Gas Chromatographic Peaks Research Articles

A METHOD FOR MEASURING OCTANOL:WATER PARTITION COEFFICIENTS OF HIGHLY TOXIC ORGANOPHOSPHORUS COMPOUNDS

Octanol:water partition coefficients (log P) can be used to provide invaluable information for the overall understanding of the uptake, distribution, biotransformation, and elimination of a wide variety of chemicals. This is especially important in the study of certain classes of organophosphorus compounds, e.g., chemical warfare agents. Although several analytical methods currently exist for providing the necessary data for determination of log P values, their use in the study of chemical warfare agents has been limited due to the rapid hydrolysis and biolethality of this class of compounds. To overcome those limitations a rapid, convenient, and safe method for generating the required data was developed using a computer-controlled gas chromatograph (GC) equipped with a nitrogen phosphorus detector. For method development four organophosphorus compounds were used:diisopropyl methyl phosphonate (DIMP), diethyl p -nitrophenyl phosphate (paraoxon), diisopropyl fluorophosphate (DFP), and pinacolyl methylphosphonofluoridate (soman). The method was capable of determining the concentration of DIMP in hexane over the concentration range of 1 to 1 × 10-4 mg / m L by comparing the GC peak areas to analytic standards (r2=0.999). The latter concentration was found to be the limiting concentration for qualitative analysis. A concentration of analyte of 10-2 mg / m L was found to provide excellent quantitative results. The intra- and interanalysis coefficient of variation (CV) for DIMP, over the concentration range 10-1 to 10-2 mg / m L in octanol subsequently diluted 1:100 in hexane or in water subsequently extracted with an equal volume of octanol followed by a 1:100 dilution in hexane, were 7.3 and 3.5%, respectively. The intra- and interanalysis CV for paraoxon, DFP, or soman, each at a fixed concentration of 10-2mg / m L in the same two solvents and treated in the same manner as DIMP, were 4.0 and 5.6% for paraoxon, 3.3 and 3.6% for DFP, and 0.7 and 1.1% for soman, respectively. Log P values were determined and found to be 0.478 for DIMP, 1.600 for paraoxon, 1.173 for DFP, and 1.824 for soman. All results were in excellent agreement with previously determined theoretical or experimental values. Based on the log P determined for DFP and soman it was possible to estimate the fluorophosphate and fluorophosphonate fragment values to be 2.14 and 1.97, respectively. Using this method toxic organophosphorus compounds at a concentration of 10-2 mg / m L could be analyzed directly in octanol, the organic solvent used for log P determinations. This analytical method was reproducible, safe, and sensitive where determination of the presence of organophosphorus compounds is a desired endpoint for comparative in vitro analytical studies.

Determination of total long-chain fatty acids in human plasma and lipoproteins, before and during copper-stimulated oxidation, by high-performance liquid chromatography.

An improved method for the high-performance liquid chromatographic determination (HPLC) of total or free long-chain (> C12) fatty acids in small volumes (10 microL) of human plasma and lipoprotein samples is described. The method is based on the formation of 2-nitrophenyl-hydrazine (2-NPH) derivatives and offers an alternative to gas chromatographic (GC) fatty acid determination. The retention of 2-NPH fatty acid derivatives on the HPLC system differs from the typical pattern produced by GC separation, thus offering a powerful tool for confirmation of peak identification where GC peak resolution is poor. Fatty acids determined include saturates [myristic acid, C14:0; palmitic acid, C16:0; stearic acid, C18:0; eicosanoic acid, C20:0; docosanoic acid, C22:0; and tetracosanoic acid, C24:0], monounsaturates [palmitoleic, C16:1; petroselenic, C18:1n12; oleic, C18:1n9; and erucic, C22:1], and polyunsaturates [linoleic, C18:2; linolenic, C18:3n3; gamma-linolenic acid, C18:3n6; eicosatrienoic, C20:3; arachidonic, C20:4; eicosapentanoic, C20:5; docosahexanoic, C20:6; and docosatetraenoic, C22:4]. Mean recoveries of fatty acids added to LDL samples were 94.1-109.4%, and intraassay coefficients of variation for the major fatty acids in human plasma were 2.7-6.9%. The potential of the method for further development is discussed. Long-chain fatty acid profiles are given for plasma and very low-, low, and high-density lipoprotein (before and during copper-stimulated oxidation) from human blood.

