Fragmentation reactions of phenoxide anions: deprotonated Dinoterb and related structures

Dinoterb (6-t-butyl-2,4-dinitrophenol), 1, Dinoseb (6-secbutyl-2,4-dinitrophenol), 2, TBP (2-t-butylphenol), 3, and DNP (2,4-dinitrophenol), 4, have been analyzed by electrospray ionization in the negative mode (ESI-N) - tandem mass spectrometry. Nominal laboratory collision energy was varied from zero to 60 eV during the experiments. Apparent fragmentation energies were estimated from a parametric fitting of the collision efficiency curves. In parallel, fragmentation mechanisms of the deprotonated molecules [M-H](-) were explored using quantum chemistry modeling at the B3LYP/6-31 + G(d,p) level. A major fragmentation of the [M-H](-) ions of Dinoterb and Dinoseb is elimination of an alcohol molecule. This reaction is shown to involve one oxygen atom originating from a nitro group rather than the phenoxide moiety. Eliminations of NO, C(4) and CH(2) = C(CH(3))(2), i.e. reactions involving significant rearrangements, constitute the major part of the other fragmentation pathways observed from [3-H](-) and [4-H](-) ions.     

Analysis of polynitrophenols and hexyl by liquid chromatography-mass spectrometry using atmospheric pressure ionisation methods and a volatile ion-pairing reagent

An LC-MS method for the determination of picric acid (2,4,6-trinitrophenol), its reductive transformation products picramic acid (2-amino-4,6-dinitrophenol) and iso-picramic acid (4-amino-2,6-dinitrophenol) and hexyl (2,2',4,4',6,6'-hexanitrodiphenylamine) has been developed. The analytes were separated using ion-pairing chromatography with a volatile ion-pairing reagent suitable for subsequent MS detection. The performance of an atmospheric pressure chemical ionisation (APCI) and an electrospray ionisation (ESI) interface was compared. ESI-MS is more sensitive for the analytes, especially for hexyl and picric acid, APCI-MS delivered more fragments necessary for unequivocal identification. With LC-ESI-MS limits of detection using single ion monitoring (SIM) mode are 4 ng (iso-picramic acid), 800 pg (picramic acid), 400 pg (picric acid) and 80 pg (hexyl). For quantification, 15N-picric acid was used as an internal standard. Using this new method, the degradation of picric acid in soil was monitored in a laboratory study. Furthermore, the presence of picramic acid was for the first time verified in soil samples from a former ammunition plant.     

Trace level determination of phenols as pentafluorobenzyl derivatives by gas chromatography-negative-ion chemical ionization mass spectrometry.

A method for the trace level determination of 11 phenols as pentafluorobenzyl (PFB) derivatives by gas chromatography-mass spectrometry (GC-MS) with negative-ion chemical ionization (NICI) is described. First, the conditions for the PFB derivatisation of phenols were optimized and were found to be reaction temperature 80 degrees C and reaction time 5 h. Second, the detection limits using selected ion monitoring (SIM) were compared between trimethylsilylated (TMS) derivatives in the electron ionization (EI) mode and PFB derivatives in the NICI mode. The responses for the PFB derivatives in the NICI mode were 3.3-61 times higher than those of the TMS derivatives in the EI mode. The instrumental detection limits using NICI-SIM ranged from 2.6 to 290 fg. This method was applied to the analysis of phenols in river water using solid-phase extraction. The recoveries of the phenols from a river water sample spiked with standards at 100 ng l-1 with 2-chlorophenol, 4-chloro-3-methylphenol and pentachlorophenol and at 1000 ng l-1 with phenol, 2,4-dimethylphenol, 2,4-dichlorophenol, 2-nitrophenol, 2,4,6-trichlorophenol and 4-nitrophenol were 81.2-106.3% (RSD 5.1-8.0%), except for 2-methyl-4,6-dinitrophenol and 2,4-dinitrophenol, for which the recoveries were 5.8 and 4.2%, respectively, because water contained in the acetone eluate interfered with the derivatisation of these compounds with two electrophilic nitro groups.     

