Analyte-matrix Interactions Research Articles

Investigation of modifier effects in supercritical CO 2 extraction from various solid matrices

In the present study, the internal interactions of an analyte/modifier/matrix complex were investigated systematically. Target compounds (ethylphenol, quinoline, and nitronaphthalene), test matrices (silica gel, silica modified with aniline, and XAD-2), and modifying solvents (acetic acid, tetrahydrofuran, and toluene) were chosen based on their α, β, or π ∗ characteristics. The SFE recoveries of target solutes were variable depending on the type of matrix, suggesting that the most critical factor in the SFE process is to eliminate the matrix–analyte interaction. The action of the modifier appears to be competition with the analyte for sorption sites. From α-matrix (silica gel), ethylphenol ( α) was quantitatively extracted by α-modifier (acetic acid) and tetrahydrofuran ( β) was the most effective modifier for quinoline ( β) extraction (73% recovery). From β-matrix (silica modified with aniline), 104% recovery of nitronaphthalene ( π ∗) was achieved with π ∗-modifier (toluene). The modifier effect of tetrahydrofuran ( β) for extracting quinoline ( β) was superior to acetic acid ( α) and toluene ( π ∗), although quantitative recovery was not obtained even with tetrahydrofuran (73% recovery). From XAD-2 substrate ( π ∗), all three target compounds were readily extracted with unmodified supercritical fluid (SCF) CO 2, showing there were no significant interactions between the target analytes and the XAD-2 matrix. The preliminary study was applied to alumina and it was found that combined solvatochromic parameters of modifiers work more effectively and thus methanol, having relatively high solvatochromic values ( α, β, and π ∗), remains the best in most cases.

Extraction of polar and hydrophobic pollutants using accelerated solvent extraction (ASE)

Accelerated solvent extraction (ASE) shows higher recoveries for PAHs in comparison with traditional Soxhlet extraction, but in a fraction of time and with less solvent consumption. Better recoveries are especially achieved with PAHs of high reactivity, the latter being expressed by the structure-to-count ratio (SCR). To estimate polar pollutants including phenols/benzenediols, the sample was subjected to a combined in-situ derivatization/extraction approach using 2% v/v acetic anhydride in toluene. The main reason for the better recovery obtained in this way, in comparison with the classical ASE approach, is to overcome strong matrix-analyte interactions. Analogously, fatty acids were analyzed as methyl esters obtained by in-situ derivatization/extraction using boron trifluoride.

Effects of various binary and ternary supercritical phases on the extraction of polycyclic aromatic hydrocarbons from contaminated soils

Polycyclic aromatic hydrocarbons (PAHs) were extracted from two real environmental soil samples using supercritical fluid extraction (SFE) with organic modifiers. The modifiers ( n-pentane, acetone, toluene, dichloromethane, methanol) as well as modifier mixtures (methanol containing acetone, toluene, acetic acid; acetone containing toluene, ethanolamine; n-pentane containing acetic acid) were used in two different concentrations. In general, extraction efficiency increased with reduced polarity of the modifier used and at higher concentrations of co-solvent. Extraction of PAHs from the minor polluted soil with a high content of humic acids was enhanced greatly by the use of modifiers in comparison with pure CO 2 and Soxhlet extraction utilising dichloromethane or n-pentane. The highest PAH yields were received with CO 2 containing 10 or 2 mol% pentane followed by CO 2 with 10 mol% toluene. In contrast, for the soil highly contaminated with mineral oil products but with a low content of humic substances, the extraction efficiency was only increased a little by the addition of modifiers in comparison with pure CO 2 and decreased in comparison with Soxhlet extraction. The results lead to the conclusion that the use of modifiers during SFE is well appropriated to break strong matrix–analyte interactions, like humic acids–PAH interactions in the low contaminated soil. But for soils highly contaminated with aliphatic hydrocarbons, SFE is less suitable, because of the viscosity and solubility of the hydrocarbons for PAHs.

Chemometric Protocol to Validate an Analytical Method in the Presence of Corrigible Constant and Proportional Systematic Errors

Abstract A statistical methodology to verify the trueness of an analytical method in the presence of corrigible systematic errors is presented. This protocol enables detection of constant and proportional components of error. By using the data set obtained in the Youden calibration with different sample test portions, the constant component of the error (Youden blank) can be determined. An analysis of covariance was applied to 3 calibration curves established with standard solutions and with standard additions to 2 different sample test portions. The slopes were compared, and the presence of any matrix-analyte interaction was detected. A method for removing the numerical components of systematic errors is proposed: a calculation procedure to obtain a correct analytical result and a statistical test to verify the correctness of analyte contents obtained from different calibrations. For demonstration purposes, the protocol was applied to spectrofluorometric determination of oxalates in spinach leaves.

