Alternative plugs flow liquid chromatography (APFLC), a type of gas-liquid two-phase flow chromatography (2PFC) is achieved by the agglomeration of unformed condensates in an open tubular capillary column with intermittently alternating condensates in which the supply of the liquid is maintained at a level that provides a hypothetical homogeneous film, 25 to 150 nm thick, on the inner surface. To support stable plugs, the flow mode should normally be maintained at a liquid-to-gas volume ratio (β) of between 0.0006 and 0.004 and below 70°C. Alternative plugs flow is formed as a result of the hydrophobic property of the stationary phase, which is induced at a water contact angle above 75°, as derived from the solubility parameter (δ) of a coated resin of less than 18.3 MPa1/2. Diffusion and mixing between the vicinal partition fields is substantially avoided in alternating disconnected nano-volume plugs, which makes it possible to obtain a very large number of theoretical plates of separation. The plugs flow as a mobile phase can be enhanced by applying sonic vibrations to the column. Since the liquid/gas phases ratio (β) is maintained in the range of 10−4 to 10−3, the extremely small volumes of solvent offer the advantages of compatibility with an EI-ionizing mass spectrometer and the use of commercially available mass search systems. The APFLC/EI-MS system, which employs commercially-available GC/MS equipment, is potentially applicable to both GC and LC analysis containing sparingly volatile components, giving close to the theoretical resolution and eliminating the need for numerous derivatization procedures.
Four saponins were isolated from the leaves of Aralia elata, and established using NMR and other spectroscopic methods, as well as data reported in the literature. Three Aralia saponins from the leaves of Aralia elata sharing the same structures as those isolated from the root bark suggested that the leaves would be a good substitute for the root bark of Aralia elata. These four Aralia saponins were then extensively investigated using complementarily positive and negative electrospray ionization multistage tandem mass spectrometry (ESI-MSn). Two isomers of saponins with different sugar linkages were then successfully differentiated by positive ESI-MSn and verified with different retention times and the collision-induced dissociation (CID) spectra by LC-MS. A simple and effective LC-MS method was thus developed for the rapid identification and screening of these saponins in plant extracts from leaves of Aralia elata.
The optimal conditions for the separation and detection of a mixture of 18 phenethylamine/tryptamine derivatives were determined, using liquid chromatography/UV-absorption (LC/UV) and liquid chromatography/electrospray ionization mass spectrometry (LC/ESI MS) methods, respectively. Complete separation could be achieved within ∼25 min using gradient elution (A, 0.1% formic acid aqueous solution/pH 2.5; B, acetonitrile). The limit of detection (LOD at S/N = 3) obtained by LC/UV-absorption (absorption wavelength, 280 nm) was in the range from 0.3 to 3 μg/mL. In contrast, when the LC/ESI MS method was used, the LODs for primary, secondary and tertiary amines were in the ranges 0.1 − 3.0, 0.1 − 0.2, and 0.05 − 1.8 μg/mL, respectively. The lower LOD obtained for a tertiary amine can be attributed to the fact that its ionization efficiency (during the ESI process) is better than the others. In order to improve the LOD of a primary/secondary amine, a derivatization procedure was used in which the chemical structure was altered to a secondary/tertiary amine, via a reaction with acetic anhydride. As a result, the LODs for primary/secondary amines could be significantly improved. The characteristic mass fragmentations of the 18 phenethylamine/tryptamine derivatives, as well as the products of the reaction with acetic anhydride, were investigated, and the data were reported. A urine sample was obtained by spiking urine from a volunteer with the 18 derivatives, and after liquid-liquid extraction the sample was examined by LC/UV and LC/ESI MS, respectively. The extraction procedures used for the urine sample and the experimental conditions for the separation and detection were optimized.
Pressurized liquid extraction (PLE) has been used for the extraction of β-sitosterol from food samples. In the present work, extractions were performed by mixing ground samples with Hydromatrix celite with methanol at 50°C and 110 bar, using two 5-min cycles. After extraction, β-sitosterol was analyzed directly by liquid chromatography with diode array detection. The extraction recovery was 92%, with relative standard deviations of 3.7% for intra-day precision and 4.3% for inter-day precision. The presence of β-sitosterol was confirmed by gas chromatography-mass spectrometry. The proposed method was validated and applied successfully to the determination of free β-sitosterol in seed and nut samples (almonds, pistachios, peanuts, and sunflower seeds).