Photolytic derivatization for improved LCEC determinations of pharmaceuticals in biological fluids

Although electrochemical (EC) methods have been demonstrated to be sensitive and selective, wide use of EC detection in high performance liquid chromatographic (HPLC) assays in forensic and clinical toxicology laboratories has not been forthcoming. This fact is due to the general difficulty involved with the use of reductive EC detection methods, as well as to the lack of EC response in either the oxidative or reductive mode, for a number of classes of drugs having substantial clinical and forensic importance. The use of an on-line, post-column, continuous photolytic derivatization step, followed by conventional oxidative amperometric detection, alleviates many of these problems. In this report, the use of HPLC-photolysis-EC (HPLC-h nu-EC) for the trace determination of a number of controlled substances in biological fluids is presented. Following system optimization, the determination of phenobarbital, cocaine, methylphenidate, and several 1,4-benzodiazepines (and metabolites) is linear over three orders of magnitude. In addition, HPLC-h nu-EC offers a sensitive approach for these compounds, in that limits of detection (LODs) are all below 1 microgram/ml, ranging from 1 ng/ml to 750 ng/ml. The validity of this newer method is demonstrated in collaborative studies involving the trace determinations of phenobarbital in human serum, and chlordiazepoxide and its major metabolite, norchlordiazepoxide, in human urine. Finally, the authors' view of the role of HPLC-h nu-EC in the clinical and forensic toxicology laboratory is presented.     

Determination of Aztreonam by liquid Chromatography with UV and Amperometric Detection

Abstract

A comparative study of UV and amperometric detection of aztreonam after HPLC separation is presented. At pH 2.0 and a detection potential of +1.15 V (vs. Ag/AgCl), the detection limits with amperometric detection are about two times higher (3–5 ng) than those obtained with UV detection (1–3 ng) for aztreonam and its main decomposition products, the E-isomer and open-ring aztreonam. With the advantage of specificity for the aminothiazole group of the aztreonam molecule, amperometry can be used as an alternative or complementary mode to UV detection for the determination of aztreonam in injectable formulations and in human serum.     

Determination of free nitrilotriacetic acid in environmental water samples by ion chromatography with potentiometric and amperometric detection with a copper electrode

An ion-chromatographic method is described for the analysis of free nitrilotriacetic acid in water samples. Separations are achieved on a polymer-based anion-exchange column with 6 mM nitric acid as eluent. Both potentiometric and amperometric detection have been applied using metallic copper as the indicator electrode. Detection limits are at about 500 ng injected in the potentiometric mode and 100 ng in the amperometric mode. On-line sample preconcentration is possible for volumes up to 2 ml of river water samples. The response of the detector to other aminopolycarboxylic and aminopolyphosphonic acids has been investigated.     

Dual electrode electrochemical detector for HPLC

Summary A method for determination of phenolic compounds in distilled alcoholic beverages has been developed. After separation by reversed phase chromatography these compounds are detected coulometrically in a dual electrode detector. The hydrodynamic electrochemical behaviour of the substances in oxidative and reductive mode was investigated. For quantitative determination phenolic compounds are oxidized at the first working electrode (+0.65 V); then the oxidation products are reduced at the second working electrode (0.0 V). The current due to these processes is recorded. By the high selectivity of the detection mode matrix interferences can be eliminated in several alcoholic beverages. In this way qualitative information is improved. The detection limits of phenolic acids and aldehydes are between 0.01 and 1 ng (S/N=3).

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Dualelektroden-Detektor für die HPLCBestimmung von phenolischen Verbindungen in Spirituosen

     

Evaluation of suspected interferents for TNT detection by ion mobility spectrometry

The use of ion mobility spectrometry systems to detect explosives in high security situations creates a need to determine compounds that interfere and may compromise accurate detection. This is the first study to identify possible interfering air contaminants common in airport settings by IMS. Seventeen suspected contaminants from four major sources were investigated. Due to the ionization selectivity gained by employing chloride reactant ion chemistry, only 7 of the 17 compounds showed an IMS response. Of those seven compounds, only 4,6-dinitro-o-cresol (4,6DNOC) was found to have a similar mobility to 2,4,6-trinitrotoluene (TNT) with K(o) values of 1.55 and 1.50 cm(2) V(-1) s(-1), respectively. Although baseline resolution between TNT and 4,6DNOC was not achieved, the drift time for TNT was still easily identified. Alkyl-nitrated phenols, due to acidic fog, responded the strongest in the IMS. The effect of contamination on TNT sensitivity was investigated. Charge competition between TNT and 2,4-dinitrophenol (2,4DNP) was found to occur and to effect TNT sensitivity.     

