Polar Aliphatic Compounds Research Articles

How soil organic matter composition controls hexachlorobenzene–soil-interactions: Adsorption isotherms and quantum chemical modeling

Hazardous persistent organic pollutants (POPs) interact in soil with the soil organic matter (SOM) but this interaction is insufficiently understood at the molecular level. We investigated the adsorption of hexachlorobenzene (HCB) on soil samples with systematically modified SOM. These samples included the original soil, the soil modified by adding a hot water extract (HWE) fraction (soil+3 HWE and soil+6 HWE), and the pyrolyzed soil. The SOM contents increased in the order pyrolyzed soil<original soil<soil+3 HWE<soil+6 HWE. For the latter three samples this order was also valid for the HCB adsorption. The pyrolyzed soil adsorbed more HCB than the other samples at low initial concentrations, but at higher concentrations the HCB adsorption became weaker than in the samples with HWE addition. This adsorption combined with the differences in the chemical composition between the soil samples suggested that alkylated aromatic, phenol, and lignin monomer compounds contributed most to the HCB adsorption. To obtain a molecular level understanding, a test set has been developed on the basis of elemental analysis which comprises 32 representative soil constituents. The calculated binding energy for HCB with each representative system shows that HCB binds to SOM stronger than to soil minerals. For SOM, HCB binds to alkylated aromatic, phenols, lignin monomers, and hydrophobic aliphatic compounds stronger than to polar aliphatic compounds confirming the above adsorption isotherms. Moreover, quantitative structure–activity relationship (QSAR) of the binding energy with independent physical properties of the test set systems for the first time indicated that the polarizability, the partial charge on the carbon atoms, and the molar volume are the most important properties controlling HCB–SOM interactions.

Linearized Equations for the Reduced Ion Mobilities of Polar Aliphatic Organic Compounds

Over the years, ion mobility spectrometry has evolved into a powerful technique for rapid identification of analytes in very complex sample matrixes such as human breath. Every analyte detected has a characteristic ion mobility value (and a retention time when additional preseparation techniques are employed) which is used to identify the peaks in a spectrum either by comparison with reference analytes or by simultaneous mass spectrometric measurements. In this study, the mass-mobility correlations between compounds in three different homologous series are used to predict the mobilities of the other substances in the same series in a medium of synthetic air. The results show a very high accuracy (>99.5%) of the prognosis. The linear trend equations of ion mobilities, as a function of the number of carbon atoms, obtained from the different series were then generalized into one linear equation for the reduced ion mobility for the polar aliphatic compounds and is validated by comparing it with the traditional Mason-Schamp equation. To compare the empirical equation obtained from the prognosis and the Mason-Schamp equation, the collision integral term in the latter was split into two terms to linearize it. The resulting novel ion mobility equation could be the starting step to completely describe the relationship between ion collision integral and the ion mobility for polar aliphatic compounds. The splitting of the collision integral into two terms will also give new inputs to describe the various ion models and the different forces that act on the ions and the neutral gas molecules upon which the collision integral is dependent on. This prognosis method could, furthermore, be extended to all other classes of organic compounds and could serve as a useful tool for identification of unknowns in ion mobility spectra, thereby considerably reducing the time-consuming and costly reference measurements and other coupling techniques that are currently employed.

Microelectrode Applications of Pulsed Electrochemical Detection

Pulsed electrochemical detection (PED) has been applied to the direct (i.e., requires no derivatization), sensitive and reproducible detection of numerous polar aliphatic compounds (e.g., carbohydrates, amines, and thiols). These compounds, many of which have biological significance, typically have been classified as non-electroactive for detection under constant applied potentials and have poor optical detection properties. PED exploits the electrocatalytic activity of noble metal (e.g., Au and Pt) electrode surfaces to oxidize various polar functional groups using multi-step potential-time waveforms to realize amperometric/coulometric detection while maintaining uniform and reproducible electrode activity. The response mechanisms in PED are dominated by the surface properties of the electrode, and as a consequence, members of each chemical class of compounds produce virtually identical voltammetric responses. Thus, the full analytical potential is achieved when combined with an a priori separation. Although popularized in combination with high performance liquid chromatography, the combination of PED with highly efficient microseparation techniques offer the analyst unique advantages. This paper reviews the fundamental aspects of PED especially at microelectrodes, and its application in microchromatographic and electrophoretic separation techniques, including microchip devices.

Applications of a copper-modified gold electrode for amperometric detection of polar aliphatic compounds by anion-exchange chromatography

An Au/Cu bimetallic electrode is investigated as a sensor in amperometric flow-through detection in alkaline solutions. Experimental conditions such as detection mode, pH, copper loading and flow-rate on sensitivity and stability are evaluated. Constant-potential detection at 0.550 V or 0.350 V and pulsed detection modes are tested after anion-exchange chromatography of alditols, carbohydrates and amino acids. The detection limits (S/N=3) of carbohydrates at 0.550 V and 0.350 V ranged between 4.6–9.0 pmol and 1.0–4.0 pmol, respectively. Those of amino acids were 9.0–39 pmol. In pulsed amperometric detection mode, molar sensitivities and detection limits were higher than those obtained in DC mode. Linear dynamic ranges spanned over four (alditols and carbohydrates) and three (amino acids) orders of magnitude. The Au/Cu CME retained 94–96% of its response after several hours operations in flowing streams. As an application, some alditols, carbohydrates and amino acids were determined in soluble coffee and red wine.

Pulsed electrochemical detection of nonchromophoric compounds following capillary electrophoresis.

