Determination of the solubility of inorganic salts by headspace gas chromatography

This work reports a novel method for determination of salt solubility using headspace gas chromatography. A very small amount of volatile compound (such as methanol) is added in the studied solution. Due to the molecular interaction in the solution, the vapor-liquid equilibrium (VLE) partitioning coefficient of the volatile species will change with different salt contents in the solution. Therefore, the concentration of volatile species in the vapor phase is proportional to the salt concentration in the liquid phase, which can be easily determined by headspace gas chromatography. Until the salt concentration in the solution is saturated, the concentration of volatile compound in the vapor phase will continue to increase further and a breakpoint will appear on the VLE curve. The solubility of the salts can be determined by the identification of the breakpoint. It was found that the measured solubility of sodium carbonate and sodium sulfate in aqueous solutions is slightly higher (about 6-7%) than those reported in the literature method. The present method can be easily applied to industrial solution systems.     

Headspace Single-Drop Microextraction with Gas Chromatography for Determination of Volatile Halocarbons in Water Samples

A simple, rapid and inexpensive procedure for extraction and analysis of volatile halocarbons in water samples was presented using the headspace single-drop microextraction (HS-SDME) technique and gas chromatography with microcell electron capture detector (GC-μECD). Operation parameters. such as extraction solvent. headspace volume. organic drop volume. salt concentration. temperature and sampling time, were studied and optimized. Extraction of 10 volatile halocarbon compounds was achieved using the optimized method. Calibration curves of 10 target compounds yielded good linearity in the respective range of concentration (R ² ≥ 0.9968, chlorodibromomethane in the concentration range of 0.05–50 μg/L). The limits of detection were found between 0.002 (tetrachloroethene) and 0.374μg/L (1,1,2-trichloroethane). and relative standard deviations (RSD%) ranged between 4.3 (chloroform) and 9.7% (1,1,2,2-tetrachloroethane). Spiked recoveries of tap water and ground water agreed well with the known values between 118.97 (20.0μg/L of 1,1,2-trichloroethane) and 82.61% (10.0μg/L of tetrachloroethene), demonstrating that the HS-SDME combined GC-μECD was a useful and reliable technique for the rapid determination of volatile halocarbon compounds in water samples.     

Importance of extracting solvent vapor pressure in headspace liquid-phase microextraction

Of the many parameters that affect the enrichment factors in headspace liquid-phase microextraction, in this study, we systematically investigated the influence of the vapor pressure of the extracting solvent. Seven extracting solvents with different vapor pressures were selected and tested. It was found that the vapor pressure of the extracting solvent dramatically affects the enrichment factor and the factor was increasing by decreasing the extracting solvent vapor pressure under given experimental conditions. The result was validated for volatile organic compounds such as polynuclear aromatic hydrocarbons, organochlorine pesticides and polychlorinated biphenyls.     

Chromatomembrane Headspace Analysis of Aqueous Solutions at Elevated Temperatures

Both theoretically and experimentally, the effect of temperature has been studied and assessed on analytical characteristics of continuous chromatomembrane gas extraction of volatile organic compounds from aqueous solutions with the aim of their subsequent gas chromatographic determination. It has been found that a rise of temperature up to 80 °C enables reduction of the detection limits of alcohols, ketones, and esters by a factor of 10 to 20. If a water vapor condenser is used in the extractant gas line, then the repeatability of results does not depend on temperature. The conditions have been optimized for the continuous headspace chromatomembrane analysis in combination with gas adsorption (purge and trap) concentration of analytes.

     

Determination of partition coefficients by automatic equilibrium headspace gas chromatography by vapor phase calibration

Summary Equilibrium headspace gas chromatography has been applied to the determination of the partition coefficients of volatile compounds in water-air systems. Only techniques that are suited to a fully automatic headspace procedure using the pneumatic headspace sampling-technique have been considered. Particularly simple is the technique of vapor phase calibration —VPC where an external vapor standard is used to calibrate the concentration of the volatile analyte in the headspace, while the concentration in the sample is found from the difference in the total amount in the vial. This technique is described in detail for 2-butanone in water. Finally, the water-air partition coefficients of several selected volatile compounds at different temperatures are listed together with their temperature functions.Dedicated to Professor Leslie S. Ettre on the occasion of his 70th birthday.     

