Hydrocarbons In Environmental Water Samples Research Articles

Deep eutectic solvents with low viscosity for the determination of polycyclic aromatic hydrocarbons in environmental water samples by dispersive liquid–liquid microextraction

This paper prepared a series of low-viscosity deep eutectic solvents (DESs) using two terpenoids, menthol and thymol, as hydrogen bonding acceptors and lauryl alcohol as hydrogen bonding donors. 1H NMR and infrared spectroscopy verified hydrogen bonding between the two components. Furthermore, the developed method used a green DES, which dispersed water samples without dispersant or additional auxiliary equipment. In addition, the low viscosity of this deep eutectic solvent allows high mass transfer efficiency and fast extraction. Based on the above, a sample pretreatment method was developed for hand-shake dispersive liquid–liquid microextraction (DLLME) to determine PAHs in environmental water samples. AGREEpre was used to evaluate the greenness of the method. The results showed that the linear range (0.1–200 µg/L), the detection limits (0.027–0.035 µg/L), and the relative standard deviations (4–7 %) of the developed method are comparable with other methods reported in the literature.

Selective solid-phase microextraction of polycyclic aromatic hydrocarbons in water based on oriented phosphorus-containing titanium oxide nanofibers grown on titanium support prior to high-performance liquid chromatography with ultraviolet detection.

A novel solid-phase microextraction coating of phosphorous-containing titanium oxide composite was developed using titanium fiber as a support and a titanium source by hydrothermal oxidation in a phosphoric acid solution containing hydrogen peroxide. The morphology of the fiber coatings was controlled by the conditions of the hydrothermal oxidation reaction. The oriented nanofiber coating was employed to extract several types of representative aromatic analytes. The experimental results demonstrated that the as-prepared fiber exhibited excellent extraction efficiency toward polycyclic aromatic hydrocarbons. Combined with high-performance liquid chromatography with ultraviolet detection, main extraction conditions were optimized, including pH, ionic strength, extraction temperature, stirring rate, extraction time and desorption time. The established method presented good linearity from 0.05 to 200 μg/L with limit of detection ranging from 0.012 to 0.126 μg/L. This convenient and green procedure was suitable for the selective extraction and determination of typical polycyclic aromatic hydrocarbons in environmental water samples. The relative recoveries of 85.8-112% were obtained for the determination of target polycyclic aromatic hydrocarbons in water samples spiked with 5.0 and 15.0 μg/L. Moreover, the as-prepared fiber showed at least 210 extraction/desorption cycles due to its high mechanical and chemical stability.

Magnetic solid phase extraction of heterocyclic aromatic hydrocarbons from environmental water samples with multiwalled carbon nanotube modified magnetic polyamido-amine dendrimers prior to gas chromatography-triple quadrupole mass spectrometer

Present study described a sensitive and efficient method for determination of heterocyclic aromatic hydrocarbons using multiwalled carbon nanotubes modified magnetic polyamido-amine dendrimers (MNPs@PAMAM-Gn@MWCNTs) as adsorbent for magnetic solid-phase extraction (MSPE) coupled with gas chromatography-triple quadrupole mass spectrometer (GC–MS/MS). Some pivotal parameters including PAMAM generation, adsorbent dosage, adsorption time, elution time and volume, pH and humic acid concentration were investigated to achieve the best adsorption efficiencies. Under the optimal conditions, 7-methylquinoline, dibenzothiophene and carbazole had good linearity in the concentration range of 0.005–20 μg L − 1, 9-methylcarbazole, 4-methyldibenzothiophene and 4,6-dimethyl dibenzothiophene had good linearity in the concentration range of 0.001–20 μg L − 1. All the correlation coefficients were higher than 0.996. The detection limits of the targets were in the range of 2.2 × 10−4–1.8 × 10−3 μg L − 1 with precisions less than 8.28% (n = 6). The enrichment factors were in the range of 141–147. The spiked recoveries were in the range of 87.0%-115.1% (n = 3). These results indicated that the method could be a reliable alternative tool for monitoring trace heterocyclic aromatic hydrocarbons in environmental water samples.

In situ growth of metal-organic framework HKUST-1 in an organic polymer as sorbent for nitrated and oxygenated polycyclic aromatic hydrocarbon in environmental water samples prior to quantitation by HPLC-UV.