Source and identity of compounds in a thermomechanical pulp mill effluent inducing hepatic mixed-function oxygenase activity in fish

The source and identity of two mixed-function oxygenase (MFO)-inducing substances present in the primary-treated effluent of a thermomechanical pulp (TMP) mill producing newsprint was determined. The source was pinpointed by exposing rainbow trout (Oncorhynchus mykiss) to various process effluents sampled throughout the mill. Exposure concentrations were based on the flow of these process streams in relation to the final effluent flow. Contaminated TMP steam condensates were identified as the major process source of MFO-inducing substances. Using conventional extraction and fractionation procedures, an MFO-inducing fraction was isolated. The major gas chromatographic peaks in this fraction were identified by gas chromatography/mass spectrometry as juvabione, dehydrojuvabione, and manool, all naturally occurring extractives in balsam fix (Abies balsamea). These substances were extracted and isolated from balsam fir and TMP condensates. Trout exposed to juvabione and dehydrojuvabione responded by exhibiting significant hepatic MFO inductions. No MFO induction was observed for manool. Secondary treatment in an activated sludge system effectively eliminated the MFO-inducing potential of the combined mill effluent consistent with a corresponding 90% reduction of both juvabione and dehydrojuvabione.

Quantitative injections or calibrations in gas chromatography: Anomaly associated with syringe type

When syringes with open needles are used for quantitative injections, e.g. to calibrate gas chromatographic peak areas against the volume of liquid injected, the resultant plot may display an intercept. Current studies on cyclohexane have shown that this effect is so severe that it can make the major contribution to the total signal. Since the magnitude of the anomaly increases with the volatility of the liquid, this means that calibrations of liquid mixtures, or the use of an internal standard, will be unreliable when moderately volatile components are injected with syringes of the above type. The problem is shown to be caused by volatilisation from the needle whilst it is inserted in the heated injection port, and is exacerbated by increasing the temperature of the port or increasing the residence time of the needle. The anomaly is totally avoided if the calibration is performed using a syringe of the type in which the piston passes down the needle. Somewhat similar effects to the ones described can also arise for other reasons, and literature references to such situations are given.

Gas Chromatographic Determination of Acephate in Technical Material and Soluble Powder Formulations: Collaborative Study

Abstract A gas chromatographic (GC) method was developed for quantitation of acephate (Orthene) in technical material and soluble powder formulations. Acephate is an organophosphate with broad spectrum contact and systemic insecticidal properties. Fourteen collaborators from 8 countries participated in a collaborative study of the method. Collaborators were provided with the method; samples of technical acephate and formulated soluble powder containing 95-99% and 71-75% acephate, respectively; an acephate reference standard; and internal standard. Samples were weighed, diluted to volume with internal standard, and quantitated by using GC peak area ratios. Relative standard deviation values for reproducibility (RSDR) were 1.03-2.55 for 95-99% technical acephate and 1.36-2.73 for formulated soluble powder containing 71-75% acephate. The GC method for determination of acephate in technical material and soluble powder formulations has been adopted by AOAC INTERNATIONAL

Isotope fractionation in methane reactions studied by gas chromatography and liquid scintillation

Determination of 14C-marked methane by gas chromatography and liquid scintillation counting is shown to be useful in studies of isotope effects. Data on the specific activity is used to separate the contributions of 14CH 4 and 12CH 4 to the gas-chromatographic peak area. As an application the difference in diffusion for 12CH 4 and 14CH 4 is observed. The total efficiency is determined without resort to a radioactive standard by utilising information on the specific activity.