Pulsed amperometric detection of sulfur-containing pesticides in reversed-phase liquid chromatography

Pulsed amperometric detection at a gold electrode is demonstrated for the separation of eight sulphur-containing pesticides on a C-18 reversed-phase column with 50% (v/v) acetonitrile in acetate buffer (pH 5.0) as the mobile phase. Detection was based on a two-step potential waveform with adsorption of the analyte during cathodic polarization and subsequent amperometric detection catalyzed by oxide formation following anodic polarization. The mechanism of the anodic detection involves prior adsorption of the sulfur compounds. Hence, the shape of the calibration curve is strongly influenced by the adsorption isotherm of the analyte and, therefore, deviates from linearity at high concentrations. Detection limits below 100 ng ml^?1 in 20-μl samples (e.g., 0.8 ng of dimethoate) were obtained by using preconcentration from a larger sample onto a C-18 fore column prior to injection into the separation column.     

Construction and characterization of a high-flow, high-resolution ion mobility spectrometer for detection of explosives after personnel portal sampling

A novel analysis of explosives via the coupling of an airline passenger personnel portal with a high-flow (HF), high-resolution (HR) ion mobility spectrometry (IMS) was shown for the first time. The HF-HR-IMS utilized a novel ion aperture grid design with a (63)Ni ionization source while operating at ambient pressure in the positive ion mode at 200 degrees C. The HF-HR-IMS response characteristics of 2,4,6-trinitrotoluene (TNT), 4,6-dinitro-o-cresol (4,6DNOC), and cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX) were investigated. Modifications made to the HF-HR-IMS exhaust and ionization source created an 800% increase in the total ion current (TIC), from 0.85 to 6.8 nA. This translated into a 65% ion response increase for TNT when compared with a traditional IMS. A mixture of TNT and (4,6DNOC) was used to successfully demonstrate the resolving power of the species with similar reduced mobility constants (K(o)), 1.54 and 1.59, respectively. The reactant ion (H(2)O)(n)H(+), peak was also used to measure the resolving power of the spectrometer while varying the internal diameter of three different aperture openings from 1.00 to 3.54cm. This provided a resolving power range of 50-60, double that typically achievable by commercial IMS instruments. Most important, these changes made in this new instrumental design can be implemented to all existing and future IMS's to greatly enhance the achievable IMS resolving power.     

Untersuchungen zur HPLC-Bestimmung von Pesticiden mit amperometrisch detektierbaren Hydroxylgruppen

Summary For the sensitive determination of pesticides in soil and water samples, the possibility of using h.p.l.c. with amperometric detection was investigated. The polarographic and voltammetric behaviour of some selected compounds (bromofenoxim, bromoxynil, dimethirimol, dinoseb, dinoterb, DNOC, ethirimol, ioxynil, maleic hydrazide, pentachlorophenol) was tested.After separation by h.p.l.c. the detection of all these pesticides may be done in the voltammetric cell by the anodic signal of their hydroxyl group. The sensitivity of the determination lies in all cases in the pg-range.The simultaneous determination of the metabolites in presence of the pesticide is shown with the example of bromofenoxim and its photolytic decomposition products.

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Herrn Prof. Dr. Rolf Neeb zum 60. Geburtstag gewidmet     

Comparison of collinear and transverse photothermal deflection spectroscopy for trace analysis of pesticides in water

Summary A fiber optic modified photothermal deflection spectrometer (PTDS) was developed and tested for trace analysis of pesticides in water. A UV argon laser beam (=363.8 nm) was delivered to a sample cuvette either directly, or through a multimode optical fiber of 400 m core diameter. The HeNe-probe laser beam was guided to the same sample cuvette by a monomode optical fiber of 5 m core diameter and focussed not only transverse within an angle of 90°, but also collinear within an angle of approximately 0° to the Ar⁺-excitation beam. In transverse dual beam experiments a limit of detection (LOD) of 400 ppt for the pesticide DNOC (4,6-dinitroorthocresol) was achieved. The LODs for 2,4-dinitrophenol, dinoseb and dinosebacetate were 2 ppb, 18 pbb and 6 ppb, respectively. These LODs obtained with the transverse PTDS are up to 2 orders of magnitudes lower than the values taken with the Cary 2400 spectrophotometer. In collinear PTDS experiments with the pesticide dinosebacetate the enhancement factor of the LOD is about 4, when compared to the transverse PTDS using 1×1 cm² diameter sample cuvettes.     