Response surface optimisation applied to a headspace-solid phase microextraction-gas chromatography-mass spectrometry method for the analysis of volatile organic compounds in water matrices

A new method for the simultaneous determination of 12 volatile organic compounds (trans-1,2-dichloroethene, 1,1,1-trichloroethane, benzene, 1,2-dichloroethane, trichloroethene, toluene, 1,1,2-trichloroethane, tetrachloroethene, ethylbenzene, m-, p-, o-xylene) in water samples by headspace solid phase microextraction (HS–SPME)–gas chromatography mass spectrometry (GC–MS) was described, using a 100?µm PDMS (polydimethylsiloxane) coated fibre. The response surface methodology was used to optimise the effect of the extraction time and temperature, as well as the influence of the salt addition in the extraction process. Optimal conditions were extraction time and temperature of 30?min and ?20°C, respectively, and NaCl concentration of 4?mol?L^?1. The detection limits were in the range of 1.1?×?10^?3–2.3?µg?L^?1 for the 12 volatile organic compounds (VOCs). Global uncertainties were in the range of 4–68%, when concentrations decrease from 250?µg?L^?1 down to the limits of quantification. The method proved adequate to detect VOCs in six river samples.     

Analysis of organic compounds in environmental samples by headspace solid phase microextraction

The analysis of samples contaminated by organic compounds is an important aspect of environmental monitoring. Because of the complex nature of these samples, isolating target organic compounds from their matrices is a major challenge. A new isolation technique, solid phase microextraction, or SPME, has recently been developed in our laboratory. This technique combines the extraction and concentration processes into one step; a fused silica fiber coated with a polymer is used to extract analytes and transfer them into a GC injector for thermal desorption and analysis. It is simple, rapid, inexpensive, completely solvent-free, and easily automated. To minimize matrix interferences in environmental samples, SPME can be used to extract analytes from the headspace above the sample. The combination of headspace sampling with SPME separates volatile and semi-volatile analytes from non-volatile compounds, thus greatly reducing the interferences from non-target compounds. This paper reports the use of headspace SPME to isolate volatile organic compounds from various matrices such as water, sand, clay, and sludge. By use of the technique, benzene, toluene, ethyl-benzene, and xylene isomers (commonly known as BTEX), and volatile chlorinated compounds can be efficiently isolated from various matrices with good precision and low limits of detection. This study has found that the sensitivity of the method can be greatly improved by the addition of salt to water samples, water to soil samples, or by heating. Headspace SPME can also be used to sample semi-volatile compounds, such as PAHs, from complex matrices.     

Headspace solid phase microextraction (HSSPME) for the determination of volatile and semivolatile pollutants in soils

We have investigated the use of headspace solid phase microextraction (HSSPME) as a sample concentration and preparation technique for the analysis of volatile and semivolatile pollutants in soil samples. Soil samples were suspended in solvent and the SPME fibre suspended in the headspace above the slurry. Finally, the fibre was desorbed in the Gas Chromatograph (GC) injection port and the analysis of the samples was carried out. Since the transfer of contaminants from the soil to the SPME fibre involves four separate phases (soil-solvent-headspace and fibre coating), parameters affecting the distribution of the analytes were investigated. Using a well-aged artificially spiked garden soil, different solvents (both organic and aqueous) were used to enhance the release of the contaminants from the solid matrix to the headspace. It was found that simple addition of water is adequate for the purpose of analysing the target volatile organic chemicals (VOCs) in soil. The addition of 1 ml of water to 1 g of soil yielded maximum response. Without water addition, the target VOCs were almost not released from the matrix and a poor response was observed. The effect of headspace volume on response as well as the addition of salt were also investigated. Comparison studies between conventional static headspace (HS) at high temperature (95 degrees C) and the new technology HSSPME at room temperature ( approximately 20 degrees C) were performed. The results obtained with both techniques were in good agreement. HSSPME precision and linearity were found to be better than automated headspace method and HSSPME also produced a significant enhancement in response. The detection and quantification limits for the target VOCs in soils were in the sub-ng g(-1) level. Finally, we tried to extend the applicability of the method to the analysis of semivolatiles. For these studies, two natural soils contaminated with diesel fuel and wood preservative, as well as a standard urban dust contaminated with polyaromatic hydrocarbons (PAHs) were tested. Discrimination in the response for the heaviest compounds studied was clearly observed, due to the poor partition in the headspace and to the slow kinetics of all the processes involved in HSSPME.     