FLAVOR AND CHEMICAL COMPARISON OF PASTEURIZED AND FRESH VALENCIA ORANGE JUICES1

ABSTRACTFreshly extracted Florida Valencia orange juice was heated at 98C for 11 s to produce lightly heated juice, and for 37 s to produce heavily heated juice. Sensory panels detected a flavor difference between the fresh, unheated juice and the heavily heated juice. Analyses of these two juice samples by headspace gas chromatography (GC) showed no significant qualitative or quantitative differences. GC analyses of methylene chloride extracts of juices and of residues from distillation of the juices showed only one detectable difference in each case. Extracts of the whole juices showed one GC peak, which was significantly larger in the heavily heated juice, identified as the potential off flavor compound, octanoic acid. Extracts of the high‐boiling distillation residue showed one unidentifed late‐eluting GC peak which appeared only in the heavily heated juice. These findings supplement previous reports that more severe pasteurization conditions than required for juice processing cause detectable flavor changes in heated juice.

Accurate spectral scans of single chromatographic peaks

A detector equipped with two channels can fetch accurate luminescence spectra either from migrating chromatographic peaks or from other inputs of sharply varying concentration. To achieve this, one (the scanning) channel is continually referenced to another (the constant-wavelength) channel: the ratio of the two hence portrays correctly the intensity distribution of the spectrum. As a model system for testing this ratio approach on single gas chromatographic peaks, a flame photometric detector was connected to a 1 8 - m scanning spectrophotometer. No significant differences were found in relative spectral amplitudes between scanning the ascent versus the descent of a peak. Similar spectral constancy was obtained when scanning non-chromatographic variable inputs. With a view to practical use, three experiments were carried out that demonstrated the sensitivity and/or the spectral resolution obtainable from reference scans of single peaks. To wit, 5 pg of phosphorus produced a clearly recognizable HPO spectrum: the vibrational levels of S 2 resolved well; and the presence/absence of atomic iron lines confirmed an earlier postulated energy limit for the chemiluminescent excitation reaction.

A Multidimensional Gas Chromatographic Method for Analysis of n-Butane Oxidation Reaction Products

A multidimensional gas chromatographic (GC) method for on-line analysis of all gas-phase reaction products obtained from the selective oxidation of n-butane to maleic anhydride in an automated catalyst testing reactor is described. A dimethylpolysiloxane megabore column and a multidimensional sequence of packed columns that contain molecular sieve 5A and Hayesep R are shown to provide resolution of n-butane, maleic anhydride, various oxygen-containing byproducts, oxygen, nitrogen, and all total combustion products by using a single temperature programmed GC with simultaneous injection of two gas samples from a multiport valve. For applications where various C1-C4 hydrocarbon byproducts are either known or suspected to be present in the product gas and precise resolution is required, an Al2O3/KCl PLOT column can be connected to the dimethylpolysiloxane column by using a series-bypass arrangement with a multiport valve. All hydrocarbons and oxygen-containing hydrocarbons are detected with a flame-ionization detector, and a thermal conductivity detector is used for the permanent gases, total combustion products, and n-butane. To quantify the raw GC peak areas, both absolute and relative response factors are also derived for all species by using gas-phase and liquid-phase calibration standards. Comparisons between experimental and theoretical values for the relative effective carbon numbers of maleic anhydride with those of n-butane are also used to confirm that solute adsorption is linear and reversible on the selected megabore column. Effective carbon numbers for the other analytes detected by the flame-ionization detector are also derived and compared with the theoretical values. Statistical characterization of the response factors and effective carbon numbers is also performed so that an assessment of the errors associated with any derived quantities in future applications of the method, such as gas compositions, reactant conversions, and product yields, will be possible. Incorporation of the proposed method into an automated fixed-bed catalyst test facility and typical results obtained for n-butane oxidation to maleic anhydride over a known metal oxide catalyst are also described. Advantages of the proposed technique over previously reported GC methods for analysis of the reaction products from butane oxidation are also suggested.

