Electron Capture Detector Response Research Articles

Pixels of chemical structures correlate to chromatographic detector responses using genetic algorithm-adaptive neuro-fuzzy inference system as a novel nonlinear feature selection method

This paper introduces the genetic algorithm-adaptive neuro-fuzzy inference system (GA-ANFIS), as a novel nonlinear feature selection method. This hybrid technique combines genetic algorithms (GAs) as powerful optimization methods with ANFIS as a robust nonlinear statistical method. Multivariate image analysis whose descriptors achieved from bidimensional images coupled to principal component analysis (PCA) and the most significant principal components (PCs) were extracted. Eigen value ranking (EV), correlation ranking (CR), GA-partial least square (GA-PLS) as well the method proposed in this work were used to select the most relevant set of PCs as inputs for ANFIS to assess electron capture detector responses of 207 polychlorinated biphenyls. The results indicated that GA-ANFIS is superior over the others in both selecting the most relevant set of PCs and correlating the inputs (PCs) with the detector responses. The best model was statistically validated for its predictive power using cross-validation, applicability domain and Y-scrambling evaluation procedures. Moreover, the superiority of this model obtained from pixels of chemical structures over the nonlinear one obtained from original molecular descriptors in a previous work indicates that the image analysis is a powerful tool in quantitative structure-(chromatographic) property relationship (QSPR) studies.

Structure and origin of the natural halogenated monoterpene MHC-1 and its concentrations in marine mammals and fish

The halogenated natural product previously named mixed-halogenated compound 1 (MHC-1) was isolated from the red seaweed Plocamium cartilagineum harvested in Helgoland, Germany. A total of 1.9 mg of pure MHC-1 was obtained from 1 g air-dried seaweed. The 1H and 13C NMR data matched those reported for a natural monoterpene isolated from this species. Thus, the structure of MHC-1 was established to be (1 R,2 S,4 R,5 R,1′ E)-2-bromo-1-bromomethyl-1,4-dichloro-5-(2′-chloroethenyl)-5-methylcyclohexane. Moreover, the isolated monoterpene proved to be identical with the compound previously detected in marine mammals and fish from different locations. In addition we examined two samples of P. cartilagineum from Ireland and from the Antarctic; however MHC-1 was only present at low levels. Not only the concentrations were lower but also the pattern of polybrominated compounds differed from MHC-1. A calibrated solution of MHC-1 was used to determine correct concentrations from samples where previously only estimates existed relative to the gas chromatography–electron capture detector (GC/ECD) response of trans-chlordane, which underrated the MHC-1 concentrations by more than factor 2. The highest MHC-1 concentration determined to date in marine mammals is 0.14 mg kg −1 blubber. Significantly higher MHC-1 concentrations were determined in farmed fish with up to 2.2 mg kg −1 lipids. The samples with high concentrations of MHC-1 have in common that they were collected in proximity of the natural habitats of P. cartilagineum.

Using Gas-Phase Molecular Descriptors to Predict Dechlorination Rates of Chloroalkanes by Zerovalent Iron

Reductive dehalogenation of chlorinated compounds is the most important process occurring within the zerovalent iron (Fe0) barrier. The relative reaction rates of individual halocarbons with Fe0 can vary considerably. This variability has been the stimulus for using various chemical descriptors for a priori predictions of transformation rates via linear free-energy relationships (LFERs). Our objective was to determine the efficacy of four molecular descriptors to describe the transformation rates of three chloromethanes, three chloroethanes, and six chloropropanes by Fe0. This was accomplished by generating an internally consistent set of rate constants under controlled environmental conditions (16 degrees C, anaerobic) and regressing the surface-area normalized rate constants (k(SA)) against (i) energy of the lowest unoccupied molecular orbital (E(LUMO)); (ii) vertical attachment energies (VAE); (iii) thermal electron attachment rate constants; and (iv) the molar response from a commercial electron capture detector (ECD). Results showed good correlations between k(SA)'s and all four descriptors (r2: 0.72-1.0), but a separate trend line was required for the chloromethanes and the chloro- ethanes/ propanes. Given the availability and ease with which ECD response can be obtained, this physical measurement may provide a practical means of determining relative rates of reactivity of various halocarbons in permeable reactive iron barriers.

