Gas Chromatographic Response Research Articles

Method to predict gas chromatographic response factors for the trace‐level analysis of volatile organic compounds based on the effective carbon number concept

A procedure has been developed to estimate GC-MS response factors based on the theory of effective carbon number defined as the sum of the carbon number and carbon number equivalent for each selected molecular descriptor (multiplied by its number of occurrences) in each compound's molecular structure. As a means to validate the effective carbon number procedure for GC-MS analysis, a test suite of 19 volatile organic compounds was analyzed by the sorbent-tube thermal desorption method. In the effective carbon number procedure, the carbon number equivalent for each descriptor was determined to yield the optimal linear plots between response factor versus the effective carbon number with the maximum R(2) (>0.975) and the minimum mean absolute error (<5%). Effective carbon number analysis is validated as a potent approach to estimate response factor values for most compounds amenable to the sorbent-tube thermal desorption GC-MS method. Overall, it is concluded that the application of response factor versus effective carbon number relationship can produce fairly reliable prediction with reduced errors relative to other comparable procedures such as the response factor versus the carbon number approach.

Dissipation kinetics and assessment of processing factor for chlorpyrifos and lambda-cyhalothrin in cardamom

The dissipation kinetics and method for estimation of residues of chlorpyrifos and lambda-cyhalothrin in cardamom were studied and developed. The limit of detection and limit of quantitation arrived for the compounds were 0.01 and 0.025 μg g(-1), respectively. Gas chromatographic response of chlorpyrifos and lambda-cyhalothrin residues was linear in the range of 0.01-0.50 μg g(-1) and the mean recovery obtained was 97.3 % for chlorpyrifos and 98.9 % for lambda-cyhalothrin with satisfactory relative standard deviation values. The mean initial residues of chlorpyrifos applied at a concentration of 0.05 % in cardamom was 2.5 μg g(-1) and the residue was 8.1 μg g(-1) after processing, with a processing factor of 3.24, while lambda-cyhalothrin when applied at 0.0025 % resulted in initial residues of 1.63 μg g(-1) that magnified to 4.86 μg g(-1) on curing, with a processing factor of 2.98. The half-life of chlorpyrifos was in the range of 5.1-5.24 days while that of lambda-cyhalothrin was in the range of 4.40-4.55 days. The processing factor arrived at in the above experiment lead to the conclusion that the residues of chlorpyrifos got magnified to 3.24-3.68 times and that of lambda-cyhalothrin got magnified to 2.98-3.46 times of initial residues, consequent to loss of weight due to dehydration during curing.

Recoveries of four representative organophosphorus pesticides from 18 plant products belonging to different botanical categories: Implications for matrix effects

A study of the matrix effect was performed in order to evaluate the influence of some matrices on the gas chromatographic responses of representative organophosphorus insecticides. In particular, three fortification levels and the maximum residual level of four organophosphorus pesticide standard solutions were added into 18 matrices (15 belonging to five specific botanical categories and three botanically unclassified). The recoveries of the examined pesticides were determined using a simple official multiresidue method of detection, without any additional clean-up step. Recoveries were estimated based on standards prepared in solvent. The recovery percentages of the most polar pesticides, especially methamidophos (20.2–288.4%), were much higher than those of non-polar ones (chlorpyrifos and methidathion 25.7–136.4 and 37.6–292.6%, respectively), except for quince. Pronounced matrix effects (>120%) were observed to the lowest fortification levels (maximum residual levels established by the European Union), of dimethoate and methamidophos (i.e. the most polar analytes) in the majority of product extracts. Furthermore, it was proved that there is no correlation between the classification of plant products, according to their botanical characteristics, and the recoveries of pesticides in the equivalent extracts. Indeed, there were significant differences in the recoveries of pesticides in extracts, derived from matrices belonging to the same botanical group (especially in the categories of pome fruits and citrus). Consequently, it was shown that, under the examined conditions, obtaining recovery data, derived from only one representative matrix, with the purpose to validate an official method in its botanical category is, possibly, an erroneous practice.

