Optimizing the quadruple-potential waveform for the pulsed amperometric detection of neomycin

Determination of neomycin is important for quality control of the pharmaceutical preparation. A quadruple-potential waveform used for pulsed amperometric detection of neomycin was investigated. The waveform cleans the electrode by application of a potential more negative than the potential limit to avoid the formation of gold oxide during applying positive potential to clean gold electrode, thus decreasing the dissolution resulting recession of the gold working electrode within gold oxide formation/reduction cycles in the triple-potential waveform. Waveform parameters were optimized to maximize the signal-to-noise ratio (S/N). The detection limit of neomycin B is lower than 0.01 microg/ml. The linearity of framycetin (plotted as peak area of neomycin B) ranges from 0.05 to 100 microg/ml with correlation coefficient 0.9998. R.S.D. (n = 60) of the peak area of neomycin B is lower than 2%. The quadruple-potential waveform shows low detection limits and long-term reproducibility.     

用于硫酸庆大霉素高效液相色谱分离-电化学测定的四电位波型的优化

提出并优化了用于庆大霉素测定的四电位检测波型，建立了庆大霉素的高效液相色谱分离-电化学测定新方法。在该新检测波型中，使用了负的清洗电位，该清洗电位具有清洗效果好，对金工作电极的氧化溶解损伤小，因而建立的分析方法具有满意的精密度。该分析方法已经被应用于两种实际样品的测定。     

Determination of several sugars in serum by high-performance anion-exchange chromatography with pulsed amperometric detection

In this paper, a sensitive, simple and direct method for simultaneous determination of glucose, ribose, isomaltose and maltose in serum sample by high-performance anion-exchange chromatography coupled with integrated pulsed amperometric detection was developed. The four target analytes were easily and completely separated on an anion-exchange column at a flow-rate of 0.25 mL/min by binary step gradient elution in about 16 min and the two eluents were deionized water and 500 mM sodium hydroxide, respectively. The separated four analytes were detected directly by using a gold electrode and quadruple-potential waveform integrated pulsed amperometry without derivatization. Under the optimized conditions, when the injection volume was 25 microL, the detection limits (signal-to-noise ratio equal to 3) for glucose, ribose, isomaltose and maltose were 0.92, 7.50, 12.9 and 10.3 ng/mL, respectively. The calibration graphs of peak area for the four analytes were linear over two to three orders of magnitude with correlation coefficients greater than 0.998. R.S.D. of peak areas of the four analytes for five determinations were no more than 5.6%. The analytical method had been applied to the determination of glucose, ribose, isomaltose and maltose in real serum samples and good results with low relative standard deviation not more than 5.3% were obtained. The accuracy of the proposed method was tested by recovery measurements on spiked samples and good recovery results (98.1-107.9%) were obtained.     

Determination of neomycin in water samples by high performance anion chromatography with pulsed amperometric detection

A simple, fast and reliable method, using high performance anion chromatography with pulsed amperometric detection, had been developed for the analysis of neomycin in water samples. The elution and separation were carried out with an isocratic mobile phase, containing 10 mmol/L NaOH. The influence of the concentration and pH of the mobile phase on the separation and detection was investigated. A quadruple-potential waveform used for the detection was optimized. The detection limit of neomycin was down to 0.027μg/mL. The linearity of neomycin calibration curve ranged from 0.050 to 0.505μg/mL with correlation coefficient of 0.9997. R.S.D. (n = 11) was 4.0%.     