Electron Capture Detector Response and Dissociative Electron Attachment Cross Sections in Chloroalkanes and Chloroalkenes

Electron capture detectors (ECDs) are widely used in gas chromatography to detect electronegative compounds. In this work, we examine the connections between the ECD response and the cross sections for dissociative electron attachment (DEA) determined from low energy electron beam studies in the chloroalkane family, stressing in particular the role of temporary anion state energies. We show that attachment rate coefficients computed from these cross sections are well correlated with ECD response, and that the latter decreases exponentially with increasing energies of the lowest anion states. ECD measurements are also carried out in monochloroalkanes substituted with unsaturated ethenyl and phenyl moieties, and the response is shown to depend strongly on the mixing between the unsaturated pi* and the C-Cl sigma* temporary anions as exhibited by the vertical attachment energies (VAEs) of these states. The results show good correlations between the chloroalkene and phenyl chloride ECD responses and the VAEs for the mixed states.

Automated rotary valve injection for polybrominated diphenyl ethers in gas chromatography

AbstractA simple, automated rotary valve injector for gas chromatography was constructed and evaluated for injection of polybrominated diphenyl ethers with molecular weights ranging from 485 to 949 Dalton. These congeners, particularly those with nine and ten bromine substituents, constitute a severe problem when a split/splitless vaporizing injector is used for gas chromatographic determination of polybrominated diphenyl ethers. Volumes up to 50 μL were injected without the need for a solvent vapor exit. Discrimination of the high molecular weight brominated diphenyl ether congeners with seven to ten bromine substituents is strongly reduced. The injector exhibits both high reproducibility and low carry‐over effects. Average relative standard deviation of the absolute peak area was less than 3.5% and for the carryover it was less than 0.4%. An important observation is that the electron capture detector response factors remained constant independent of injection volume, allowing large volume injection to decrease sample concentration detection limits, which is an important feature regarding determination of high molecular weight polybrominated diphenyl ethers. A further advantage is that the rotary valve large volume injector facilitates on‐line coupling of various clean‐up systems to the gas chromatograph.

Non-polar halogenated natural products bioaccumulated in marine samples. I. 2,3,3 ′,4,4 ′,5,5 ′-Heptachloro-1 ′-methyl-1,2 ′-bipyrrole (Q1)

This presentation adds new spectroscopic and analytical data on the natural product Q1 that was recently identified by synthesis as 2,3,3 ′,4,4 ′,5,5 ′-heptachloro-1 ′-methyl-1,2 ′-bipyrrole. Solid state magic angle spinning 13C NMR data of Q1 is presented as an option for structural proof. Furthermore, the UV spectrum of neat Q1 (absorption maximum at 223 nm) was recorded and, with NMR spectroscopic data, confirmed a twisted bipyrrole ring system. A quantitative standard of Q1 was prepared which allowed to correct previous concentration estimates relative to the electron capture detector response factor of trans-nonachlor. As a result, the actual Q1 response was only 0.65 ± 15% of the response factor of trans-nonachlor. Therefore, actual Q1 levels are about 50% higher than the previous estimates. With this result the highest (corrected) Q1 concentration determined to date in the blubber of marine mammals from Australia is 14 mg/kg lipid. Analysis of Q1 and trans-nonachlor in specimens from the German North Sea coast suggests that harbor seals are more able to metabolize Q1 than harbor porpoises. Finally, we calculated that 79 congeners of Q1 (i.e. lower chlorinated 1 ′-methyl-1,2 ′-bipyrroles) are theoretically possible and present their structures.

Gas chromatographic determination of d-/ l-arabinitol ratio in healthy Polish children