A SIMPLE QUANTITATIVE CHEMICAL RECOVERY METHOD OF OXYFLUORFEN (2-CHLORO-1-(3-ETHOXY-4-NITROPHENOXY)-4-(TRIFLUOROMETHYL) (GOAL®)1 FROM LABORATORY FORTIFIED SOIL AND WATER SAMPLES

Oxyfluorfen herbicide chemical extraction and analyses techniques from both soil and water are time consuming or/and require the use of HPLC. Our paper reports the results of a simple method for the extraction and gas chromatographic (GC) analyses in laboratory oxyfluorfen fortified soil and water samples. A representative field sample from a Byler silt loam (Typic fragiudalf, mixed, thermic) soil site was collected from the 0–10 cm depth, and appropriately air-dried and sieved (2-mm size). Soil had an organic matter (O.M.) content (Walkley Black) of 2% w/w, and pH of 6.5. The soil sample was fortified with 0.05 to 2.0 μg/g levels of oxyfluorfen herbicide, extracted (1:5 soil: acetonitrile), and directly injected into the HP 6890 gas chromatograph (μ-ECD detector). Deionized (DI) laboratory water samples were spiked at the 0.010 and 0.100 μg/mL levels. They were intermittently shaken by hand with n-hexane for 3 min., and the supernatant directly analyzed by GC. Quantitative recoveries of the herbicide in duplicate samples were obtained in both soil and water. Recovery percentage as well as GC response suggest that soil MDLs of at least 0.005 μg/g in soil are achievable. Organic solvent/water ratio does not appear to affect the recovery of oxyfluorfen from fortified water samples. However, the solvent exchange step in soil extraction can affect the oxyfluorfen recovery. These extraction techniques represent considerable savings in time and labor compared with traditional methods. *Oxyfluorfen; 2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl) benzene, trade name Goal®, was obtained from the Rohm and Hass Company, Independence Mall West, Philadelphia, PA. 19105. Trade names and company names are mentioned with the understanding that no discrimination is intended nor endorsement implied.

Gas chromatography with electron-capture and nitrogen–phosphorus detection in the analysis of pesticides in honey after elution from a Florisil column: Influence of the honey matrix on the quantitative results

A modified procedure to extract pesticides from honey samples that involves loading the honey onto a Florisil packed column and subsequently eluting it with an n-hexane–dichloromethane mixture is proposed. Anomalous high gas chromatographic responses and subsequently very high recoveries for the pesticides in the extracts were obtained by a conventional calibration with pesticide solutions in organic solvent. This effect was attributed to the honey matrix and can be circumvented by using spiked honey extracts as calibration standards.

Determination of fenthion and its oxidative metabolites in olives and olive oil: Errors caused by matrix effects

A method for determination of fenthion residues and its oxidative metabolites (fenoxon, fenthion sulphoxide, fenthion sulphone, fenoxon sulphoxide, fenoxon sulphone) in olive pulp, oil and water obtained from olive pressing is described and evaluated. Recoveries for the improved gas chromatographic conditions were: 92–104% for fenthion, fenoxon, fenthion sulphone and fenoxon sulphone in the olive pulp matrix, whereas the mean recovery values for fenthion sulphoxide and fenoxon sulphoxide were respectively 140% and 118%. In the vegetation water matrix, the recoveries for fenthion, fenthion sulphoxide, fenoxon sulphoxide and fenoxon sulphone averaged between 91% and 103%. Higher values were obtained for fenoxon (139%) and for fenthion sulphone (115%). Recoveries for all metabolites in the oil matrix averaged between 100% and 104%. Detection limits ranged between 0.005 ng for fenthion to 0.02 ng for fenoxon sulphoxide and fenoxon sulphone. Three different gas chromatographic conditions are presented in order to underline how their choice could strongly affect the result because of the matrix effect on the gas chromatographic response. In some cases such an effect can increase the instrumental response up to three times the real value.

Matrix-induced effects: a critical point in the gas chromatographic analysis of pesticide residues

The influence of several experimental factors related to the enhanced gas chromatographic responses yielding apparent recoveries of pesticide residues greater than 100% was investigated. Optimisation of a gel permeation chromatographic clean-up step with respect to the trueness and precision of generated data was performed. An increase of relative detector response (100%=response of analyte in pure solvent solution) was evidenced to be dependent both on the concentration of the analyte and the character of the matrix: pronounced matrix-induced effects were observed particularly in orange and wheat extracts at low concentration levels of analytes (especially for GC–electron-capture detection analysis of certain pesticides). As soon as the splitless injector became contaminated after injection of large series of matrix-containing samples, a decrease of relative responses of pesticides, largely below 100%, was experienced. Although troublesome compounds tending to give matrix-induced effects can be identified, and increased recoveries may be tentatively predicted, poor accuracy of generated data can be presumed as long as quantitation is not based on a standard prepared in blank matrix extract to compensate for matrix-induced effects.

