A low-cost method for determination of calcium carbonate in cement by membraneless vaporization with capacitively coupled contactless conductivity detection

This work presents a flow analysis method for direct quantitation of calcium carbonate in cement without pretreatment of the sample. The method is based on online vaporization of CO₂ gas following acidification of the sample inside a small chamber that has a flow of acceptor solution passing around it. Solubilization of the CO₂ gas into the acceptor stream changes the conductivity of the acceptor solution causing an increase of signal at the capacitively coupled contactless conductivity detection (C⁴D) placed at the outlet of the vaporization chamber. This chamber is an adaption from previous work reported on ‘membraneless vaporization’ (MBL-VP).The method can be used in the quality control of production of mixed cement. These cement materials usually have calcium carbonate contents at high concentration range (e.g., 33-99% (w/w) CaCO₃). Analysis of samples by this method is direct and convenient as it requires no sample pretreatment. The method is low-cost with satisfactory accuracy and acceptable precision.     

A membraneless gas diffusion unit: design and its application to determination of ethanol in liquors by spectrophotometric flow injection

This work presents new design of a gas diffusion unit, called ‘membraneless gas diffusion (MGD) unit’, which, unlike a conventional gas diffusion (GD) unit, allows selective detection of volatile compounds to be made without the need of a hydrophobic membrane. A flow injection method was developed employing the MGD unit to determine ethanol in alcoholic drinks based on the reduction of dichromate by ethanol vapor. Results clearly demonstrated that the MGD unit was suitable for determination of ethanol in beer, wine and distilled liquors. Detection limit (3S/N) of MGD unit was lower than the GD unit (GD: 0.68%, v/v; MGD: 0.27%, v/v). The MGD design makes the system more sensitive as mass transfer is more efficient than that of GD and thus, MGD can perfectly replace membrane-based designs.     

Flow injection colorimetric method using acidic ceric nitrate as reagent for determination of ethanol

Ceric ammonium nitrate has been used for qualitative analysis of ethanol. It forms an intensely colored unstable complex with alcohol. In this work, a simple flow injection (FI) colorimetric method was developed for the determination of ethanol, based on the reaction of ethanol with ceric ion in acidic medium to produce a red colored product having maximum absorption at 415 nm. Absorbance of this complex could be precisely measured in the FI system. A standard or sample solution was injected into a deionized water donor stream and flowed to a gas diffusion unit, where the ethanol diffused through a gas permeable membrane made of plumbing PTFE tape into an acceptor stream to react with ceric ammonium nitrate in nitric acid. Color intensity of the reddish product was monitored by a laboratory made LED based colorimeter and the signal was recorded on a computer as a peak. Peak height obtained was linearly proportional to the concentration of ethanol originally presented in the injected solution in the range of 0.1-10.0% (v/v) (r² = 0.9993), with detection limit of 0.03% (v/v). With the use of gas diffusion membrane, most of the interferences could be eliminated. The proposed method was successfully applied for determination of ethanol in some alcoholic beverages, validating by gas chromatographic method.     

Evaluation of dual sample introduction systems by comparison of cyclonic spray chambers with different entrance angles for ICP-OES

The operating characteristics of a number of locally constructed inductively coupled plasma (ICP) dual sample introduction designs used for the determination of volatile and non volatile elements have been compared. Four cyclonic spray chamber arrangements with entrance angles of 0, 45, 90, and 180° were tested. The operating characteristics studied included instrumental conditions, chemical conditions for the hydride generation reaction and analytical figures of merit. Analytical performance of the nebulization systems was characterized by determination of the signal to background ratio (SBR), the limits of detection (LODs), and the precision (RSDs). In general, the results suggest that the dual system with 45° configuration was found to be the best indicator of the analytical performance of the dual pneumatic nebulizer-cyclonic chamber arrangement studied, giving the best SBR, precision (1.0–3.5%) and in most cases the lowest detection limits (0.3–24.6 ng L^{− 1}) for both, the hydride and non-hydride forming elements. Long-term reproducibility from the 45 degree-dual system was superior to those of different angles configuration (ranging from 4.2 to 5.1% for the non- and hydride forming elements respectively). The applicability of the dual system was proven by analyzing NIST 1573 Tomato Leaves with satisfactory results.     