The d-/ l-arabinitol enantiomers ratio (a marker of disseminated candidiasis of Candida species) in urine was determined by gas chromatography (GC) in 198 healthy Polish children ranging in age from 0 to 18 years. The urine samples were dry and trifluoroacetic anhydride (TFAA)-treated. Enantiomers derivatives were separated on a chiral column (β-Dex 120, 60 m×0.25 mm I.D.). A glass “solid-phase” injector and electron capture detector (ECD) were used. The ECD response was linear with correlation coefficients 0.999. The limit of detection was 0.02 μmol/l. Good results in terms of reproducibility, accuracy (RSD<10%, bias<6%), and linearity were obtained from real urine samples containing up to 400 μmol/l d-arabinitol. TFA–arabinitol derivatives in biological samples were stable from 1 to 5 days (depending on the arabinitol contents), while TFA–arabinitol standard derivatives were stable for 2 weeks. The identity of d- and l-arabinitol were confirmed by GC–MS analysis. The mean d-/ l-arabinitol ratios ranged from 2.48 to 1.65 in the examined groups. The d-/ l-arabinitol ratio was found to be exponentially regressive with age. A few cases of diagnosis of disseminated candidiasis by the GC method and confirmed by blood culture are described. The described GC method was also used for monitoring antifungal treatment of patients with disseminated candidiasis.

Determination of nitroaromatic, nitramine, and nitrate ester explosives in soil by gas chromatography and an electron capture detector

Hazardous waste site characterization, forensic investigations, and land mine detection are scenarios where soils may be collected and analyzed for traces of nitroaromatic, nitramine, and nitrate ester explosives. These thermally labile analytes are traditionally determined by high-performance liquid chromatography (HPLC); however, commercially available deactivated injection port liners and wide-bore capillary columns have made routine analysis by gas chromatography (GC) possible. The electron-withdrawing nitro group common to each of these explosives makes the electron capture detector (ECD) suitable for determination of low concentrations of explosives in soil, water, and air. GC-ECD and HPLC-UV concentration estimates of explosives residues in field-contaminated soils from hazardous waste sites were compared, and correlation ( r>0.97) was excellent between the two methods of analysis for each of the compounds most frequently detected: 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), 2,4-dinitrotoluene (2,4-DNT), 1,3-dinitrobenzene (1,3-DNB), 1,3,5-trinitrobenzene (TNB), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The analytes were extracted from soils with acetonitrile by 18 h of sonication in a cooled ultrasonic bath. Two soil-to-solvent ratios were evaluated: 2.00 g:10.00 ml and 25.0 g:50.0 ml. GC-ECD method detection limits were similar for the two soil-to-solvent ratios and were about 1 μg kg −1 for the di- and trinitroaromatics, about 10 μg kg −1 for the mono-nitroaromatics, 3 μg kg −1 for RDX, 25 μg kg −1 for HMX, and between 10 and 40 μg kg −1 for the nitrate esters (nitroglycerine [NG] and pentaerythritol tetranitrate [PETN]). Spike recovery studies revealed artifacts introduced by the spiking procedure. Recoveries were low in some soils if the amount of soil spiked was large (25.0 g) compared to the volume of spike solution added (1.00 ml). Recoveries were close to 100% when 2.00-g soil samples were spiked with 1.00 ml of solution. Analytes most frequently found in soils collected near buried land mines were the microbial transformation products of TNT (2-amino-4,6-dinitrotoluene [2-Am-DNT] and 4-amino-2,6-dinitrotoluene [4-Am-DNT]), manufacturing impurities of TNT (2,4-DNT, 2,6-DNT, and 1,3-DNB), and TNT. The microbial reduction products of the isomers of DNT and of 1,3-DNB were also detected, but the ECD response to these compounds is poor.

Comparison of gas chromatography detection limits and relative responses of common alternative fluorocarbons using electron capture, atomic emission, and mass spectrometry detection

Detection limits for several common alternative fluorocarbons (AFCs) using electron capture detector (ECD), mass spectrometry (MS) and atomic emission detector (AED) gas chromatography (GC) are determined and compared. Mass detection limits for chlorofluorocarbon (CFC) CFC-12, and hydrochlorofluorocarbons (HCFCs) HCFC-22, HCFC-123 and HCFC-124 are found to be in the picogram range. Those of hydrofluorocarbons (HFCs) HFC-125, HFC-134a and HFC-152a are found to be in nanograms using ECD and in the picogram range for the MS and AED. HFC-152a was not detected by the ECD. Relative ECD responses for all AFCs are at least one order of magnitude lower than that for CFC-12. The relative MS responses are found to be in the same order of magnitude of that for CFC-12. For the AED, all AFCs had somewhat higher responses than CFC-12 with the exception of HFC-152a, which showed the same response than CFC-12. The mass detection limits for 1 L sample with a signal-to-noise of 3 σ is in the low ppt (v/v) for AED and MS. ECD has a limit of detection of about one order of magnitude higher than other detectors for the HCFCs, and a limit of detection in the nanogram range for the HFCs. MS has slightly higher sensitivity than the AED, and both detector proved to be far better for the detection of the AFCs than the ECD. Preconcentration of whole-air sample is required for all tested methods to make measurements at predicted levels. An amount of 1–10 L samples are needed for AED and MS while ECD requires 10–100 L samples for HCFCs and over 100 L for the HFCs. A more sensitive detector is still needed to perform in situ analysis without preconcentration for the current atmospheric concentrations.