Characterization of catalysts for methane coupling by means of thermal methods

Abstract The activity of two catalysts for methane coupling, 1% K 2 O/ZnO and 2% Li 2 O/Y 2 O 3 , has been in- vestigated in the temperature range 680–830°C by using temperature-programmed reaction techniques, either following the gas chromatographic response or the electrical conductivity response: in that way it was possible to obtain the progress of the catalytic activity and of the related selectivity to carbon oxides and to C 2 hydrocarbons. Furthermore, temperature-programmed desorption curves have been studied, either performing adsorption of methane and specifically of oxygen, or performing adsorption of carbon oxides and of C 2 hydrocarbons: the results obtained from the two catalysts allow us to deduce that only a small amount of methane and of oxygen are still anchored on the surface of 1% K 2 O/ZnO at the reaction temperatures, while a considerable presence of adsorbed species in the same range of tem- peratures was observed on 2% Li 2 O/Y 2 O 3 . These results, as well as those obtained by the kinetic analysis of the desorption peaks allow us to stress that homogeneous gas-phase processes are markedly prevailing when zinc oxide is used, while heterogeneous processes play an important role when yttrium oxide is used.

Prediction of gas chromatographic response factors by the PLS method

“Dietz” response factors (RF) have been measured under standard conditions for 100 substituted benzenes and pyridines. The data have been treated by the partial least squares (PLS) method, using as explanatory variables the molecular weight together with structural features such as the numbers of atoms of each element and of multiple bonds, functional groups, etc. The “Dietz” RF are explained to 84% of the variance by three PLS components. It is shown that Dietz RF can be predicted from the structural formulae for many classes of compounds with an average deviation of 0.05 within the model and 0.09 out of the model. This should be of considerable utility in the quantitative analysis of complex product mixtures by GC/MS, especially for those cases where some or all of the products are unavailable. The PREDICT.EXE program for the PC and Fortran code, PREDICT.FOR, as well as the raw data set used to derive the model are included on disk.

Gas Chromatographic Determination of Capsaicinoids in Green Capsicum Fruits

Abstract A new gas chromatographic (GC) method is described that uses a simple on-column methylation for the quantitative determination of the major naturally occurring capsaicinoids. Capsaicinoids are extracted from the dry sample with acetone in a Soxhlet apparatus, purified by extraction with petroleum ether and water, mixed in a microsyringe with trimethylanilinium hydroxide solution, and injected onto the GC column. The methylated capsaicinoids thus formed require 16 min for elution. The gas chromatographic response is linear with increased concentration of capsaicinoids

Variations Among Gas Chromatographic Measurements of Ethylene Standards

Abstract A survey was conducted to measure the variability among gas chromatographic analyses of ethylene and the variability among the ethylene gas standards used in 22 laboratories in the United States and Canada. The linearity of gas chromatographic responses to injections of 10.0, 1.0, and 0.1 μl/l ethylene in air mixtures had an average coefficient of linear determination (R2) of 0.9984, with a SD among the R2 values of only 0.33%. The ethylene concentrations calculated for the laboratory standards varied from 11.3% higher to 21.7% lower than the value assumed by the participant; with an average variance of about 6% lower.

Determination of cyanide ion by homogeneous catalysis and gas chromatography

A simple procedure for the determination of trace levels of cyanide ions is described and evaluated. The procedure is based on the cyanide-catalyzed cleavage of benzil in the presence of methanol to produce benzaldehyde and methyl benzoate. The concentrations of these products are determined by gas chromatography. Linearity between gas chromatographic response and cyanide concentration is observed from 0.05 to 3 ppm cyanide; a detection limit of 1 ppb is calculated. A series of anions is shown not to interfere.

Comparison of electron-capture and electrolytic conductivity detection for the gas-liquid chromatographic analysis of some perfluoro derivatives of four agricultural chemicals

The gas chromatographic response of four perfluoro derivatives of four agricultural chemicals [diethylstilbestrol (DES), clopidol, linuron and carbofuran] was examined. The derivatives varied in fluorine content from 3 to 30 atoms per molecule. The sensitivities of the derivatives were found to be ca. 10–100 times greater by electron-capture detection than by electrolytic conductivity detection (halogen mode). The sensitivity also was found to increase with increasing fluorine content for all derivatives by electrolytic conductivity, whereas by electron capture, DES and clopidol exhibited similar responses with either the penta-, hepta- or pentadeca-fluoro derivatives. The sensitivity of linuron and carbofuran derivatives by electron capture varied with increasing fluorine content. For detection by electron capture, the heptafluorobutyryl derivatives were preferred for DES and clopidol, and either the heptafluorobutyryl or the pentadecafluorooctanoyl derivatives for linuron and carbofuran. For detection by electrolytic conductivity, the pentadecafluorooctanoyl derivatives were superior for all four compounds.