Effects of analyte-matrix interactions on supercritical fluid extraction efficiencies of polycyclic aromatic hydrocarbons

The effects of different variables for analyte-matrix interactions on supercritical fluid extraction (SFE) efficiencies of polycyclic aromatic hydrocarbons (PAHs) were investigated. The results indicated that the main factors for describing a matrix, e.g., the chemical properties of the matrix surface, surface area, and pore size, could significantly influence the analyte-matrix interactions. The polarizability of an analyte was demonstrated to be yet another significant factor for influencing analyte-matrix interactions. Either elevating the temperature or adding modifiers could also increase the SFE recoveries. The underlying mechanisms of these variables are discussed.

Optimization of a Microwave-assisted Extraction Method for Phenol and Methylphenol Isomers in Soil Samples Using a Central Composite Design

A simple and rapid microwave-assisted extraction (MAE) procedure was developed and optimized for phenol, o-cresol, m-cresol and p-cresol in soil samples. The spiked soil used was prepared 25 d before treatment to simulate weathering processes and to allow for the formation of analyte-matrix interactions. The samples, immersed in hexane–acetone, were irradiated with microwaves in a closed-vessel system. Optimization of the method was achieved by using a factorial design approach on parameters such as the volume of solvent, the percentage of acetone and the extraction temperature. The MAE procedure yielded extracts that could be analyzed directly using a GC–flame ionization detection system without any preliminary clean-up or concentration steps.

Phenol and methylphenol isomers determination in soils by in-situ microwave-assisted extraction and derivatisation

A simultaneous microwave-assisted extraction-derivatisation procedure was developed and optimised for phenol and methyl phenol isomers from soil samples. Both spiked and real soil samples (carbon content of 18%) were irradiated with microwaves in a closed-vessel system while immersed in hexane that had been previously doped with acetic anhydride and pyridine so as to effect an in situ catalytic acetylation of the target phenolic compounds. Spiked samples were prepared at least 20 days before treatment to simulate natural weathering processes and allow for the formation of analyte-matrix interactions. Optimization of the method was achieved by using a factorial design approach on parameters such as the volume of solvent, the quantity of acetic anhydride and the extraction temperature. Comparison with ultrasonic extraction procedures indicated that microwave-assisted methods gave superior recoveries (ca. two-fold) and greater precision in addition to being characterised by shorter extraction times. The procedure using microwaves yielded samples that could be analyzed directly on a GC-MS system without any preliminary clean-up or concentration steps. The detection limit of this novel method was found to be in the low-ppb range.

Aerosol Matrixassisted Laser DesorptionIonization Mass Spectrometry: I.Effect of Solvent on Ion Signal

The effects of methanol, ethanol, propan-2-ol and acetonitrile on the aerosol matrix-assisted laser desorption/ionization (MALDI) ion signal of bradykinin, bovine insulin and myoglobin were examined. The effect of these solvents mixed with water was also studied. The aerosol MALDI ion signal for each analyte depends on the solvent, but the solvent dependence is different for the matrices 4-nitroaniline and α-cyano-4-hydroxycinnamic acid. The data suggest that matrix–analyte–solvent interactions are an important consideration for the aerosol MALDI ion signal. It appears that the ion signal is the result of a competition between solvent polarity and volatility. The solvent polarity affects matrix–analyte interactions whereas solvent volatility affects the aerosol drying process.

Aerosol Matrix-assisted Laser Desorption/Ionization Mass Spectrometry: I.—Effect of Solvent on Ion Signal

The effects of methanol, ethanol, propan-2-ol and acetonitrile on the aerosol matrix-assisted laser desorption/ionization (MALDI) ion signal of bradykinin, bovine insulin and myoglobin were examined. The effect of these solvents mixed with water was also studied. The aerosol MALDI ion signal for each analyte depends on the solvent, but the solvent dependence is different for the matrices 4-nitroaniline and α-cyano-4-hydroxycinnamic acid. The data suggest that matrix–analyte–solvent interactions are an important consideration for the aerosol MALDI ion signal. It appears that the ion signal is the result of a competition between solvent polarity and volatility. The solvent polarity affects matrix–analyte interactions whereas solvent volatility affects the aerosol drying process.

