Noise and Drift Phenomena in Amperometric and Coulometric Detectors for HPLC and FIA

Abstract

Noise and drift phenomena in electrochemical detectors with solid electrodes are discussed. A relationship between the capacity of the working electrode and the noise of the detector is demonstrated in three different ways, using direct correlation of noise with capacity, time correlation functions and electrical simulation of the cell properties. Conclusions are drawn with respect to the prospects of various measures to improve the detection limit.     

高效液相色谱-脉冲积分安培电化学检测牛奶中的妥布霉素

采用C18键合硅胶柱和脉冲积分安培电化学检测器对妥布霉素样品进行了分离检测。对两种检测电位波形进行了对比，并选取了表现优良的检测电位。方法对妥布霉素的检出限19μg／L，在0．1～25mg／L范围内表现良好线性（r=0．9999）。该方法应用于市售鲜奶的检测，妥布霉素的回收率为98％。     

Dual frequency liquid crystals: a review

We review the recent progress in developing high performance dual frequency liquid crystal (DFLC) materials. A new figure of merit is derived for evaluating the DFLC performance. The dielectric relaxation and electro-optical properties of several high performance compounds, which possess a positive dielectric anisotropy at low frequencies and a relaxation frequency in the kilohertz region, are presented. Potential applications of these compounds are discussed. Compounds with a negative dielectric anisotropy were also compared and discussed. Two experimental DFLC mixtures were demonstrated with one intended for room temperature operation and another for elevated temperature operation.     

用于气相色谱的微波等离子体原子发射光谱检测器的发展

分别介绍和评价了用于气相色谱的微波诱导等离子体、电容耦合微波等离子体和微波等离子体炬等3种微波等离子体原子发射光谱检测器的发展、应用以及局限性。对用于气相色谱的微波等离子体原子发射光谱检测器的发展作了展望。     

HPLC—ECD法检测小鼠体内多巴胺及其代谢产物

建立了HPLC-电化学检测法同时测定小鼠血浆及脑组织中多巴胺及其代谢产物的方法。采用安捷伦水相柱，流动相为V（甲醇）＋V（水）=10＋90，其中水相中含NaH2PO4·H2O，KCl，EDTA，辛烷磺酸钠，流量0．25mL·min^-1。，检测电压0．52V。结果表明，线性范围：多巴胺（DA）25～750ng，多巴烯（DOPAC）25～750ng，高香草酸（HVA）50～1000ng，线性关系与精密度良好；该法操作简单、快速、准确，可用于检测小鼠血浆及脑组织中多巴胺及其代谢产物的含量。     

Determination of phenolic antioxidants in pharmaceutical formulations by liquid chromatography and migration study on HDPE packagings

Summary The performance of UV diode-array, spectrometric and electrochemical detectors was compared in the chromatographic analysis of trace amounts of six phenolic antioxidants. Quantitative validation was undertaken; the linearity range was wider using UV detection although the limits of detection were lower with electrochemical detection. UV detection was applied both to identification of an antioxidant in a hydrophilic suspension and to a migration study.     

Use of Principal Component and Cluster Analysis for the Comparison of Reversed-Phase HPLC Columns

Abstract

The retention of ethoxylated nonylhylphenyl surfactants was determined in reversed-phase high-performance liquid chromatography (RP-HPLC) using various supports (C1, C2, C6, C8, C18, polyethylene-coated silica, and polyethylene-coated alumina). The retention data matrix was evaluated both by principal component analysis and cluster analysis. The retention characteristics of both polyethylene-coated supports were similar to that of C1, the retention capacity of RP columns for surfactants increased with the increasing length of the covalently bonded hydrocarbon chain. The retention of ethoxylated surfactants depended nonlinearly on the length of ehtyleneoxide chain suggesting that the polar ethyleneoxide chains are in folded state under the chromatographic separation.     

Comparison of two ultrasound‐enhanced microextractions combined with HPLC for determining acaricides in water

An ultrasound‐enhanced in situ solvent formation microextraction has been developed first time and compared with ultrasound‐enhanced ionic‐liquid‐assisted dispersive liquid–liquid microextraction for the HPLC analysis of acaricides in environmental water samples. A ionic liquid ([C₈MIM][PF₆]) was used as the green extraction solvent through two pathways. The experimental parameters, such as the type and volume of both of the extraction solvent disperser solvent, ultrasonication time, and salt addition, were investigated and optimized. The analytical performance using the optimized conditions proved the feasibility of the developed methods for the quantitation of trace levels of acaricides by obtaining limits of detection that range from 0.54 to 3.68 μg/L. The in situ solvent formation microextraction method possesses more positive characteristics than the ionic‐liquid‐assisted dispersive liquid–liquid microextraction method (except for spirodiclofen determination) when comparing the validation parameters. Both methods were successfully applied to determining acaricides in real water samples.     