Identification and bioassay of kairomones forHelicoverpa zea

Hexane extracts of leaves of 307 accessions from 73 host plant species ofHelicoverpa zea were analyzed by gas chromatography (GC) and used forH. zea oviposition and neonate larvae orientation bioassays. The gas chromatographic (GC) retention times of compounds statistically associated with behavioral activity were identified by correlation of GC peak area with oviposition and larval orientation preferences. Although taxonomically diverse in their origin, compounds for study were purified from extracts of species of the genusLycopersicon, due to their relative abundance. The structures of eight long-chain alkanes associated with oviposition preference were assigned by mass spectrometry, and the structures of five similarly associated organic acids and a terpenoid alkene were identified by(1)H and(13)C nuclear magnetic resonance spectroscopy. The structures of a number of other phytochemicals from the plant leaves were identified for comparative purposes, including a previously unknown terpene, 7-epizingiberene. Bioassays were performed on the isolated acids and on the alkane wax fractions of severalLycopersicon species, and significant differences were found in oviposition stimulation for both classes of compounds. Of the hundreds of compounds found in the extracts, none were observed to act as oviposition deterrents. The results of these bioassays may be useful in explaining the broad host range ofH. zea, as well as the process and evolution of host plant selection for oviposition.

Volatile Compounds Produced by the Reaction of Leucine and Valine with Glucose in Propylene Glycol

Equimolecular amounts of amino acids (l-leucine and l-valine) and d-glucose were refluxed at 140 °C for 2 h in propylene glycol. The volatiles were isolated by simultaneous steam distillation and extraction. The extracts were separated in advance by column chromatography on silica gel with a benzene−ether gradient. The resulting fractions were analyzed by GC and GC-MS. Twenty-seven compounds, including 13 pyrazines, 6 dioxolanes, 2 aldehydes, 2 acids, and 2 pyridines, were identified. Dioxolanes, aldehydes, and pyrazines are the major volatiles, and the percentages of their total gas chromatographic peak areas are 41.13, 23.67, and 17.69 respectively. Formation pathways and sensory properties are described. Keywords: Volatile compounds; Maillard reaction; model reaction system

Simple quantitative transesterification of lipids 1. introduction

AbstractA simple and reproducible quantitative method of acid catalysed transesterification of lipids is described, utilisable for sample quantities from several hundreds of μg to several hundreds of mg. It consists in heating the sample with a transesterification mixture (chloroform, methanol and acetylchloride) in a sealed glass ampoule or in a vial and subsequent neutralization of the hydrogen chloride formed with silver carbonate. After centrifugation the supernatant can be analyzed directly by GC, GC‐MS, or also further separated by means of TLC, CC or HPLC Quantitative studies can be carried out especially with lipids containing short chain fatty acids in their molecule, beginning with 2‐methyl propanoic acid (i‐4:0). Description of all significant gas chromatography peaks of individual components of the transesterification mixture is presented, before and after the reaction, in relation to the peaks of methyl esters of short chain fatty acids.

Chlorobornanes in Sediments and Fish 30 Years after Toxaphene Treatment of Lakes

Chlorobornanes (CHBs) were isolated from recent and older sediments as well as from rainbow trout muscle in two prairie lakes that were treated with toxaphene as a pesticide in the early 1960s. Cored sediments representing the years of treatment (1961/1962) contained high concentrations of hepta-, octa-, and nonachlorobornanes (ΣCHBs, 1602 and 500 ng g -1 ) with a similar gas chromatographic peak pattern to technical toxaphene. In contrast, negative ion mass spectrometry confirmed that the near-surface sediments contained hexa- and heptachlorobornanes at levels not normally observed in environmental samples. This suggests that some of the original toxaphene mixture had been dechlorinated and was redistributed by porewater diffusion and/or sediment focusing. Analyses of fish revealed a toxaphene fingerprint similar to that of the dechlorinated material in sediments, suggesting that the chemical was both water soluble and bioavailable. Significant residual CHBs in surface sediments and in fish indicate that other toxaphene-treated lakes should be reexamined.

