Procedural variables

Each step ia the method should be iavestigated to determine the extent to which environmental, matrix, material, or procedural variables, from time of collection of material until the time of analysis and including the time of analysis, may affect the estimation of analyte ia the matrix. VariabiUty of the matrix owiag to its physiological nature should be considered. 

Regioselectivity in Friedel-Crafts acylations can be quite sensitive to the reaction solvent and other procedural variables.51 In general, para attack predominates for... 

Earlier in this chapter, it was pointed out that prior treatment and procedural variables can affect the kinetics of reactions of a solid substance. Although a large number of studies have been conducted to evaluate these factors, two such studies will be summarized here. 

Garst, J.E., Wilson, W.C. (1984) Accurate, wide range, automated, high performance liquid chromatographic method for the estimation of octanol/water partition coefficients. II Effects of chromatographic method and procedure variables on accuracy and reproducibility of the method. J. Pharm. Sci. 73(11), 1616-1623. 

Regioselectivity in Friedel-Crafts acylations can be quite sensitive to the reaction solvent and other procedural variables.45 In general, para attack predominates for alkylbenzenes.46 The percentage of ortho attack increases with the electrophilicity of the acylium ion, and as much as 50% ortho product is observed with the formylium and... 

With the above procedure, variables such as reaction temperature, mole ratio of catalyst to dihydroanthracene, reaction solvent, and oxygen concentration were examined. 

The following principles should be used to establish a valid analytical method A specific detailed description and protocol should be written (standard operating procedure (SOP)). Each step in the method should be investigated to determine the extent to which environmental, matrix, material, or procedural variables, from time of collection of material until the time of analysis and including the time of analysis, may affect the estimation of analy te in the matrix. A method should be validated for its intended use with an acceptable protocol. Wherever possible, tire same matrix should be used for validation purposes. The concentration range over which the analyte will be determined must be defined in the method, on the basis of actual standard samples over the range (standard curve). It is necessary to use a sufficient number of standards to adequately define the relationship between concentration and response. Determination of accuracy and precision should he made by analysis of replicate sets of analyte samples of known concentration from equivalent matrix. 

Overall, key takeaways from the nonpoly meric paclitaxel delivery studies were that despite their improvement of angiographic parameters, paclitaxel-eluting stents without a polymer carrier did not demonstrate a positive effect on clinical outcomes, as seen with polymer-based paclitaxel elution (65), discussed in the next section. Potential reasons for the failure of such an approach could be loss of drug to the systemic circulation prior to reaching the target site during the stent deployment procedure, variability associated with the dose delivered to the lesion, and lack of control over drug-release kinetics due to the absence of a polymer carrier. 

In this same vein, the synthesis of 25m and 2621) has been attempted by initial formation of the cyclic sulfides, then extrusion of sulfur by irradiation in the presence of triethylphosphite. In both cases the limited sample size precluded formation of the desired cyclophanes (25 and 26) by this photolytic procedure. Variable temperature H NMR studies have been performed on 27 and 28 27 has a barrier to conformational inversion of 12.5 kcal/mole, while 28 has a corresponding barrier of 10.4 kcal/mole. Phane 28 exhibits a shift of the aromatic protons to higher field as compared to 29 the most dramatic shift is observed for H-3, which absorbs at 7.72 ppm for 28 and at 8.37 ppm for 29. This shift has been explained in terms of an approximately syn orientation of the pyridine rings in 28. 

Experimental determination of the optimum values for these instrumental and procedural variables makes ETAA a tedious and unattractive method when compared with faster multielement procedures. However, once the procedures and optimum conditions have been determined, samples can be analyzed in about 10 min per element per sample. The procedures and instrumental conditions reported in this chapter permit the... 

Designing an Experiment How would you design an experiment to evaluate the effectiveness of a new and improved chemical fertilizer on bean plants Be sure to describe your hypothesis, procedure, variables, and control. 

After initial tests, the next step is to optimize steps of the analytical process, including nucleic acid extraction, amplification, detection, and quantification. For amplification procedures, variables to be optimized may include buffer pH, nucleotide concentration, MgCb concentration, type of polymerase, primer concentration, and annealing temperature. 