An in situ preparation of metal-organic framework (HKUST-1) on the surface of an organic polymer is reported. The hybrid material was evaluated as sorbent for solid-phase extraction (SPE) of oxygenated and nitrated polycyclic aromatic hydrocarbons (PAHs). The growth of HKUST-1 on MAA-based polymer was accomplished using a layer-by-layer assembly strategy. The HKUST-1-polymer was characterized using powder X-ray diffraction and scanning electron microscopy to demonstrate the incorporation of MOF crystals onto the polymer surface. The MOF (nano)crystals were characterized using high-resolution transmission electron microscopy and high-angle annular-dark-field scanning transmission electron microscopy. The water-stable MOF-polymer was evaluated as SPE sorbent, and several variables that can influence the extraction recoveries of PAH derivatives were investigated. Under the selected conditions, the detection limits varied between 4 and 21ng·L-1, and the precision (relative standard deviation) was below 12%. The recovery values for spiked solutionsranged from 95 to 104%, and the enrichment factor achieved was close to 1000-fold using a high sample volume (100mL) and very low desorption volume (100μL). The SPE enrichment combined with HPLC and DAD detector was successfully applied to the extraction and determination of polycyclic hydrocarbons in environmental water samples. Graphical abstract A layer-by-layer synthesis of HKUST-1 onto polymer surface was performed and applied to isolation of PAH derivatives in environmental water samples.

Hybrid nanocomposites prepared from a metal-organic framework of type MOF-199(Cu) and graphene or fullerene as sorbents for dispersive solid phase extraction of polycyclic aromatic hydrocarbons.

Different types of hybrid nanocomposites were prepared from a copper-based metal-organic framework (MOF-199) and graphene (Gr) or fullerene (Fl). The porosity and quality of the nanocomposites were studied by scanning electron microscopy, transmission electron microscopy and BET surface area analysis. The nanocomposites are shown to be viable sorbents for the dispersive micro solid phase extraction of polycyclic aromatic hydrocarbons (PAHs) from environmental water samples. This is due to (a) the presence of MOF-199 which leads to improved adsorption capacity, and (b) the presence of Gr or Fl on the surface of MOF-199 which enhances the interaction with PAHs. Specifically, acenaphthene, anthracene, benz[a]anthracene, fluorene, naphthalene, 2-methylnaphthalene, and pyrene were studied. A comparison of the sorbents shows MOF-199/Gr to possess the highest adsorption affinity and to be most durable, probably a result of the high porosity of graphene. Following desorption with acetonitrile, the PAHs were quantified by GC with FID detection. Under the optimum conditions, limits of detection (at an S/N ratio of 3) range from 3 to 10pgmL-1, and the analytical ranges are linear at 0.01-100ngmL-1 of PAHs. The relative standard deviations for five replicates at two spiking levels (0.03 and 50ngmL-1) range from 5.0 to 7.4%. The applicability of this method was confirmed by analyzing spiked real water samples, and recoveries are between 91.9 and 99.5%. Graphical abstract Different types of the hybrid nanocomposites of the copper-based metal-organic framework MOF-199 with graphene or fullerene were synthesized and used as sorbent for the dispersive micro solid phase extraction of polycyclic aromatic hydrocarbons in environmental water samples.

Purification and enrichment of polycyclic aromatic hydrocarbons in environmental water samples by column clean-up coupled with continuous flow single drop microextraction

A novel method for the determination of sixteen polycyclic aromatic hydrocarbons (PAHs) in environmental water samples was developed using column clean-up (CCU) coupled with continuous flow single drop microextraction (CF-SDME) prior to the determination by gas chromatography-mass spectrometry (GC–MS). In this method, purification, extraction and enrichment were carried out in a single step, which reduced the effect of most organic interferents on the determination of target analytes, greatly simplified the operation process and shortened the whole pre-treatment time. The CCU-CF-SDME system was first used, and shortcomings such as long extraction time, instability of suspended microdrop of the traditional SDME system were overcome. The influences of several experimental parameters, including the type and amount of column packing material, the type and volume of extraction solvent, the flow rate of sample solution, extraction temperature, pH and ionic strength of sample solution, on the extraction efficiency were investigated. Under the optimal experimental conditions, the good linearities were obtained in the concentration range of 0.02–10 μg L−1 with correlation coefficients ranging from 0.9912 to 0.9995 for all the analytes. The limits of detection and quantification of the analytes were in the range of 0.0012–0.0101 μg L−1 and 0.0041–0.0336 μg L−1, respectively. The pre-concentration factors in the range of 777–978 for the PAHs were obtained. The proposed method was applied on the analysis of environmental water samples, and the recoveries of target analytes were in the range of 81.8–105.8% with the relative standard deviations ranging from 0.5% to 6.4%. The results showed that the developed method was a simple, quick and feasible method for the determination of PAHs in environmental water samples.