Simulation of chromatographic peaks by simple functions

The quality of bigaussian (BG) and gaussian-lorentzian (GL) simulations of theoretical exponentially modified gaussian (EMG) chromatographic peaks is compared over a range of relaxation time/standard deviation ratios. The GL simulation has superior features above a ratio of about two. A further comparison is made of “best fit” BG, GL and EMG peaks to experimental points for selected gas chromatographic peak types.

Irradiation‐induced off‐odour in chicken and its possible control

1. Volatiles isolated from irradiated raw chicken were analysed by gas chromatography (GC) in conjunction with olfactory assessment of the effluent carrier gas to locate compounds with strong smells. 2. Sixteen odours of differing intensities were registered, some, but not others, coinciding with recognisable GC peaks. Identifications were made on the basis of retention data, mass spectrometric information and odour quality agreement. 3. Dimethyltrisulphide was found to be the most potent and obnoxious compound (foul gas, sulphurous), followed by cis-3- and trans-6-nonenals (soapy), oct-1-en-3-one (mushroom) and bis(methylthio-)methane (foul). With the exception of oct-1-en-3-one, none of these compounds has been reported before in irradiated raw chicken. 4. alpha-Tocopherol and ascorbic acid induce stability in tissues in vivo and post mortem. Chickens were reared on diets supplemented with high concentrations (800 mg/kg food) of each of these vitamins. Yields of irradiation volatiles from the tissues of these birds were very much reduced, compared to yields from similar tissues from birds fed unsupplemented diets. 5. Concomitantly with the reduced yield of volatiles, less odour was associated with the samples when analysed by GC-olfactory analysis. 6. The use of enhanced concentrations of the two vitamins in combination in the diet of poultry may provide a means of controlling development of off-odour in irradiated raw chicken, thus improving acceptability to the consumer.

Automated determination of s-triazine herbicides using solid-phase microextraction

Solid-phase microextraction (SPME) allows the determination of pollutants in aqueous solution by the adsorption of analytes onto stationary-phase coated fused-silica fibres followed by thermal desorption in the injection system of a capillary column gas chromatograph. This technique has been fully automated using a Varian 8100 autosampler and 3400 gas chromotograph fitted with a nitrogen-phosphorus flame thermionic detector. Fibres coated with 7-μm and 10-μm film thicknesses were used to evaluate the adsorption and desorption of four s-triazines. The resulting gas chromatographic peaks desorbed from the fibres were shown to be comparable to those obtained with direct manual injection. The 7-μm fibre, designed for the analysis of semi-volatile analytes was used to investigate the effect of desorption temperature and on-column focusing temperatures on peak response. The desorption temperature was found to be non-critical and an optimum focusing temperature of 40°C was used throughout the analysis. Evaluation of the 100-μm film fibre demonstrated its potential to adsorb greater quantities of analyte from solution and this study established that an adsorption time of 15 min gave an equilibrium distribution of the solutes between the stationary and liquid phases. With the thicker film fibre it was noted that the effectiveness of the desorption process was reduced at temperatures below 140°C. The linear dynamic range of the technique was evaluated over three orders of magnitude. To enhance method sensitivity, the fibre was used to extract a 0.1 ppb solution of herbicide by repeatedly adsorbing and desorbing from the same solution and focusing the combined solutes at the front of the analytical column prior to elution and analysis.

Quantitation of flavor volatiles in oxidized soybean oil by dynamic headspace analysis

A dynamic headspace procedure was developed for isolating the volatiles from oxidized soybean oil and trapping them on an adsorbent under conditions that gave minimal decomposition of hydroperoxides (50°C for 30 min at a helium flow of 75 mL/min). The volatiles were desorbed from the adsorbent and separated by gas chromatography (GC) on a methyl silicone capillary column. Equations were derived from theoretical considerations that allowed the actual concentration of each flavor component in the oxidized oil to be calculated from the area of the GC peaks. The reliability of the method and calculations was demonstrated by recovery experiments. The concentration of 2‐heptanone in a mineral oil emulsion, equivalent in flavor intensity to each component, was calculated and summed to estimate the overall flavor intensity of the samples. The overall estimations were compared with the concentrations of 2‐heptanone observed to be equivalent in flavor intensity to the oxidized oil samples when these were tasted in emulsion. The concentrations of individual components calculated from the headspace volatiles data were all present at concentrations below their flavor thresholds, and the simple sum of the intensities of their flavors generally accounted for less than half of the flavor intensities of the oil samples. The differences in the headspace and sensory analyses might be attributed to the flavor of the unoxidized oil, synergistic interactions, and/or the presence of unmeasured flavors components.