The effect of salt and temperature on the infinite dilution activity coefficients of volatile organic chemicals in water

We have measured the infinite dilution activity coefficients for five volatile organic compounds (dichloromethane, chloroform, 1,2-dichloroethane, trichloroethylene and benzene) in aqueous salt solutions over the temperature range of 10°C to 40°C, and sodium chloride concentrations from 0 to 2.5 M using headspace gas chromatography. These data were easily fit as a function of salt concentration using the Setschenow expression, and as a function of temperature. By taking derivatives of the fitted data as a function of temperature, one obtains estimates of the partial molar excess enthalpy and entropy at infinite dilution of the organic chemicals as a function of temperature and salt concentration.     

Low‐temperature headspace‐trap gas chromatography with mass spectrometry for the determination of trace volatile compounds from the fruit of Lycium barbarum L.

The total saccharides content of Lycium barbarum L. is very high, and a high temperature would result in saccharide decomposition and the emergence of a large amount of water. Moreover, the volatile compounds from the fruit of L. barbarum L. are rather low in concentration. Hence, it is difficult for a conventional headspace method to study the volatile compounds from the fruit of L. barbarum L. Since headspace‐trap gas chromatography with mass spectrometry is an excellent method for trace analysis, a headspace‐trap gas chromatography with mass spectrometry method based on low‐temperature (30°C) enrichment and multiple headspace extraction was developed to explore the volatile compounds from the fruit of L. barbarum L. The headspace of the sample was extracted in 17 cycles at 30°C. Each time, the compounds extracted were concentrated in the trap (Tenax TA and Tenax GR, 1:1). Finally, all the volatile compounds were delivered into the gas chromatograph after thermal desorption. With the method described above, a total of 57 compounds were identified. The identification was completed by mass spectral search, retention index, and accurate mass measurement.     

Probing the microwave degradation mechanism of phenol-containing polymeric compounds by sample pretreatment and GC-MS analysis

We have detected the volatile organic species obtained after microwave digestion of poly(hydroxystyrene) with nitric acid in closed vessels at various reaction temperatures by using headspace solid-phase microextraction (HS-SPME) and gas chromatography/mass spectrometry (GC-MS). We probed the digestion reaction by measuring the response of the detectable species - we detected a total of 20 volatile organic compounds - with respect to the temperature over the range from room temperature to 80 °C. These compounds can be classified into alkane, alkanol, and aromatic species. The alkane species decreased monotonically, whereas the alkanol and aromatic compounds first increased and then decreased, as the digestion temperature increased. We have established the degradation mechanisms, which involve bond scission, recombination, adduct formation, and ring opening, on the basis of product analyses and bond length simulations.     

Headspace solid phase microextraction and gas chromatography-quadrupole mass spectrometry methodology for analysis of volatile compounds of marine salt as potential origin biomarkers

