Determination of amino acids and peptides without their pre-column derivatization by capillary electrophoresis with ultraviolet and contactless conductivity detection. An overview.

This review provides an overview of recent works focusing on the determination of amino acids (AAs) and peptides using capillary electrophoresis with contactless conductivity detection and ultraviolet (UV) detection, which is the most widespread detection in capillary electromigration techniques, without pre-capillary derivatization. Available options for the UV detection of these analytes, such as indirect detection, complexation with transition metal ions, and in-capillary derivatization are described. Developments in the field of direct detection of UV-absorbing AAs and peptides as well as progress in chiral separation are described. A separate section is dedicated to using on-line sample preconcentration methods combined with capillary electrophoresis-UV.

Vertical Barrier Heterostructures for Reliable, Robust, and High-Performance Ultraviolet Detection.

Photodetectors based on low-dimensional materials usually suffer from serious optical power-dependent photoresponse and low reliability, particularly in the ultraviolet regime. The barrier photodetector is an effective and reliable strategy where the barrier layer can block the low-energy charge carriers while allowing for a flow of the high-energy photocarriers. Here, vertical barrier heterostructure photodetectors (VBHPs), consisting of a graphene bottom electrode, a MoS2 light absorber, and an h-BN energy barrier, for reliable, robust, and high-performance ultraviolet detection are reported. The asymmetric barrier distribution in the conduction/valence band at the MoS2 /h-BN interface results in an ultralow noise current of 1.69 × 10-15 A Hz-1/2 at room temperature, stable photo on/off states exceeding 104 cycles at 300 K and 400 K, a light power-independent high responsivity of 416.2 mA W-1 at 360 nm, a high photo on-off ratio of 1.2 × 105 at 360 nm, high measured detectivities (3.2 × 1010 Jones at 266 nm and 9.9 × 1010 Jones at 360 nm), and wide linear dynamic ranges. The VBHPs show a high potential for new-type reliable ultraviolet detection.

Epitaxial Growth of 2D Ultrathin Metastable γ-Bi2 O3 Flakes for High Performance Ultraviolet Photodetection.

Ultraviolet detection is of great significance due to its wide applications in the missile tracking, flame detecting, pollution monitoring, and so on. The nonlayered semiconductor γ-Bi2 O3 is a promising candidate toward high-performance UV detection due to the wide bandgap, excellent light sensitivity, environmental stability, nontoxic and elemental abundance properties. However, controllable preparation of ultrathin 2D γ-Bi2 O3 flakes remains a challenge, owing to its nonlayered structure, metastable nature, and other competing phases. Moreover, the UV photodetectors based on 2D γ-Bi2 O3 flake have not been implemented yet. Here, ultrathin (down to 4.8 nm) 2D γ-Bi2 O3 flakes with high crystal quality are obtained via a van der Waals epitaxy method. The as-synthesized single-crystalline γ-Bi2 O3 flakes show a body-centered cubic structure and grown along (111) lattice plane as revealed by experimental observations. More importantly, photodetectors based on the as-synthesized 2D γ-Bi2 O3 flakes exhibit promising UV detection ability, including a responsivity of 64.5 A W-1 , a detectivity of 1.3 × 1013 Jones, and an ultrafast response speed (τrise  ≈ 290 µs and τdecay  ≈ 870 µs) at 365 nm, suggesting its great potential for various optoelectronic applications.

Ion chromatograph with three-dimensional printed absorbance detector for indirect ultraviolet absorbance detection of phosphate in effluent and natural waters.

An ion chromatography system employing a low-cost three-dimensional printed absorbance detector for indirect ultraviolet detection towards portable phosphate analysis of environmental and industrial waters has been developed. The optical detection cell was fabricated using stereolithography three-dimensional printing of nanocomposite material. Chromatographic analysis and detection of phosphate were carried out using a CS5A 4 × 250 mm analytical column with indirect ultraviolet detection using a 255 nm light-emitting diode. Isocratic elution using a 0.6 mM potassium phthalate eluent combined with 1.44 mM sodium bicarbonate was employed at a flow rate of 0.75 mL/min. A linear calibration range of 0.5 to 30 mg/L PO4 3- applicable to environmental and wastewater analysis was achieved. For retention time and peak area repeatability, relative standard deviation values were 0.68 and 4.09%, respectively. Environmental and wastewater samples were analyzed with the optimized ion chromatography platform and the results were compared to values obtained by an accredited ion chromatograph. For the analysis of environmental samples, relative errors of <14 % were achieved. Recovery analysis was also carried out on both freshwater and wastewater samples and recovery results were within the acceptable range for water analysis using standard ion chromatography methods.