Comparison of Different Clean-Up Procedures for the Analysis of Deoxynivalenol in Cereal-Based Food and Validation of a Reliable HPLC Method

Three different clean-up methods for the analysis of deoxynivalenol in cereals and cereal-based foods are described and compared. DON was extracted with water or acetonitrile/water (84/16, v/v) and the extracts were cleaned-up by passing either through two different immunoaffinity columns (DONtest, Vicam; DONPrep, Rhone) or a charcoal/alumina column (MycoSep, Romer Labs). Determinative analysis was performed by high performance liquid chromatography (HPLC) with diode array detection (DAD) and fluorescenic detection (FLD) after post-column derivatisation. Different parameters were optimised and possible reasons for diverse results are discussed. The final method was validated and adopted to different matrices. Best results were obtained using immunoaffinity columns for clean-up in combination with HPLC-FLD detection of the DON-derivative.     

A comparison of some electronic filters for use in noise suppression circuitry of HPLC detectors

Summary Four electronic filters suitable for baseline noise suppression in HPLC detectors were compared for their response and peak distortion characteristics under conventional HPLC conditions. The filters were designed to be white noise equivalent and the types of filter used were lag, lag-lag, critically damped lag and chain, these being representative of common, simple filter configurations. These filters were evaluated for speed of response to an optical step input signal and also for the effects of their level of noise suppression on peak height, retention time, peak asymmetry and apparent column efficiency. The critically damped lag filter was superior to the other filters in almost all respects and its use considerably enhanced detector performance.     

Comparative Study of the Determination of Parabens in Shampoos by Liquid Chromatography with Amperometric and Coulometric Detection

The simultaneous determination of four para‐hydroxybenzoic acid esters (parabens) in shampoos was studied by liquid chromatography (LC) with amperometric (LC‐AD) and coulometric (LC‐CD) detection. The parabens were separated on an ODS C18 reversed column by isocratic elution with a mobile phase based on methanol‐0.1 M acetic acid (60 : 40%, v/v) with 0.02 M NaClO₄ at a flow rate of 0.8 mL min^?1. The limit of detection (S/N>3) for the analytes was in the 15–25 pg (injected mass) range at an applied potential of 1.20 V vs. Ag/AgCl using the LC‐AD and in the 2–3 pg range at a potential of 0.790 V vs. Pd using the LC‐CD. The peak ratio of the internal standard peak (IS: 4‐hydroxybenzoic acid sec‐butyl ester) versus the analyte peak was found to be related to the amount injected from 0.1 ng to 100ng (r=0.996–0.999) with the LC‐AD and from 0.050 ng to 100 ng range (r=0.999–1.000) with the LC‐CD. The relative standard deviation (RSD, n=10) was comprised between 1.8 to 3.5% by LC‐AD ( 5 ng injected) and between 2.0 to 2.4% by LC‐CD (0.5 ng injected). The determination of four most used parabens in ten different shampoos was successfully realized.     

Inertness and stability of a maltose-silica for the separation of proteins by HPLC

Summary A stationary phase consisting of maltose on a silica support, designed for the separation of proteins, is described. When maltose is coupled to aminopropylsilica an effective shielding of the silica surface can be obtained. A size exclusion experiment is used to investigate pore structure and the behaviour of proteins. The stability of the surface layer is studied, as influenced by silica pretreatments and cross-linking reactions. Inertness is studied via the denaturation of trypsin in columns of different length at a number of temperatures. An example of a derivatization of the maltose layer is given which demonstrates its use as an interactive separation medium.     

Determination of the lipophilicity of some anti-hypoxia drugs: Comparison of TLC and HPLC methods

Summary The lipophilicity of 14 anti-hypoxia drugs has been determined by reversed phase thin-layer (RPTLC) and reversed phase high performance liquid chromatography (RPHPLC) in eluent systems containing different concentrations of acetonitrile and potassium dihydrogen phosphate. There was significant correlation between lipophilicity and the specific hydrophobic surface area of the drugs in RPTLC, indicating that the drugs behave as an homologous series of compounds. In RPTLC the concentration of buffer has a negligible effect on the retention of the drugs whereas in RPHPLC the buffer concentration influenced the retention. This discrepancy can be explained by the lower sensitivity of RPTLC. There was strong correlation between lipophilicity values determined by both methods, proving that both are suitable for the determination of molecular lipophilicity.     