Magnetic Solvent Bar Liquid-Phase Microextraction Followed by Gas Chromatography-Flame Ionization Detection for the Trace Determination of Selected Polycyclic Aromatic Hydrocarbons in Environmental Water Samples

A novel and efficient hollow fiber-based method, viz. magnetic solvent bar liquid-phase microextraction (MSB-LPME) combined with gas chromatography-flame ionization detection (GC-FID) was successfully developed for the trace determination of selected polycyclic aromatic hydrocarbons (PAHs) in environmental water samples. The target analytes were extracted from sample solution to the organic solvent immobilized in a fiber. After extraction, the analyte-adsorbed magnetic solvent bar could be readily isolated from the sample solution by a magnet which could greatly simplify the operation and also decline the total pretreatment time. The bar was first eluted with methanol, evaporated to dryness while the residue was dissolved in toluene and finally injected into GC-FID. Begin with, effective parameters controlling the performance of the microextraction were evaluated and optimized. The values of the detection limit of the method were in the range of 0.05-0.08 µg L -1 and the RSD% values for the analysis of 10.0 µg L -1 of the analytes was below than 5.8% (n= 6). A good linearity (0.998 ≥ r 2 ≥ 0.994) and a broad linear range (0.1-200 µg L -1 ) were obtained. The method was eventually utilized for the preconcentration and determination of the PAHs in environmental water samples and satisfactory results were obtained.

Basalt fibers functionalized with gold nanoparticles for in-tube solid-phase microextraction.

Basalt fibers were functionalized with gold nanoparticles and characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. An in-tube solid-phase microextraction device was developed by packing the functionalized basalt fibers in a polyether ether ketone tube. The device was connected into high performance liquid chromatography equipment with a diode array detector to build online enrichment and analysis system. Eight polycyclic aromatic hydrocarbons were used as model analytes, important factors including sampling rate, sampling volume, organic solvent content in sample, and desorption time were investigated. Linear range (0.01-20 μg/L), detection limits (0.003-0.015 μg/L), and enrichment factors (130-1628) were given by the online analysis method. Relative standard deviations (n=5) of extraction repeatability on one tube and tube-to-tube repeatability were less than 5.2 and 14.7%, respectively. The analysis method was applied to detect polycyclic aromatic hydrocarbons in environmental water samples, and relative recoveries ranged from 87 to 128%.

Carbon nanotube-based benzyl polymethacrylate composite monolith as a solid phase extraction adsorbent and a stationary phase material for simultaneous extraction and analysis of polycyclic aromatic hydrocarbon in water

A composite of multi-walled carbon nanotubes incorporated into a benzyl methacrylate-co-ethylene dimethacrylate porous monolith was prepared, characterized and used as solid phase adsorbent and as stationary phase for simultaneous extraction and separation of ten polycyclic aromatic hydrocarbons, followed by nano-liquid chromatography analysis. The extraction and chromatographic parameters were optimized with regard to the extraction efficiency and the quality of chromatographic analytes separation. Under the optimized conditions, all PAHs were separated in 13 min with suitable resolution values (Rs = 1.74–3.98). Addition of a small amount of carbon nanotubes (0.1% with respect to monomers) to the polymerization mixture increased the efficiency for the separation column to over 41,700 plates m−1 for chrysene at flow rate of 0.5 μL min−1. The method showed a wide linear range (1–500 μg L−1 with R2 more than 0.9938), acceptable extraction repeatability (RSDs < 6.4%, n = 3) and reproducibility (RSDs < 12.6%, five parallel-made solid phase extraction cartridges) and satisfactory detection limits (0.02–0.22 μg L−1). Finally, the proposed method was successfully applied to the detection of polycyclic aromatic hydrocarbons in environmental water samples. After a simple extraction procedure with preconcentration factor equal to 100, the average recovery values in ultra-pure, tap and sea water samples were found to be in the range 81.3–95.4% with %RSD less than 6.4. Again, the presence of carbon nanotubes (0.3% relatively to monomers) in native polymer enhanced the extraction performance for the solid phase adsorbent up to 78.4%. The application of the monoliths modified with CNTs in extraction and nano-scale liquid chromatography for analysis of environmental samples offered several advantages; it demonstrated an acceptable precision, low detection limits, good reproducibility, satisfying recoveries and wide dynamic linear ranges.