Analytical, Risk Assessment, and Remedial Implications Due to the Co-Presence of Polychlorinated Biphenyls and Terphenyls at Inactive Hazardous Waste Sites

Investigations conducted at three inactive hazardous waste sites in New York State have confirmed the co-presence of polychlorinated hiphenyls (PCBs) and polychlorinated terphenyls (PCTs) in soils, sediments, and biota. The PCTs at all three sites were positively identified as Aroclor 5432, with the most probable source being the hydraulic fluid Pydraul 312A utilized for high-temperature applications. The identification of the lower-chlorinated PCT formulations in environmental samples is problematical, since PCT Aroclors 5432 and 5442 are not chromatographically distinct from the higher-chlorinated (PCB) Aroclors 1254, 1260, 1262, and 1268 using conventional gas chromatography–electron capture detection. Results from this study indicate that U.S. Environmental Protection Agency (USEPA) approved PCB methods routinely utilized by most commercial laboratories based on Florisil adsorption column chromatography cleanup are inadequate to produce valid chromatographic separation and quantitative results with soils, sediment, and biota samples containing both PCBs and PCTs. The presence of co-eluting PCBs and PCTs precludes accurate quantitation due to significant differences in PCB/PCT electron capture detector response factors, and the potential for misidentification of PCT Aroclors as higher chlorinated PCB Aroclors. A method based on alumina column adsorption chromatography was used, allowing for the accurate identification and quantitation of PCB and PCT Aroclors. The results of this study suggest that the utilization of alumina adsorption column separation may have applicability and regulatory significance to other industrially contaminated sites which historically used Pydraul 312A. Inferences.

An Automated System for Near-Real-Time Monitoring of Trace Atmospheric Halocarbons

A new gas chromatographic method developed to quantitatively determine important atmospheric halocarbons is described. Target compounds include replacement CFCs, chlorinated solvents, and biosynthesized (naturally produced) organohalogens, all trace gases in the atmosphere at concentrations ranging from 0.1 to 600 pptv (where pptv = 1 part in 10(-)(12) by volume). A combination of ultralow concentrations and relatively small electron attachment cross sections renders these compounds very difficult to routinely measure in the background air typical of remote atmospheric monitoring stations. Detection is achieved by preconcentration of a 200-mL air sample using an adsorbent-filled microtrap and enhancement of electron capture detector response by oxygen doping one of two detectors connected in series. Oxygen doping specifically targets halocarbons with relatively poor electron attachment rate coefficients. The work described here details construction of a novel analytical system, laboratory trials, and optimization followed by an extended field campaign at a remote atmospheric monitoring station, Mace Head, Ireland. A calibration standard or ambient air sample was acquired every hour using a cyclic, automated procedure without employing cryogenic preconcentration or refocusing. Overall precision of the analytical method for the target compounds is between 0.3 and 1.5%.

Determination of physiological levels of volatile organic compounds in blood using static headspace capillary gas chromatography with serial triple detection.