The HPLC method for the determination of sotalol (SOT), metoprolol (MET) and α-hydroxymetoprolol metabolite (MET-H), paracetamol (PAR), paracetamol glucuronide (PAR-G) and paracetamol sulfate (PAR-S) in human urine is described. Analyses were carried out on a reversed-phase LiChroCART® Purospher® C18e column (125 mm×3 mm, 5 μm particles) (Merck) with gradient elution as well as spectrophotometric and fluorometric detection. Good resolution of the analyzed substances was obtained within a time range of no longer than 15 min. The linearity ranges of the callibration curves in human urine (as matrix) were: 3.25 − 45 μg ml−1 (SOT), 0.75 − 40 μg ml−1 (MET), 0.6 − 40 μg ml−1 (MET-H), 4.6 − 60 μg ml−1 (PAR-G), 4.95 − 50 μg ml−1 (PAR-S), 1.95 − 45 μg ml−1 (PAR). An application to human urine samples was performed.
Water-soluble CdTe nanoparticles and hemoglobin (Hb) were immobilized on a glassy carbon (GC) electrode with Nafion. The direct electrochemistry of Hb on this surface was studied. The results indicated that CdTe nanoparticles could effectively promote the direct electron transfer of Hb at the interface of a electrode. The average surface coverage of Hb on the surface could be calculated as 2.63×10−9 mol/cm2, the heterogeneous electron transfer rate constant, k, was calculated as 0.068 s−1 and the transfer coefficient, α, was 0.59, further study indicated that immobilized Hb still kept its catalytic activity to H2O2 reduction. The apparent Michaelis-Menten constant was calculated to be 17.7 μM. It was also found that the modified electrode could be used as a sensor for H2O2; the linear range of detection was 5.0×10−6 − 4.5×10−5 M, with a detection limit of 8.4×10−7 M. The sensor exhibited high sensitivity, reproducibility and stability.
Electrochemical biosensors for phenolic compound determination were developed by immobilization of tyrosinase enzyme in a series of methacrylic-acrylic based biosensor membranes deposited directly using a photocuring method. By modifying the hydrophilicity of the membranes using different proportions of 2-hydroxyethyl methacrylate (HEMA) and butyl acrylate (nBA), we developed biosensor membranes of different hydrophilic characters. The differences in hydrophilicity of these membranes led to changes in the sensitivity of the biosensors towards different phenolic compounds. In general biosensors constructed from the methacrylic-acrylic based membranes showed the poorest response to catechol relative to other phenolic compounds, which is in contrast to many other biosensors based on tyrosinase. The decrease in hydrophilicity of the membrane also allowed better selectivity towards chlorophenols. However, phenol biosensors constructed from the more hydrophilic membrane materials demonstrated better analytical performance towards phenol compared with those made from less hydrophilic ones. For the detection of phenols, these biosensors with different membranes gave detection limits of 0.13 − 0.25 μM and linear response range from 6.2 − 54.2 μM phenol. The phenol biosensors also showed good phenol recovery from landfill leachate samples (82 − 117%).
Efficient quenching electrochemiluminescence (ECL) of the tris(2,2'-bipyridine)ruthenium(II)/tripropylamine (Ru(bpy)32+/TPrA) system by a novel quencher, sodium diphenylamine-4-sulfonate (SDS), has been investigated. The quenching behaviors can be observed with a 100-fold excess of SDS over Ru(bpy)32+. The mechanism of quenching is believed to involve energy transfer from the excited-state Ru(bpy)32+* to the dimer of SDS, which was formed after the electrochemical oxidation of SDS at the electrode surface. Photoluminescence experiments coupled with bulk electrolysis support formation of the SDS dimer upon electrochemical oxidation.
A potentiometric sensor is reported for the mercury(II) detection, which uses substituted thiourea-functionalized nanoporous silica (FTU-LUS-1) as the sensitive material. Substituted thiourea (FTU) and FTU-LUS-1 were first prepared and then characterized by 1H NMR, 19F NMR, 13C NMR, FTIR, XRD, TG and CNS elemental analysis. The electrodes with FTU-LUS-1 proportion of 10.0 wt% demonstrated very stable potentials. The prepared electrodes exhibit a Nernstian slope of 28.4 ± 1.0 mV decade−1 for mercury(II) ion over a wide concentration range of 1.0 × 10−7 to 1.0 × 10−1 mol dm−3. The electrode exhibited a detection limit of 7.0 × 10−8 mol dm−3. Moreover, the selectivity coefficient, response time, performance, sensitivity and stability of the modified electrode were investigated. The electrode presented a response time of about 35 s, a high performance and sensitivity in a wide range of cation activities as well as good long term stability (more than 9 months). The method was satisfactory and could also be used to monitor the mercury(II) ion concentration in waste water and fish samples.
A theory of detection limit, developed in analytical chemistry, is applied to public health surveillance to detect an outbreak of national emergencies such as natural disaster and bioterrorism. In this investigation, the influenza epidemic around the Tokyo area from 2003 to 2006 is taken as a model of normal and large-scale epidemics. The detection limit of the normal epidemic is used as a threshold with a specified level of significance to identify a sign of the abnormal epidemic among the daily variation in anti-influenza drug sales at community pharmacies. While auto-correlation of data is often an obstacle to an unbiased estimator of standard deviation involved in the detection limit, the analytical theory (FUMI) can successfully treat the auto-correlation of the drug sales in the same way as the auto-correlation appearing as 1/f noise in many analytical instruments.
