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.     

Determination of acidic pharmaceutical products and carbamazepine in roughly primary-treated wastewater by solid-phase extraction and gas chromatography–tandem mass spectrometry

A gas chromatography–tandem mass spectrometry (GC–MS/MS) method has been developed for the determination of selected pharmaceutical residues (carbamazepine, salicylic acid, clofibric acid, ibuprofen, 2-hydroxy-ibuprofen, fenoprofen, naproxen, ketoprofen, diclofenac, and triclosan) in sewage influent and roughly primary-treated effluent. The method involved solid-phase extraction (SPE) with polymeric sorbents, and two SPE cartridges were compared for the extraction and elution of the targeted compounds in complex matrices. A successful chemical derivatization of carbamazepine and acidic compounds using N,O-bis(trimethylsilyl) trifluoroacetamide +10% trimethylchlorosilane is also described. The quantification limits of the analytical procedure ranged from 30 to 60?ng?L^?1 for 500?mL of wastewater. The best recovery rates (72–102%) in spiked effluent samples were obtained with Phenomenex Strata-X? cartridges. Detection limits (S/N?=?3) were estimated at between 1 and 18?ng?L^?1. The reported GC–MS/MS method significantly reduces the strong matrix effects encountered with more expensive LC-MS/MS techniques. Application of the developed method showed that most selected analytes were detected at concentrations ranging from low µg?L^?1 to trace level ng?L^?1 in Montreal's wastewater treatment plant effluent and influent, as well as in the receiving waters at more than 8?km downstream of the effluent outfall. The rugged alternative analytical method is suitable for the simultaneous analysis of carbamazepine and pharmaceutical acidic residues in wastewater samples from influents and effluents that have undergone rough primary treatment.     

Simultaneous determination of trichloroethylene,perchloroethylene and trichloroacetic acid in human urine using solid-phase microextraction fibre coated with single-walled carbon nanotubes

A reliable and sensitive method for simultaneous determination of perchloroethylene (PCE), trichloroethylene (TCE), and trichloroacetic acid (TCA) in human urine by gas chromatography-mass spectrometry (GC/MS) is described after extraction and preconcentration by a new solid-phase microextraction (SPME) adsorbent. The potential of single-walled carbon nanotubes (SWCNTs) as SPME adsorbent for the pre-concentration of environmental pollutants has been investigated in recent years. This work was carried out to investigate the feasibility of SWCNTs as a headspace SPME adsorbent for the determination of chloroethylenes in human urine. SWCNTs were attached onto a stainless steel wire through an organic binder. Potential factors affecting the extraction efficiency, including extraction time, extraction temperature, desorption time, desorption temperature, and salinity were optimized. The developed method showed good performance. For PCE and TCE, calibration curves were linear (r ^{2 ?}≥?0.994) over the concentration ranges from 15 to 8000?ng?L^?1 and the limit of detection (LOD) at signal-to-noise (S/N) ratio of 3 was 5?ng?L^?1. The analytical procedure also involves derivatization of TCA with dimethyl sulfate, before headspace sampling. For TCA the linear range and LOD were 45-8000 (r ^{2 ?}≥?0.992) and 15?ng L^?1, respectively. In addition, a comparative study between the SWCNT and a commercial carboxen/polydimethylsiloxane (CAR/PDMS) SPME fibre for the determination of chloroethylenes in human urine was carried out. SWCNT fibre showed higher extraction capacity, better thermal stability (over 350°C) and longer life span (over 200 times) than the commercial CAR/PDMS fibre. The developed method was successfully applied to determine chloroethylenes in human urine samples. As the results indicated, the mean concentrations of TCE, PCE and TCA in exposed workers (dry-cleaning industry workers) were significantly greater than that of control group.     

An evaluation of antifouling booster biocides in Gran Canaria coastal waters using SPE-LC MS/MS

A solid phase extraction (SPE) technique for seawater samples coupled to quantification using liquid chromatography tandem-mass spectrometry (LC-MS/MS) is described to quantify relevant antifouling booster biocides of ecotoxicological concern (Diuron, TCMTB, Irgarol 1051 and Dichlofluanid). The optimised methodology provides a sensitive, easy to use and efficient analytical procedure with detection limits in the range of between 0.1 and 0.2?ng?L^?1 and appropriate reproducibility (with analytical standard deviations of less than 10%). Spiked recoveries for all compounds exceeded 72%. The method was tested through a thorough monitoring regime of commercial port and marinas on the island of Gran Canaria (off the north-west coast of Africa) over a period of several months in 2008. Results provide the first data for antifouling booster biocides in the Canary Islands. Concentrations of Diuron and Irgarol 1051 in samples ranged between 2 and 195?ng?L^?1 and 2 and 146?ng?L^?1, respectively. TCMTB and Dichlofluanid were not detected.     