A static capillary gas chromatographic method using three different detectors [photoionization detector (PID), electron capture detector (ECD) and flame ionization detector (FID)] switched in series is presented for the determination of volatile organic compounds (VOCs) in sub microgram l-1 levels. The method was applied for the analysis of selected environmentally and occupationally relevant non-halogenated and chlorinated aromatic hydrocarbons (e.g., benzene, toluene, xylenes, dichlorobenzenes) as well as chlorinated aliphatic hydrocarbons (e.g., trichloroethene, tetrachloroethene) in blood samples. Detailed investigations, in respect to the figures of merit were carried out. For most of the selected VOCs detection limits (calculated as the three-fold standard deviation of low level calibration standards) in the range from 26 (benzene) to 67 ng l-1 (m/p-xylene) were achieved which are comparable with those reported for dynamic headspace techniques in combination with mass spectrometric detection. For the individual VOCs the within-series precision varied from 4 to 19% and the day-to-day precision from 11 to 28%. Regarding PID as well as FID the calibration graphs for all substances were linear up to at least 10 micrograms l-1 while the ECD response was linear up to concentrations of about 0.6 microgram l-1 for the halogenated compounds. Our method is applicable for the quantitative determination of VOCs in blood in the occupationally as well as in the physiologically relevant (normal) concentration range.

Calibration of the electron-capture detector for the determination of polychlorinated biphenyls

The response of the constant current electron capture detector (CC-ECD) to 21 polychlorinated biphenyls is discussed. Responses were non-linear from the detection limit upwards, showing that the CC-ECD is a fundamentally non-linear detector. The extra sum of squares principle was used to assess systematically the validity of several non-linear calibration functions with their respective calibrated ranges. The power function could be applied in the small amount range. The maximum amount to which the power function applied was of the order of several tens to several hundreds of picograms, depending on the compound. Using a slightly modified power function, the calibrated range could be extended by a compound specific factor of 2–45 (maximum amount of the order of several hundred to >1000 pg). Logarithmic and linear interpolation could be used to increase the calibrated range substantially without significant loss of accuracy, from the detection limit to >1000 pg injected. Model calculations showed that the use of appropriate internal standards can limit the effect of volume errors on the quantification to less than 1%.

Temperature-Dependent Electron Capture Detector Response to Common Alternative Fluorocarbons

The relative electron capture detector (ECD) response to alternative fluorocarbons (AFCs) using gas chromatography are found to be at least 1 order of magnitude lower than that for CFC-12. Detection limits for the chlorofluorocarbons CFC-12, HCFC-22, HCFC-123, and HCFC-124 are found to be 2.5, 90, 30, and 90 pg, respectively. Those for the hydrofluorocarbons are significantly poorer; 14 and 45 ng for HFC-125 and HFC-134a, respectively. HFC-152a was not detected using ECD. Since atmospheric concentrations of these compounds are in the low part-per-trillion level, GC-ECD is apparently not sensitive enough to be used for AFC analysis without substantial preconcentration. Two columns are evaluated for the AFC separation. The Poraplot Q WPLOT column showed good separation ability, though column bleed limits detection performance. A Carboxen 1004 packed column exhibits much lower interference. But separations are time consuming and peak broadening adversely affects limits of detection. Mechanisms for the ECD response are proposed based on thermodynamics and temperature-dependent ECD responses. CFC-12, HCFC-123, and HFC-125 apparently undergo ion-forming dissociative electron capture. The electron capture process for HCFC-22 and HFC-134a appear to form molecular ions. Both mechanisms appear to be operative for HCFC-124 electron capture. Dissociative electron capture rate constants for HCFC-123, HCFC-124, and HFC-125 are estimated to be 3.5 × 10(-)(10), 1.0 × 10(-)(10), and 5.6 × 10(-)(13) cm(3) s(-)(1), respectively at 300 °C.

Effect of ammonia on the response of the electron capture detector (ECD)

AbstractThe effect of ammonia on the electron capture detector (ECD) response has been investigated. Nitrogen with different ammonia concentrations (5–20%) was used as make‐up gas. Compared to pure nitrogen, the ECD response decreased when the ammonia concentration in the make‐up gas was 5%. However, the response increased when the ammonia concentration was 20%. The response factor of 4‐chlorophenol increased 4 fold when ammonia was 20%. Also, di‐ and tri‐chlorophenols increased by 30–50%. The nitroaromatic compounds responses increased by about 2–3.7 times with 20% ammonia in the make‐up gas. The signal‐to‐noise (S/N) increased when 20% ammonia in nitrogen was used as make‐up gas compared to pure nitrogen. Also, the detector linearity increased by 50% with ammonia.

