Pre-concentration step

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

Improved sensitivities can be attained by the use of longer collection times, more efficient mass transport or pulsed wavefomis to eliminate charging currents from the small faradic currents. Major problems with these methods are the toxicity of mercury, which makes the analysis less attractive from an eiivironmental point of view, and surface fouling, which coimnonly occurs during the analysis of a complex solution matrix. Several methods have been reported for the improvement of the pre-concentration step [17,18]. The latter is, in fact. 

The direct mode is used when the concentration of the SEE extract is enough for direct analysis in the CE instrument without the need for a pre-concentration step. In this case, the sample concentrator is by-passed and the SEE extract goes directly to the CE instrument. The extract is collected in a CE vial containing an appropriate solvent and is thus ready for the CE analysis (Eigure 6.12). 

Surfactants are produced on very large or medium technical scales. Their analysis by manufacturers in products and their formulations sometimes may be complicated because of the great variety of surfactants [5]. After use as directed in aqueous systems they were discharged mainly with wastewaters. Their analysis in environmental samples then becomes quite difficult because analysis must be performed at trace concentrations with limited sample amounts after essential matrix-dependent pre-concentrations steps. In addition, homologues and isomers that exist for many surfactants, besides metabolites which are generated in biochemical processes, complicate their specific determination [6]. 

In a further application of MI-SPE, theophylline could be separated from the structurally related caffeine by combining the specific extraction with pulsed elution, resulting in sharp baseline-separated peaks, which on the other hand was not possible when a theophylline imprinted polymer was used as stationary phase for HPLC. A detection limit of 120 ng mb1 was obtained, corresponding to a mass detection limit of only 2.4 ng [45]. This combination of techniques was also used for the determination of nicotine in tobacco. Nicotine is the main alkaloid in tobacco and is the focus of intensive HPLC or GC analyses due to its health risk to active and passive consumers. However, HPLC- and GC-techniques are time-consuming as well as expensive, due to the necessary pre-purification steps required because the sample matrices typically contain many other organic compounds besides nicotine. However, a simple pre-concentration step based on MI-SPE did allow faster determination of nicotine in tobacco samples. Mullett et al. obtained a detection limit of 1.8 jig ml 1 and a mass detection limit of 8.45 ng [95]. All these examples demonstrate the high potential of MI-SPE to become a broadly applicable sample pre-purification tool. 

There is no need for a pre-concentration step at the head of the secondary column. 

Becanse of the complexity of the environmental matrices and becanse of the general trace concentration levels of micropollutants, a pre-concentration step is necessary to isolate the target compounds from the matrix and to achieve the LOD required. 

Wang and Freiha have designed a carbon-paste electrode, based on a piston principle that enables an outer layer of paste to be extruded and removed, and used this in investigating possible extractive accumulation and further characteristics of the pre-concentration step in voltammetric analysis [163]. Chlorpromazine was used as a model compound. 

Standardized ecotoxicity tests (bioassays) have been developed and optimized over the last few years and encompass the effects on bacteria, daphnia and fish (DIN 38 412, parts 30, 31 and 34). These tests are designed to assess the toxicity on aquatic organisms. They are quick to perform, easy to handle and comparatively inexpensive, with the goal of allowing the toxicity of a complex water matrix to be estimated. However, they use pre-concentration steps so that it is possible that not all byproducts are recovered (which itself is hard to prove). 

A number of analytical methods were developed for determination of elemental mercury. The methods are reviewed in Refs. [1-4]. They include traditional analytical techniques, such as atomic adsorption spectroscopy (AAS), atomic fluorescence spectroscopy (AFS), and atomic emission spectroscopy (AES). The AAS is based on measurements of optical adsorption at 253.7 or 184.9 nm. Typical value of the detection limit without pre-concentration step is over 1 pg/l. The AEF is much more sensitive and allows one to detect less than 0.1ng/l of mercury... 

Figure 6.12 Schematic diagram of the interface used for direct SFE-CEST coupling without a sample pre-concentration step 1, micro-LC pump 2, heated restrictor 3, six-port valve 4, direct by-pass to the CE unit 5, three-port valve 6, CE instrument, (from ref. 58).

Up to 30 elements can be determined by voltammetric procedures, but when the metals of interest are present at 10-9 to 10 12 mol dm-3 levels, the introduction of a pre-concentration step becomes essential. The concentration process may disturb the distribution pattern of labile molecular species but the status of stable species should change little. Some pre-concentration procedures can be selective in respect to the species retained, and this can limit the value of total element content analyses (i.e. procedures need to be validated). 

The ICP-MS detector offers excellent selectivity and sensitivity for HPLC and HPLC-ICP-MS is a rapidly expanding technique. The high sensitivity of this method means that sample preparation steps are simplified and pre-concentration steps tend not to be necessary. Isotopic dilution measurement techniques can also be used to improve accuracy at low concentrations. Heumann et al. (1994), for example, could determine iodate concentrations in the range of 0.5-20 ngml-1 with RSDs of 2%. 