Comparison of pulsed amperometric detection and integrated voltammetric detection for inorganic sulfur compounds in liquid chromatography

A Variety of potential–time waveforms are useful in pulsed electrochemical detection (PED) when applied for the amperometric detection of numerous polar organic compounds following their separation by liquid chromatography (LC). Here, we compare the waveforms for pulsed amperometric detection (PAD) and integrated voltammetric detection (IVD) applied for detection of organosulfur compounds at Au electrodes in acidic media. In PAD waveforms, electrodes response is measured at a constant detection potentials. In IVD waveforms, electrodes current is integrated throughout a fast cyclic scan of the detection potential. As a consequence of this difference in detection strategy, the background signal for IVD is significantly smaller for PAD in the detection of organosulfur compounds whose response mechanisms require the concomitant formation of surface oxides on Au electrodes. Furthermore, in comparison to Pad, IVD has a larger sensitivity and a diminished system peak from 0₂ dissolved in the sample. Use of a preadsorption step increases detection sensitivity in both PAD and IVD. The limit of detection (S/N=3)for cysteine in LC-IVD is ca. 6 nM for a 50-μl injection (i.e., 300 fmol) using a detection waveform that includes a 1000-ms preadsorption period.     

新型三维可调共聚焦激光诱导荧光检测器的研制与评价

共聚焦结构是激光诱导荧光检测器(LIFD)中使用最广泛的光路结构之一,增强光路系统同轴精度和降低系统杂散光是降低仪器基线噪声水平、提高其信噪比(S/N)的两种有效手段。采用精密三维反光镜调节架和模块化设计光路系统,研制了一种高效液相色谱用高精度共聚焦激光诱导荧光检测器,对异硫氰酸荧光素(fluorescein isothiocyanate, FITC)的检出限为1×10~12 mol/L。基线噪声与漂移较改进前降低了一个数量级,分别达到8.0×10~3 mV和1.4×10~3 mV/h,且稳定性好,5×10~9 mol/L FITC样品连续5次进样分析的峰面积和峰高相对标准偏差(RSD)均小于0.5%。进一步用FITC衍生化生物胺对研制的检测器进行评价,检出限(S/N=3)达到0.01～0.02 nmol/L。新研制的LIFD噪声低、稳定性好、灵敏度高,适用于生物、食品和环境样品中痕量物质的分析。     

Capillary electrophoresis of some tetracycline antibiotics coupled with reductive fast cyclic voltammetric detection.

The separation and quantitative performance parameters for tetracycline, chlortetracycline and oxytetracycline antibiotics were investigated by capillary zone electrophoresis coupled with fast cyclic voltammetric detection. Optimization of pH and complexation with a boric acid-sodium tetraborate buffer provided good resolution of all compounds. Detection by electrochemical reduction using fast on-line cyclic voltammetric detection with a Hg-film-microm electrode gave detection limits (2 x peak-to-peak baseline noise) of 7 x 10(-7) mol/l for tetracycline and chlortetracycline, and 1.5 x 10(-6) mol/l for oxytetracycline. The influence of electrode material, potential range and scan rate was examined and discussed. Optimal electrochemical detection was obtained at a Hg-film electrode with a waveform that consisted of an initial constant potential of -0.6 V for 200 ms followed by a cyclic voltammetry (CV) scan at 300 V/s from - 0.6 V to a vertex potential of 1.7 V. The analytical signal was obtained by plotting the integrated values of the CV current from each applied waveform as a function of time. The calibration plot (peak areas) for each separated peak was found to be linear over three-orders of magnitude.     

A pulsed potential waveform displaying enhanced detection capabilities towards sulfur-containing compounds at a gold working electrode

Pulsed electrochemical detection of sulfur-containing compounds was successfully investigated by applying a four-step potential waveform at a gold working electrode. This potential waveform called APAD, which stands for activated pulsed amperometric detection, is composed of an activation potential step added to a conventional three-step potential waveform. A key advantage of the APAD at the Au electrode is the ability to enhance sensitivity through the use of a short potential pulse (E(ACT) = +750 mV versus Ag/AgCl and tACT approximately 90 ms) during which the formation of redox active species (presumably OH*) are able to efficiently oxidize organosulfur compounds. The APAD waveform parameters were optimized to maximize the signal-to-noise ratio (S/N) and successfully applied for the sensitive detection of lipoic acid, biotin, iminobiotin, methionine, cystine, cysteine, homocysteine, homocystine, N-acetylcysteine and glutathione, following their separations by high-performance anion-exchange chromatography (HPAEC) using alkaline mobile phases. The detection limits (S/N = 3, 10 microL injected) ranged from 0.3 for cysteine (400 pg) to 0.02 micromol/L for biotin (50 pg) and methionine (30 pg). The response of sulfur-, amine- and alcohol-based compounds was compared by using four selected pulsed potential waveforms. It was found that the APAD exhibits excellent sensitivity for thiocompounds outperforming all other pulsed potential waveforms. Ratios of the peak areas for APAD and the six-step potential integrated waveform increased from 3.2 +/- 0.4 to 13.5 +/- 0.6 for lipoic acid and biotin, respectively.     