A MICROTITER GAS CHROMATOGRAPHIC ASSAY FOR STUDY OF THEOPHYLLINE PHARMACOKINETICS

Therapy of childhood asthma with theophyllines has been controversial and less than optimal because of a variable dose response relationship and unpredictable toxicity. Theophylline plasma concentration appears to correlate directly with pharmacologic effect and toxicity. The standard spectrophotometric (SP) method for theophylline is time consuming, requires large volumes of blood and is inaccurate in the presence of caffeine and other xanthines. An accurate gas chromatographic (GC) assay was developed and compared with the SP method. The GC response was linear from 0.5 to 100μg/ml of plasma. Recovery of theophylline was 95%. Ten children receiving theophylline preparations by oral and intravenous routes were studied. Peak plasma theophylline levels with the SP method ranged from 7.6 to 30.1 μg/ml, mean 18.2 ± 7.6; with the GC method the range was 4.8 to 18.4 μg/ml, mean 11.2 ± 4.6. Caffeine and other xanthine compounds did not interfere with the GC determinations. Assuming a two compartment open system model and first order kinetics, plasma half time (t½) and elimination constants were calculated individually to determine a dose which would insure maximum therapeutic response and avoid toxic symptoms. This rapid, accurate, specific, microtiter gas chromatographic assay has direct application to patient care.

Gas chromatography of substituted phenylureas by flash-heater methylation with trimethylanilinium hydroxide

A simple method for the methylation of substituted phenylureas with trimethylanilinium hydroxide (methelute) is described. The methylation reaction product was the 3-N-methyl analog of the substituted phenylurea. Seven phenylureas with a variety of structures were analyzed using six different chromatographic columns. The gas chromatographic response was linear with increased concentration of the 3-N-methyl analogs of monuron and diuron for both synthetic standards and flash-heater reaction products. Maximum methylation was obtained when the ratio of methylating reagent to phenylurea was approximately 2.5:1. This methylation technique provides for rapid qualitative and quantitative chromatography of substituted phenylureas without side-chain decomposition.

Gas chromatographic response to injections of acetic acid in water.

Gas chromatographic response to injections of acetic acid in water.

Comparative gas chromatographic response of organophosphorus compounds containing nitrogen and nitrogen compounds with alkali flame and electrolytic conductivity detectors

Both the alkali flame detector and the electrolytic conductivity detector can be used for the gas chromatographic determination of nitrogen compounds. While the electrolytic conductivity detector is specific, the alkali flame detector responds to various hetero atoms. The response for a series of organonitrogen and organophosphorus compounds contianing other hetero atoms were determined under the same experimental conditions, and the results were compared. The response of the alkali flame detector is grater for phosphorus than for nitrogen and the results obtained suggest that the magnitude of response is dependent on structural effects. In the absence of phosphorus, compounds containing only nitrogen showed no obvious structural correlation. With the electrolyte conductivity detector the response is purely a function of the amount of nitrogen in the molecule. The responses of these two detectors to nitrogen compounds were of the same order.

The gas-chromatographic determination of cyclohexylamine in blood as the trifluoroacetyl derivative.

A method is described for the determination of small concentrations of cyclohexylamine in blood. The procedure involves the conversion of cyclohexylamine into its trifluoroacetyl derivative after extraction from blood, followed by gas chromatography, with a flame ionisation detector. The trifluoroacetyl derivative has an increased gas-chromatographic response, and its stability enables the blood extracts to be concentrated to small volume, factors that facilitate the detection of cyclohexylamine in blood at levels down to 0·1 µg ml–1.

Determination of Flavor Threshold Levels and Sub‐Threshold, Additive, and Concentration Effects

SUMMARY— The flavor threshold concentrations were determined for 23 compounds composing an artificial peach beverage base. The threshold levels ranged from 52 ppm to 0.4 ppb. The threshold values were used in sub‐threshold, additive sub‐threshold, and concentration trials. Sub‐threshold and additive sub‐threshold effects resulted from only a few of the flavor combinations tried. Change in concentration of one compound in a mixture of six compounds was not readily detectable organoleptically. The difference in concentration could be detected gas chromatographically, but only if the mixtures were extracted with pentane and concentrated.By dividing the respective threshold values into the amount of each compound present in the beverage, a factor was calculated, called the “unit flavor base,” which indicated the relative importance of each compound as a flavor substance in the peach beverage. By dividing the gas chromatographic peak heights by the respective threshold values, gas chromatographic response was weighted for flavor importance. When the unit flavor base and weighted gas chromatographic response were converted into logarithms, the relation between the values was nearly linear. The correlation coefficient was 0.8691 (significant @ 0.01).

An Improved Method for Cleanup of Brassica for Parathion Analysis

Abstract An improved cleanup procedure has been developed for the recovery of parathion from crops, with special reference to cole crops. Parathion is separated from many chlorinated hydrocarbon pesticides that could cause interfering gas chromatographic responses. Electron capture techniques were used to detect the chlorinated hydrocarbon pesticides, and colorimetry and electron capture were used to determine parathion.