The establishment of geographic origin chemical biomarkers for the marine salt might represent an important improvement to its valorisation. Volatile compounds of marine salt, although never studied, are potential candidates. Thus, the purpose of this work was the development of a headspace solid phase microextraction (SPME) combined with gas chromatography-quadrupole mass spectrometry (HS-SPME/GC-qMS) methodology to study the volatile composition of marine salt. A 65 μm carbowax/divinylbenzene SPME coating fibre was used. Three SPME parameters were optimised: extraction temperature, sample quantity, and presentation mode. An extraction temperature of 60 °C and 16 g of marine salt in a 120 mL glass vial were selected. The study of the effect of sample presentation mode showed that the analysis of an aqueous solution saturated with marine salt allowed higher extraction efficiency than the direct analysis of salt crystals. The dissolution of the salt in water and the consequent effect of salting-out promote the release of the volatile compounds to the headspace, enhancing the sensitivity of SPME for the marine salt volatiles. The optimised methodology was applied to real matrices of marine salt from different geographical origins (Portugal, France, and Cape Verde). The marine salt samples contain ca. 40 volatile compounds, distributed by the chemical groups of hydrocarbons, alcohols, phenols, aldehydes, ketones, esters, terpenoids, and norisoprenoids. These compounds seem to arise from three main sources: algae, surrounding bacterial community, and environment pollution. Since these volatile compounds can provide information about the geographic origin and saltpans environment, this study shows that they can be used as chemical biomarkers of marine salt.     

Determination of volatile organic compounds by headspace trap

A new analytical method for the determination of halogenated and aromatic volatile organic compounds in groundwater, mineral water, and drinking water at concentrations ranging between 1-10000 ng/L is developed. A new type of headspace sampler that combines static headspace sampling with a trap is used, yielding very low detection limits and good repeatability without carryover effects. An unexpected transformation of 1,1,2,2-tetrachloroethane into trichloroethene is observed and explained.     

新型水蒸气顶空富集装置在饮用水中痕量挥发性有机物非目标筛查中的应用

以饮用水中痕量挥发性有机物(VOCs)非目标筛查为目的,构建了一种新型的大体积水样高倍富集装置。对其精馏管长度、回收冷凝液体积、吸收介质等影响富集效果的关键因素进行了优化。该装置以水蒸气为吹扫气,同时以水作为吸收剂,将1 L水样富集浓缩至5 mL后,可使原有吹扫捕集-气相色谱-质谱法(P&;T-GC-MS)分析VOCs的灵敏度提高1～2个数量级。用该方法对某净水厂的源水与出厂水进行了痕量VOCs的定性分析与比较。与传统P&;T-GC-MS方法相比,本方法对两种水样的污染物检出数目由原来的无检出和5种分别提高至16种和35种。分析结果表明饮用水消毒前后污染物的种类及含量存在显著差异。     

Roles of hydrophobicity and volatility of organic substrates on sonolytic kinetics in aqueous solutions

The aquasonolytic rate constants of cyclic C6H(X), aliphatic C6H(X), thioethers, thiophenes, and N-heterocyclic compounds show over a 90-fold variation under identical conditions of ultrasonic irradiations. Henry's Law constant of the substrate has a substantial effect on the aquasonolytic rate; a higher Henry's Law constant leads to a aquasonolytic rate constant, which indicating the transfer process of organic substrate between bulk liquid and cavitational bubbles is essential for aquasonolysis. The aquasonolytic rate constants, however, dramatically show an irregular variation with increasing vapor pressure among various substrates. Although the volatility of substrate has been widely regarded as a basic factor influencing aquasonolysis, it seems that vapor pressure of substrate is not a determining one that accounts for the difference of aquasonolytic rate constants. In contrast, the hydrophobic parameters of volatile substrate such as water solubility and octanol-water partition coefficient have shown obvious correlation with the aquasonolytic rate constant for the model compounds; a higher hydrophobicity of volatile substrate results in a higher aquasonolytic rate constant. It could be concluded that the transfer process from bulk liquid to cavitational bubbles and the aquasonolytic kinetics of organic substrate are jointly controlled by the hydrophobicity and volatility; therein the hydrophobicity dominates the transfer process and the aquasonolysis of volatile substrate.     

Direct screening and confirmation of priority volatile organic pollutants in drinking water

A screening tool was proposed for the rapid detection of eight priority volatile organic pollutants according to European standards in drinking water. The method is based on the direct coupling of a headspace sampler with a mass spectrometer, using a chromatographic column heated to 175 degrees C as an interface. The water sample was subjected to the headspace extraction process and the volatile fraction was introduced directly into the mass spectrometer, without prior chromatographic separation, achieving low detection limits (0.6-1.2 ng/ml) for all compounds. The mass spectrum resulting from the simultaneous ionization and fragmentation of the mixture of molecules constitutes the volatile profile of each sample. An appropriate chemometric treatment of these signals permitted them to be classified, on the basis of their volatile composition, as contaminated or uncontaminated with respect to the legally established concentration levels for these compounds in drinking water, and providing no false negatives. A conventional confirmation method was carried out to analyze positive water samples by using the same instrumental setup as in the screening method, but using an appropriate temperature program in the chromatographic column to separate, identify and quantify each analyte.     