Amino functionalized magnetic covalent organic framework for magnetic solid-phase extraction of sulfonylurea herbicides in environmental samples from tobacco land.

An amino-functionalized magnetic covalent organic framework composite TpBD-(NH2 )2 @Fe3 O4 (Tp=Tp1,3,5-triformylphloroglucinol, BD-(NH2 )2 is 3,3',4,4'-biphenyltetramine) was prepared by post-synthesis modification. Due to its abundant benzene rings and amino groups, large specific surface area and porous structure, the prepared TpBD-(NH2 )2 @Fe3 O4 exhibits high extraction efficiency toward sulfonylurea herbicides. Based on this, a new method of magnetic solid-phase extraction with TpBD-(NH2 )2 @Fe3 O4 as the sorbent combined with high-performance liquid chromatography and ultraviolet detection was developed for trace analysis of sulfonylurea herbicides in environmental water, soil and tobacco leaves samples from tobacco land. Under the optimized conditions, the limits of detection within 0.05-0.14 µg/L were achieved with a high enrichment factor of 217-260-fold, and the relative standard deviations were 4.9-7.5% (n = 7, c = 0.5 µg/L). The linear range was around three orders of magnitude with the square of correlation coefficient higher than 0.9936. The method was applied to analyze five sulfonylurea herbicides in the environmental water, soil, and tobacco leave samples collected from tobacco land. No sulfonylurea herbicides were detected in these samples. The recoveries of target sulfonylurea herbicides in spiked environmental water, soil, and tobacco leaf samples were found in the range of 90.7-104, 70.7-99.0, and 59.3-97.8%, respectively. The results illustrate that the established TpBD-(NH2 )2 @Fe3 O4 -magnetic solid-phase extraction- high-performance liquid chromatography-ultraviolet detection method is efficient for the analysis of trace sulfonylurea herbicides in environmental samples.

Magnetic Molecularly Imprinted Polymers for the Rapid and Selective Extraction and Detection of Methotrexatein Serum by HPLC-UV Analysis.

In this work, novel selective recognition materials, namely magnetic molecularly imprinted polymers (MMIPs), were prepared. The recognition materials were used as pretreatment materials for magnetic molecularly imprinted solid-phase extraction (MSPE) to achieve the efficient adsorption, selective recognition, and rapid magnetic separation of methotrexate (MTX) in the patients' plasma. This method was combined with high-performance liquid chromatography-ultraviolet detection (HPLC-UV) to achieve accurate and rapid detection of the plasma MTX concentration, providing a new method for the clinical detection and monitoring of the MTX concentration. The MMIPs for the selective adsorption of MTX were prepared by the sol-gel method. The materials were characterized by transmission electron microscopy, Fourier transform-infrared spectrometry, X-ray diffractometry, and X-ray photoelectron spectrometry. The MTX adsorption properties of the MMIPs were evaluated using static, dynamic, and selective adsorption experiments. On this basis, the extraction conditions were optimized systematically. The adsorption capacity of MMIPs for MTX was 39.56 mgg-1, the imprinting factor was 9.40, and the adsorption equilibrium time was 60 min. The optimal extraction conditions were as follows: the amount of MMIP was 100 mg, the loading time was 120 min, the leachate was 8:2 (v/v) water-methanol, the eluent was 4:1 (v/v) methanol-acetic acid, and the elution time was 60 min. MTX was linear in the range of 0.00005-0.25 mg mL-1, and the detection limit was 12.51 ng mL-1. The accuracy of the MSPE-HPLC-UV method for MTX detection was excellent, and the result was consistent with that of a drug concentration analyzer.

Streamlined three step total vitamin C analysis by HILIC-UV for laboratory testing.

