Interfering species

The response of the immobilized enzyme electrode can be made independent of the enzyme concentration by using a large excess of enzyme at the electrode surface. The electrode response is limited by the mass transport of the substrate. Using an excess of enzyme often results in longer electrode lifetimes, increased linear range, reduced susceptibiUty to pH, temperature, and interfering species (58,59). At low enzyme concentrations the electrode response is governed by the kinetics of the enzyme reaction. 

Corrosion inhibitors are substances which slow down or prevent corrosion when added to an environment in which a metal usually corrodes. Corrosion inhibitors are usually added to a system in small amounts either continuously or intermittently. The effectiveness of corrosion inhibitors is partiy dependent on the metals or alloys to be protected as well as the severity of the environment. For example, the main factors which must be considered before apphcation of a corrosion inhibitor to an aqueous system are the compatibility of the inhibitor and the metal(s), the salt concentration, the pH, the dissolved oxygen concentration, and the concentration of interfering species such as chlorides or metal cations. In addition, many inhibitors, most notably chromates, are toxic and environmental regulations limit use. Attention is now being given to the development of more environmentally compatible inhibitors (37). 

Polyethylene glycol [25322-68-3] various, from -200 to -35,000. May be contaminated with aldehydes and peroxides. Methods are available for removing interfering species. [Ray and Purathingal Anal Biochem 146 307 7955.]... 

Coulometry measures the amount of cunent flowing dirough a solution in an electrochemical oxidation or reduction reaction and is capable of measuring at ppm or even ppb levels of reactive gases. Thus a sample of ambient air is drawn through an electrolyte in a cell and the required amount of reactant is generated at the electrode. This technique tends to be non-specific, but selectivity can be enhanced by adjustment of pH and electrolyte composition, and by incorporation of filters to remove interfering species. 

Several characteristics of the metal beam have been studied in detail. It is well known that metal clusters and metal oxides are formed as a result of the ablation process. However, these potentially interfering species have been studied in detail130 and it has been concluded that they do not introduce any doubt as to the validity of the experimental results. Much more important than cluster or oxide formation are the atomic electronic state populations of the metal beams. For each metal reactant, these have been characterized using laser-induced fluorescence (LIF) excitation spectroscopy. For Y, only the two spin-orbit states of the ground electronic state (a Dz/2 and a D-3,/2) were observed.123... 

Only in exceptional cases is it possible to estimate all or part of the influences of interfering species because their amount is mostly unknown. In such cases, the sum of SAiXj is considered an additional term of deviation e, caused by the interferents ... 

Summation of the instrumental back- IUPAC [1995] Currie [1999] ground plus signals in the analyte (peak) region of interest due to interfering species . 

To date, electrochemical (amperometric) detection of NO is the only available technique sensitive enough to detect relevant concentrations of NO in real time and in vivo and suffers minimally from potential interfering species such as nitrite, nitrate, dopamine, ascorbate, and L-arginine. Also, because electrodes can be made on the micro- and nano-scale these techniques also have the benefit of being able to measure NO concentrations in living systems without any significant effects from electrode insertion. 

The greatest area of applications of this type of ECL has been in the analysis of pharmaceutical compounds with amine functionality. The reader is directed toward the previously mentioned review articles and Table 1 for further details [12, 14-16], Many methods have also been successfully applied to real samples in the form of body fluids or pharmaceutical preparations, although sample pretreatment such as deproteinization, centrifugation, and neutralization followed by a chromatographic step to remove interfering species is often required. Limits of detection are typically in the range 10-9—10 12 M. Figure 4 shows examples of some classes of pharmaceutical compounds that have been determined by Ru(bpy)32+ ECL. 

It is unfortunate that many analytical chemists are required to work in laboratories which are far from suitable for the type of tests that they are required to perform. This can ultimately influence the quality of the results they produce. There are a number of factors that may influence the quality of analytical work. One important factor is that when a sample is being analysed to detect, e.g. very small amounts of the analyte of interest, it is essential to avoid all other sources of the analyte and other potentially interfering species which might contaminate the sample and distort the result. 

Second, leucovorin is known to dehydrate under acidic conditions to form anhydroleucovorin, 5,10-methenyl H, F, which absorbs at 352-353 nm. In the absence of interfering species, leucovorin may be analyzed by acidification with 0.1N HC1 followed by uv measurement after 1.5-2.0 hours. Purity may be determined relative to a sample of known purity or relative to literature values e352 = 2.39-2.41 x 10 M 1 cm"1.21... 

Matrix interferences can be observed in a number of different forms. Components in the extract may interfere with excitation of electrons. Absorbance of light by unexpected metals or organic compounds, generally or specifically, would also cause interference. Another source of interference is the complex-ation of the analyte with extract components such that the metal of interest is protected from the heat source. Other interferences such as changes in the viscosity of the extract are also possible, although they are less common. Whenever soil samples are analyzed by atomic spectroscopic methods, it is essential to make sure that no interfering species are present in the soils. If they are, then steps must be taken to correct for these interferences. 

This type of interference normally takes place when the absorption of an interfering species either overlaps or lies veiy near to the analyte absorption, with the result that resolution by the monochromator almost becomes impossible, Hollow-cathode-source invariably give rise to extremely narrow emission-lines, hence interference caused due to overlap of atomic spectral lines is rather rare. 

Remedy The addition of an excess of strontium (Sr), or lanthanum (La), or thorium (Th) ion remarkably minimizes the interference of P043 ion in the determination of Mg, and Ca by replacing the analyte in the analyte in the compound formed with the respective interfering species. In short, these ions do combine preferentially with P043 ions. 

Electrokinetic injection is useful when the analyte is in the presence of interfering species (with different mobilities), qualitative applications, or when viscous buffers or gels are being used. It is usually not suitable for quantitative applications since the variability caused by conductivity, microenvironments, and matrix differences significantly reduces the reproducibility. Since sample depletion can be a significant issue, it is recommended that different samples are used when repeated injections are needed. 

For higher accuracy in the low polymer concentration range, two different methods were used. In the case of PAA, potentiometric titrations of solutions of PAA were performed with 0.01 N NaOH using a Brinkman model, Westbury, NY, automated titrator. Blank tests indicated no interfering species. Known amounts of PAA were used to prepare a calibration curve immediately after titration of the samples containing unknown amounts of polymer. The starting point of the titration was pH 4.0, and the end point was reached near pH 8. Total volumes of 75 or 100 cc were used for the titrations, and the ionic strength was controlled at 0.01 M NaCl. 

Improve selectivity, by removal of interfering species from the sample matrix... 

Continuous separation systems, optional elements such as dialysers, hquid-hquid extractors, sorption or ion-exchange columns and filters, that can be placed before the reaction coils to remove potentially interfering species. 

As with the classical example, ail calculations are performed without knowledge about the presence of the interfercnt ( int" in Equation 5-21). It is only by chance that the interferent is represented in the calibration design. In practice, many samples would be collected in order to increase the likelihood that the interfering species would be adequately represented in the design. 

The advantage of estimating a model with stepwise MLR rather than with the full-spectrum techniques (e.g., PLS and PCR, Section 5-3-2) is that the MLR model is simple. It does not add variables whose variability is described by previously entered variables or that are not linearly related to the anal te of interest (e.g., have large contributions from interfering species). With the full-spectrum techniques, all sources of variation are implicitly accounted for in the model. This is a more complicated way of dealing with the variation not related to the anahte of interest. 

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