Accurate nano-injection system for capillary gas chromatography

The first version of nano-injection device for capillary gas chromatography (cGC) based on inkjet microchip was developed. The nano-injector could accurately control the injection volume in nano-liter, even pico-liter range. Its configuration and mechanism were discussed in detail. Adopting photolithography and plasma etching technology, we firstly fabricated the inkjet microchip and stuck to a piezoelectric device to eject droplets. Then, a special feedback tube was added to make it function as a nano-injector for cGC, which was an important design to compensate pressure difference between the evaporation chamber of cGC and the sample extrusion chamber of inkjet microchip. The injected volume can be precisely controlled by the number of injected droplets. Excellent precision (RSDs were below 10.0%, n = 5) was observed for the injection of ethanol at elevated pressure. Minimum injection volume was about 1.25 nL at present. Additionally, good repeatability of the calibration curves for the hydrocarbons ethanolic solution (the RSDs of all components were below 5.30%, n = 5) confirmed its feasibility in quantitative analysis regardless of concentration. These results suggested that it can be an accurate nano-injector for cGC.     

Sol–gel-based silver nanoparticles-doped silica – Polydiphenylamine nanocomposite for micro-solid-phase extraction

A nanocomposite of silica-polydiphenylamine doped with silver nanoparticles (Ag–SiO₂-PDPA) was successfully synthesized by the sol–gel process. For its preparation, PDPA was mixed with butanethiol capped Ag nanoparticles (NPs) and added to the silica sol solution. The Ag NPs were stabilized as a result of their adsorption on the SiO₂ spheres. The surface characteristic of nanocomposite was investigated using scanning electron microscopy (SEM). In this work the Ag–SiO₂-PDPA nanocomposite was employed as an efficient sorbent for micro-solid-phase extraction (μ-SPE) of some selected pesticides. An amount of 15 mg of the prepared sorbent was used to extract and determine the representatives from organophosphorous, organochlorine and aryloxyphenoxy propionic acids from aqueous samples. After the implementation of extraction process, the analytes were desorbed by methanol and determined using gas chromatography–mass spectrometry (GC–MS). Important parameters influencing the extraction and desorption processes such as pH of sample solution, salting out effect, type and volume of the desorption solvent, the sample loading and eluting flow rates along with the sample volume were experimentally optimized. Limits of detection (LODs) and the limits of quantification (LOQs) were in the range of 0.02–0.05 μg L⁻¹ and 0.1–0.2 μg L⁻¹, respectively, using time scheduled selected ion monitoring (SIM) mode. The relative standard deviation percent (RSD %) with four replicates was in the range of 6–10%. The applicability of the developed method was examined by analyzing different environmental water samples and the relative recovery (RR %) values for the spiked water samples were found to be in the range of 86–103%.     

Method development for liquid chromatographic/triple quadrupole mass spectrometric analysis of trace level perfluorocarboxylic acids in articles of commerce

An analytical method to identify and quantify trace levels of C5–C12 perfluorocarboxylic acids (PFCAs) in articles of commerce (AOCs) was developed and rigorously validated. Solid samples were extracted in methanol, and liquid samples were diluted with a solvent consisting of 60:40 (v/v) methanol and 2 mM ammonium acetate (NH₄Ac) aqueous solution. In both cases, the samples were spiked with an isotopically labeled recovery check standard. The samples were concentrated in a nitrogen atmosphere (solid samples only), filtered, and then analyzed by HPLC coupled with a tandem mass spectrometer. Method evaluation included selection of the extraction solvent and the sample preparation solvent used to facilitate sample injection into the analytical system, method comparison for extraction and sample concentration, determination of extraction efficiency, instrument and method detection limits, and determination of potential sample loss during filtration and sample storage. Results of consecutive extractions demonstrated that a single extraction step accounts for 70–100% of the “total” PFCAs in the AOCs with the exception of cookware. The instrument's detection limit was ≤0.05 ng/mL, and the method detection limit were 1.0–3.9 ng/g for solid AOCs and 1.1–6.8 ng/g for liquid AOCs. The method has been used to determine the PFCA content in a wide range of AOCs containing or treated with fluoropolymers and fluorotelomers.     