A sensitivity increase and lower detection limit can be achieved in various ways with the use of voltammetric detectors rather than amperometry at fixed potential or with slow sweep. The principle of some of these methods was already mentioned application of a pulse waveform (Chapter 10) and a.c. voltammetry (Chapter 11). There is, nevertheless, another possibility—the utilization of a pre-concentration step that accumulates the electroactive species on the electrode surface before its quantitative determination, a determination that can be carried out by control of applied current, of applied potential or at open circuit. These pre-concentration (or stripping) techniques24"26 have been used for cations and some anions and complexing neutral species, the detection limit being of the order of 10-10m. They are thus excellent techniques for the determination of chemical species at trace levels, and also for speciation studies. At these levels the purity of the water and of the... 

The use of pulse techniques for electroanalytical determinations has been much publicized, and is applicable to both solid electrodes and the HMDE/SMDE. The development in recent years of square wave voltammetry (SWV)39 widens the possibilities beause of its rapidity (Section 10.9) it is especially useful because the time necessary to do an experiment is only 2 s, which means that a SMDE drop in the dropping mode can also be used for micromolar determinations. Progress obtained with pulse techniques40,41 has meant that applications of a.c. voltammetry have been few, but there is no theoretical reason for this. The very low detection limits achieved in stripping voltammetry result not only from the pre-concentration step but also from the use of pulse waveforms in the determination step. 

The new process includes an upstream nanofiltration twofold pre-concentration step. Thereafter, the regular process including an UF concentration step to achieve the final dry matter value follows. In this case, however, the calcium losses into the serum are much lower as compared to the conventional process, because the final concentration step only has to remove a small amount of serum including the ionic calcium. The final UF step, that is, a partial calcium removal is still necessary to prevent the product to assume a bitter taste during shelf-life. 

Toxic trace elements were isolated from water samples by extraction with di-ethyldithiocarbamate (Table 2.1.2). Following this pre-concentration step the metal ions were adsorbed on a cation-exchange resin using a mixture of tetrahydro-furan-methylglycol-6 M HCl as sorption solution. The succesive elution was treated with 6 M HCl, 1 M HCl and 2 M HNO3 for fractional separation. In another application hexane-isopropanol-HCl mixture was used as the adsorption medium An analytical scheme which provides quantitative results, is described for ion-exchange separation of fifteen major, minor and trace elements in silicates For concentration and separation of copper, chromium, lead and iron an ion-exchanger in phosphate or OH -form was used in various combinations ... 

The aliquot of digested sample placed in the electrochemical vessel is purged for 15-20 minutes with N2, then it is rapidly transferred and screwed under the cell head of the electrochemical device where the Hg film electrode has already been prepared, tested and rinsed. The electrolytic pre-concentration step during the analytical measurement is carried out by constant potential electrolysis. The deposition potential is set at -0.95 V for Cd and Pb and at -0.85 V for Cu. The deposition time depends on the metal concentration, a time of 20 min normally being sufficient to determine Cd and Pb concentration in samples collected in... 

The most important Tropical Fruit Scenting Sulphur Volatiles (TFSV) in wine are 3-mercaptohexan-l-ol (3-MH), 3-mercaptohexanol acetate (3-MFLA) and 4-methyl-4-mercaptopentan-2-one (4-MPP). The main problem for quantification of TFSV is their possible very low concentration which may be only a few ng/L. Therefore, only the use of selective isolation and pre-concentration steps allows this difficulty to be overcome. Another problem occurring in thiols analysis is their possible reactivity towards oxidation induced by the working conditions (Rapp et al., 1985) and/or by some components of the matrix in analysis (Murat et al., 2003 Blanchard et al., 2004). 

Three methods are reported to quantify these TFSV-thiols analytes with separation and pre-concentration steps (Tominaga et al., 1998c Schneider et al., 2003 Mateo-Vivaracho et al., 2007). For all such approaches, the first step of the analysis is their recovery from... 

The sensitivity of any analytical technique can be greatly increased by introducing a preliminary pre-concentration step, eg solvent extraction. In stripping voltammetry an electrochemical preconcentration technique is used. The analyte is concentrated, from very dilute solutions, by electrolysis to an insoluble product which collects at the electrode and can be subsequently determined with a very high sensitivity. The method is applicable only to a limited number of important analytes. Stripping voltammetry requires the use of solid or stationary electrodes, (2.7). 

Stripping voltammetry is an important, but limited technique, which uses a pre-concentration step to enhance the sensitivity of the voltammetric analysis step. This pre-concentration step is electrochemical using the same instrumentation as the subsequent voltammetric analysis. The simplest polarographic equipment can be used to obtain the highest sensitivity which depends on the length of the pre-concentration step as well as on the sensitivity of the voltammetric stripping step. Since the pre-concentration step requires the production of an insoluble product which can be reproducibly stripped from the electrode surface in the determination step, the use of stripping voltammetry is limited to a few analytes, eg transition metal ions, halides and pseudo-halides. 

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