Determination of trace levels of aquaculture chemotherapeutants in seawater samples by SPME-GC-MS/MS

A sensitive and efficient solid-phase microextraction (SPME) method for the determination of organophosphorous (OPPs) and pyrethroid pesticides (Pyrs) in aquaculture-seawater samples by using GC with MS/MS (GC-MS/MS) was developed. Dichlorvos and chlorpyrifos (OPPs); permethrin, alpha-cypermethrin and deltamethrin (Pyrs) were selected according to their use as chemotherapeutants in the aquaculture industry. Different parameters affecting extraction efficiency such as fibre coating, agitation, pH and extraction time profiles were investigated. An experimental central composite design (alpha = 1) and desirability functions were used for the simultaneous optimization of extraction temperature and sample volume. Finally, a method based on direct SPME in 40 min at 75 degrees C using 100-microm-thick poly(dimethyl)siloxane (PDMS) fibre and 20 mL of sample volume is proposed. The method was validated, exhibiting good linearity, precision and accuracy parameters with picogram per millilitre LODs. The proposed methodology was applied to determine the ultratrace levels of OPPs and Pyrs in aquaculture-seawater samples by the standard addition approach, which proved to be reliable and sensitive, in addition to requiring only small amounts of sample.     

Uncovering seasonal patterns of 56 pesticides in surface coastal waters of the Ria Formosa lagoon (Portugal), using a GC-MS method

This study describes the simultaneous quantification of 56 pesticides in surface coastal water, supported by the development and validation of a gas chromatography (GC)–ion trap (IT) mass spectrometry (MS) method. Samples (500 mL) were pre-concentrated 2500 times by solid phase extraction (OASISTMHLB). The compounds were identified and quantified, within 35 minutes, by GC tandem mass spectrometry (GC-MS/MS) and GC-MS, respectively. The methodology proved to be highly specific for all target pesticides, with an average linearity of 0.99. Detection limits and recovery rates ranged from 0.4 to 1.3 ng L^?1 and 71% to 120%, respectively. The performance of the method was checked using water samples collected from nine sampling sites along the Ria Formosa Lagoon Natural Park (south of Portugal, n = 54) in each season (2010). The total annual concentrations of all pesticides in each category (fungicides, herbicides and insecticides) were 1.4, 0.6 and 9.0 µg L^?1, respectively. Moreover, 89% of the pesticides tested for were detected, 84% could be quantified and 25% had concentrations above the European recommended levels (2013/39/EU). The highest total loads of pesticides were found in the spring, which is in agreement with their seasonal application. Physicochemical parameters such as, nitrites, nitrates, ammonia and phosphates, also indicate poor water quality, supporting the fact that the Ria Formosa lagoon actually needs an effective monitoring programme for effective preservation of its natural reserve status.     

Optimization of a SPME/GC/MS Method for the Simultaneous Determination of Pharmaceuticals and Personal Care Products in Waters

A headspace solid phase microextraction (HS-SPME) method coupled with gas chromatography and MS detection (GC/MS) was optimized for the simultaneous determination of 21 target Pharmaceuticals and Personal Care Products (PPCPs) in water samples. The analytes included fragrances, UV-filters, antiseptics, estrogens, anti-inflammatory drugs, and pesticides. An on-fiber SPME derivatization, using silyl reagents, was performed for the analysis of more polar acidic compounds. An experimental design approach was applied to systematically investigate and optimize the operative parameters affecting the extraction recovery, namely: extraction temperature and time, derivatization time, desorption temperature and time. The optimum operating conditions were: extraction time of 125?min at a temperature of 40?°C; derivatization time of 30.5?min; desorption time of 2?min at a temperature of 300?°C. Under these conditions, good reproducibility was assessed as RDS% values ≤10% for underivatized PPCPs and ≤20% for derivatized compounds. The method detection limits (LOD) were between 0.7 and 9.0?ng?L^?1, with the highest values in the range 2.5–9.0?ng?L^?1 for the derivatized analytes. Method accuracy was evaluated on spiked tap water samples: recoveries varied from 85 to 103% and from 75 to 110% for non-derivatized and derivatized compounds, respectively.     

