Data modelling with neural networks: Advantages and limitations

The origins and operation of artificial neural networks are briefly described and their early application to data modelling in drug design is reviewed. Four problems in the use of neural networks in data modelling are discussed, namely overfitting, chance effects, overtraining and interpretation, and examples are given of the means by which the first three of these may be avoided. The use of neural networks as a variable selection tool is shown and the advantage of networks as a nonlinear data modelling device is discussed. The display of multivariate data in two dimensions employing a neural network is illustrated using experimental and theoretical data for a set of charge transfer complexes.     

Differential linear scan voltammetry: analytical performance in comparison with pulsed voltammetry techniques

We report here on differential linear scan voltammetry, DLSV, that combines the working principles of linear scan voltammetry, LSV, and the numerous existing pulsed voltammetry techniques. DLSV preserves the information from continuous interrogation in voltage and high accuracy that LSV provides about electrochemical processes, and the much better sensitivity of differential pulsed techniques. DLSV also minimizes the background current compared to both LSV and pulsed voltammetry. An early version of DLSV, derivative stationary electrode polarography, DSEP, had been proposed in the 1960s but soon abandoned in favor of the emerging differential pulsed techniques. Relative to DSEP, DLSV takes advantage of the flexibility of discrete smoothing differentiation that was not available to early investigators. Also, DSEP had been explored in pure solutions and with reversible electrochemical reactions. DLSV is tested in this work in more challenging experimental contexts: the measurement of oxygen with a carbon fiber microelectrode in buffer, and with a gold microdisc electrode exposed to a live biological preparation. This work compares the analytical performance of DLSV and square wave voltammetry, the most popular pulsed voltammetry technique.

Advantages and limitations of HPLC in environmental analysis

Summary The application of HPLC to environmental analysis is often hindered by difficulty not experienced in other areas of analysis. Usually the components being determined are at parts per million levels or less and are usually in sample matrices that can yield many interferences. In order to develop successful methodology the prime requirements for an HPLC system are column efficiency and the sensitivity and selectivity of the detection system. In this presentation, examples are given to illustrate how HPLC can be used to advantage by comparing it to gas chromatographic (GC) methods and even TLC methods. For many classes of compounds, such as halogenated hydrocarbons (pesticides, PCB’s, dioxins), the GC methods may be preferred. However, for polycyclic aromatic hydrocarbons (PAH) HPLC with fluorescence detection has proven to be excellent for trace environmental analysis. Comparisons of HPLC with TLC for aflatoxins and with bioassays for paralytic shellfish toxins are made. Novel combinations such as headspace-HPLC analyses for SO₂ and HPLC-AA for organometallic compounds are discussed.     

Proteins and peptides voltammetry: Trends,potential, and limitations

Numerous biological processes are connected with the efficient electron transfer reactions in proteins and peptides. In this review, we discuss briefly the relevance and current challenges associated with the voltammetric analysis of peptides and proteins with and without a metal redox center. Special attention is paid to the integration of electrochemical methods with new nanomaterials which offers amplification of multiplexing capabilities for simultaneous and very sensitive examination of various proteins. After critically discussing the most interesting approaches in the proteins/peptides voltammetric analysis reported so far, for the single or multiplexed examination of such biomolecules with demonstrated applicability in the real-sample analysis, existing challenges still need to be addressed and future directions in this field will be pointed out.     

Coupling immunoassays with chromatographic separation techniques

Coupling immunoassays with HPLC separation techniques is becoming increasingly useful in the analysis of biological and nonbiological samples of both large and small molecules. This is because it provides both sensitivity and selectivity for molecular analysis at relatively low cost, low maintenance and with excellent potential for automation. This paper reviews application of this hyphenated approach both in the pre-column immunoextraction and post-column immunodetection modes. Systems in which immunoassays are interfaced to chromatographic separations in order to separate bound and free fractions of the immunoassay will not be included since these systems do not provide the enhanced selectivity common to hyphenated systems. Post-column immunodetection is based on various immunoassay formats such as direct detection, one-site, competitive and sandwich immunoassays. Homogeneous immunodetectors are more convenient than heterogeneous immunodectors since there are no separation and column regeneration steps involved in homogeneous immunoassays. On the other hand, heterogeneous immunoassays are generally more sensitive than homogeneous immunoassays since interfering substances are removed prior to immunodetection. Advantages and limitations for the various approaches will be discussed.     