Optimization of multiwalled carbon nanotubes reinforced hollow-fiber solid-liquid-phase microextraction for the determination of polycyclic aromatic hydrocarbons in environmental water samples using experimental design.

A novel design of hollow-fiber liquid-phase microextraction containing multiwalled carbon nanotubes as a solid sorbent, which is immobilized in the pore and lumen of hollow fiber by the sol-gel technique, was developed for the pre-concentration and determination of polycyclic aromatic hydrocarbons in environmental water samples. The proposed method utilized both solid- and liquid-phase microextraction media. Parameters that affect the extraction of polycyclic aromatic hydrocarbons were optimized in two successive steps as follows. Firstly, a methodology based on a quarter factorial design was used to choose the significant variables. Then, these significant factors were optimized utilizing central composite design. Under the optimized condition (extraction time=25min, amount of multiwalled carbon nanotubes=78mg, sample volume=8mL, and desorption time=5min), the calibration curves showed high linearity (R2 =0.99) in the range of 0.01-500ng/mL and the limits of detection were in the range of 0.007-1.47ng/mL. The obtained extraction recoveries for 10ng/mL of polycyclic aromatic hydrocarbons standard solution were in the range of 85-92%. Replicating the experiment under these conditions five times gave relative standard deviations lower than 6%. Finally, the method was successfully applied for pre-concentration and determination of polycyclic aromatic hydrocarbons in environmental water samples.

Graphene‐based molecularly imprinted polymer for separation and pre‐concentration of trace polycyclic aromatic hydrocarbons in environmental water samples

ABSTRACTA composite of reduced graphene oxide and pyrene‐imprinted polymer was synthesized and employed as a solid phase for extraction of five selected polycyclic aromatic hydrocarbons (PAHs) from water samples. Gas chromatography‐time of flight/mass spectrometry was employed in the analysis of the extracts. The composite was prepared by a free radical polymerization of methacrylic acid and 4‐vinylpyridine as monomers and ethylene glycol dimethacrylate as a crosslinker. The adsorption studies were carried out through batch binding studies. The binding capacity for the imprinted and non‐imprinted composite was 101.83 and 68.21 µg g−1, respectively. The adsorption followed the pseudo 2nd order and well fitted the Langmuir isotherm. Mean recoveries ranging from 73% to 105.4% for both spiked deionized water and environmental water samples were obtained when the imprinted composites were employed in solid phase extraction of the PAHs. The composites could be re‐used for five times without a significant loss in recoveries. The proposed method was employed for the analysis of spiked environmental water samples and did not show significant changes in the recoveries showing there were no matrix interferences. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45300.

Magnetic metal–organic framework MIL‐100(Fe) microspheres for the magnetic solid‐phase extraction of trace polycyclic aromatic hydrocarbons from water samples

In this work, a magnetic metal-organic framework designated as MIL-100(Fe) was prepared and applied as a magnetic solid-phase extraction sorbent for the determination of trace polycyclic aromatic hydrocarbons in environmental water samples by coupling with high-performance liquid chromatography and fluorescence detection. The magnetic microspheres exhibited large surface areas and high extraction ability, making them excellent candidates as sorbents for enrichment of trace polycyclic aromatic hydrocarbons. Under the optimized experimental conditions, good sensitivity levels were achieved with low detection limits ranging from 32 to 2110 pg/mL and good linearities with correlation coefficients higher than 0.9990 for the investigated 13 polycyclic aromatic hydrocarbons. The proposed method has been validated in the analysis of real water samples with mean recoveries in the range of 81.4-126.9% at four spiked levels and the relative standard deviations in the range of 1.3-17.0%. The magnetic MIL-100(Fe) microspheres were stable enough for 150 extractions without a significant loss of extraction performance.

