Homogeneity determination methods

After sampling, homogenization, extraction, cleanup, concentration and possible derivatization, use a suitable determination method which provides sufficient selectivity or specificity and sufficient sensitivity. 

The homogeneous catalysis method is suitable to measure rate constants over a very wide range, up to the diffusion limit. The lower limit is determined by interferences, such as convection, which occur at very slow scan rates. It is our experience that, unless special precautions are taken, scan rates below lOOmV/s result in significant deviations from a purely diffusion-controlled voltammetric wave. For small values of rate constants (down to 10 s ), other potentiostatic techniques are best suited, such as chronoamperometry at a rotating disk electrode UV dip probe and stopped-flow UV-vis techniques. ... 

G. T. Carter, R.E. Schiesswohl, H. Burke, and R. Young, Peak homogeneity determination for the validation of high-performance liquid chromatography assay methods, J. Pharm. Sci., 77 317 (1982). 

In 1996, Nemet and Kozma showed the emission spectrometry of gold laser-produced plasma to be of interest for analytical purposes a delay time of 800-1000 ns was found to ensure nni/-thermal equilibrium and thorough atomization in the plasma. The line profiles obtained under such conditions (both resonant and Stark-broadened) were fitted to a symmetric Lorentzian curve [170]. Recently, LIBS was used in combination with effective chemometric tools to develop a determination method for gold in homogeneous samples that allows the characterization of jewellery products. The results confirmed the LIBS technique as an effective alternative to the hallmark official methods [143,144,171]. 

The use of NIR for homogeneity determination requires no calibration. The method could be applied to any mixture without the extensive preliminary work of building a quantitative calibration as required for most near-IR work. On the other hand, homogeneity determinations could be carried out by using either quantitative or qualitative calibrations. Quantitative calibrations involve prediction of the drug level at various locations or times with an appropriate calibration using spectra collected in one of the methods named previously. The... 

The Lowry method was used to determine the protein concentration in brain and tumour cell homogenates. The method was found to be reproducible for both preparations and for different dilutions of the preparations. The... 

Mass spectrometry is one of the most powerful multielemental determination methods, allowing the estimation of non-pollution levels in a multielement run. In field-desorption MS, 10 pg Tl could be detected in 2, L of non-ashed sample solution (homogenized rat brain) by means of an isotope dilution technique (Schulten et al., 1978), offering possibilities of microlocal trace analysis. Ionization of CHCI3 extracts from plant cytosols with Ar" (secondary ion MS) enabled the detection of dimethylthallium-t- directly in the mass spectrum (Gunther and Umland, 1989). 

These books remain the reference works in measuring dielectric and magnetic constants of homogeneous materials. Methods used at that time were limited in frequency band measurements, permitting only the determination of the complex permittivity and permeability at fixed frequencies and sometimes versus temperature variations. 

Perhaps the most often overlooked aspect of the application of EIA methods to pesticide analysis of foods is the size of the analytical portion taken for analysis. As mentioned above, about 20 pounds of food is usually collected. After a compositing step to assure homogeneity, traditional methods typically require analysis of 100 g of this mixture to obtain a representative portion and to provide the quantitation limits needed for regulatory monitoring. Analytical portion size is being studied to determine the minimum quantity that is statistically representative of the composite (11). Preliminary indications are that the analytical portion must be at least 10 g. 

Separation using UF and MF membranes is most selective, however, if soluble reagents are added. Such techniques may supplement two-phase distribution methods (e.g., liquid-liquid extraction, sorption, and precipitation), which are frequently used to extract species from dissolved matrices, industrial fluids, or natural waters. Although many such methods have been developed and successfully used, their application is sometimes troublesome. Some problems are caused by heterogeneous reactions and transfer between phases. Other problems can arise from the composition of the solution finally obtained, which is analyzed using the final determination method. In such cases, additional procedures may be required, e.g., back-extraction or desorption, which make the analytical procedure more complex and can cause additional contamination. Membrane separation can yield a homogeneous aqueous phase suitable for subsequent analysis using a number of methods. 

After collecting spectral data at different locations in the blender and at various hme points, Sanchez et al. [7] evaluated several qualita-hve blend endpoint methods. A mean standard deviation for the spectra collected at each hme point was compared over time to determine homogeneity. This method allowed authors to determine the character-ishcs of the mixture spectrum, corresponding to the desired blend state. Using that mixture spectrum, they defined a dissimilarity index based on the orthogonal projection of spectra collected at various hmes points onto the mixture spectrum. The norm of the resulting vector was then employed to evaluate the blend state. 

CO2) isochores = straight lines, is presented on Fig. 5 as the "two homogenization points method". The isochore is determined by two points A and B A, at low temperature, i.e. within the miscibility gap of the system, corresponds to Tji CO2 (Fig. k.l) (the contribution to the pressure of HpO liquid (vapour pressure near room temperature can be neglected). The temperature of total homogenization defines B as the intersection of Th and the "critical" curve for a given Xc02 ([Pg.217]

Distillation is by far the most commonly used method for the separation of homogeneous fluid mixtures. The cost of distillation varies with operating pressure, which, in turn, is mainly determined by the molecular weight of the materials being separated. Its widespread use can be attributed to its ability to... 

The concentration of insulin in a production vat is determined by the method of isotope dilution. A 1.00-mg sample of insulin labeled with with an activity of 549 cpm, is added to a 10.0-mL sample taken from the production vat. After homogenizing the sample, a portion of the insulin is separated and purified, yielding 18.3 mg of pure insulin. The activity for the isolated insulin is measured at 148 cpm. How many milligrams of insulin are in the original sample ... 

The importance of between-laboratory variability on the results of an analytical method can be determined by having several laboratories analyze the same sample. In one such study seven laboratories analyzed a sample of homogenized milk for a selected alfatoxin. The results, in parts per billion, are summarized in the following table. 

It is extremely difficult to know values for all of these parameters precisely. Therefore, absolute quantitation is almost never attempted. The determination of relative atomic ratios is an inherently more tractable approach, however. This method is best illustrated by consideration of a binary material composed exclusively of atoms A and B that is perfectiy homogeneous up to the surface. In this case, independent equations can be developed relating the number of atoms sampled to the xps intensity for each atom as follows ... 

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