OBJECTIVES: In the clinical setting, the analysis and quantification of vitamin C (ascorbic acid) poses several challenges including analyte instability and poor retention by reverse phase HPLC systems. In this article we describe a rapid hydrophilic interaction chromatography ultraviolet method for the measurement of total vitamin C in plasma which overcomes these issues. METHODS: Ascorbic acid and the internal standard were separated under isocratic conditions using a Waters BEH-Amide column and a mobile phase containing 0.005 M potassium phosphate in 80% acetonitrile. RESULTS: The proposed method was validated and showed good precision (coefficient of variation <5%), accuracy (>99%), and analyte stability after extraction (>24 h). CONCLUSIONS: The simple sample preparation allows full automation and rapid analytical run times of the assay and is therefore suitable for a high-throughput clinical chromatography laboratory.

Bioanalytical RP-HPLC method for the determination of ponazuril in plasma.

The goal of this investigation was to establish a reliable technique for the quantitation of ponazuril in limited sample volumes. Samples were extracted with chloroform and separation was achieved with a Symmetry RP18 column. Ultraviolet absorption was measured at 254 nm. A combination of 0.1% formic acid and acetonitrile (50:50) was used as the mobile phase. The calibration curve was linear from 0.1-25 µg/mL, with a lower limit of quantification of 0.1 µg/mL with a 100 µL sample. The precision and accuracy were well within the range set by the Food and Drug Administration and the recovery was over 95%. This technique was used to analyze ponazuril samples and found to be appropriate for pharmacokinetic studies.

Simultaneous separation and indirect ultraviolet detection of chlorate and perchlorate by pyridinium ionic liquids in reversed-phase liquid chromatography.

A novel approach for the simultaneous separation and indirect ultraviolet detection of chlorate and perchlorate using pyridinium ionic liquids as mobile phase additives in reversed-phase liquid chromatography was developed. Pyridinium ionic liquids and imidazolium ionic liquids as the mobile phase additives were compared. The effects of pyridinium ionic liquids, methanol, column temperature, and detection wavelength on the separation and detection of chlorate and perchlorate were investigated. The role of ionic liquids, retention rules and relevant mechanisms were discussed. Pyridinium ionic liquids mainly acted as ultraviolet absorption reagent and ion-pair reagent. The successful separation and indirect ultraviolet detection of chlorate and perchlorate were achieved by using a common reversed-phase column, 0.2 mmol/L N-octylpyridinium bromide aqueous solution/methanol (90/10, v/v) as mobile phase and at the detection wavelength of 210 nm. The retention times of chlorate and perchlorate were 30.51 and 37.06 min, respectively. The detection limits of chlorate and perchlorate were 0.16 and 0.29 mg/L, respectively. The linearity and repeatability of the method were satisfactory. The approach was used to the analysis of river water samples with accurate and reliable results. This method is easy to popularize due to the use of common reversed-phase column and ultraviolet detector in liquid chromatography.

A Simplified Method for Simultaneous Determination of α-Lipoic Acid and Low-Molecular-Mass Thiols in Human Plasma.

α-Lipoic acid, glutathione, cysteine, and cysteinylglycine can be applied as therapeutic agents in civilization diseases such as diabetes mellitus, cardiovascular diseases, and cancers. On the other hand, a higher concentration of homocysteine can result in health problems and has been indicated as an independent risk factor for cardiovascular disease and accelerated atherosclerosis. Here, the first simplified HPLC-UV assay that enables simultaneous determination of α-lipoic acid and low-molecular-mass thiols in plasma, reduces the number of steps, shortens the total time of sample preparation, and limits the amount of single-use polypropylene laboratory materials is described. The assay is based on reversed-phase high performance liquid chromatography with UV detection and simultaneous reduction of disulfide bound with tris(2-carboxyethyl)phosphine and the selective pre-column derivatization of the thiol group with 1-benzyl-2-chloropyridinium bromide. Linearity in the detector responses for plasma samples were observed in ranges: 0.12-5.0 nmol mL-1 for α-lipoic acid; 2.0-20.0 nmol mL-1 for glutathione, cysteinylglycine, and homocysteine; and 40.0-400.0 for cysteine. The LODs for α-lipoic acid and low-molecular-mass thiols were 0.08 and 0.12 nmol mL-1, respectively, while LOQs were 0.12 and 0.16 nmol mL-1, respectively. The usefulness of the proposed method has been proven by its application to real samples.