An efficient and fast microwave-assisted extraction method developed for the simultaneous determination of 18 organochlorine pesticides in sediment

An efficient and fast microwave-assisted extraction (MAE) method followed by gas chromatographic separation with mass spectrometric detection (GC–MS) was developed for the extraction of 18 organochlorine pesticides (OCPs) from sediment. Parameters affecting the MAE procedure such as the type and volume of the extraction solvent, irradiation power, temperature and irradiation time were successfully optimised. Under the optimal conditions, extraction efficiencies in the range of 73.4–119% were obtained with THF–HEX (9:1, v/v) for all OCPs studied. The method was linear over the range of 2.9–5000 ng g^?1 with determination coefficients (r²) higher than 0.992 for all analytes. The limits of detection, LODs (S/N = 3), obtained varied from 1.0 to 2.2 ng g^?1 and limits of quantification, LOQs (S/N = 10) were between 2.9 and 6.8 ng g^?1. The proposed method was successfully applied to the analysis of real sediment samples and acceptable recoveries from 70.1 to 124% with RSDs ≤14.8% were obtained. 10 OCPs were determined below their LOQ and 8 OCPs in the range of 124–2830 ng g^?1. The MAE method was compared with Soxhlet, shake flask and ultrasonic solvent extraction techniques. Thus, the MAE–GC–MS method could efficiently be used for selective extraction and quantification of the target analytes from the complex sediment matrices.     

Ultrasound‐assisted dispersive liquid–liquid microextraction combined with gas chromatography‐mass spectrometry in negative chemical ionization mode for the determination of polybrominated diphenyl ethers in water

A simple and economical method for the determination of eight polybrominated diphenyl ethers (BDE‐28, 47, 99, 100,153,154,183, and 209) in water was developed. This method involves the use of ultrasound‐assisted dispersive liquid–liquid microextraction combined with GC‐MS in negative chemical ionization mode. Various parameters affecting the extraction efficiency, including the type and volume of extraction and dispersive solvents, salt concentration, extraction time, and ultrasonic time, were investigated. A volume of 1.0 mL of acetone (dispersive solvent) containing 10 μL tetrachloroethylene (extraction solvent) was injected into 5.0 mL of water samples and then emulsified by ultrasound for 2.0 min to produce the cloudy solution. Under the optimal condition, the enrichment factors for the eight PBDEs were varied from 845‐ to 1050‐folds. Good linearity was observed in the range of 1.0–200 ng L^?1 for BDE‐28, 47, 99, and 100; 5.0–200 ng L^?1 for BDE‐153, 154, and 183; and 5.0–500 ng L^?1 for BDE‐209. The RSD values were in the range of 2.5–8.4% (n = 5) and the LODs ranged from 0.40 to 2.15 ng L^?1 (S/N = 3). The developed method was applied for the determination of eight BPDEs in the river and lake water samples, and the mean recoveries at spiking levels of 5.0 and 50.0 ng L^?1 were in the range of 70.6–105.1%.     

Selective Determination of Se4+ and Se6+ Using SPME and GC/MS

A method for the selective determination of Se⁴⁺ and Se⁶⁺ using solid phase microextraction (SPME) and GC/MS analysis is presented. Se⁴⁺ is selectively derivatized by reaction with 4,5-dichloro-1,2-phenylenediamine to form the corresponding piazselenolo complex, extracted by the SPME fiber, and determined by GC/MS. The RSD at a 5 μg/L concentration was 9.88% and the theoretical detection limit 6 ng/L. The method was employed to test real matrices; tap and river water were analyzed before and after spiking giving a recovery rate of 102% in river water and 97% in tap water.     

Analysis of odorous trichlorobromophenols in water by in-sample derivatization/solid-phase microextraction GC/MS