Application of headspace single-drop microextraction coupled with gas chromatography for the determination of short-chain fatty acids in RuO4 oxidation products of asphaltenes

In this study, headspace single-drop microextraction (HS-SDME) coupled with gas chromatography-flame ionization detection (GC-FID), was employed to determine short-chain fatty acids (SCFAs) in ruthenium tetroxide (RuO₄) oxidation products of asphaltenes. Several significant parameters, such as drop solvent type, drop volume, sample solution ionic strength, agitation speed, extraction time, and ratio of headspace volume to sample volume were optimized. Under optimum extraction conditions (i.e., a 3-μL drop of 1-butanol, 20 min exposure to the headspace of a 6 mL aqueous sample placed in a 10 mL vial, stirring at 1000 rpm at room temperature, and 30% (w/v) NaCl content), the reproducibility and accuracy of the method have been tested and found to be satisfactory. The analysis of a real asphaltene sample using this method proved that HS-SDME can be a promising tool for the determination of volatile SCFAs in complex matrices.     

Temperature-controlled headspace liquid-phase microextraction device using volatile solvents

A novel temperature-controlled headspace liquid-phase microextraction (TC-HS-LPME) device was established in which volatile solvents could be used as extractant. In this device, a PTFE vial cap with a cylindrical cavity was used as the holder of the extraction solvent. Up to 40 μl of extraction solvent could be suspended in the cavity over the headspace of aqueous sample in the vial. A cooling system based on thermoelectric cooler (TEC) was used to lower the temperature of extractant in PTFE vial cap to reduce the loss of volatile solvent during extraction process and increase the extraction efficiency. The selection of solvents for HS-LPME was then extended to volatile solvents, such as dichloromethane, ethyl acetate and acetone. The use of volatile extraction solvents instead of semi-volatile solvent reduced the interference of the large solvent peak to the analytes peaks, and enhanced the compatibility of HS-LPME with gas chromatograph (GC). Moreover, the use of larger volume of extractant solvent increases the extraction capacity and the injection volume of GC after extraction, thus improving detection limits. Several critical parameters of this technique were investigated by using chlorobenzenes (CBs) as the model analytes. High enrichment factors (498–915), low limits of detection (0.004–0.008 μg/L) and precision (3.93–5.27%) were obtained by using TC-HS-LPME/GC-FID. Relative recoveries for real samples were more than 83%.     

Novel sample preparation technique with needle-type micro-extraction device for volatile organic compounds in indoor air samples

A novel needle-type sample preparation device was developed for the effective preconcentration of volatile organic compounds (VOCs) in indoor air before gas chromatography–mass spectrometry (GC–MS) analysis. To develop a device for extracting a wide range of VOCs typically found in indoor air, several types of particulate sorbents were tested as the extraction medium in the needle-type extraction device. To determine the content of these VOCs, air samples were collected for 30 min with the packed sorbent(s) in the extraction needle, and the extracted VOCs were thermally desorbed in a GC injection port by the direct insertion of the needle. A double-bed sorbent consisting of a needle packed with divinylbenzene and activated carbon particles exhibited excellent extraction and desorption performance and adequate extraction capacity for all the investigated VOCs. The results also clearly demonstrated that the proposed sample preparation method is a more rapid, simpler extraction/desorption technique than traditional sample preparation methods.     

