Capillary electrophoretic determination of ammonia using headspace single-drop microextraction

A new method involving headspace single-drop microextraction (SDME) and capillary electrophoresis (CE) is developed for the preconcentration and determination of ammonia (as dissolved NH₃ and ammonium ion). An aqueous microdrop (5 μL) containing 1 mmol/L H₃PO₄ and 0.5 mmol/L KH₂PO₄ (as internal standard) was used as the acceptor phase. Common experimental parameters (sample and acceptor phase pH, extraction temperature, extraction time) affecting the extraction efficiency were investigated. Proposed SDME-CE method provided about 14-fold enrichment in about 20 min. The calibration curve was linear for concentrations of NH₄⁺ in the range from 5 to 100 μmol/L (R² = 0.996). The LOD (S / N = 3) was estimated to be 1.5 μmol/L of NH₄⁺. Such detection sensitivity is high enough for ammonia determination in common environmental and biological samples. Finally, headspace SDME was applied to determine ammonia in human blood, seawater and milk samples with spiked recoveries in the range of 96-107%.     

Homogeneous, unmodified gold nanoparticle-based colorimetric assay of hydrogen peroxide

An unmodified gold nanoparticle-based colorimetric assay system in homogeneous format has been developed using hydrogen peroxide (H₂O₂) as a model analyte. H₂O₂ is added to o-phenylenediamine/horseradish peroxidase solution, and allowed to react for 10 min. Then, unmodified gold nanoparticles that serve as “reaction indicators” are added to the reaction solution. The resulting mixture color changes dramatically from red to blue. The reason is that azoaniline, a horseradish peroxidase-catalyzed oxidation product, induces the nanoparticle aggregation. Using this approach, H₂O₂ can be semiquantitatively determined over the concentration range of ∼4 orders of magnitude by the naked eye. If the observed peak intensity at 420 nm is used for the construction of the calibration plot, hydrogen peroxide can be accurately determined down to concentration levels of 1.3 × 10⁻⁶ M. Compared with the conventional electrochemical protocol, this sensing system offers several important advantages: (1) ability to be monitored by the naked eye, (2) avoiding the need of surface modification of electrodes or gold nanoparticles and (3) detection in homogeneous solution. It is worthy of note that this efficient and convenient strategy is also suitable for the detection of other species, such as glucose and cholesterol.     

Label-free colorimetric detection of picomolar amounts of hydrazine using a gold nanoparticle-based assay

We report the development of a simple and rapid colorimetric detection method for hydrazine in boiler feed water using label-free aggregation-based gold nanoparticles as probe. This assay relies upon the distance-dependent optical properties of gold nanoparticles. Hydrazine could rapidly induce the aggregation of gold nanoparticles, thereby resulting in visual color change from red to blue (or purple). The concentration of hydrazine can be determined by the naked eye or by a UV–vis spectrometer. Calibration curve was linear ranging from 1.0 × 10^?11 to 1.0 × 10^?7 M of hydrazine. The present limit of detection for hydrazine was 1.0 × 10^?12 M. The method is rather simple, and the whole process including sample pretreatment takes only 15 min at room temperature. The merits (such as simplicity, rapidity, low cost and visual colorimetry) make the proposed method especially useful for on-site screening of hydrazine levels well in boiler feed water.     

Ionic liquid-based headspace single-drop microextraction coupled to gas chromatography for the determination of chlorobenzene derivatives

A novel method, based on the coupling of ionic liquid-based headspace single-drop microextraction (SDME) with gas chromatography (GC), is developed for the determination of chlorobenzene derivatives. For the SDME of five chlorobenzene derivatives, a 1.0 μL 1-octyl-3-methylimidazolium hexafluorophosphate microdrop is exposed for 20 min to the headspace of a 15 ml aqueous sample containing 20% (w/v) NaCl placed in 25 ml vial at 40 °C. Then, the extractant is directly injected into the injector block of the GC instrument. To avoid ionic liquid leaking into the chromatographic column, a small glass tube is placed in the injection block. Under optimized operation conditions, linear relation between peak areas and analyte concentrations up to 1.5 mg L^?1 has been obtained The detection limits range from 0.1 to 0.5 μg L^?1 for the various analytes. The relative standard deviations at 1.0 μg L^?1 range from 7.7 to 12.4%, and the enrichment factors from 41 to 127. The method is simple and sensitive, and does not suffer from the influence of a solvent peak. Its applicability is demonstrated by the determination of chlorobenzenes in wastewater samples.     