For rate processes in which the Arrhenius parameters are independent of reaction conditions, it may be possible to interpret the magnitudes of A and ii, to provide insights into the chemical step that controls the reaction rates. However, for a number of reversible dissociations (such as CaCOj, Ca(OH)2, LijSO Hp, etc.) compensation behaviour has been foimd in the pattern of kinetic data measured for the same reaction proceeding under different experimental conditions. These observations have been ascribed to the influence of procedural variables such as sample masses, pressure, particle sizes, etc., that affect the ease of heat transfer in the sample and the release of volatile products. The various measured values of A and cannot then be associated with a particular rate controlling step. Galwey and Brown [52] point out that few studies have been specifically directed towards studying compensation phenomena. However, many instances of compensation behaviour have been recognized as empirical correlations applicable to kinetic data... 

Recognizing this sensitivity of reaction rates to prevailing conditions, several studies have reported systematic measurements of the influences of procedural variables on kinetic parameters. Wilburn et al. [62] used TG and DTA (1 and 7 K min ) data to measure CaCOj decomposition rates and peak temperahues. DTA and DTG curves were shown to depend on sample mass, heating rate and the partial pressure of COj. (A generally similar pattern of behaviour was reported for the... 

Values of reported by Kubas et a/. [31], 140 to 175 kJ mol, were comparable with those cited above, but (as with calcite) in the presence of CO2, E was increased to 234 kJ mol. Addition of ZnO or NiO to MgCOj decreased the value of E slightly. The much greater decrease (to ii, = 54 kJ mol ) which resulted from the addition of CaO was ascribed to the formation of CaCOj as an intermediate. Doi and Kato [32] identified MgO.MgCOj as an intermediate in the decomposition of magnesite in N2 or CO2. The effects of procedural variables on the non-isothermal decompositions of magnesite and of dolomite have been reported by Sharp et al. [12]. 

The value of a study increases considerably when it reports the influences on the rate processes of changing the experimental conditions (also called procedural variables). Further value is added by attempts to relate these results to processes occurring at the molecular level, i.e. to formulate a reaction mechanism. 

Studies of the influences of procedural variables (sample mass, partial pressure of COj, heating rate, etc.) on the kinetics of calcite powder dissociation continue to be published [49-51]. Such kinetic observations are the sum of different reaction rates... 

Simple salt reactants (131 entries). Articles concerned with decompositions of simple salts were often concerned with kinetic characteristics, many used non-isothermal data, and stoichiometric information was provided for some of these chemical changes. Several of these studies were concerned with determining trends of behaviour through comparisons between related salts. Detailed descriptions of the chemical steps and identifications of the rate-controlling processes in the mechanisms were less frequently provided. A small proportion of the papers was concerned with previously well-studied reactions such as the dissociations of carbonates (13 entries), including the effects of procedural variables on the decompositions of CaCOj (4 entries) and of dolomite (5 entries). 

Kinetic parameters. The hterature contains numerous reports of the rate equations identified for particular crystolysis reactions, together with the calculated Arrhenius parameters. However, reproducible values of (Section 4.1.) have been reported by independent researchers for relatively few solid state decompositions. Reversible reactions often yield Arrhenius parameters that are sensitive to reaction conditions and can show compensation effects (Section 4.9.4.). Often the influences of procedural variables have not been carefully identified. Thus, before the magnitudes of apparent activation energies can be compared, attempts have to be made to relate these values to particular reaction steps. 

It is important to emphasize that the situations described above are different synthesis strategies that expectedly lead to composite and porous materials that have distinctly different properties. In the preparation of mesoporous materials, procedural variables define a very complex system in which kinetic parameters (time, basic operations sequence) may play a determinant role. 

Species Study Design/procedure/variable Dependent variable Main findings o 3... 

Up to now several methods have been used to prepare iron molybdates, the most part of them based on coprecipitation techniques. Previous studies [11] have evidenced that the catalytic behavior of Mo-Fe oxides depends on many variables of the coprecipitation procedure starting compounds, concentration of parent solutions, pH and temperature of coprecipitation step, order of addition of parent solutions, ripening etc. In a typical preparation procedure iron molybdate is coprecipitated from solutions of ferric chloride or ferric nitrate and ammonium molybdate [8]. The control of all the above mentioned procedure variables, strongly difficult the preparation of these catalysts and deviations from the preparation recipe can have very adverse effects on the performances of the catalyst from the standpoint of activity, selectivity and stability. 

Reaction kinetics, particularly for many solid-state endothermic and reversible rate processes, are sensitive to reaction conditions [the procedural variables (1)]. Because of the participation of secondary controls, the reaction rate measured is not necessarily that of the limiting chemical (or rate-determining) step. 

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