Development of multi‐response optimization and quadratic calibration curve for determination of ten pesticides in complex sample matrices using QuEChERS dispersive liquid–liquid microextraction followed by gas chromatography

In this study, QuEChERS combined with dispersive liquid‐liquid microextraction is developed for extraction of ten pesticides in complex sample matrices of water and milk. In this regard, effective factors of proposed extraction technique combined with gas chromatography with flame ionization detector were designed, modeled, and optimized using central composite design, multiple linear regression, and Nelder–Mead simplex optimization. Later, univariate calibration model for ten pesticides was developed in concentration range of 0.5–100 ng/mL. Surprisingly, quadratic calibration behavior was observed for some of the pesticides. In this regard, Mandel's test was used for evaluating linearity and types of calibration equation. Finally, four pesticides followed linear calibration curve with sensitivity (0.23–0.66 mL/ng), analytical sensitivity (0.20–0.32), regression coefficient (0.988–0.995), limit of detection (0.39–1.83 ng/mL), and limit of quantitation (1.30–6.10 ng/mL) and six of them followed quadratic calibration curve with sensitivity (0.18–0.93 mL/ng), analytical sensitivity (0.25–0.86), regression coefficient (0.944–0.999), limit of detection (0.59–1.92 ng/mL), and limit of quantitation (1.96–6.40 ng/mL). The calculated limits of detection were below the maximum residue limits according to European Union pesticides database of European Commission. Finally, the proposed analytical method was used for determination of ten pesticides in water and milk samples.     

Voltammetric study of 2-methyl-4,6-dinitrophenol at a modified carbon paste electrode

The voltammetric behavior of 2-methyl-4,6-dinitrophenol at a modified carbon paste electrode has been studied. Among the modifiers tested, hidepowder was found to give the best results. Cyclic voltammetry and differential pulse voltammetry were used to study the nature of the reaction; the possibility of accumulation of the analyte onto the electrode was studied by differential pulse voltammetry. An irreversible behavior and the adsorption of 2-methyl-4,6-dinitrophenol on the electrode were confirmed. The reduction signal shows two peaks. A linear relationship between the first peak height and the concentration of 2-methyl-4,6-dinitrophenol was obtained in the range 0.001–5 mg l⁻¹, with a detection limit of 3.2 μg l⁻¹ and a relative standard deviation of 3.20%. The interferences with the reduction peak of 2-methyl-4,6-dinitrophenol of several nitro- and chloro-phenols and inorganic species were tested.     

Amperometric Detection of Catecholamine Neurotransmitters Using Electrocatalytic Substrate Recycling at a Laccase Electrode

An enzyme electrode based on the coimmobilization of an osmium redox polymer and laccase on glassy carbon electrodes has been applied to ultra sensitive amperometric detection of the catecholamine neurotransmitters dopamine, epinephrine and norepinephrine, resulting in nanomolar detection limits, as low as 4 nM for dopamine. The sensitivity of the electrode is due to signal amplification via oxidation of the catecholamine by the immobilized laccase, which is regenerated by concomitant reduction of oxygen to water, coupled to the electrocatalytic re‐reduction of the oxidized catecholamine by the osmium redox complex: electrocatalytic substrate recycling. In addition because the sensor can be operated in reductive mode at ?0.2 V (vs. Ag/AgCl), noise and interferences are diminished. Combined with its high sensitivity this enzyme electrode also exhibited excellent selectivity allowing the detection of catecholamines in the presence of ascorbic acid. However, differentiation between the current responses achieved for the three catecholamines is not possible. The effective mode of constant recycling, resulting in amplification of the current response, of the laccase enzyme electrode sensor combined with the inherent advantages of using electrochemical techniques holds great promise for the future of catecholamine detection and monitoring.     