Aptamer enzymatic cleavage protection assay for the gold nanoparticle-based colorimetric sensing of small molecules

A label-free, homogeneous aptamer-based sensor strategy was designed for the facile colorimetric detection of small target molecules. The format relied on the target-induced protection of DNA aptamer from the enzymatic digestion and its transduction into a detectable signal through the length-dependent adsorption of single-stranded DNA onto unmodified gold nanoparticles (AuNPs). The proof-of-principle of the approach was established by employing the anti-tyrosinamide aptamer as a model functional nucleic acid. In the absence of target, the aptamer was cleaved by the phosphodiesterase I enzymatic probe, leading to the release of mononucleotides and short DNA fragments. These governed effective electrostatic stabilization of AuNPs so that the nanoparticles remained dispersed and red-colored upon salt addition. Upon tyrosinamide binding, the enzymatic cleavage was impeded, resulting in the protection of the aptamer structure. As this long DNA molecule was unable to electrostatically stabilize AuNPs, the resulting colloidal solution turned blue after salt addition due to the formation of nanoparticle aggregates. The quantitative determination of the target can be achieved by monitoring the ratio of absorbance at 650 and 520 nm of the gold colloidal solution. A limit of detection of ∼5 μM and a linear range up to 100 μM were obtained. The sensing platform was further applied, through the same experimental protocol, to the adenosine detection by using its DNA aptamer as recognition tool. This strategy could extend the potentialities, in terms of both simplicity and general applicability, of the aptamer-based sensing approaches.     

Ligand displacement, headspace single-drop microextraction, and capillary electrophoresis for the determination of weak acid dissociable cyanide

A new method involving ligand displacement, headspace single-drop microextraction (SDME) with in-drop derivatization and capillary electrophoresis (CE) was developed for the determination of weak acid dissociable (WAD) cyanide. WAD metal-cyanide complexes (Ag(CN)(2)(-), Cd(CN)(4)(2-), Cu(CN)(3)(2-), Hg(CN)(2), Hg(CN)(4)(2-), Ni(CN)(4)(2-) and Zn(CN)(4)(2-)) are decomposed with ligand-displacing reagent and the released hydrogen cyanide is extracted from neutral solution (pH 6.5) with an aqueous microdrop (5 microl) containing Ni(II)-NH(3) as derivatization agent. The hydrogen cyanide extracted reacts with Ni(2+) to form a stable and highly UV absorbing tetracyanonickelate anion which is then determined by CE. Among the three different ligand-displacing reagents (i.e., ethylenediamine, dithizone and polyethileneimine) studied none of the reagents used alone releases cyanide completely from all WAD cyanide complexes. Complete recoveries were obtained by the extraction of WAD cyanide from 0.4 mol l(-1) ethylenediamine chloride buffer (pH 6.5) containing 0.001% (wt) dithizone. Proposed system was applied to determine WAD cyanide in industrial wastewater and river waters samples with spiked recoveries in the range of 95.8-104.7%.     

Selective determination of homocysteine levels in human plasma using a silver nanoparticle-based colorimetric assay

The first use of silver nanoparticles (AgNPs) for the rapid, simple, and selective determination of homocysteine (Hcy) levels in human plasma was studied. Hcy and five other amino acids, including cysteine (Cys), could be distinguished by their different aggregation kinetics, which caused a change in the visible color and a shift in the UV-vis absorption spectra. The difference in the cross-linking (aggregation) rate between Hcy and Cys was used as the basis for developing a selective probe for Hcy and allowed the detection of Hcy in the linear range of 2-12 μM (R² = 0.9936). The limits of detection and quantification were found to be 0.5 μM and 1.7 μM, respectively. To investigate its selectivity and potential applicability, this AgNP-based method was successfully applied for the determination of Hcy levels in actual biological (human plasma) samples, where the determined levels of Hcy were within the error range of the measured level using the traditional chemiluminescence microparticle immunoassay (CMIA). Thus, the use of AgNPs is a feasible and potentially reliable method for the determination of Hcy levels in biological samples.     