Isolation and preconcentration of volatile organic compounds from water

Methods for the isolation and/or concentration of volatile organic compounds from water samples for trace organic analysis by gas chromatography are reviewed. The following basic groups of methods are discussed: liquid-liquid extraction, adsorption on solid sorbents, extraction with gas (gas stripping and static and dynamic headspace techniques) and membrane processes. The theoretical bases of these methods are discussed. Experimental arrangements for the isolation and/or concentration of volatile compounds from water are presented and discussed with respect to their efficiency. The applicability of the described methods to the isolation and/or concentration of various organic compounds from waters of various origins is discussed.     

Rapid removal of selected volatile organic compounds from gaseous mixtures using a new dispersive vapor extraction technique: A feasibility study

A new dispersive vapor extraction (DVE) technique for rapid removal of selected volatile organic compounds (VOCs) from gaseous mixtures was investigated. In this technique, less than 1.0 mL of a volatile solvent was vaporized for 8 min in a 250-mL flask containing a gaseous mixture. The flask was then cooled under running tap water for 2-3 min to induce condensation of the vapor and co-extraction of the VOCs from the headspace. The technique was tested over a concentration range of 4-23 ppb, and resulted in extraction efficiencies ranging from 80 to 97% for the VOCs tested. Because of its simplicity and the relatively short sampling time, DVE could potentially lead to high sample throughput and rapid air analysis.     

Sampling volatile organic compounds using a modified solid phase microextraction device

Headspace solid phase microextraction (headspace SPME) has been demonstrated to be an excellent solvent-free sampling method. One of the major factors contributing to the success of headspace SPME is the concentrating effect of the fiber coating toward organic compounds. The affinity of the fiber coating toward very volatile analytes, such as chloromethane, may, however, not be large enough for detection at the parts per trillion concentration level. Static headspace analysis, on the other hand, is very effective for these very volatile compounds. As analyte volatility decreases, the sensitivity of static headspace analysis drops. The complementary nature of these two sampling methods can be exploited by combining the SPME device with a gastight syringe. The sensitivity of the new sampling device is better than that of SPME for very volatile compounds or that of static headspace analysis for less volatile compounds. This new method can sample a wide range of compounds from chloromethane (b.p. −24°C) to bromoform (b.p. 149°C) with estimated limits of detection at the low parts per trillion level.     

Effect of inorganic salt on partition of high‐polarity parishins in two‐phase solvent systems and separation by high‐speed counter‐current chromatography from Gastrodia elata Blume

Parishins are high‐polarity and major bioactive constituents in Gastrodia elata Blume. In this study, the effect of several inorganic salts on the partition of parishins in two‐phase solvent systems was investigated. Adding ammonium sulfate, which has a higher solubility in water, was found to significantly promote the partition of parishins in the upper organic polar solvents. Based on the results, a two‐phase solvent system composed of butyl alcohol/acetonitrile/near‐saturated ammonium sulfate solution/water (1.5:0.5:1.2:1, v/v/v/v) was used for the purification of parishins by high‐speed counter‐current chromatography. Fractions obtained from high‐speed counter‐current chromatography were subjected to semi‐preparative high‐performance liquid chromatography to remove salt and impurities. As a result, parishin E (6.0 mg), parishin B (7.8 mg), parishin C (3.2 mg), gastrodin (15.3 mg), and parishin A (7.3 mg) were isolated from water extract of Gastrodia elata Blume (400 mg). These results demonstrated that adding inorganic salt that has high solubility in water to the two‐phase solvent system in high‐speed counter‐current chromatography was a suitable approach for the purification of high‐polarity compounds.