Simple-QuEChERS Nano结合GC-MS/MS同时检测全血中的97种农药

建立了Simple-QuEChERS Nano结合气相色谱一串联质谱(GC-MS/MS)同时检测血液中97种农药的方法,并对基质条件、提取溶剂以及净化材料进行了优化.0.5 mL血液样品经3倍水稀释混匀,使用2.0 mL乙酸乙酯提取后振荡、离心,过Simple-QuEChERS Nano净化柱及0.22μm有机微孔滤膜...     

Rapid determination of chlorinated pesticides in fish by freezing-lipid filtration, solid-phase extraction and gas chromatography-mass spectrometry

An analytical method has been developed for measuring 24 chlorinated pesticides in fish tissue samples. Extraction of chlorinated pesticides was carried out by ultrasonication using an acetone-n-hexane (5:2, v/v) mixture. Most of the lipids in the extract were eliminated by freezing-lipid filtration, prior to solid-phase extraction (SPE) cleanup. During freezing-lipid filtration, about 90% of the lipids extracted from the fish samples were easily removed without any significant losses of chlorinated pesticides. For purification, SPE using Florisil was shown to be more effective than silica. Quantification was performed using gas chromatography-mass spectrometry in the selected ion monitoring mode. Spiking experiments were carried out to determine the recovery, precision, and limits of detection (LODs) of the method. The overall recovery was above 80% in the spiked fish tissue sample at 100 ng/g level. The detection limits for chlorinated pesticides were ranged from 0.5 to 5 ppb, except for endosulfan I and II which was 20 ppb. The newly developed method is demonstrated to give efficient recoveries and LODs for detecting chlorinated pesticides spiked into fish tissue with high lipid content.     

固相萃取/高效液相色谱-串联质谱法同时检测环境水样中24种农药残留

建立了一种固相萃取/高效液相色谱-串联质谱(SPE/HPLC-MS/MS)同时检测水体中24种农药的分析方法。样品用乙腈提取后,经固相萃取小柱富集净化。以乙腈-0.1%(体积分数)甲酸水溶液为流动相梯度洗脱,在电喷雾离子源正离子模式下(ESI+)采用多反应监测(MRM)模式检测。结果显示,24种农药在1~200μg/L范围内具有良好的线性关系,相关系数(r2)均不小于0.998,水样中3个添加水平(5、20、100μg/L)下的回收率为65.9%~127.8%,相对标准偏差(RSD)为0.7%~14.2%;方法检出限为0.05~0.71 ng/L。采用该方法对大连地区10个河流入海口及2个水库的水样进行了检测,12个站位的样品中共检出10种农药,质量浓度为0.2~558.3 ng/L。结果表明,所建立的SPE/HPLC-MS/MS方法高效、灵敏、可靠,可用于实际水体中多种农药的同时检测。     

Separation of pesticides by high-performance liquid chromatography with amperometric detection

The possibility of the amperometric detection of a number of pesticides, such as benomyl, thiram, linuron, metoxuron, desmedipham, dicuron, lenacil, and fludioxonil, widely used in agrochemical practice was studied. The effect of the working electrode material (glassy carbon, nickel, and gold) and the type of the electrochemical cell on the value of the analytical signal was studied using the example of thiram. It was found that the optimum potential of the working electrode in analyzing a pesticide mixture was 1400 mV. The dependence of the analytical signal on the pesticide concentration was shown to be linear. The detection limits for the analytes were calculated. Using a 100-μL sample loop, all of the studied pesticides can be determined at the level of the maximum permissible concentration (MPC). The amperometric determination of seven pesticides at the level of MPC in real samples was shown by the examples of model mixtures dissolved in tap water and beetroot juice.     

固相萃取-GC/MS法测定水样中20种有机氯农药

建立了用固相萃取小柱提取和净化、GC/MS定性定量同时测定水样中20种有机氯农药的方法。方法采用OasisHLB固相萃取小柱萃取富集水样,二氯甲烷洗脱,加入菲-d₁₀作为内标,利用GC/MS进行定性定量,步骤简便,线性响应良好,干扰小,方法检出限为0.21～0.72 ng/L（按水样1L计）,加标回收率为64.8%～122%,RSD为1.2%～11.0 %。成功利用该方法对广西实际河水样品进行了检测。结果表明方法可以同时满足环境水样中20种痕量有机氯农药的测定。