Fast determination of Z-ligustilide in plasma by gas chromatography/mass spectrometry following headspace single-drop microextraction

Angelica sinensis (danggui in Chinese) is a common traditional Chinese medicine (TCM), and its essential oil has been used for the treatment of many diseases such as hepatic fibrosis. Z-Ligustilide has been found to be an important active component in the TCM essential oil. In this work, for the first time, headspace single-drop microextraction (HS-SDME) followed by gas chromatography-mass spectrometry (GC-MS) was developed for the determination of Z-ligustilide in rabbit plasma after oral administration of essential oil of danggui. The extraction parameters of solvent selection, solvent volume, sample temperature, extraction time, stirring rate, and ion strength were systemically optimized. Furthermore, the method linearity, detection limit, and precision were also investigated. It was shown that the proposed method provided good linearity (0.02-20 microg/mL, R2 = 0.997), low detection limit (10 ng/mL), and good precision (RSD value less than 9%). Finally, HS-SDME followed by GC/MS was used for fast determination of Z-ligustilide in rabbit plasma at different time intervals after oral administration of danggui essential oil. The experimental results suggest that HS-SDME followed by GC/MS is a simple, sensitive, and low-cost method for the determination of Z-ligustilide in plasma, and a low-cost approach to pharmacokinetics studies of active components in TCMs.     

Development of microwave-assisted extraction followed by headspace single-drop microextraction for fast determination of paeonol in traditional Chinese medicines

Paeonol is an important active component present in traditional Chinese medicines (TCMs), which was used for the treatment of many diseases such as eczema. In this work, microwave-assisted extraction (MAE) was firstly combined with headspace single-drop microextraction (HS-SDME), and applied to rapid determination of paeonol in two TCMs of Cynanchum paniculatum and Paeonia suffruticosa. In the proposed method, paeonol in TCMs was isolated by using MAE, followed by extraction and concentration by HS-SDME, and detected by gas chromatography-mass spectrometry (GC-MS). The experiment parameters of MAE and HS-SDME were discussed, and the method precision, recovery and detection limit were also studied. To further demonstrate the reliability of the quantification, both the proposed method and a standard method of steam distillation (SD) were simultaneously applied to quantitative analysis of paeonol in TCM samples from different growing areas. The experimental results show that MAE-HS-SDME is a simple and rapid method for the quantitative analysis of paeonol in TCMs, and is also a potential and alternative tool for quality monitoring for the two TCMs of C. paniculatum and P. suffruticosa.     

Active single-drop microextraction for the determination of gaseous diisocyanates

Heterocyclic amines (HAs) were analysed in meat extract samples using a new method based on pressurised liquid extraction (PLE) and liquid chromatography-tandem mass spectrometry. This method combines the use of a pressurised fluid with a triple quadrupole MS/MS system, resulting in benefits from both systems: high extraction efficiency and sensitivity. The effects of solvent type and PLE operational parameters, such as temperature and extraction time, were studied to obtain maximum recovery of the analytes with minimum contamination. HA extraction was best achieved using dichloromethane/acetone (50/50, v/v) at 80 degrees C for 10 min. Recoveries ranged from 45% to 79% with good quality parameters: limit of detection values between 0.02 and 1 ng g(-1), linearity (r(2)>0.997), and run-to-run and day-to-day precisions with relative standard deviations lower than 13% achieved at both low (0.20 microg g(-1)) and medium (1.0 microg g(-1)) concentrations. This method reduces sample manipulation and total extraction time by nearly four-fold compared to conventional solid phase extraction. The optimised method was validated using laboratory reference material based on a meat extract, and was successfully applied to HA analysis in several cooked beef samples.     

Multiple headspace single-drop microextraction coupled with gas chromatography for direct determination of residual solvents in solid drug product

This paper proposed a multiple headspace single-drop microextraction (MHS-SDME) method coupled to gas chromatography with flame-ionization detection (GC-FID) for direct determination of residual solvents in solid drug product. The MHS-SDME technique is based on extrapolation to an exhaustive extraction of consecutive extractions from the same sample which eliminates the matrix effect on the quantitative analysis of solid samples. The total peak area of analyte is calculated with a beta constant which can be obtained from the slope of the linear regression that related to the peak area of each extraction and the number of extraction times. In this work, a model drug powder was chosen and the amounts of residues of two solvents, methanol and ethanol, were investigated. The factors influencing the extraction process including extraction solvent, microdrop volume, extraction time, sample amount, thermostatting temperature and incubation time were studied. 10 mg of drug powder was incubated for 3 h at 140 °C prior to the first extraction and thermostatted for 15 min at 140 °C between each extraction. Extraction was carried out with 2 μL of dimethyl sulfoxide (DMSO) as the microdrop for 5 min. The features of the method were established using standard solutions. Validation of the proposed method showed good agreement with the traditional dissolution method for analysis of residual solvents in drug product. The results indicated that MHS-SDME has a great potential for the quantitative determination of residual solvents directly from the solid drug products due to its low cost, ease of operation, sensitivity, reliability and environmental protection.     