小波变换用于示波信号中有用信息提取的研究

示波分析是近年来在我国发展起来的一个新的电化学分析研究领域［１～４］．它根据阴极射线示波器荧光屏上示波图及其变化进行分析测试,从原理上可以将其分为示波电位法和示波计时电位法;从测定方式上可以将其分为示波滴定和示波测定．关于示波电位法的一些理论问题（如...     

Optimization of the design and operation of FAIMS analyzers

Field asymmetric waveform ion mobility spectrometry (FAIMS) holds significant promise for post-ionization separations in conjunction with mass-spectrometric analyses. However, a limited understanding of fundamentals of FAIMS analyzers has made their design and operation largely an empirical exercise. Recently, we developed an a priori simulation of FAIMS that accounts for both ion diffusion (including anisotropic components) and Coulomb repulsion, and validated it by extensive comparisons with FAIMS/MS data. Here it is corroborated further by FAIMS-only measurements, and applied to explore how key instrumental parameters (analytical gap width and length, waveform frequency and profile, the identity and flow speed of buffer gas) affect FAIMS response. We find that the trade-off between resolution and sensitivity can be managed by varying gap width, RF frequency, and (in certain cases) buffer gas, with equivalent outcome. In particular, the resolving power can be approximately doubled compared to "typical" conditions. Throughput may be increased by either accelerating the gas flow (preferable) or shortening the device, but below certain minimum residence times performance deteriorates. Bisinusoidal and clipped-sinusoidal waveforms have comparable merit, but switching to rectangular waveforms would improve resolution and/or sensitivity. For any waveform profile, the ratio of two between voltages in high and low portions of the cycle produces the best performance.     

小波变换用于二次微分交流示波计时电位信号中有用信息的提取

小波分析是８０年代发展起来的一种新的数学分支。由于小波变换具有许多其它的信号处理手段所不具备的优良特性，如正交性，可变的时－频分辨率和可调节的局部支持等，使它成为信号处理的一种强有力的工具。     

An argon-ion laser fluorometer optimal for concentration detection of 5-bromomethyl fluorescein derivatized carboxylic acids

A continuous wave, argon ion laser-induced fluorescence detector was designed for use with conventional high performance liquid chromatography. Palmitic acid was selected as a model analyte to evaluate the performance of the detector. Using the intense 488.0 nm emission line, a concentration detection limit of 7.5 × 10^–10 M (38 femtomoles on-column) was established for palmitic acid at a signal to noise ratio of three. The linear dynamic range extended over two orders of magnitude with a correlation coefficient of 0.9998. The contribution of scattered radiation to the analytical signal was minimal. The concentration detection limit achieved with the constructed laser fluorometer was superior to other reports for fatty acids.     

Application of power functions to chromatographic data for the enhancement of signal to noise ratios and separation resolution

Chromatographic detection responses are recorded digitally. A peak is represented ideally by a Guassian distribution. Raising a Guassian distribution to the power ‘n’ increases the height of the peak to that power, but decreases the standard deviation by √n. Hence there is an increasing disparity in detection responses as the signal moves from low level noise, with a corresponding decrease in peak width. This increases the S/N ratio and increases peak to peak resolution. The ramifications of these factors are that poor resolution in complex chromatographic data can be improved, and low signal responses embedded at near noise levels can be enhanced. The application of this data treatment process is potentially very useful in 2D-HPLC where sample dilution occurs between dimension, reducing signal response, and in the application of post-reaction detection methods, where band broadening is increased by virtue of reaction coils. In this work power functions applied to chromatographic data are discussed in the context of (a) complex separation problems, (b) 2D-HPLC separations, and (c) post-column reaction detectors.     