State of the art of shear driven chromatography. Advantages and limitations

The present paper reports on the experimental difficulties encountered when trying to realize the full potential of shear-driven chromatography in nanochannels. While it theoretically offers the possibility to yield over 10,000 plates per centimetre in a few seconds, the practical realization of this potential requires a detector miniaturisation that is carried to the extreme combined with very high sampling rates. In the present study, a charge coupled device camera and a photomultiplier tube combined with pinhole were tested as detector. Despite the fact that the photomultiplier tube could offer a higher sampling rate and a better sensitivity, the charge coupled device turned out to be better suited for the current set-up because of inevitable problems with the stray-light transported through the glass channel wall. The chemistry of the separation surface was additionally studied getting more homogenous coating, thus higher separation efficiency. Having also carried out a number of mechanical improvements, it is now possible to measure separations at a distance of 8mm downstream from the injection point. This is four times further downstream than ever before while realizing a four components mixture separation in less than 1.5s, with a plate generation velocity of about 2000-7000 plates per second depending on the sample.     

Separation of nucleotides by reversed-phase high-performance liquid chromatography: Advantages and limitations

Summary The determination of nucleotides by reversedphase high-performance liquid chromatography with 0.1 M (NH₄)₃PO₄ as an elution buffer is described. Effects of pH, type of RP packing and column efficiency are discussed. Currently used columns with efficiencies corresponding to 7,000–15,000 theoretical plates, containing 15–20% of bound carbon, have been shown not to be sufficient for the separation of nucleotides from tissue samples in the ionsuppression mode, but they provide excellent separation of 2-, 3-, and 5-monophosphates, deoxytriphosphates or synthetic derivatives of adenosine and guanosine.

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Trennung von Nucleotiden mit der RP-Hochleistungsflüssigkeits-Chromatographie: Vorteile und Beschränkungen

Zusammenfassung Die Bestimmung von Nucleotiden mit reversed-phase HPLC mit 0,1 M (NH₄)₃PO₄ als Elutionspuffer wird beschrieben. Der Einfluß des pH sowie der Art der RP-Belegung wird diskutiert, Säulen mit einer theroretischen Bodenzahl von 7000–15000, die 15–20% gebundenen Kohlenstoff enthalten, erwiesen sich als nicht geeignet für die Trennung von Nucleotiden aus Gewebeproben mittels Ionen-Verdrängung. Sie ergeben aber eine ausgezeichnete Trennung von 2-, 3- und 5-Monophosphaten, Desoxytriphosphaten oder synthetischen Derivaten von Adenosin und Guanosin.

     

Anion-exchange separation techniques with methanol-water solutions of hydrochloric and nitric acids

Mixed methanol-water systems were shown to be of use in the analysis of samples containing 500-mg amounts of metallic impurities for rare earths and actinides. Detailed study of the hydrochloric acid-methanol system led to improved separation of einsteinium and californium from americium and curium as well as to lanthanideactinide separations. Comparisons of elution orders are also drawn between these systems and the corresponding lithium salt systems, with emphasis on ion-hydration theories.     

Advantages and limitations of thermal lens spectrometry over conventional spectrophotometry for absorbance measurements

This review considers the advantages and the limitations that thermal lens spectrometry has over conventional spectrophotometry for the measurement of optical absorption in specific applications. The photothermal method is characterized by its intrinsic sensitivity resulting from the indirect nature of the measurement and amplified by physical and thermo-optical parameters which are not effective in absorbance measurements. Other advantages include a weak dependence on light scattering and the complementary nature of photothermal spectra with respect to absorption and emission spectra for speciation studies at very low concentrations. The main drawbacks are the convective noise, the background absorbance and the complexity of the experimental set-up, especially when differential or wavelength scanning measurements are required.     

Determination of caprolactam by adsorptive voltammetry after separation by thin-layer chromatography

Caprolactam (2-oxohexamethyleneimine) can be determined in wastewaters and natural waters by adsorptive stripping voltammetry after separation of the product of the reaction between caprolactam and p-(N,N-dimethylamino)benzene-p′-azobenzoyl chloride. When a hanging mercury drop electrode is used with an accumulation time of 60 s in stirred solution, caprolactam can be determined from a lower limit of 0.2 μg ml^?1. With a 360-s accumulation time, linear calibration plots are obtained for 8 × 10^?10?8 × 10^?9 mol l^?1 caprolactam. The effect of interfering sample components is eliminated by the TLC separation.     