Evaluation of the performance of various universal and selective detectors for sulfur determination in natural gas

AbstractIn the present contribution, the performance of a number of commercially available selective and universal detectors that can be used in the analysis of sulfur components in natural gas is evaluated in terms of sensitivity, selectivity, reproducibility, quenching effect, stability, and compound dependence of the sulfur response. Investigated detectors include the sulfur chemiluminescnce, the flame photometric, the electron capture, the mass spectrometric, the thermal conductivity, and the flame ionization detector. The sulfur chemiluminescence detector was found to have the best overall performance, e.g., low picogram amounts of sulfur can be detected accurately and the linear dynamic range is more than five orders of magnitude. After careful optimization, the sulfur response of this detector was found to be almost compound independent. All other detectors, including the flame photometric and the electron capture detector, have more or less compound dependent responses.

A Continuous Fast-Response Dual-Tracer Analyzer for Halogenated Atmospheric Tracer Studies

An apparatus for the simultaneous measurement of two tracers, sulfur hexafluoride (SF6) and a perfluorocarbon compound, is introduced. The new instrument is a modification of a commercially available fast-response, continuous analyzer for single halogenated atmospheric tracer studies. A two-channel flow system was implemented consisting of an alumina cartridge in one channel and a glass beads cartridge of equal flow resistance in the second channel. The alumina passes only sulfur hexafluoride, while the glass beads pass both SF6 and the perfluoroarbon tracer. The SF6 is quantified directly from the electron capture detector (ECD) signal in the alumina channel, and the perfluorocarbon concentration is obtained from the difference of the ECD responses in the two channels. The dual-tracer analyzer is field portable for mobile operations or fixed-location monitoring, has a response time of 1.2 s, and has limits of detection of about 15 pptv for SF6 and 10 pptv for perfluoro-methylcyclohexane, which was the principal perfluorocarbon tracer used in this study. The present instrument configuration, which requires periodic purging of the adsorbent trap, can obtain continuous measurements for a 10-15-min segment in every half hour of operation. Dual-tracer data from a field demonstration test are presented.

Comment on ‘‘Temperature-enhanced electron detachment from C6F−6 negative ions’’ [J. Chem. Phys. 98, 7875 (1993)

The results of three recent, independent measurements of the electron affinity of C6F6 are discussed. These utilize an electron capture detector (ECD), an atmospheric pressure ionization mass spectrometer (APIMS), and a photoelectron spectrometer (PES). Seven independent measurements of the temperature dependence of the ECD response of C6F6 give an average value of 0.86±0.02 eV for the electron affinity of C6F6 in agreement with an earlier published value. The APIMS and PES studies have been recently published and support the ECD value. On this basis, it is suggested that the value of the electron affinity of about 0.5 eV presented in the subject article refers to an excited electronic state of the anion of C6F6. Morse potential energy curves are calculated for four negative ions states of C6F6 using experimental data.

Relative electron-capture detector response of selected polychlorinated biphenyl congeners influence of detector temperature and design

Molar and mass responses for polychlorinated biphenyls (PCBs) IUPAC Nos. 28, 52, 77, 101, 126, 138, 153, 169 and 180 were determined for electron-capture detectors of two different designs, Model 400 (Carlo Erba) and PU 4400/03 (Philips Scientific). The detector temperature influences the response level of different PCB congeners in different ways. The course of the dependences of the molar responses on temperature in the range 150–360°C varies for different congeners. It is also influenced by the design of the detector. The presence of oxygen in the gas passing through the detector influences the response of PCB congeners. The application of the relative responses determined at one detector temperature to another temperature can produce a relative error of several tens per cent. The same is valid for the application of the relative responses from one detector design to another design. The error with different oxygen contents in the carrier gas can also be 100%.

Determination of patulin by capillary gas chromatography of the heptafluorobutyrate derivative

Formation and capillary gas chromatography of patulin heptafluorobutyrate is reported for the first time. The derivative was identified by gas chromatography—mass spectrometry (M + = 350). Electron-capture detector response for patulin heptafluorobutyrate was linear in the range 0.05 to 0.5 ng. This sensitive and reproducible derivatization technique was applied to the determination of patulin in apple juice by capillary gas chromatography—electron-capture detection. The overall method recovery averaged 84% and ⪕10 μg/l could be detected.