Automated Agitation-Assisted Demulsification Dispersive Liquid–Liquid Microextraction

Dispersive liquid-liquid microextraction (DLLME) is an extremely fast and efficient sample preparation procedure. For its capability and applicability to be fully exploited, full automation of its operations seamlessly integrated with analysis is necessary. In this work, for the first time, fully automated agitation-assisted demulsification (AAD)-DLLME integrated with gas chromatography/mass spectrometry was developed for the convenient and efficient determination of polycyclic aromatic hydrocarbons in environmental water samples. The use of a commercially available multipurpose autosampler equipped with two microsyringes of different capacities allowed elimination or significant reduction of manpower, labor, and time with the large-volume microsyringe used for liquid transfers and the small-volume microsyringe for extract collection and injection for analysis. Apart from enhancing accessibility of DLLME, the procedure was characterized by the application of agitation after extraction to break up the emulsion (that otherwise would need centrifugation or a demulsification solvent), further improving overall operational efficiency and flexibility. Additionally, the application of low-density solvent as extractant facilitated the easy collection of extract as the upper layer over water. Some parameters affecting the automated AAD-DDLME procedure were investigated. Under the optimized conditions, the procedure provided good linearity (ranging from a minimum of 0.1-0.5 μg/L to a maximum of 50 μg/L), low limits of detection (0.010-0.058 μg/L), and good repeatability of the extractions (relative standard deviations, below 5.3%, n = 6). The proposed method was applied to analyze PAHs in real river water samples.

Fast automated dual-syringe based dispersive liquid–liquid microextraction coupled with gas chromatography–mass spectrometry for the determination of polycyclic aromatic hydrocarbons in environmental water samples

An automated procedure, combining low density solvent based solvent demulsification dispersive liquid-liquid microextraction (DLLME) with gas chromatography–mass spectrometry analysis, was developed for the determination of polycyclic aromatic hydrocarbons (PAHs) in environmental water samples. Capitalizing on a two-rail commercial autosampler, fast solvent transfer using a large volume syringe dedicated to the DLLME process, and convenient extract collection using a small volume microsyringe for better GC performance were enabled. Extraction parameters including the type and volume of extraction solvent, the type and volume of dispersive solvent and demulsification solvent, extraction and demulsification time, and the speed of solvent injection were investigated and optimized. Under the optimized conditions, the linearity ranged from 0.1 to 50μg/L, 0.2 to 50μg/L, and 0.5 to 50μg/L, depending on the analytes. Limits of detection were determined to be between 0.023 and 0.058μg/L. The method was applied to determine PAHs in environmental water samples.

QSAR aided design and development of biopolymer-based SPE phase for liquid chromatographic analysis of polycyclic aromatic hydrocarbons in environmental water samples

A solid-phase extraction method, using a starch based biopolymer as an adsorbent, coupled with high performance liquid chromatography-fluorescence/UV detectors was developed for the determination of 16 polycyclic aromatic hydrocarbons in environmental water samples.

In situ hydrothermal growth of a dual-ligand metal–organic framework film on a stainless steel fiber for solid-phase microextraction of polycyclic aromatic hydrocarbons in environmental water samples

In situ hydrothermal growth of bio-MOF-1 film on stainless steel fiber for solid-phase microextraction of polycyclic aromatic hydrocarbons.

Development of a magnetic solid-phase extraction coupled with high-performance liquid chromatography method for the analysis of polyaromatic hydrocarbons.

A novel magnetic solid phase extraction coupled with high-performance liquid chromatography method was established to analyze polyaromatic hydrocarbons in environmental water samples. The extraction conditions, including the amount of extraction agent, extraction time, pH and the surface structure of the magnetic extraction agent, were optimized. The results showed that the amount of extraction agent and extraction time significantly influenced the extraction performance. The increase in the specific surface area, the enlargement of pore size, and the reduction of particle size could enhance the extraction performance of the magnetic microsphere. The optimized magnetic extraction agent possessed a high surface area of 1311 m(2) /g, a large pore size of 6-9 nm, and a small particle size of 6-9 μm. The limit of detection for phenanthrene and benzo[g,h,i]perylene in the developed analysis method was 3.2 and 10.5 ng/L, respectively. When applied to river water samples, the spiked recovery of phenanthrene and benzo[g,h,i]perylene ranged from 89.5-98.6% and 82.9-89.1%, respectively. Phenanthrene was detected over a concentration range of 89-117 ng/L in three water samples withdrawn from the midstream of the Huai River, and benzo[g,h,i]perylene was below the detection limit.