Optimizing the Triple‐Potential Waveform for the Pulsed Amperometric Detection of Streptomycin

Abstract

A triple‐potential waveform used for amperometric detection of streptomycin (STP) is investigated. The mobile phase is an aqueous solution of 75 mM phosphoric acid. Waveform parameters are optimized to maximize the signal‐to‐noise ratio (S/N). The potentials of the waveform are designated E1, E2, and E3, and potentials are maintained for time periods t1, t2, and t3. Unlike the normal triple‐potential waveform, the cleaning function is accomplished at negative potential instead of positive potential in our triple‐potential waveform. The detection limit of STP is <0.04 µg mL^?1 (S/N=3). The linearity of STP ranges from 0.1 to 5 µg mL^?1 (r=0.9986). RSD (n=30) of the peak area of STP is 5.8%. The triple‐potential waveform shows low detection limits and long‐term reproducibility.     

Fast gas chromatographic/mass spectrometric determination of diuretics and masking agents in human urine: Development and validation of a productive screening protocol for antidoping analysis

An analytical procedure was developed for the fast screening of 16 diuretics (acetazolamide, althiazide, amiloride, bendroflumethiazide, bumetanide, canrenoic acid, chlorthalidone, chlorthiazide, clopamide, ethacrynic acid, furosemide, hydrochlorthiazide, hydroflumethiazide, indapamide, triamterene, trichlormethiazide) and a masking agent (probenecid) in human urine. The whole method involves three analytical steps, including (1) liquid/liquid extraction of the analytes from the matrix, (2) their reaction with methyl iodide at 70 degrees C for 2 h to form methyl derivatives, (3) analysis of the resulting mixture by fast gas chromatography/electron impact mass spectrometry (fast GC/EI-MS). The analytical method was validated by determining selectivity, linearity, accuracy, intra and inter assay precision, extraction efficiencies and signal to noise ratio (S/N) at the lowest calibration level (LCL) for all candidate analytes. The analytical performances of three extraction procedures and five combination of derivatization parameters were compared in order to probe the conditions for speeding up the sample preparation step. Limits of detection (LOD) were evaluated in both EI-MS and ECNI-MS (electron capture negative ionization mass spectrometry) modes, indicating better sensitivity for most of the analytes using the latter ionization technique. The use of short columns and high carrier gas velocity in fast GC/MS produced efficient separation of the analytes in less than 4 min, resulting in a drastic reduction of the analysis time, while a resolution comparable to that obtained from classic GC conditions is maintained. Fast quadrupole MS electronics allows high scan rates and effective data acquisition both in scan and selected ion monitoring modes.     

A coincidence transmission electron microscope with digital waveform measuring system for analytical microscopy

We have been developing a new analytical transmission electron microscope (TEM), called a coincidence TEM, which in principle enables elemental mapping images to be observed at a high signal‐to‐noise (S/N) ratio under very low dose radiation conditions. In this paper, we report the development of a coincidence TEM with a digital waveform measuring system for obtaining a coincidence elemental mapping image. In this system, analog signals detected by a Si(Li) detector and a multianode, position‐sensitive photomultiplier (PSPM) are continuously converted into 12‐bit digital waveform data at a rate of 100 MHz, and transferred to a PC. From the transferred digital waveform data, information on X‐ray photon energy, electron incident position, and detection times of both X rays and electrons are calculated by digital waveform measurement, which lead to the observation of a successful coincidence elemental mapping image. Copyright © 2005 John Wiley & Sons, Ltd.     