Gas chromatographic determination of carbonyl compounds in biological and oil samples by headspace single-drop microextraction with in-drop derivatisation

A suitable method for the gas chromatographic determination of 10 characteristic carbonyls in biological and oil samples based on the in-drop formation of hydrazones by using 2,4,6-trichlorophenylhydrazine (TCPH), has been developed. The derivatisation-extraction procedure was optimized separately for aqueous and oil samples with respect to the appropriate organic drop solvent, drop volume, in-drop TCPH concentration, sample stirring rate, temperature during single-drop microextraction (SDME), reaction time and headspace-to-sample volume ratio. The optimization showed differentiation of optimum values between the studied matrices. The limits of detection were found to range from 0.001 to 0.003 μg mL⁻¹ for the aqueous biological samples and from 0.06 to 0.20 μg mL⁻¹ for the oil samples. The limits of quantification were in the range of 0.003-0.010 μg mL⁻¹ and 0.020-0.059 μg mL⁻¹ for aqueous and oil samples, respectively. The overall relative standard deviations of the within-day repeatability and between-day reproducibility were <4.4% and <8.2% for the aqueous biological samples and <3.9% and <7.4% for the oxidized oil samples.     

Gold nanoparticle-based competitive colorimetric assay for detection of protein-protein interactions

A gold nanoparticle-based competitive colorimetric assay uses the ensemble of Concanavalin (ConA) and mannopyranoside-encapsulated gold nanoparticles (Man-GNPs) to identify the binding partners for ConA and the binding constants are determined based on the wavelength shifts.     

Coupling of ionic liquid-based headspace single-drop microextraction with GC for sensitive detection of phenols

A headspace single-drop microextraction (SDME) based on ionic liquid (IL) has been developed for the gas chromatographic determination of phenols. The volume of IL microdrop used was 1 microL. After extraction, the analytes were desorbed from the drop in the injection port and the involatile IL was withdrawn into the microsyringe. To facilitate the withdrawal of IL the upper diameter of the split inlet liner was enlarged to some extent. Some parameters were optimized for the determination of phenols. Under the selected conditions, i.e., desorption for 100 s at 210 degrees C after extraction for 25 min at 50 degrees C in solutions (pH 3) containing 0.36 g/mL sodium chloride, the LODs, RSDs, and the average enrichment factors of phenols were 0.1-0.4 ng/mL, 3.6-9.5% (n=5), and 35-794, respectively. The proposed procedure was applied to the determination of phenols in lake water and wastewater samples, and the spiked recoveries were in the range of 81-111% at a spiked level of 0.4 microg/mL. This method is a promising alternative for the sensitive determination of phenolic compounds.     

Rapid and sensitive detection of cholera toxin using gold nanoparticle-based simple colorimetric and dynamic light scattering assay

Herein, a rapid and simple gold nanoparticle based colorimetric and dynamic light scattering (DLS) assay for the sensitive detection of cholera toxin has been developed. The developed assay is based on the distance dependent properties of gold nanoparticles which cause aggregation of antibody-conjugated gold nanoparticles in the presence of cholera toxin resulting discernible color change. This aggregation induced color change caused a red shift in the plasmon band of nanoparticles which was measured by UV–Vis spectroscopy. In addition, we employed DLS assay to monitor the extent of aggregation in the presence of different concentration of cholera toxin. Our assay can visually detect as low as 10 nM of cholera toxin which is lower than the previously reported colorimetric methods. The reported assay is very fast and showed an excellent specificity against other diarrhetic toxins. Moreover, we have demonstrated the feasibility of our method for cholera toxin detection in local lake water.     

Use of single-drop microextraction for determination of fentanyl in water samples

Fentanyl is a very potent synthetic narcotic analgesic. Because of its strong sedative properties, it has become an analogue of illicit drugs such as heroin. Its unambiguous detection and identification in environmental samples can be regarded as strong evidence of its illicit preparation. In this paper we report application of single-drop microextraction (SDME) for analysis of water samples spiked with fentanyl. Experimental conditions which affect the performance of SDME, for example the nature of the extracting solvent, sample stirring speed, extraction time, ionic strength, and solution pH, were optimized. The method was found to be linear in the concentration range 0.10–10 ng mL⁻¹. The limits of quantitation and detection of the method were 100 pg mL⁻¹ and <75 pg mL⁻¹, respectively. This technique is superior to other sample-preparation techniques because of the simple experimental set-up, short analysis time, high sensitivity, and minimum use of organic solvent.     