The simultaneous determination of several odorous trichlorobromophenols in water has been carried out by an in-sample derivatization headspace solid-phase microextraction method (HS-SPME).The analytical procedure involved their derivatization to methyl ethers with dimethyl sulfate/NaOH and further HS-SPME and gas chromatography-mass spectrometry (GC/MS) determination. Parameters affecting both the derivatization efficiency and headspace SPME procedures, such as the selection of the SPME fiber coating, derivatization–extraction time and temperature, were studied. The commercially available polydimethylsiloxane (PDMS) 100 μm and Carboxen-polydimethylsiloxane-divinylbenzene (CAR-PDMS-DVB) fibers appeared to be the most suitable for the simultaneous determination of these compounds. The precision of the HS-SPME/GC/MS method gave good relative standard deviations (RSDs) run-to-run between 9% and 19% for most of them, except for 2,5-diCl-6-Br-phenol, 2,6-diCl-3-Br-phenol and-2,3,6-triBr-phenol (22%, 25% and 23%, respectively). The method was linear over two orders of magnitude, and detection limits were compound dependent but ranged from 0.22 ng/l to 0.95 ng/l. The results obtained for water samples using the proposed SPME procedure were compared with those found with the EPA 625 method, and good agreement was achieved. Therefore, the in-sample derivatization HS-SPME/GC/MS procedure here proposed is a suitable method for the simultaneous determination of odorous trichlorobromophenols in water.     

Sol-Gel-based SPME and GC–MS for Trace Determination of Geosmin in Water and Apple Juice Samples

A headspace solid-phase microextraction (HS-SPME) method was developed for the trace determination of geosmin in water and apple juice samples. A poly(dimethylsiloxane) (PDMS) fiber was synthesized as coated fiber using sol-gel methodology. After performing the extraction in the headspace, the fiber was introduced directly into the injection port of a gas chromatography-mass spectrometry (GC–MS) using electron impact (EI) ionization mode. One-at-the-time optimization strategy was applied to investigate and optimize important extraction parameters such as extraction temperature and time, ionic strength, headspace volume, and desorption temperature and time. The analytical data exhibited an RSD of 2–15%, a linear calibration range of 1–1,000 ng L^?1, and a detection limit of 0.30 ng L^?1 using selected ion monitoring (SIM) mode. The proposed method was successfully applied to the extraction and determination of geosmin in the spiked real water and apple juice samples and a relative recovery of 95–102% were achieved. The developed method offers the advantage of being simple to use, with shorter analysis times, lower cost of equipment, very little matrix effect, and high relative recovery in comparison to conventional methods of analysis.     

Recently developed GC/MS and LC/MS methods for determining NSAIDs in water samples

Pharmaceuticals have become major targets in environmental chemistry due to their presence in aquatic environments (following incomplete removal in wastewater treatment or point-source contaminations), threat to drinking water sources and concern about their possible effects to wildlife and humans. Recently several methods have been developed for the determination of drugs and their metabolites in the lower nanogram per litre range, most of them using solid-phase extraction (SPE) or solid-phase microextraction (SPME), derivatisation and finally gas chromatography mass spectrometry (GC-MS), gas chromatography tandem mass spectrometry (GC-MS/MS) and liquid chromatography electrospray tandem mass spectrometry (LC-ES/MS/MS). Due to the elevated polarity of non-steroidal anti-inflamatory drugs (NSAIDs), analytical techniques based on either liquid chromatography coupled to mass spectrometry (LC-MS) and gas chromatography coupled to mass spectrometry (GC-MS) after a previous derivatisation step are essential. The most advanced aspects of current GC-MS, GC-MS/MS and LC-MS/MS methodologies for NSAID analysis are presented.     

Automated analysis of geosmin, 2-methyl-isoborneol, 2-isopropyl-3-methoxypyrazine, 2-isobutyl-3-methoxypyrazine and 2,4,6-trichloroanisole in water by SPME-GC-ITDMS/MS

This paper describes a method of determining the following compounds in water characterised by complex matrices (raw waters and drinking waters): geosmin, 2-methylisoborneol (2-MIB), 2-isobutyl-3-methoxypyrazine (IBM), 2-isopropyl-3-methoxypyrazine (IPM) and 2,4,6-trichloroanisole (TCA). The method is carried out using headspace solid-phase microextraction (HS-SPME) combined with gas chromatography (GC) and ion trap mass spectrometry (ITMS). Several parameters of extraction and desorption were optimised through the use of a Combi PAL autosampler to automate various tasks (temperature extraction, extraction time, stir speed). Quantities of NaCl and the liquid volume/total volume ratio were also optimised. Double fragmentation (tandem MS/MS) was optimised on the target compounds. The method resulted in good linearity obtained for concentrations of 1 to 100?ng?L^?1 and provided detection limits of approximately below 1?ng?L^?1. Good precision (1–8%) was obtained. This method was successfully applied to the analysis of earthy and musty odours in municipal raw source waters with high concentrations of natural organic matter and in the corresponding treated waters. This is the first time MS/MS has been used to analyse odorous compounds in waters destined for human consumption. In addition, the method as developed is simple to use and lends itself to easy interpretation of chromatograms.     

