Detection modes

The simplest, and by far the most common, detection scheme is measurement of the current at a constant potential. Such fixed-potential amperometric measurements have the advantage of being free of double-layer charging and surface transient effects. As a result, extremely low detection limits—on the order of l-100pg ( 10 14 mol of analyte)—can be achieved. In various situations, however, it may be desirable to change (scan, pulse, etc.) the potential during the detection. 

The power of electrochemical detection can be improved by using more than one working electrode (89). Different strategies, based primarily on dualelectrode detection, can thus be employed. For example, in the series mode (Fig. 3.30, top) the first upstream electrode can be used to generate an electroactive species that is then more easily detected at the downstream electrode. Discrimination against compounds with irreversible redox chemistry can also be achieved. Significantly improved qualitative information can be 

Example 3.1 Voltammogram a was obtained for adsorptive stripping measurements of Fe(III) in seawater. Voltammograms b and c show successive standard additions of 4ppb Fe(III). Find the concentration of Fe(III) in the sample. 

Solution The resulting current peaks lead to the following standard addition plot  

From this plot, an Fe(III) concentration of 1.47ppb can be obtained for the sample. 

Pulsed amperometric detection (PAD), introduced by Joluison and LaCourse (64, 65) has greatly enhanced the scope of liquid cluomatography/electrochemistry (66). This detection mode overcomes the problem of loss of activity of noble metal electiodes associated with the fixed-potential detection of compounds such as caibohydrates, alcohols, amino acids, or aldehydes. Pulsed amperometric detection couples file process of anodic detection with anodic cleaning and cathodic reactivation of a noble metal eleetrode, thus assuring a continuously cleaned and active 

FIGURE 3-26 Dual-electrode thin-layer detector configurations for operation in the series (a) and parallel b) amperometric modes. 

FIGURE 3-27 Tliree-dimeiisional chromatogram for oxidizable biological compoiuids at a multichaimel amperometric detection system, consisting of an array of 16 carbon-paste electrodes held at different potentials. AA = ascorbic acid NE = norepineplirine DOPAC = 3,4-dihydroxyphenylacetic acid 5-HIAA = 5-hydroxyindole-3-acetic acid DA = dopamine HVA = homovanillic acid. (Reproduced with pemiission from reference 68.) 

Ouziel, C. Yamitzky aiid M Ariel, Anal. Chim. Acta, 102, 99 (1978). 

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Naphthalenedisulfonate-acetonitrile as the only mobile phase with a silica column coated with a crosslinked aminofluorocarbon polymer has proven to be an effective combination for the separation of aliphatic anionic surfactants. Indirect conductivity and photometric detection modes are used to monitor these analytes. The retention of these surfactants is found to depend on both the ionic strength and the organic solvent content of the mobile phase. The mechanism of retention is considered to be a combination of both reverse phase and ion exchange processes. Selective separation of both alkanesulfonates and... 

By CH4-chemical ionization two predominant ions from DDS were formed in the ion source m/z 163 (M + 1) and m/z 165 (M + 3), which were accelerated and separated in the first quadrupole. Decomposition by collision activation with argon then occurred in the second quadrupole and the resulting daughter ions were separated in the third quadrupole to give rise to collision-induced spectra (equations 41-43). When carrying out the analysis with a solid inlet MS-MS system (TSQ) it was observed that, when operating TSQ in the multiple ion detection mode and selecting the ions m/z 63, 83, 85,99 and 101, only in the third quadrupole was there no interference from other m/z 163 and 165 precursor ions . ... 

To ensure that the detector electrode used in MEMED is a noninvasive probe of the concentration boundary layer that develops adjacent to the droplet, it is usually necessary to employ a small-sized UME (less than 2 /rm diameter). This is essential for amperometric detection protocols, although larger electrodes, up to 50/rm across, can be employed in potentiometric detection mode [73]. A key strength of the technique is that the electrode measures directly the concentration profile of a target species involved in the reaction at the interface, i.e., the spatial distribution of a product or reactant, on the receptor phase side. The shape of this concentration profile is sensitive to the mass transport characteristics for the growing drop, and to the interfacial reaction kinetics. A schematic of the apparatus for MEMED is shown in Fig. 14. 

