiked into 20 g of homogenized salmon fillets and the mixture hydrolysed in NaOH solution for 2 h in a stoppered distillation flask. This was followed by the addition of CaCl 2 in solution to reduce foaming and 2 ml of dichloromethane as a co-distillation solvent. The mixture was then steam distilled with the dichloromethane recovered in the distillate as the hydrocarbon carrier. Such distillates contained negligible amounts of non-hydrocarbon material, and were therefore suitable for direct GC analysis without the need of any clean-up step. Recoveries for all hydrocarbons analyzed were high and reproducibility acceptable in the higher spiking levels. The major advantage of the method is a reduced background level of non-hydrocarbon volatiles from control salmon flesh. This increases the sensitivity of determining bioaccumulated tainting hydrocarbons while maintaining the basic simplicity of steam distillation.

A Method for the Direct Isolation and Gas Chromatographic Analysis of Milk Flavor Components Using a Programmed Temperature Vaporizer

A simple and sensitive method has been developed for the analysis of the volatile flavor components of milk. The volatiles are purged from samples at low temperature and then concentrated in the liner of a programmed temperature vaporizer; the liner is packed with porous polymers. Subsequently, on-line thermal desorption and capillary gas chromatography are used for analysis. The variables have been optimized by using the simplex optimization technique. To illustrate its potential usefulness, the proposed method has been applied to the differentiation of milks that were subjected to heat treatments.