Separation and indirect ultraviolet detection of ferrous and trivalent iron ions by using ionic liquids in ion chromatography.

A method of simultaneous separation and indirect ultraviolet detection of different valence iron ions Fe2+ and Fe3+ by using ionic liquids as mobile phase additives and ultraviolet absorption reagents on a cation exchange column functionalized with carboxylic acid group was developed. The effects of ionic liquids, organic acids, detection wavelength, etc. on separation and detection of Fe2+ and Fe3+ were investigated and the mechanism was discussed. The pyridinium and imidazolium ionic liquids were not only ultraviolet absorption reagents of indirect ultraviolet detection but also effective components for separating Fe2+ and Fe3+ . The separation and detection of Fe2+ and Fe3+ can be achieved using 0.5 mmol/L pyridinium ionic liquid-1.2 mmol/L methanesulfonic acid as the mobile phase. The determination of Fe2+ and Fe3+ had a good linear relationship in the concentration range of 1-100 mg/L. The limits of detection of Fe2+ and Fe3+ were 0.12 and 0.09 mg/L, respectively. This method was applied to the actual sample detection in the field of medical analysis. The spiked recoveries were between 97.3 and 99.5%, and the relative standard deviations were less than 0.6%. The method is simple, accurate, and reliable, and is an analytical method with universal and practical value.

Ionic liquids as mobile phase additives for determination of thiocyanate and iodide by liquid chromatography.

An analytical method was developed for the simultaneous determination of thiocyanate and iodide by reversed-phase liquid chromatography with UV detection using imidazolium ionic liquids as mobile phase additives. The chromatographic behaviors of the two anions on a C18 column were studied and compared with four types of reagents including imidazolium ionic liquids, pyridinium ionic liquids, 4-aminophenol hydrochloride and tetrabutylammonium as mobile phase additives. The effects of the concentrations of imidazolium ionic liquids, organic solvents and detection wavelength on separation and detection of the anions were investigated. The role of ionic liquids, retention rules and mechanisms were discussed. The separation of the anions was performed on the C18 reserved-phase column using acetonitrile-0.3 mmol/L 1-amyl-3-methylimidazolium tetrafluoroborate (10:90, v/v) as mobile phase, with column temperature of 35°C, flow rate of 1 mL/min and detection wavelength of 210 nm. Under these conditions, the two anions can be completely separated within 6 min. The limits of detection were 0.05 mg/L. The method was applied for the determination of thiocyanate and iodide in ionic liquid samples and iodide drugs, and the spiked recoveries ranged from 97 to 101%. The method is simple, accurate and meets the requirements of quantitative analysis for thiocyanate and iodide.

Analysis of fentanyl derivatives by ultra high performance liquid chromatography with diode array ultraviolet and single quadrupole mass spectrometric detection.

Fentanyl has become pervasive as a drug of abuse and as adulterant in seized drugs. Positional isomers analyzed by gas chromatography with mass spectrometry can follow the same fragmentation pathway and therefore may not be differentiated. Additionally, electron ionization leads to lack of discernible molecular ion for most fentanyl related compounds. Liquid chromatography may be used as an orthogonal identification technique with diode array ultraviolet and mass spectrometric detection. Here we provide a chromatographic method for the separation of 20 different fentanyl analogues, homologues and positional isomers using ultra high performance liquid chromatography with photodiode array ultraviolet and mass spectrometry detection. Five different columns were investigated utilizing reverse phase chromatography and hydrophilic interaction chromatography. Chromatographic systems were evaluated to determine which could separate the most compounds overall, as well as the most positional isomers. We found that isocratic elution, with a methanol modifier (35%) and formic acid (0.1%) as an additive, on a C18 column at a temperature of 25°C could resolve 10/20 compounds overall and 16/20 positional isomers. Using electrospray ionization, compounds with different masses could easily be distinguished based on their pseudo molecular ions. Ultraviolet detection facilitated differentiation of positional isomers that could not be distinguished by either electron ionization or electrospray ionization mass spectrometry alone.