Valence properties of tellurium in different chemical systems and its determination in refractory environmental samples using hydride generation – Atomic fluorescence spectroscopy

Using HG – AFS as a powerful tool to study valence transformations of Te, we found that, in presence of HCl and at high temperature, Te can form volatile species and be lost during sample digestion and pre-reduction steps. It was also noticed that the chemical valences of Te can be modified under different chemical and digestion conditions and even by samples themselves with certain matrices. KBr can reduce Te(VI) to Te(IV) in 3.0 M HCl at 100 °C, but when HNO₃ was >5% (v/v) in solution, Br₂ was formed and caused serious interference to Te measurements. HCl alone can also pre-reduce Te(VI) to Te(IV), only when its concentration was ≥6.0 M (100 °C for 15min). Among 10 studied chemical elements, only Cu²⁺ caused severe interference. Thiourea is an effective masking agent only when Cu²⁺ concentration is equal or lower than 10 mg/L. Chemical reagents, chemical composition of sample, as well as the modes of digestion can greatly affect Te valences, reagent blanks and analytical precisions. A protocol of 2–step–digestion followed by an elimination of HF is proposed to minimize reagent blank and increase the signal/noise ratios. It is important to perform a preliminary test to confirm whether a pre-reduction step is necessary; this is especially true for samples with complex matrices such as those with high sulfide content. The analytical detection limits of this method in a pure solution and a solid sample were 100 ng/L and 0.10 ± 0.02 μg/g, respectively.     

Membraneless vaporization unit for direct analysis of solid sample

A new apparatus, called ‘membraneless vaporization’ (MBL-VP) unit was designed and developed for direct analysis of solid samples. Solid analyte was converted into a gaseous form which then reacts with an indicator reagent. Change in absorbance was used to quantitate the analyte. Stirring with a magnetic bar was employed to facilitate the evaporation of the gas. Unlike the pervaporation technique, hydrophobic membrane was not required for this MBL-VP technique.Application of the membraneless technique for direct determination of calcium carbonate in calcium supplements, has shown to be very precise (R.S.D. = 2.5% for 0.16 mmol CO₃²⁻), with detection limit of 0.5 mg CaCO₃. Results by this method agreed well with flame atomic absorption spectrometric method. Sample throughput was 20 samples h⁻¹.     

In-line coupling headspace liquid-phase microextraction with capillary electrophoresis

An analytical technique of in-line coupling headspace liquid-phase microextraction (HS-LPME) with capillary electrophoresis (CE) was proposed to determine volatile analytes. A special cover unit of the sample vial was adopted in the coupling method. To evaluate the proposed method, phenols were used as model analytes. The parameters affecting the extraction efficiency were investigated, including the configuration of acceptor phase, kind and concentration of acceptor solution, extraction temperature and time, salt-out effect, sample volume, etc. The optimal enrichment factors of HS-LPME were obtained with the sample volume of about half of sample vials, which were confirmed by both the theoretical prediction and experimental results. The enrichment factors were obtained from 520 to 1270. The limits of detection (LODs, S/N = 3) were in the range from 0.5 to 1 ng/mL each phenol. The recoveries were from 87.2% to 92.7% and the relative standard deviations (RSDs) were lower than 5.7% (n = 6). The proposed method was successfully applied to the quantitative analysis of the phenols in tap water, and proved to be a simple, convenient and reliable sample preconcentration and determination method for volatile analytes in water samples.     

Enhancing the Sensitivity of Full Evaporation Technique Using Multiple Headspace Extraction Analysis

This article reports on the development of a new full evaporation (FE) headspace technique based on multiple headspace extraction (MHE). Using multiple headspace extraction procedures, the sample volume used in the headspace can be dramatically increased, thereby significantly enhancing the sensitivity. The technique was applied to the quantification of ethanol. The results showed that up to 0.2 mL of ethanol solution can be used in full evaporation HS-GC analysis by adding multiple headspace extraction procedures. The sensitivity for ethanol content was ten times higher than that in conventional full evaporation HS-GC measurement without using multiple headspace extraction procedures. The present MHE-FE headspace analytical technique is accurate and automated and has great potential for the application in determining volatile analytes in aqueous samples.     