Resonant ac dipolar excitation for ion motion control in the Orbitrap mass analyzer

A dipolar ac signal applied to the split outer electrode of an Orbitrap mass spectrometer at the axial resonance frequency causes excitation of ion axial motion and either eventual ion ejection from the trap, if applied in phase with ion motion, or de-excitation, if applied 180 degrees out of phase. Both de-excitation and excitation may be achieved mass-selectively. The extent of ion axial de-excitation depends on the ac amplitude and on the number of cycles applied; sufficient de-excitation can be accomplished such that the ion signal cannot be observed above baseline noise. After de-excitation, the ions remain trapped and in rapid orbital (but not axial) motion, which allows them to be re-excited coherently by application of a second ac waveform allowing the signal again to be observed. Both broad-band and narrow-band waveforms have been used to de-excite and to re-excite ion motion. Using narrow-band waveforms, selective de-excitation and re-excitation can be performed with unit mass selection, leaving an adjacent 13C isotopic peak unaffected. The origin and potential applications of these new capabilities is delineated.     

Non-linear signal response functions and their effects on the statistical and noise cancellation properties of isotope ratio measurements by multi-collector plasma mass spectrometry

A nebulizer-centric response function model of the analytical inductively coupled argon plasma ion source was used to investigate the statistical frequency distributions and noise reduction factors of simultaneously measured flicker noise limited isotope ion signals and their ratios. The response function model was extended by assuming i) a single gaussian distributed random noise source (nebulizer gas pressure fluctuations) and ii) the isotope ion signal response is a parabolic function of the nebulizer gas pressure.Model calculations of ion signal and signal ratio histograms were obtained by applying the statistical method of translation to the non-linear response function model of the plasma. Histograms of Ni, Cu, Pr, Tl and Pb isotope ion signals measured using a multi-collector plasma mass spectrometer were, without exception, negative skew. Histograms of the corresponding isotope ratios of Ni, Cu, Tl and Pb were either positive or negative skew. There was a complete agreement between the measured and model calculated histogram skew properties.The nebulizer-centric response function model was also used to investigate the effect of non-linear response functions on the effectiveness of noise cancellation by signal division. An alternative noise correction procedure suitable for parabolic signal response functions was derived and applied to measurements of isotope ratios of Cu, Ni, Pb and Tl. The largest noise reduction factors were always obtained when the non-linearity of the response functions was taken into account by the isotope ratio calculation.Possible applications of the nebulizer-centric response function model to other types of analytical instrumentation, large amplitude signal noise sources (e.g., lasers, pumped nebulizers) and analytical error in isotope ratio measurements by multi-collector plasma mass spectrometry are discussed.     

改进计时电流法的数学模型和电路实现

当采用电化学计时电流法对样品进行检测时,由于检测系统的延迟,采集到的电化学信号会产生偏差,这种现象在多通道电化学信号快速采集时尤为明显.为克服该问题,我们提出一种新的电化学方法来快速产生电流峰值,通过测量峰值快速准确地测出被测物浓度.本文首先为该方法建立理论模型,推导出电流峰值与被测物的浓度关系;再引入经典控制理论中的控制环节来改进传统电化学电路以实现该方法,并加入峰值检测电路来准确获取电流峰值信号;最后,利用该方法研究K3[Fe(CN)6]和3,3′,5,5′-四甲基联苯胺(TMB)溶液的电化学反应,证明了本方法相比传统计时电流法具有更高的信噪比和灵敏度,电流峰值与浓度呈线性关系,并且检测结果不受采样时间延迟的影响,克服了计时电流法的不足.     

New algorithms for processing and peak detection in liquid chromatography/mass spectrometry data

Two new algorithms for automated processing of liquid chromatography/mass spectrometry (LC/MS) data are presented. These algorithms were developed from an analysis of the noise and artifact distribution in such data. The noise distribution was analyzed by preparing histograms of the signal intensity in LC/MS data. These histograms are well fit by a sum of two normal distributions in the log scale. One new algorithm, median filtering, provides increased performance compared to averaging adjacent scans in removing noise that is not normally distributed in the linear scale. Another new algorithm, vectorized peak detection, provides increased robustness with respect to variation in the noise and artifact distribution compared to methods based on determining an intensity threshold for the entire dataset. Vectorized peak detection also permits the incorporation of existing algorithms for peak detection in ion chromatograms and/or mass spectra. The application of these methods to LC/MS spectra of complex biological samples is described.