Simultaneous determination of short-chain fatty acids in human feces by HPLC with ultraviolet detection following chemical derivatization and solid-phase extraction segmental elution.

Short-chain fatty acids are currently the most studied metabolites of gut microbiota, but the analysis of them, simultaneously, is still challenging due to their unique property and wide concentration range. Here, we developed a sensitive and versatile high-performance liquid chromatography with ultraviolet detection method, using pre-column derivatization and solid-phase extraction segmental elution, for the quantification of both major and trace amounts of short-chain fatty acids in human feces. Short-chain fatty acids were converted to 3-nitrophenylhydrazine-derived analytes, and then solid-phase extraction segmental elution was used for extraction of major analytes and enrichment of trace analytes. The method validation showed limits of quantitation ˂0.04 mM, and coefficient of determination > 0.998 at a wide range of 0.04-8.0 mM. The intra- and interday precision of analytes were all within accepted criteria, and the recoveries were 96.12 to 100.75% for targeted analytes in fecal samples. This method was successfully applied in quantification of eight analytes in human feces, which therefore could provide a sensitive and versatile high-performance liquid chromatography with ultraviolet detection method for precise and accurate quantitation of short-chain fatty acids in human feces.

Imidazolium ionic liquids as mobile phase additives in reversed phase liquid chromatography for the determination of iodide and iodate.

An analytical method for the simultaneous determination of iodide and iodate by reversed phase liquid chromatography with ultraviolet detection using imidazolium ionic liquids as mobile phase additives was developed in this paper. Imidazolium ionic liquids, pyridinium ionic liquids, and common ion pair reagent tetrabutylammonium hydroxide as mobile phase additives were compared. The results indicated that imidazolium ionic liquids as the mobile phase additives were better than any tetrabutylammonium hydroxide or pyridinium ionic liquids. The relevant mechanisms of separation and detection for the anions were discussed. Imidazolium ionic liquids acted as ion pair reagents and ultraviolet absorption reagents in the separation and detection of the anions. The optimized mobile phase of methanol/0.3 mmol/L 1-hexyl-3-methylimidazolium tetrafluoroborate aqueous solutions (12/88, v/v), detection wavelength of 210 nm, and column temperature of 35 °C were used for the determination of iodide and iodate. Under these conditions, the detection limits of iodide and iodate were 0.02 and 0.03 mg/L, respectively. To evaluate the practicability of the method, the determination of iodide and iodate in medicines and beverages was performed by this method. The results are that the spiked recoveries were greater than 95% and RSD was less than 3.0%. The method was simple, accurate, and reliable and could provide a reference for the analysis of iodide and iodate. Graphical abstract The simultaneous determination of iodide and iodate by reversed phase liquid chromatography with ultraviolet detection using imidazolium ionic liquids as mobile phase additives.

Simultaneous determination of alcohols including diols and triols by HPLC with ultraviolet detection based on the formation of a copper(II) complex.

We developed a reversed-phase high-performance liquid chromatography method with ultraviolet detection using on-line complexation with Cu(II) ion for analysis of five alcohols including diols and triol (methanol, ethanol, 1,2-propanediol, 1,3-propanediol, and glycerol). The Cu(II) ion concentration in the mobile phase had a great influence on the peak areas of these alcohols, but not on their retention times. Column temperature (25-40°C) and pH of the mobile phase did not affect the separation of analytes. The optimum separation conditions were determined as 5 mM CuSO4 , 3 mM H2 SO4 , and 3 mM NaOH at 30°C. The ratio of the peak areas for three alcohols (methanol, 1,2-propanediol, and glycerol) was in good agreement with that calculated from the obtained stability constants, molar absorption coefficients for the 1:1 Cu(II) complexes with the three alcohols, and the injected molar quantities. This fact strongly suggests that the observed high-performance liquid chromatography signals resulted from formation of the 1:1 Cu(II)-alcohol complexes. Using the proposed method, these five alcohols in spirit, liquid for electronic cigarette, mouthwash, and nail enamel remover samples were successfully analyzed with only a simple pretreatment.

Simultaneous determination of tricarboxylic acid cycle metabolites by high-performance liquid chromatography with ultraviolet detection.