Photochemical vapor generation of carbonyl for ultrasensitive atomic fluorescence spectrometric determination of cobalt

UV photochemical vapor generation (photo-CVG) as sample introduction was first adapted for determination of ultratrace cobalt by atomic fluorescence spectrometry (AFS). Cobalt volatile species can be generated when the buffer system of formic acid and formate containing Co (II) is exposed to UV radiation. The generated gaseous products were separated from liquid phase within a gas–liquid separator and then transported to AFS for determination of cobalt. Factors affecting the efficiency of photo-CVG were investigated in detail, including type and concentration of low molecular weight (LMW) organic acid, buffer system, UV irradiation time, reaction temperature, carrier gas flow rate and hydrogen flow rate. With 4% (v/v) HCOOH and 0.4 mol L^{− 1} HCOONa buffer solution, 150 s irradiation time and 15 W low pressure mercury lamp, a generation efficiency of 23–25% was achieved. A limit of detection (LOD) of 0.08 ng mL^{− 1} without any pre-concentration procedure and a precision of 2.2% (RSD, n = 11) at 20 ng mL^{− 1} were obtained under the optimized conditions. The proposed method was successfully applied in the analysis of several simple matrix real water samples.     

Optimisation of a simple and reliable method based on headspace solid-phase microextraction for the determination of volatile phenols in beer

A simple, accurate and sensitive method based on headspace solid-phase microextraction (HS-SPME) coupled to gas chromatography–tandem mass spectrometry (GC–MS/MS) was developed for the analysis of 4-ethylguaiacol, 4-ethylphenol, 4-vinylguaiacol and 4-vinylphenol in beer. The effect of the presence of CO₂ in the sample on the extraction of analytes was examined. The influence on extraction efficiency of different fibre coatings, of salt addition and stirring was also evaluated. Divinylbenzene/carboxen/polydimethylsiloxane was selected as extraction fibre and was used to evaluate the influence of exposure time, extraction temperature and sample volume/total volume ratio (V_s/V_t) by means of a central composite design (CCD). The optimal conditions identified were 80 °C for extraction temperature, 55 min for extraction time and 6 mL of beer (V_s/V_t 0.30). Under optimal conditions, the proposed method showed satisfactory linearity (correlation coefficients between 0.993 and 0.999), precision (between 6.3% and 9.7%) and detection limits (lower than those previously reported for volatile phenols in beers). The method was applied successfully to the analysis of beer samples. To our knowledge, this is the first time that a HS-SPME based method has been developed to determine simultaneously these four volatile phenols in beers.     

Microwave-assisted extraction combined with gel permeation chromatography and silica gel cleanup followed by gas chromatography–mass spectrometry for the determination of organophosphorus flame retardants and plasticizers in biological samples

An analytical method for the determination of 14 organophosphorus flame retardants (OPFRs), including halogenated OPFRs, non-halogenated OPFRs and triphenyl phosphine oxide (TPPO) in biological samples was developed using gas chromatography–mass spectrometry (GC/MS). Biological samples were extracted using microwave-assisted extraction (MAE) with hexane/acetone (1:1, v/v) as the solvent; then, a two-step clean-up technique, gel permeation chromatography (GPC) combined with solid phase extraction (SPE), was carried out before GC/MS analysis. Experimental results showed that the developed method efficiently removed the lipid compounds and co-extract interferences. Moreover, using the relatively “narrow” column (with an i.d. of 10 mm) significantly decreased the elution volume and, therefore, prevented the loss of the most volatile OPFRs, especially trimethyl phosphate (TMP) and triethyl phosphate (TEP). The method detection limits (MDLs) for OPFRs in the biological samples ranged from 0.006 to 0.021 ng g⁻¹ lw, and the recoveries were in the range of 70.3–111%, except for TMP (38.9–55.6%), with relative standard deviations (RSDs) of less than 14.1%. The developed method was applied to determine the amount of the target OPFRs in biological samples (i.e., fish and domestic birds) that were collected from the Pearl River Delta (PRD) region in southern China. Of the 14 OPFRs, tri-n-butyl phosphate (TnBP), tris(2-chloroethyl) phosphate (TCEP), tris(chloropropyl) phosphate (TCPP) and tributoxyethyl phosphate (TBEP) were present in all of the biological samples that were analyzed, and dominated by TnBP, TCEP and TBEP. The concentrations of OPFRs in the biological samples that were collected from the PRD region were higher than those reported in other locations.     