A novel technique, corona discharge ion mobility spectrometry (CD-IMS), was developed for the qualitative and quantitative determination of 2-furfural (F) and 5-methyl-2-furfural (MF) in aqueous solutions. The limits of detection (LODs) were 5.3 × 10−3 μg/mL for F and 6.7 × 10−3 μg/mL for MF. The linear dynamic ranges of 1.16 × 10−2 to 1.04 μg/mL and 2.20 × 10−2 to 1.10 μg/mL were obtained for F and MF, respectively. The relative standard deviation was below 12% for both compounds. In addition to analysis of the individual compound, simultaneous determination of F and MF was also investigated. It was realized that F imposes a matrix effect on the MF signal and vice versa. The standard addition method was used to deal with the matrix effect. The recovery of the compounds in the synthetic samples validates the capability of the method.
A highly sensitive and selective method has been proposed for speciation determination of trace amounts of Cr(III) and Cr(VI) in water samples using 1,4-diaminoanthraquinone (1,4-DAAQ). The method is based on mixing an organic phase containing 1,4-DAAQ with an acidic solution (pH 1) of Cr(VI), which is accomplished with absorbance decreasing or luminescence quenching of the organic phase due to a partial oxidation of the 1,4-DAAQ content. After oxidation of Cr(III) to Cr(VI) and determining the total chromium content, Cr(III) content is obtained by subtracting. All of the variables were studied in order to optimize the reaction conditions. No considerable interference was observed due to the presence of coexisting anions and cations. The calibration graphs were linear in the range 3.0×10−7 − 3.0×10−6 and 1.0×10−7 − 4.0×10−6 M with detection limits of 3.5×10−8 and 2.1×10−9 M for absorptiometry and fluorometry, respectively. Validation of the measurements was confirmed using standard reference materials and a certificated method.
Osmium was determined spectrophotometrically after coprecipitation with 5-chloro-2-hydroxythiobenzhydrazide at room temperature in the pH range 2.5 − 5.0 and collection on microcrystalline naphthalene. Beer's law was obeyed in the concentration range 1.8 − 14.4 ppm of osmium in a chloroform solution at 510 nm. The molar absorptivity and Sandell sensitivity were 1.056 × 104 l mol−1 cm−1 and 0.018 ppm, respectively. Six replicate analyses of a solution containing 7.2 ppm of osmium gave a mean absorbance of 0.405 with a standard deviation of 0.013 and a relative standard deviation of 0.92%. The complex was stable for over 72 h. The metal-to-ligand ratio in the complex was 1:2. Interference from various ions was studied, and the method was applied to the determination of osmium in various synthetic mixtures containing commonly associated metals and corrosponding to the alloy composition. The sequential separation and determination of osmium and platinum is reported.
Simple instrumentation and procedure with stopped flow system coupled to hydrodynamic injection and spectrophotometric detection were developed for automatic osmotic fragility test (OFT). OFT is a test for abnormal red blood cells based on the kinetics of their rupturing in a hypotonic saline solution. A portion of red cells was merged on-line with a hypotonic saline solution. They were mixed while flowing into the detector and were stopped for a short period for continuously monitoring the change of turbidity based on a transmission signal. In this work, a possible application of the system for screening of beta-thalassemia trait is demonstrated. Descriptions of instrumentation development and parameter optimization are presented. The system offers advantages over the conventional batch-wise OFT in terms of automation, precision, analysis time and sample volume.
An amperometric biosensor for the detection of glucosamine (GlcN) and chitosan oligosaccharides ((GlcN)n) was introduced for an activity measurement of chitosanase. By using the biosensor, an increase in the anodic current due to the production of GlcN and (GlcN)n by chitosanase was measured in a chitosan solution. The maximum value of the slope of the current increase was proportional to the enzyme concentration up to 1.4 μg mL−1. The present method had the advantages of being simple and rapid over the conventional Elson-Morgan method.
An LC postcolumn derivatization method for determination of polythiols has been developed. This method involves separation using reversed-phase LC, postcolumn derivatization with N-(1-pyrenyl)maleimide, and excimer fluorescence detection. Analytes with a polythiol structure were converted into corresponding polypyrene-labeled derivatives, and the derivatives exhibited intramolecular excimer fluorescence (440 − 520 nm). In this study, dimercaprol and dithiothreitol were used as model polythiols. This polythiol analysis method is simple; it is also highly selective and sensitive and yields good calibration curves.