Determination of chlorinated toluenes in soils using gas chromatography tandem mass spectrometry

A method for the analysis of chlorotoluenes (CTs) in soil has been developed based on ultrasonic assisted extraction with a low volume of organic solvent and determination by gas chromatography-tandem mass spectrometry (GC–MS/MS). A simultaneous clean-up on an alumina–anhydrous sodium sulphate mixture was carried out to remove soil interferences. However, an additional clean-up with graphitised carbon was needed for some very dirty samples. Several solvents were assayed and a mixture of ethyl acetate:hexane (80?:?20, v/v) was selected to carry out soil extractions. Recovery studies were performed at 0.2, 0.1, 0.05 and 0.02?ng?g^?1 fortification levels, and recoveries obtained for all the compounds and concentrations were higher than 81% with standard deviations fulfilling the requirements of the IUPAC. LODs from 0.7 to 5.2?ng?kg^?1 and LOQs from 2.2 to 17.5?ng?kg^?1 were achieved for the analysed compounds, being pentachlorotoluene the compound with the highest limits, followed by the monochlorinated toluenes. The proposed analytical method was applied to determine CT levels in agricultural and industrial soils. These compounds were found in all the industrial soils analysed and some CTs were present in agricultural soils at lower levels.     

Quantification of Transformation Products of Unsymmetrical Dimethylhydrazine in Water Using SPME and GC-MS

Quantification of trace concentrations of transformation products of rocket fuel unsymmetrical dimethylhydrazine (UDMH) in water requires complex analytical instrumentation and tedious sample preparation. The goal of this research was to develop a simple and automated method for sensitive quantification of UDMH transformation products in water using headspace (HS) solid-phase microextraction (SPME) in combination with GC-MS and GC-MS/MS. HS SPME is based on extraction of analytes from a gas phase above samples by a micro polymer coating followed by a thermal desorption of analytes in a GC inlet. Extraction by 85 µm Carboxen/polydimethylsiloxane fiber at 50 °C during 60 min provides the best combination of sensitivity and precision. Tandem mass spectrometric detection with positive chemical ionization improves method accuracy and selectivity. Detection limits of twelve analytes by GC-MS/MS with chemical ionization are about 10 ng L^?1. GC-MS provides similar detection limits for five studied analytes; however, the list of analytes detected by this method can be further expanded. Accuracies determined by GC-MS were in the range of 75–125% for six analytes. Compared to other available methods based on non-SPME sample preparation approaches (e.g., liquid–liquid and solid-phase extraction), the developed method is simpler, automated and provides lower detection limits. It covers more UDMH transformation products than available SPME-based methods. The list of analytes could be further expanded if new standards become available. The developed method is recommended for assessing water quality in the territories affected by space activities and other related studies.     

Optimisation of an in-tube solid phase microextraction method coupled with HPLC for determination of some oestrogens in environmental liquid samples using different capillary columns

A simple on-line method for simultaneous determination of some oestrogens including oestriol (E3), norethisterone (NORE), ethynylestradiol (EE2), D-norgestrel (NORG) and bisphenol A (BPA), in environmental liquid samples was developed by coupling in-tube solid phase microextraction (in-tube SPME) to high-performance liquid chromatography with diode array (DAD) and fluorescence (FLD) detectors. Two capillary chromatographic columns (Supel-Q? and Carboxen? 1006 porous layer open tubular) were selected to develop this method. To achieve optimum extraction performance, several parameters were investigated including number of draw/eject cycles and the sample volume for each of the columns. Reproducibility was satisfactory for inter- and intra-day precision, yielding % RSDs of less than 10% and 7.6%, respectively. Limits of detection (LODs) and quantification (LOQs) for the proposed method using a DAD detector were achieved in the ranges of 0.04–0.63?ng?mL^?1 and 0.12–1.9?ng?mL^?1, depending of the capillary column used. Fluorescence detection improved these parameters for E3, BPA and EE2, obtaining LODs of 0.005–0.03?ng?mL^?1 and LOQs of 0.015–0.08?ng?mL^?1 using Supel-Q and LODs of 0.01–0.015?ng?mL^?1 and LOQs of 0.025–0.04?ng?mL^?1 using Carboxen. The proposed method was successfully applied to spiked environmental waters obtaining recoveries greater than 80%.     