A molecular probe with dual output signals offers two detection modes allowing use of the same probe in different environments. We have demonstrated how an AB2 self-immolative dendron with double quinone methide release mechanism can be applied to create a molecular probe with UV-Vis and fluorescence modes for the detection of a specific catalytic activity.15 The molecular probe is illustrated in Fig. 5.36. The central unit of the probe (the molecular adaptor) is linked to an enzymatic substrate that acts as a trigger and to two different reporter molecules. Cleavage of the enzymatic substrate triggers the release of the two reporters and a consequent activation of their signals. 

Online detection using 4H nuclear magnetic resonance (NMR) is a detection mode that has become increasingly practical. In a recent application, cell culture supernatant was monitored on-line with 1-dimensional NMR for trehalose, P-D-pyranose, P-D-furanose, succinate, acetate and uridine.33 In stopped-flow mode, column fractions can also be analyzed by 2-D NMR. Reaction products of the preparation of the neuromuscular blocking compound atracurium besylate were separated on chiral HPLC and detected by 4H NMR.34 Ten isomeric peaks were separated on a cellulose-based phase and identified by online NMR in stopped-flow mode. 

Mayer, W. J. and Greenberg, M. S., A comparison of differential pulse and D. C. amperometric detection modes for the liquid chromatographic determination of oxalic acid, /. Chromatogr Sci., 17, 614, 1979. 

Alternative approaches consist in heat extraction by means of thermal analysis, thermal volatilisation and (laser) desorption techniques, or pyrolysis. In most cases mass spectrometric detection modes are used. Early MS work has focused on thermal desorption of the additives from the bulk polymer, followed by electron impact ionisation (El) [98,100], Cl [100,107] and field ionisation (FI) [100]. These methods are limited in that the polymer additives must be both stable and volatile at the higher temperatures, which is not always the case since many additives are thermally labile. More recently, soft ionisation methods have been applied to the analysis of additives from bulk polymeric material. These ionisation methods include FAB [100] and LD [97,108], which may provide qualitative information with minimal sample pretreatment. A comparison with FAB [97] has shown that LD Fourier transform ion cyclotron resonance (LD-FTTCR) is superior for polymer additive identification by giving less molecular ion fragmentation. While PyGC-MS is a much-used tool for the analysis of rubber compounds (both for the characterisation of the polymer and additives), as shown in Section 2.2, its usefulness for the in situ in-polymer additive analysis is equally acknowledged. 

The most important area for packed column use involves modified mobile phases (MPs). Consequently, pSFC needs detection systems in which the MP modifier and possible additive(s) do not interfere, and in which detection of low or non-UV-absorbing molecules is possible in combination with pressure/modifier gradients. The disadvantage of adding even small amounts of modifier is that FID can no longer be used as a detector. In the presence of polar modifiers in pSFC the detection systems are restricted basically to spectroscopic detection, namely UVD, LSD, MSD (using PB and TSP interfaces as in LC). ELSD can substitute FID and covers the quasi-universal detection mode, while NPD and ECD cover the specific detection mode in pSFC on a routine basis. As ELSD detects non-UV absorbing molecules dual detection with UV is an attractive option. 

Many IC techniques are now available using single column or dual-column systems with various detection modes. Detection methods in IC are subdivided as follows [838] (i) electrochemical (conductometry, amper-ometry or potentiometry) (ii) spectroscopic (tJV/VIS, RI, AAS, AES, ICP) (iii) mass spectrometric and (iv) postcolumn reaction detection (AFS, CL). The mainstay of routine IC is still the nonspecific conductometric detector. A significant disadvantage of suppressed conductivity detection is the fact that weak to very weak acid anions (e.g. silicate, cyanide) yield poor sensitivity. IC combined with potentiometric detection techniques using ISEs allows quantification of selected analytes even in complex matrices. The main drawback... 

Development and validation of protocols for solid-phase extraction coupled to LC and LC-MS have been described [136]. Hennion [137] has reviewed online SPE-HPLC coupling followed by various detection modes. 

Compared with other LC detection modes (UV, MS), the use of IR detection in LC is still rather limited. 

In HPLC-TLC coupling, the crucial aspect is the maintenance of the chromatographic integrity during the deposition process. The chromatogram is preserved after LC separation, and is available for further separation and/or investigation. LC-TLC coupling increases the separation efficiency, and allows detection modes which are incompatible with LC (e.g. spectroscopic techniques... 