HPLC evaluation of mono-ol,butylphenyl and perfluorinated columns prepared via olefin hydrosilation on a silica hydride intermediate

Summary Three columns, mono-ol, butylphenyl and perfluorinated synthesized through the reaction of an olefin with a silica hydride intermediate, are evaluated for the separation of various model compounds. Two of the columns, mono-ol and perfluorinated, can operate in both the normal phase and reversed phase modes. Some effects of the hydride surface are detected, particularly for the perfluorinated phase. A four-component steroid mixture is tested on both the mono-ol and perfluorinated phases in the normal mode.     

Determination of carbofuran in a nutrient solution by GLC/NPD and HPLC

Summary Two analytical methods using GC/NPD and HPLC to analyse carbofuran in a nutrient solution used in hydroponic culture are described. Both methods show a high recovery, (greater than 90%), and their limits of detection are low.     

Comparison of Micellar Electrokinetic Chromatography with HPLC for the Separation of Hematoporphyrin Derivatives

ABSTRACT

A mixed SDS micelle and BSA buffer system was used in the micellar electrokinetic chromatography (MEKC) separation of hematoporphyrin derivatives (HPD) at pH 8.0 with untreated capillaries. The effects of altering the composition of sodium dodecyl sulfate (SDS) electrolyte solution on the separation efficiency of the hematoporphyrin derivatives were presented. The results show that separation efficiencies were enhanced by using a mixture of SDS and BSA. The results demonstrated that CE methodology can compete with well-established techniques such as HPLC for the separation of biomedical and pharmaceutical samples with regard to time and expense of analysis.     

Fabrication of a Novel Polymeric Scaffold for Amperometric Laccase Biosensor

Present work displays the preparation of an electrochemical biosensor using a conjugated polymer and laccase enzyme for catechol quantification in samples. The biosensing system is based on an enzyme immobilization on polymer modified graphite transducer surface. For that purpose, a random conjugated polymer, thienothiophene‐benzoxadiazole‐alt‐benzodithiophene (BOTT), was coated onto a graphite electrode surface via drop casting method followed by immobilization of a biomolecule (laccase) for sensing experiments. Herein, for the first time, we proposed a BOTT polymer as an inexpensive and effective way to fabricate highly sensitive and fast response biosensors. The proposed sensing system possessed superior properties with 0.38 μM limit of detection and 110.81 μA mM^?1 sensitivity. Furthermore, cyclic voltammetry and scanning electron microscopy techniques were used to examine the surface modifications. The proposed system could be useful for many future studies for catechol quantification in environmental samples.© 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2333–2339     

Development of a micellar electrokinetic capillary chromatography method for the determination of four naphthalenediols in cosmetics and a comparison with a HPLC method

A micellar electrokinetic capillary chromatography (MEKC) method with ultraviolet visible (UV) detection was used for the determination of 1,7-naphthalenediol, 2,3-naphthalenediol, 1,5-naphthalenediol, and 2,7-naphthalenediol in cosmetics. The current method for their determination in various cosmetics is high-performance liquid chromatography (HPLC). Separation conditions affecting the MEKC method were optimized as 20 mM Na₂B₄O₇–50mM SDS, pH 9.8, with 22 kV applied voltage and UV detection at 230 nm. Under optimal conditions, electrophoretic analysis was completed in less than 6 min, with limit of detection (LOD) of 0.070–0.19 μg/mL and limit of quantitation (LOQ) of 0.23–0.63 μg/mL. A good linear relationship (r² > 0.99) was obtained at the range of 0.75–20 μg/mL. Recoveries for the four naphthalenediols in lotion, loose powder, and sun cream are between 91.2–107.2% with relative standard deviation (RSD) less than 4.04%. The method has been successfully applied to the determination of the four naphthalenediols in different kinds of cosmetics. A comparison with HPLC-UV method was also carried out according to the National Standards of the People's Republic of China. The results obtained by MEKC and HPLC methods are comparable, but the proposed MEKC method can help us obtain a much shorter detection time and low cost.