Determination of organotin compounds in-water by headspace solid phase microextraction with gas chromatography-mass spectrometry

This investigation evaluates headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS) to determine trace levels of organotins in water. The organotins were derivatized in situ with sodium tetraethylborate and adsorbed on a poly(dimethysiloxane) (PDMS)-coated fused silica fiber. The SPME experimental procedures to extract organotins in water were at pH 5, with extraction and derivatization simultaneously at 45 degrees C for 30 min in a 2% sodium tetraethylborate solution and a sample solution volume in the ratio of 1:1, and desorption in the splitless injection port of the GC at 260 degrees C for 2 min. Detection limits are determined to be in the low ng/L range. According to the analysis, the linearity range is from 10 to 10,000 ng/L with R.S.D. values below 12% except triphenyltin (24%). The proposed method was tested by analyzing surface seawater from the harbors on the Taiwanese coast for organotins residues. Some organotins studied were detected in the analyzed samples. Results of this study demonstrate the adequacy of the headspace SPME-GC-MS method for analyzing organotins in sea water samples.     

Simultaneous Determination of Free Cortisol,Cortisone and their Tetrahydrometabolites in Urine by Single Solvent Extraction and Liquid Chromatography–Tandem Mass Spectrometry

Abstract

A fast, efficient and low-cost high performance liquid chromatography–tandem mass spectrometry methodology was developed and validated for the simultaneous determination of free urinary cortisone, cortisol and their tetrahydro-metabolites. The developed method comprises a simple liquid-liquid extraction with CH₂Cl_2, followed by reversed-phase liquid chromatography–tandem mass spectrometry (LC–MS/MS) with electrospray ionization (ESI) in positive mode. The baseline chromatographic separation of the analytes, including the stereoisomers tetrahydrocortisol (THF) and allo-THF, was achieved on a Hypersil Gold C₁₈ column with a mobile phase consisting of 0.05%v/v formic acid in water—acetonitrile, using a gradient elution program. The influence of the mobile phase composition and the ESI parameters on the sensitivity of the method was extensively studied. Sample preparation was also optimized, testing two techniques: solid phase extraction (SPE) and liquid-liquid extraction (LLE). Recoveries ranged from 74.7% (a-THF) to 93.5% (cortisol) and the method limits of detection (MLD) ranged from 0.34?ng mL^?1 (cortisol) to 1.37?ng mL^?1 (THF). Intra- and inter-day coefficient of variation of the assay varied from1.5% (allo-THF) to 13% (tetrahydrocortisone) and from 3.6% (allo-THF) to 14.9% (tetrahydrocortisone), respectively. The method was applied for the analysis of urine samples from 53 healthy individuals with a mean age of 13.96?years in order to estimate the concentration of the five corticosteroids and the ratio of the metabolites. Associations between urinary cortisol/cortisone and serum cortisol/cortisone values were also characterized.     

Time‐of‐flight secondary ion mass spectrometry and gas chromatography–mass spectrometry studies of alkanethiol self‐assembled monolayers on nanoporous gold surfaces

The dimerization of alkanethiol mixtures (hexanethiol, octanethiol, and dodecanethiol) to form self‐assembled monolayers (SAMs) from headspace on nanoporous gold surfaces was studied for the first time using gas chromatography (GC/MS) and time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). The nanoporous gold surfaces were obtained by an acidic etching of a 585‐gold alloy. Field emission scanning electron microscopy (FE‐SEM) was utilized to study the change of the surface geometry and porosity of the gold surfaces before and after etching. Alkanethiols were deposited from the vapor phase above the thiol solutions (headspace) on nanoporous gold plates and nanoporous gold solid‐phase vmicroextraction (SPME) fibers. The nanoporous gold substrates were analyzed by TOF‐SIMS and GC/MS, respectively. The TOF‐SIMS spectra exhibited various gold–sulfur ion clusters and specific peaks related to the adsorption of thiols such as deprotonated monomers, thiolate–Au, dimers (e.g., dialkyl sulfides–Au and dialkyl disulfides–Au). The GC/MS analysis of headspace extractions of alkanethiol mixtures by nanoporous gold SPME fibers showed a high extraction efficiency of alkanethiol, dialkyl sulfide, and dialkyl disulfide when compared with the commercial SPME fibers (DVB‐CAR‐PDMS and CAR‐PDMS). Different GC/MS optimization factors were studied including the extraction time and desorption temperature.