Determination of 3-nitroaniline in water samples by directly suspended droplet three-phase liquid-phase microextraction using 18-crown-6 ether and high-performance liquid chromatography

Liquid–liquid–liquid microextraction (LLLME) with directly suspended droplet in high-performance liquid chromatography (HPLC) has been applied as a new, rapid and easy method for the determination of 3-nitroaniline in environmental water samples. The target compound was extracted from the aqueous sample solution (donor phase, pH 13) into an organic phase and then was back-extracted into a directly suspended droplet of an acidic aqueous solution (acceptor phase, pH 0.3). In this method, without using a microsyringe as supporting device, an aqueous large droplet is freely suspended at the top-center position of an immiscible organic solvent, which is laid over the aqueous sample solution while being agitated. Then, the droplet was withdrawn into the microsyringe and directly was injected into the HPLC system with UV detection at 227 nm. Up to 148-fold enrichment of the analyte could be obtained under the optimal conditions [i.e. donor phase: 0.1 M sodium hydroxide solution (4.5 mL); organic phase: o-xylene/1-octanol (90:10, v/v; 250 μL); acceptor phase: 0.5 M hydrochloric acid and 500 mM 18-crown-6 ether (6 μL); extraction time: 60 s; back-extraction time: 6 min and stirring rate: 600 rpm]. The limit of detection was 1 μg/L (n = 7) and the relative standard deviation (RSD, n = 5) was 4.9 at S/N = 3. The calibration graph was linear in the range of 5–1500 μg/L with r = 0.9983. All experiments were carried out at room temperature (22 ± 0.5 °C).     

Hollow fiber supported liquid membrane extraction for ultrasensitive determination of trace lead by portable tungsten coil electrothermal atomic absorption spectrometry

Hollow fiber supported liquid membrane extraction (HF-SLME) was used to separate and enrich trace lead from a large volume of 250 mL water sample to a final tiny volume of 30 μL of 1-octanol, 5 μL of which was inject into a tungsten coil electrothermal atomic absorption spectrometer (W-coil ET-AAS) for determination of lead. Some important parameters that influenced the extraction and determination were investigated in detail, such as the concentration of ammonium pyrrolidine dithiocarbamate (APDC), pH of sample solution, stirring rate, extraction time, pyrolysis current, atomization current, carrier gas flow rate, as well as interferences. Under the optimized conditions, a practical enrichment factor of 499 and a limit of detection (3σ) of 0.2 ng mL^{− 1} were obtained. The calibration curve was linear in the range of 0.5–10 ng mL^{− 1}. The relative standard deviation (RSD) was 5.6% for five measurements of a 4 ng mL^{− 1} lead standard solution. The accuracy of this method was examined by the analysis of certified reference water samples (GBW(E)080398 and GSBZ(E) 50009-88) for lead. Finally, the proposed method was applied to the determination of lead in local tap water, pond water and river water, with recoveries in the range of 96–109% for spiked samples.     

Headspace solid-phase microextraction for the determination of volatile organic sulphur and selenium compounds in beers,wines and spirits using gas chromatography and atomic emission detection

A rapid and solvent-free method for the determination of eight volatile organic sulphur and two selenium compounds in different beverage samples using headspace solid-phase microextraction and gas chromatography with atomic emission detection has been developed. The bonded carboxen/polydimethylsiloxane fiber was the most suitable for preconcentrating the analytes from the headspace of the sample solution. Volumes of 20 mL of undiluted beer were used while, in the case of wines and spirits, sample:water ratios of 5:15 and 2:18, respectively, were used, in order to obtain the maximum sensitivity. Quantitation was carried out by using synthetic matrices of beer and wine, and a spiked sample for spirits, and using ethyl methyl sulphide and isopropyl disulphide as internal standards. Detection limits ranged from 8 ng L⁻¹ to 40 ng mL⁻¹, depending on the compound and the beverage sample analyzed, with a fiber time exposure of 20 min at ambient temperature. The optimized method was successfully applied to different samples, some of the studied compounds being detected at concentration levels in the 0.04–152 ng mL⁻¹ range.