Advantages and limitations of coupling isotachophoresis and comprehensive isotachophoresis-capillary electrophoresis to time-of-flight mass spectrometry

Capillary isotachophoresis (ITP) and comprehensive isotachophoresis-capillary electrophoresis (ITP-CE) were successfully coupled to electrospray ionization (ESI) orthogonal acceleration time-of-flight mass spectrometry (TOF-MS) using angiotensin peptides as model analytes. The utility of ITP-TOF-MS and ITP-CE-TOF-MS for the analysis of samples containing analyte amounts sufficient to form flat-top ITP zones (30 microM) as well as for samples with trace analyte amounts (0.3 microM) was studied. Separations were performed in 150 microm internal diameter (I.D.) capillaries for the ITP experiments, and in 200 microm I.D. (ITP) and 50 microm I.D. (CE) capillaries for ITP-CE experiments. The fused-silica columns were coated with poly(vinyl alcohol) to suppress electroosmotic flow that can disrupt ITP zone profiles. The sample loading capacity in both ITP and comprehensive ITP-CE was greatly enhanced (up to 10 microl) compared with typical nanoliter-sized injection volumes in CE. It was concluded that ITP-TOF-MS alone was adequate for the separation and detection of high concentration samples. The outcome was different at lower analyte concentrations where mixed zones or very sharp peaks formed. With formation of mixed zones, ion suppression and discrimination could occur, complicating quantitative determination of the analytes. This problem was effectively overcome by inserting a CE capillary between the ITP and TOF-MS. In such an arrangement, samples were preconcentrated in the high load WTP capillary and then injected into a CE capillary where they were separated into non-overlapping peaks prior to their detection by TOF-MS. The advantage of this comprehensive arrangement, which we have described previously, is that there is no need to discard portions of the sample in order to avoid overloading of the CE capillary. The whole sample is analyzed by multiple injections from ITP to CE. Thus, this method can be used for the analysis of complex samples with wide ranges of component concentrations.     

Ionic liquids in separation techniques

The growing interest in ionic liquids (ILs) has resulted in an exponentially increasing production of analytical applications. The potential of ILs in chemistry is related to their unique properties as non-molecular solvents: a negligible vapor pressure associated to a high thermal stability. ILs found uses in different sub-disciplines of analytical chemistry. After drawing a rapid picture of the physicochemical properties of selected ILs, this review focuses on their use in separation techniques: gas chromatography (GC), liquid chromatography (LC) and electrophoretic methods (CE). In LC and CE, ILs are not used as pure solvents, but rather diluted in aqueous solutions. In this situation ILs are just salts. They are dual in nature. Too often the properties of the cations are taken as the properties of the IL itself. The lyotropic theory is recalled and the effects of a chaotropic anion are pointed out. Many results can be explained considering all ions present in the solution. Ion-pairing and ion-exchange mechanisms are always present, associated with hydrophobic interactions, when dealing with IL in diluted solutions. Chromatographic and electrophoretic methods are also mainly employed for the control and monitoring of ILs. These methods are also considered. ILs will soon be produced on an industrial scale and it will be necessary to develop reliable analytical procedures for their analysis and control.     

Combining immunoassays with chromatographic and electrophoretic separation techniques — a review

The combination of immunoassays with separation techniques such as chromatography and electrophoresis can provide both selectivity and sensitivity that is competitive with any method currently available for molecular analysis. Immunoassays can be carried out on-line and off-line, pre and post separation. The on-line post separation mode is the most promising for routine analysis because of the high throughput that can be achieved but also provides the greatest challenge with regard to compatibility of the interfaced systems. This paper reviews the various approaches that have been researched from a practical immunochemical point of view with emphasis on the special problems incurred with matrix compatibility for on-line post separation systems.     

Advantages of using a mercury coated, micro-wire, electrode in adsorptive cathodic stripping voltammetry

A mercury coated, gold, micro-wire electrode is used here for the determination of iron in seawater by catalytic cathodic stripping voltammetry (CSV) with a limit of detection of 0.1 nM Fe at a 60 s adsorption time. It was found that the electrode surface is stable for extended periods of analyses (at least five days) and that it is reactivated by briefly (2 s) applying a negative potential prior to each scan. Advantages of this electrode over mercury drop electrodes are that metallic mercury use is eliminated and that it can be readily used for flow analysis. This is demonstrated here by the determination of iron in seawater by continuous flow analysis. It is likely that this method can be extended to other elements. Experiments using bismuth coated, carbon fibre, electrodes showed that the bismuth catalyses the oxidation of the important oxidants bromate and hydrogen peroxide, which makes it impossible to use bismuth based electrodes for catalytic CSV involving these oxidants. For this reason mercury coated electrodes retain a major advantage for catalytic voltammetric analyses.     

Rapid automated batchwize radiochemical separation techniques

The basic principles and specific techniques of rapid, automated radiochemical separation techniques that use batchwize separation methods are reviewed.