Multiple headspace single-drop micro-extraction for quantitative determination of lactide in thermally-oxidized polylactide

Single-drop micro-extraction (SDME), an emerging micro-extraction technique, was combined with multiple headspace (MHS) extraction to allow quantitative determination of lactide in thermally-oxidized polylactide. Different solvents, drop sizes and extraction times were tested to obtain best extraction efficiency and the method was further developed to obtain a linear regression plot for the multiple extractions. The combination of SDME and MHS extraction offered several advantages over traditional liquid-solid and headspace extraction techniques. No concentration step was needed and loss of volatiles was prevented as the ageing and extraction were performed in a closed system. Matrix effects, that disturb the quantitation of analytes in solid samples, were removed by the multiple headspace extraction.     

Development of gas chromatography-mass spectrometry following microwave distillation and simultaneous headspace single-drop microextraction for fast determination of volatile fraction in Chinese herb

In this work, for the first time, microwave distillation (MD) coupled with simultaneous headspace single-drop microextraction (HS-SDME) was developed for the determination of the volatile components in the Chinese herb, Artemisia capillaris Thunb. The volatile components were rapidly isolated by MD, and simultaneously extracted and concentrated by using a dodecane microdrop. The volatile oil extracted in the microdrop solvent was analyzed by gas chromatography-mass spectrometry (GC-MS). The experimental parameters of solvent selection, microdrop volume, microwave power, irradiation time and sample amount were investigated, and the method precision was also studied. The optimal parameters were extraction solvent of dodecane, solvent volume of 2.0 microL, microwave power of 400 W, irradiation time of 4 min, and sample amount of 2.0 g. Thirty-five volatile compounds present in Artemisia capillaris Thunb. were identified by using the proposed method, which were identical with those obtained by the conventional steam distillation method. The experimental results showed that MD-HS-SDME is a simple, rapid, reliable, and solvent-free technique for the determination of volatile compounds in Chinese herbs.     

Development of gas chromatography-mass spectrometry following headspace single-drop microextraction and simultaneous derivatization for fast determination of short-chain aliphatic amines in water samples

In this work, for the first time, gas chromatography-mass spectrometry (GC-MS) following headspace single-drop microextraction (HS-SDME) and simultaneous derivatization was developed for fast determination of short-chain aliphatic amines (SCAAs) in water samples. In the proposed method, SCAAs in water samples were headspace extracted and concentrated by suspending a microdrop of solvent, and SCAAs extracted in the microdrop of solvent were simultaneously and rapidly reacted with pentafluorobenzaldehyde (PFBAY). The formed SCAA derivatives were analyzed by GC-MS. The HS-SDME parameters of solvent selection, solvent volume, sample temperature, extraction time and stirring rate were studied, and the method linearity, precision and detection limits, were also studied. The results show that the proposed method provided good linearity (R(2)>0.99, 5.0-500 ng/ml), low detection limit (0.6-1.1 ng/ml), and good precision (RSD value less than 10%). The proposed method was further tested by its application to quantitative analysis of SCAAs in four wastewater samples. The experiment results have demonstrated that GC-MS following HS-SDME and simultaneous derivatization is a simple, rapid and low-cost method for the determination of SCAAs in water samples.     

Headspace single-drop microextraction coupled to microvolume UV-vis spectrophotometry for iodine determination

Headspace single-drop microextraction has been combined with microvolume UV-vis spectrophotometry for iodine determination. Matrix separation and preconcentration of iodide following in situ volatile iodine generation and extraction into a microdrop of N,N′-dimethylformamide is performed. An exhaustive characterization of the microextraction system and the experimental variables affecting iodine generation from iodide was carried out. The procedure employed consisted of exposing 2.5 μL of N,N′-dimethylformamide to the headspace of a 10 mL acidic (H₂SO₄ 2 mol L⁻¹) aqueous solution containing 1.7 mol L⁻¹ Na₂SO₄ for 7 min. Addition of 1 mL of H₂O₂ 1 mol L⁻¹ for in situ iodine generation was performed. The limit of detection was determined as 0.69 μg L⁻¹. The repeatability, expressed as relative standard deviation, was 4.7% (n = 6). The calibration working range was from 5 to 200 μg L⁻¹ (r² = 0.9991). The large preconcentration factor obtained, ca. 623 in only 7 min, compensate for the 10-fold loss in sensitivity caused by the decreased optical path, which results in improved detection limits as compared to spectrophotometric measurements carried out with conventional sample cells. The method was successfully applied to the determination of iodine in water, pharmaceutical and food samples.