Enhanced selectivity in ion mobility spectrometry analysis of complex mixtures by alternate reagent gas chemistry

Ion mobility spectrometry (IMS) analysis of a complex mixture of volatile organic compounds (VOCs) and organophosphorus compounds (OPCs) at vapor levels of 10–40 mg/m 3 produced mobility spectra with broad profiles illustrating limitations of ion mobility spectrometry (IMS) for screening such mixtures. Preseparation of this mixture with a gas Chromatograph inlet to an ion mobility spectrometer enhanced analytical selectivity although OPC detection was complicated by co-elution with other VOCs. Water reagent gas in the ion source of the ion mobility spectrometer yielded 46 gas Chromatographic peaks in a mixture of 45 VOCs and 19 OPCs. Co-elution of two materials was observed in eight of the Chromatographic peaks and co-elution of three materials occurred in four instances. Further selectivity was gained using reagent gases of elevated proton affinity in the ion source. Reagent gas chemistry for acetone and dimethylsulfoxide reduced the number of GC peaks to 26 and 20, respectively. Moreover, spectral integrity and quantitative response for OPCs were retained at 50 to 1000 pg levels with these reagent gases. For OPCs, analyte ions were shown to be of the type M 2H + under these conditions of analysis and the mobilities of the product ions were independent of reagent gas. Reduced mobility values were assigned to OPC spectra using a well-characterized OPC ion as the reference. Spectral profiles and reduced mobilities suggested that the OPC product ions were not clustered with reagent gas molecules at 100 °C.

On the conversion of a mass selective detector from gas chromatography/mass spectrometry application to stand-alone, on-line, real-time mass spectrometry application

Development of initial versions of hardware and software for a Hewlett-Packard HP-5971A mass selective detector (MSD) was performed in order to improve its capabilities for monitoring gaseous waste streams. The development allowed for initial evaluation of the feasibility of the conversion of the MSD from an integrated gas chromatography/mass spectrometry (GC-MS) detector to a stand-alone, on-line, real-time mass spectrometry (SOR-MS) detector. Chemical compound quantitation is accomplished via mass spectrometric step heights, rather than gas chromatographic peak areas or heights. Contributions of more than one chemical compound to a single M/Z ratio are accounted for by nonlinear deconvolution of the mass spectrum, provided a calibration can be obtained for all contributing chemical compounds. The enhanced MSD was tested by a number of calibration and monitoring experiments in order to evaluate the potential and limitations of the conversion. With sufficient care and awareness of the potential and limitations, it appears to be possible to convert the GC-MS detector to a SOR-MS detector. In particular, this conversion could allow for simple monitoring of reactive chemical compounds.

Volatile compounds produced from peanut oil heated with different amounts of cysteine

Volatile compounds produced in the headspace of peanut oil heated with cysteine (0.5-10.0 g) at 200 o C for 5 h were collected by a simultaneous purging and solvent extraction apparatus. Volatiles in the extracts were analyzed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS). Ninety-eight compounds were positively identified among over 150 gas chromatographic peaks. The majority of these were sulfur-containing compounds such as thiophenes, thiazoles, thiazolines, thiazolidines, and cyclic polysulides. The two most abundant sulfur-containing compounds were 2-methylthiazolidine and 2-methyl-2-thiazoline. Decarboxylation of cysteine at high temperatures and subsequent reaction with aldehydes was proposed as a formation mechanism of both 2-alkylthiazolidines and 2-alkyl-2-thiazolines