Pulsed electrochemical detection (PED) following capillary electrophoresis (CE) has been applied to the direct detection of polar aliphatic compounds. These compounds typically have poor optical detection properties and are considered to be electroinactive under constant applied potentials. The voltammetric responses at microelectrodes of model compounds were studied under various buffers and pH conditions. The detection of unsubstituted carbohydrates requires highly alkaline conditions, whereas amine-containing compounds (e.g., glycopeptides, peptides, and amino acids) and thiocompounds can best be detected at mildly alkaline (i.e., pH 9.0) and mildly acidic (i.e., pH 5.5) conditions, respectively. In a comparative study of pulsed amperometric detection (PAD) and integrated pulsed amperometric detection (IPAD), IPAD is determined to be better suited to manage the large oxide-formation backgrounds, which accompany oxide-catalyzed detections of amine- and sulfur-containing compounds. IPAD results in more stable baselines, eliminates oxide-induced artifacts, and often yields lower limits of detection. Mass detection limits under optimal conditions are typically 10 fmole or less. This paper delineates PED waveform parameters for the optimal detection of underivatized nonchromophoric compounds of biological significance in various operating buffers and electrolytes.

Pulsed electrochemical detection at noble metal electrodes in liquid chromatography

AbstractThe direct electrochemical detection of numerous polar aliphatic compounds is achieved at Au and Pt electrodes under the control of multistep potential‐time waveforms. In this technique, called “Pulsed Electrochemical Detection (PED),” the waveforms manage the sequential processes of sampling the faradaic signal, from the oxidation of analyte, followed by oxidative cleaning and reductive reactivation of the electrode surfaces. The response mechanisms in PED are strongly influenced by surface properties of the electrodes and, therefore, members of each chemical classification detected are observed to produce virtually identical voltammetric response. Consequently, applications of PED have their greatest analytical significance when coupled with Liquid Chromatography (LC) to achieve a priori resolution of complex mixtures. The basic concepts of PED are reviewed and LC‐PED results are presented for a variety of samples containing mixtures of alcohols, carbohydrates, alkanolamines, amines, and sulfur compounds.

Ozonation of pyrene in aqueous solution

The reaction of pyrene with ozone was studied in aqueous solution at low (10–200 μg l −1) substrate concentration and with ozone concentrations (5 mg l −1) approximating to those used in potable water treatment. Degree of reaction and the formation of polyaromatic/aromatic reaction products were assessed using a technique of solvent extraction and derivatisation with gas chromatographic (GC), gas chromatographic-mass spectrometric (GC-MS), high performance liquid chromatographic (HPLC) and u.v. spectrometric analysis of extracts. A freeze drying/derivatisation technique with GC and GC-MS analysis was employed for the isolation/identification of short chain polar aliphatic reaction products. Substantial destruction of pyrene was recorded within 1 min of reaction with ozone. Polyaromatic/aromatic compounds did not persist; reaction products are likely to be short chain polar aliphatic compounds. Products of this type were isolated but not identified in the study.

The occurrence of plant wax constituents in airborne particulate matter in an urbanized area

Airborne particulate matter was collected by filtration in an urbanized area at Delft (The Netherlands) during 1981. The samples were extracted by means of cyclohexane and analysed by GC/MS. Differences were observed between summer and winter samples. Besides a different n-alkane pattern, the summer samples contained large quantities of long-chain polar aliphatic compounds. The identification of these compounds was carried out by means of electron impact, chemical ionization and high resolution mass spectrometry. These compounds are known plant was constituents. The contribution of the plant wax constituents to the total amount of analysed compounds in the cyclohexane extracts was estimated at about 40 % in the summer.

Solvent effects on 13C nuclear magnetic resonance shifts of polar aliphatic compounds. A charge separation model

A model is proposed which explains the large solvent effects on the shielding of carbon atoms bearing polar bonds (ΔCα) with an increased charge separation in solvents of a higher dielectric constant. Using cis-1-iodo-1-methyl-4-t-butylcyclohexane and t-butyl chloride as conformationally homogeneous solutes, one obtains for a series of solvents a linear correlation (r > 0.95) for ΔCα in the expected range of 415 p.p.m./electron charge unit and 320 p.p.m./e, respectively. In a second set of experiments, the sensitivity of ΔCα against solvent changes for differing Cα–X bonds is obtained, again yielding a linear correlation (r > 0.99) with 380 p.p.m./e. In addition, a ΔCα enhancement with increasing carbon substitution at Cα is found for several alkyl iodides and rationalized in terms of an increasing contribution of the charged form to the resonance hydride.

Effect of solvent on the spectrum of chlorophyll

Visible spectra of chlorophyll in forty solvents have been recorded and the following parameters tabulated for each: position of the red and Soret bands, extinction coefficient and half-width of the red band, the Soret/red band and the Soret/Soret satellite band extinction coefficient ratios. The position of the red band depends more on the refractive index than on the dielectric constant. Interpretation of other spectral properties is assisted by division of solvents into three classes: (I) polar aliphatic compounds with single bonds only; (II) polar aliphatic compounds with multiple bonds; (III) mainly aromatic compounds. With the aid of this division, correlations were found between the half-width and the part of the red shift that depends on dielectric constant, between the position of the Soret and red bands, and between the Soret/red and the Soret/Soret satellite absorptivity ratios.

Interaction of Anionic Detergents and Certain Polar Aliphatic Compounds in Foams and Micelles

Interaction of Anionic Detergents and Certain Polar Aliphatic Compounds in Foams and Micelles