Additive interference

An analytical method vahdation study should include demonstration of the accuracy, precision, specificity, limits of detection and quantitation, linearity, range, and interferences. Additionally, peak resolution, peak tailing, and analyte recovery are important, especially in the case of chromatographic methods (37,38). 

With As(III) as the reductant and Ce(IV) as the oxidant in sulfuric acid, the situation is reversed. The oxidized indicator, ferriin, is reduced hardly at all by excess As(III) even in the presence of osmium tetroxide as a catalyst. If a drop of Ce(IV) solution is added, however, the red color of ferroin is rapidly developed, evidently because of an induced reaction. In hydrochloric acid the induced reaction does not occur, ferroin is oxidized by the first drop of Ce(IV), and so the titration fails. A small amount of chloride (for example, 0.1 Af hydrochloric acid in 0.5 Af sulfuric acid) does not interfere. Addition of excess Hg(II) perchlorate prevents the interference by complexation of the chloride, ... 

Atomic spectrometric tests in air/acetylene flames a number of ions (like phosphate, sulphate, aluminium) decrease the emission and absorption signals of barium (and all of the alkaline earth metals). Adding an equal volume of a 25% solution of lanthanum chloride heptahydrate LaCl3 7H2O eliminates these interferences. Addition of lanthanum is not required if the nitrous oxide/acetylene flame is used. 

Constructive Interference Addition of two or more waves that are in phase, leading to a larger overall wave. 

The first term in this relationship is the same for all volume elements at every instant. It therefore makes no overall contribution to scattering because it corresponds to perfect order and complete destructive interference. Additionally, the time-average of the second term is zero because equivalent positive and negative values of dAa are equally probable. On this basis Equation (3.123) can be modified to... 

The detection system (in its full scope) must determine the analytical gases individually and without interference. Additionally, for each analyte, either... 

If Aroclor identification is prevented by the presence of interferences, additional sample preparation is required. All composites having such interferences shall be analyzed as individual samples. Individual samples may be diluted prior to analysis, but it must be remembered that the detection limit of the analysis has been changed. Used oil samples shall not be diluted beyond 1 100 during initial screening analysis to meet the regulated level of interest (2 ug/mL). 

The interest in vesicles as models for cell biomembranes has led to much work on the interactions within and between lipid layers. The primary contributions to vesicle stability and curvature include those familiar to us already, the electrostatic interactions between charged head groups (Chapter V) and the van der Waals interaction between layers (Chapter VI). An additional force due to thermal fluctuations in membranes produces a steric repulsion between membranes known as the Helfrich or undulation interaction. This force has been quantified by Sackmann and co-workers using reflection interference contrast microscopy to monitor vesicles weakly adhering to a solid substrate [78]. Membrane fluctuation forces may influence the interactions between proteins embedded in them [79]. Finally, in balance with these forces, bending elasticity helps determine shape transitions [80], interactions between inclusions [81], aggregation of membrane junctions [82], and unbinding of pinched membranes [83]. Specific interactions between membrane embedded receptors add an additional complication to biomembrane behavior. These have been stud-... 

Due to the rather stringent requirements placed on the monochromator, a double or triple monocln-omator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm and the linewidths are only tens of cm it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monocln-omator to collect the spectrum. In fact, this is the approach taken with Fourier transfonn Raman spectrometers. 

Furthennore, the magnetic diffraction and the nuclear diffraction do not interfere with one another, and the nuclear and magnetic intensities simply add together, although in many cases the magnetic unit cell is larger than the nuclear unit cell, which produces additional diffraction peaks. 

As already discussed, transparent specimens are generally only weakly visible by their outlines and flat areas caimot be distinguished from the surroundings due to lack of contrast. In addition to the phase contrast teclmiques, light interference can be used to obtain contrast [8, 9]. 

Note Additional procedural information plus interferences and general remarks will be found in J. A. Dean, ed.. Analytical... 

Compensating for an interference in the sample s matrix is more difficult. If the identity and concentration of the interferent are known, then it can be added to the reagent blank. In most analyses, however, the identity or concentration of matrix interferents is not known, and their contribution to S stead, the signal from the interferent is included as an additional term... 

The following sources provide additional information on preparing samples for analysis, including the separation of analytes and interferents. 

Inclusions, occlusions, and surface adsorbates are called coprecipitates because they represent soluble species that are brought into solid form along with the desired precipitate. Another source of impurities occurs when other species in solution precipitate under the conditions of the analysis. Solution conditions necessary to minimize the solubility of a desired precipitate may lead to the formation of an additional precipitate that interferes in the analysis. For example, the precipitation of nickel dimethylgloxime requires a plT that is slightly basic. Under these conditions, however, any Fe + that might be present precipitates as Fe(01T)3. Finally, since most precipitants are not selective toward a single analyte, there is always a risk that the precipitant will react, sequentially, with more than one species. 

The formation of these additional precipitates can usually be minimized by carefully controlling solution conditions. Interferents forming precipitates that are less soluble than the analyte may be precipitated and removed by filtration, leaving the analyte behind in solution. Alternatively, either the analyte or the interferent can be masked using a suitable complexing agent, preventing its precipitation. 

When possible, a quantitative analysis is best conducted using external standards. Unfortunately, matrix interferences are a frequent problem, particularly when using electrothermal atomization. Eor this reason the method of standard additions is often used. One limitation to this method of standardization, however, is the requirement that there be a linear relationship between absorbance and concentration. 

Accuracy The accuracy of a fluorescence method is generally 1-5% when spectral and chemical interferences are insignificant. Accuracy is limited by the same types of problems affecting other spectroscopic methods. In addition, accuracy is affected by interferences influencing the fluorescent quantum yield. The accuracy of phosphorescence is somewhat greater than that for fluorescence. 

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