Determination of Polycyclic Aromatic Hydrocarbons in Environmental Water Samples by Microwave-Assisted Headspace Solid-Phase Microextraction

Microwave-assisted headspace solid-phase microextraction (MA-HS-SPME) with a polydimethylsiloxane/divinylbenzene (PDMS/DVB) and PDMS fibers were applied as a single step before determination of polycyclic aromatic hydrocarbon (PAH) concentrations in water using gas chromatography–mass spectrometry (GC-MS). PDMS/DVB was found better in performance and was chosen for the rest of the investigations. Dissolved organic matter (DOM) was found to not effect PAH analysis significantly. This study (MA-HS-SPME) presents the first finding in the literature that total (both freely dissolved and DOM-associated) PAH concentration was determined. To optimize the extraction efficiency of PAHs by MA-HS-SPME, influence of various parameters, including fibers, temperature, duration of thermal desorption, microwave irradiation power and duration, and the temperature of the circulating cooling water system, was studied. Under optimal operating conditions, good linearity of a calibration curve was observed over the studied concentration range for 16 PAHs (0.1–5 μg/L) in aqueous solution. PAH detection limit varied from 26.8 to 128 ng/L. A protocol was then proposed for estimating field binding constant between PAH and DOM through using our proposed MA-HS-SPME method and traditional SPME technique (freely dissolved PAH concentration determined). The proposed method was demonstrated applicable to environmental water samples, especially contaminated aquatic environment samples.

Magnetic N-doped carbon nanotubes: A versatile and efficient material for the determination of polycyclic aromatic hydrocarbons in environmental water samples

This paper describes a new, efficient and versatile method for the sampling and preconcentration of PAH in environmental water matrices using special hybrid magnetic carbon nanotubes. These N-doped amphiphilic CNT can be easily dispersed in any aqueous matrix due to the N containing hydrophilic part and at the same time show high efficiency for the adsorption of different PAH contaminants due to the very hydrophobic surface. After adsorption, the CNT can be easily removed from the medium by a simple magnetic separation. GC/MS analyses showed that the CNT method is more efficient than the use of polydimethylsiloxane (PDMS) with much lower solvent consumption, technical simplicity and time, showing good linearity (range 0.18–80.00μgL−1) and determination coefficient (R2>0.9810). The limit of detection ranged from 0.05 to 0.42μgL−1 with limit of quantification from 0.18 to 1.40μgL−1. Recovery (n=9) ranged from 80.50±10 to 105.40±12%. Intraday precision (RSD, n=9) ranged from 1.91 to 9.01%, whereas inter day precision (RSD, n=9) ranged from 7.02 to 17.94%. The method was applied to the analyses of PAH in four lake water samples collected in Belo Horizonte City, Brazil.

Extraction and enrichment of polycyclic aromatic hydrocarbons by ordered mesoporous carbon reinforced hollow fiber liquid-phase microextraction.

A novel microextraction method, ordered mesoporous carbon reinforced hollow fiber liquid-phase microextraction coupled with high-performance liquid chromatography and fluorescence detection, was developed for the determination of some organic pollutants in water samples. Four polycyclic aromatic hydrocarbons (fluorene, anthracene, fluoranthene, and pyrene) were selected to validate this new method. Main parameters that could influence the extraction efficiency such as extraction time, fiber length, stirring rate, the type of the extraction solvent, pH value, the concentration of ordered mesoporous carbon, and salt effect were optimized. Under the optimal extraction conditions, good linearity was observed in the range of 2-1000 ng/L, with the correlation coefficients of 0.9954-0.9986. The recoveries for the spiked samples were in the range of 88.96-100.17%. The limits of detection of the method were 0.4-4 ng/L. The relative standard deviations varied from 4.2-5.9%. The results demonstrated that the newly developed method was an efficient pretreatment and enrichment procedure for the determination of polycyclic aromatic hydrocarbons in environmental water samples.