Here we describe a simple high-performance liquid chromatography (HPLC) procedure for the simultaneous detection and quantitation in standard solutions of 13 important metabolites of cellular energy metabolism, including 9 tricarboxylic acid (TCA) cycle components and 4 additional metabolites. The metabolites are detected by their absorbance at 210 nm. The procedure does not require prior derivatization, and an analysis can be carried out at ambient temperature within 15 min. The significance of the current work is that the current HPLC procedure should motivate the development of simplified TCA cycle enzyme assays, isotopomer analysis, and determination of selected TCA metabolite levels in plasma/tissues.

Ion chromatography with the indirect ultraviolet detection of alkali metal ions and ammonium using imidazolium ionic liquid as ultraviolet absorption reagent and eluent.

Indirect ultraviolet detection was conducted in ultraviolet-absorption-agent-added mobile phase to complete the detection of the absence of ultraviolet absorption functional group in analytes. Compared with precolumn derivatization or postcolumn derivatization, this method can be widely used, has the advantages of simple operation and good linear relationship. Chromatographic separation of Li(+) , Na(+) , K(+) , and NH4 (+) was performed on a carboxylic acid base cation exchange column using imidazolium ionic liquid/acid/organic solvent as the mobile phase, in which imidazolium ionic liquids acted as ultraviolet absorption reagent and eluting agent. The retention behaviors of four kinds of cations are discussed, and the mechanism of separation and detection are described. The main factors influencing the separation and detection were the background ultraviolet absorption reagent and the concentration of hydrogen ion in the ion chromatography-indirect ultraviolet detection. The successful separation and detection of Li(+) , Na(+) , K(+) , and NH4 (+) within 13 min was achieved using the selected chromatographic conditions, and the detection limits (S/N = 3) were 0.02, 0.11, 0.30, and 0.06 mg/L, respectively. A new separation and analysis method of alkali metal ions and ammonium by ion chromatography with indirect ultraviolet detection method was developed, and the application range of ionic liquid was expanded.

Five-descriptor model to predict the chromatographic sequence of natural compounds.

Despite the recent introduction of mass detection techniques, ultraviolet detection is still widely applied in the field of the chromatographic analysis of natural medicines. Here, a neural network cascade model consisting of nine small artificial neural network units was innovatively developed to predict the chromatographic sequence of natural compounds by integrating five molecular descriptors as the input. A total of 117 compounds of known structure were collected for model building. The order of appearance of each compound was determined in gradient chromatography. Strong linear correlation was found between the predicted and actual chromatographic position orders (Spearman's rho = 0.883, p < 0.0001). Application of the model to the external validation set of nine natural compounds was shown to dramatically increase the prediction accuracy of the real chromatographic order of multiple compounds. A case study shows that chromatographic sequence prediction based on a neural network cascade facilitated compound identification in the chromatographic fingerprint of Radix Salvia miltiorrhiza. For natural medicines of known compound composition, our method provides a feasible means for identifying the constituents of interest when only ultraviolet detection is available.

HPLC Determination of Fexofenadine in Human Plasma For Therapeutic Drug Monitoring and Pharmacokinetic Studies.

A simple and sensitive method was developed for fexofenadine determination in human plasma by liquid chromatography with ultraviolet detection. Satisfactory separation was achieved on a Hypersil® BDS C18 column (250 × 4.6 mm, 5µm) using a mobile phase comprising 20 mm sodium dihydrogen phosphate-2 hydrate (pH adjusted to 3 with phosphoric acid)-acetonitrile at a ratio of 52:48, v/v. The elution was isocratic at ambient temperature with a flow rate of 1.0 mL/min. The UV detector was set at 215 nm for the drug and 330 nm for the internal standared (tinidazole). The total time for a chromatographic separation was ~6.5 min. Linearity was demonstrated over the concentration range 0.01-4 µg/mL. The observed within- and between-day assay precision ranged from 0.346 to 13.6%; accuracy varied between 100.4 and 111.2%. This method was successfully applied for therapeutic drug monitoring in patients treated with clinical doses of fexofenadine and for pharmacokinetic studies. Copyright © 2015 John Wiley & Sons, Ltd.