Kinetic Study of Supercritical Fluid Extraction of Organic Contaminants from Heterogeneous Environmental Samples with Carbon Dioxide and Elevated Temperatures

Supercritical fluid extraction (SFE) rates of spiked polychlorinated dibenzo-p-dioxins (PCDDs) from Florisil, spiked [ 13 C]PCDDs and native PCDDs from fly ash, and spiked [ 2 H]polycyclic aromatic hydrocarbons (PAHs) and native PAHs from marine sediment and railroad bed soil were examined at 40, 120, and 200 o C, while constant fluid density (d=0.67 g/mL) and flow rate were maintained. Over 30 min (150 void volumes of CO 2 ) was required to quantitatively remove both spiked 2,3,7,8-tetrachlorodibenzo-p-dioxin and 1,2,3,7,8-pentachlorodibenzo-p-dioxin from Florisil at 40 o C, while SFE at 200 o C significantly improved the elution rate so complete removal was achieved in ∼10 min. Elution rates of spiked PCDDs from Florisil were slower with a 5-mL vessel (12 cm long) than a 0.5-mL vessel (6 cm long). Increasing the temperature from 40 to 120 and 200 o C enhanced the SFE rates of spiked [ 13 H]PCDDs and native PCDDs from fly ash, as well as [ 2 H]PAHs and native PAHs from marine sediment and railroad bed soil. In all cases, native analytes were extracted more slowly than spiked analytes, suggesting that additional processes affect SFE rates of native analytes. A kinetic model was used that could help distinguish between there processes and included terms for matrix-fluid mass transport, as well as partitioning and bulk mass transport in teh supercritical fluid. Using a three-rate constant desorption model to describe mass transport, good correlations (r 2 >0.9 in most cases) were obtained with experimental data for native analytes, and desorption rate constants suggest that analyte-matrix interactions are strong. The results of this study show that increasing the extraction temperature is a simple and effective method to increase SFE rates while still exploiting the advantages of supercritical CO 2 , and can be used regardless of whether slow SFE rates are due to poor partitioning into the fluid or limited by slow desorption due to strong analyte-matrix interactions

Factors controlling quantitative supercritical fluid extraction of environmental samples

The development of quantitative supercritical fluid extraction (SFE) methods for the recovery of organic pollutants from environmental samples requires three steps: quantitative partitioning of the analytes from the sample into the extraction fluid, quantitative removal from the extraction vessel, and quantitative collection of the extracted analytes. While spike recovery studies are an excellent method to develop the final two steps, they are often not valid for determining extraction efficiencies from complex real-world samples such as soils and sediments, exchaust particulates, and sludges. SFE conditions that yield quantitative recoveries of spiked analytes may recover < 10% of the same analytes from real-world samples, because spiked pollutants are not exposed to the same active sites as the native pollutants. Because of the heterogeneous nature of environmental samples, the partitioning step may be controlled by analyte solubility in the extraction fluid, kinetic limitations, and/or the ability of the extraction fluid to interrupt matrix-analyte interactions. While the interactions that control SFE rates from heterogenous environmental samples are not well understood, a generalized scheme for developing quantitative SFE methods is proposed based on interactive considerations of the collection efficiencies after SFE, fluid flow parameters in the extraction cell, analyte solubility, extraction kinetics, and analyte-matrix-extraction fluid interactions. The proposed development scheme includes increasing SFE extraction rates by the use of more polar fluids than CO 2 such as CHClF 2, the addition of organic modifiers to CO 2, and the use of high temperature extractions with pure CO 2. Validation of quantitative extractions based on multiple extraction methods (SFE followed by liquid solvent extractions) is also described.

Recovery of substance P and related C-terminal fragments on solid-phase extraction cartridges for subsequent high-performance liquid chromatographic separation and radioimmunoassay.

The recoveries of substance P (SP) and five related peptides were evaluated on different types of solid-phase extraction sorbent. Best results were obtained by use of a C18 silica gel cartridge. Marked differences of extraction yields occurred for the different peptide fragments and, in general, recovery increased with increasing hydrophobicity of the peptide when reversed-phase materials like C18 and C8 cartridges were used. This observation is indicative of a sorption-desorption mechanism by prevailing solvophobic interactions. A similar trend was found when phenylpropyl silica gel (CPhenyl), generally known as a reversed-phase adsorbent of lower hydrophobicity, was used. It was concluded that a substantial participation of analyte-matrix pi-pi interactions has to be taken into account when extraction yields are compared with corresponding values obtained by use of a C8 cartridge. With CN silica gel cartridges, marked differences in extraction yields were obtained by use of acetonitrile or methanol as the organic modifier. As an attempt to explain this observation, conformational effects were assumed for the sorption-desorption behaviour of the peptides on the polar matrix.

Trace analysis of anionic surfactants in mineral waters and soft drinks by spectrophotometry

The applicability of the extractive spectrophotometric method using Ethyl Violet as the chromogenic reagent for identification and quantification of anionic surfactants in different types of mineralized drinking waters, as well as in some non-alcoholic beverages, was investigated. The matrix-analyte interactions of major water constituents and soft-drink ingredients in the combinations and concentration ranges met in practice were studied individually. The method was evaluated by performing recovery tests on commercial and natural samples, with sodium dodecyl sulphate added as a model surfactant. The recoveries at the 57.6 and 86.4 μg/l levels ranged from 98% to 104%.