In both cases, GC fingerprint libraries must be built before quantitative analysis can be routinely carried out. In analysis of QTLC by laser pyrolysis scanning (LPS), the TLC plates are placed in a chamber after development, and were irradiated with an IR laser to produce a high temperature at the location of the spot. The analyte is swept by a carrier gas to a GC, and detected with FID or ECD. The technique combines the separation power of TLC and the detection modes of GC [846]. 

Many of the aforementioned techniques comprise experimentally important derivatives based on alternative X-ray sources (SR-XRF), detection modes... 

Detection mode microcoulometric titration. c Detection mode UV fluorescence. 

Microcoulometric titration is used as the detection mode in some commercial sulfur-specific analysers. Sulfur in PP and waxes (range from 0.6 to 6 ppm S) were determined by means of an oxidative coulometric procedure [537]. The coulometric electrochemical array detector was used for determining a variety of synthetic phenolic antioxidants (PG, THBP, TBHQ, NDGA, BHA, OG, Ionox 100, BHT, DG) in food and oils [538],... 

Mass spectrometry can be specific in certain cases, and would even allow on-line QA in the isotope dilution mode. MS of molecular ions is seldom used in speciation analysis. API-MS allows compound-specific information to be obtained. APCI-MS offers the unique possibility of having an element- and compound-specific detector. A drawback is the limited sensitivity of APCI-MS in the element-specific detection mode. This can be overcome by use of on-line sample enrichment, e.g. SPE-HPLC-MS. The capabilities of ESI-MS for metal speciation have been critically assessed [546], Use of ESI-MS in metal speciation is growing. Houk [547] has emphasised that neither ICP-MS (elemental information) nor ESI-MS (molecular information) alone are adequate for identification of unknown elemental species at trace levels in complex mixtures. Consequently, a plea was made for simultaneous use of these two types of ion source on the same liquid chromatographic effluent. 

Molecular weights were measured by gel permeation chromatography on a Perkin-Elmer Series 10 Liquid Chromatograph using tetrahydro-furan as solvent and refractive index as the detection mode. Standards were polystyrene, and reported molecular weights for the poly-siloxanes do not include a correction. 

The surface analysis for morphology and average particle size was carried out with JEOL JSM 6301 F scanning electron microscope (SEM). The micrographs of the samples were observed at different magnifications under different detection modes (secondary or back-scattered electrons). 

Some commercially available detectors have a number of detection modes built into a single unit. Fig. 2.4o is a diagram of the detector used in the Perkin Elmer 3D system, which combines uv absorption, fluorescence and conductivity detection. The uv function is a fixed wavelength (254 nm) detector, and the fluorescence function can monitor emission above 280 nm, based on excitation at 254 nm. The metal inlet and outlet tubes act as the electrodes in the conductance cell. The detection modes can be operated independently or simultaneously, using a multichannel recorder. In the conductivity mode, using NaCl, a linear range of 103 and a noise equivalent concentration of 5 x 10 8 g cm-3 have been obtained. 

Taxon Host Detection Mode Chemical Name(s) Ref. 

The detection modes commonly applied in enzyme-based optical fiber sensors are based on one of the following principles ... 

Optical fiber sensors that use enzymes can operate in the direct or indirect detection mode. In the first case, the optical properties of the reactives, intermediates or products of the biocatalyzed reaction can be monitored using the optical fibers. In the second type, an optochemical transducer generates the optical changes. 

In this introductory chapter, the basic principles of CL will be presented, with a brief introduction to the essential instrumentation as well as some general aspects showing that this technique is suitable as a detection mode for analytical purposes. Details on each specific topic can be found later in this book. 

Other detection modes in bright CL or BL reactions are multichannel detectors, which provide simultaneous detection of the dispersed radiation and produce a permanent image of a wide area. Photographic films or plates are emulsions that contain silver halide crystals in which incident photons produce stable clusters of silver atoms within the crystals. Internal amplification is provided in the development process by an electron donor that reduces the remaining silver ions to silver atoms within the exposed crystals. A complexing agent is used to remove the... 

In the past, general chapters and reviews have been published, related to the characteristics of CL as analytical technique [7-9], mainly in the liquid phase [10-14], and its use as detection mode in flowing streams and immunoassay [15-17]. Two extensive reviews reported on the specific application of CL reactions according to the nature of the analyte (inorganic species, enzymes and nucleotides, acids and amines, carbohydrates, steroids, polycyclic aromatic compounds, and drugs) and covering the literature from 1983 to 1991 [18] and from 1991 to mid-1995 [19]. 

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