Harry L. Pardue

Kinetic aspects of analytical chemistry

Abstract A broad view of the role of kinetics in analytical chemistry is presented. Two principal themes are that the role of kinetic methods is much greater than is generally acknowledged and that the most commonly used kinetic approaches make inadequate use of information that is available. After a brief discussion of historical developments that date back to the latter part of the 1800s, primary attention is focused on a classification of measurement and data-processing approaches that have been developed. Approaches are grouped into methods for single components without and with error compensation and methods for resolving two or more components in mixtures. They are further grouped into direct-computation and curve-fitting approaches and into integral and derivative methods within these more general categories. Correlations are then drawn between these approaches as applied to chemical processes and to physicochemical and purely physical processes such as luminescence, electrode responses, and flow systems, including chromatographic processes. It is argued that all analytical methods are either kinetic or equilibrium in character and that kinetic-based methods represent a much larger fraction of the total than is generally recognized.

Kinetic treatment of unsegmented flow systems : Part 1. Subjective and semiquantitative evaluations of flow-injection systems with gradient chamber

Abstract A variable-time kinetic model is used to evaluate a single-channel flow-injection system with gradient chamber that has been identified as a continuous-flow titration. A physical model, mathematical equations, computed concentration vs. time profiles, experimental data, and formal definitions are used to identify qualitative and quantitative features of the method that have not been apparent from the titration model for the system. It is shown that determinations can be performed with and without reactant in the flow stream and when reactant is in the flow stream, with and without reactant in the gradient chamber when the sample is introduced. It is shown that lowest concentrations with shortest cycle times can be achieved when determinations are performed without reagent in the gradient chamber initially. Characteristics unique to each of three different data processing options are used to evaluate the validity of equations presented. It is suggested that some methods previously identified as continuous-flow titrations are most accurately identified as variable-time kinetic methods, and it is shown that this semantic differentiation can provide improved insight into the methods and can expand the scope of the methods by suggesting new experimental approaches with potential advantages relative to previously described procedures.

Kinetic treatment of unsegmented flow systems : Part 2. Detailed Treatment of Flow-Injection Systems with Gradient Chamber

Abstract A kinetic model is utilized for a detailed mathematical treatment and experimental evaluation of single-channel and dual-channel flow injection systems that include a gradient chamber. The kinetic model includes three distinct stages in the process, namely clearing reactant from the gradient chamber by first portions of sample, continued entry of sample into the gradient chamber, and decrease of determinant concentration in the gradient chamber via dilution and reaction with reagent. Equations predict entirely different behavioral patterns for different conditions and these predictions are verified experimentally for a wide range of conditions. The data show that the variable-time kinetic model is superior to the titration models previously utilized to describe these flow systems. The principal limitation of the kinetic equations involves an assumption of plug flow that is not completely valid. The extent of deviations from ideal behavior depend on conditions, but are negligible at low concentrations (0–50 mmol l -1 ) and can be 10% or larger at higher concentrations (100 mmol l -1 ). While equations are not exact, they are useful in predicting performance characteristics for a variety of conditions and experimental approaches.

Effects of wavelength range on the simultaneous quantitation of polynuclear aromatic hydrocarbons with absorption spectra

Abstract The study described was designed to evaluate the use of carefully selected wavelength ranges to improve the accuracy of results when multiwavelength absorption data are used to quantity components in mixtures. Mixtures of polynuclear aromatic hydrocarbons were used as test samples. It is shown that in some cases narrow wavelength ranges can yield substantially improved results relative to broad wavelength ranges. Such improvement is most likely when the component of interest is present in low concentration relative toother components with similar absorptivities and when the component of interest has fine structure in a limited region of the spectrum. Results for components with fine structure throughout broad regions of the spectrum are not likely to be improved by selection of a narrow range that emphasizes just one of the regions of fine structure. However, careful selection of such a region seldom degrades results.

Algorithm for error-compensated kinetic determinations without prior knowledge of reaction order or rate constant

The development and evaluation of a new algorithm for error-compensating predictive kinetic determinations are described. With the new algorithm it is possible to fit kinetic data without prior knowledge of the rate constant, reaction order or initial and final values of the detector signal. A curve-fitting method is used to obtain values of these parameters that give the best fit of the model to the data. Although intended to be used primarily to compute signal changes between t=0 and ∞, the method can also be used to determine reaction orders. Although the algorithm fails for reaction orders of zero and unity, it works well for orders between these values and orders greater than unity. Simulated data are used to evaluate the effects of reaction order, signal noise and data range on computed values of signal change and, to a lesser extent, reaction order.

An all-electronic system for the automatic measurement of slopes of reaction-rate curves

Abstract A new method is described for the automatic measurement of reaction rates. Slopes of response curves are matched with output slopes from an electronic integrator. When the net output is zero, the input to the integrator is proportional to the slope of the response curve and is read directly on a meter near zero reaction time. All the necessary circuitry is wired into a blank plug-on chassis for commercially available equipment. The method is evaluated for the determination of cystine in a reaction where the slope of the response curve is proportional to the cystine concentration. Cystine is determined over the range of 1.0–24 p.p.m. with a relative standard deviation of less than 1%. Automatic direct readout of p.p.m. cystine is obtained within 15 sec after mixing sample and reagents.

Kinetic treatment of unsegmented flow systems : Part 3. Flow-injection system with gradient chamber evaluated with a linearly responding detector

Abstract A variable-time kinetic model is evaluated for a flow-injection sample-processing system that includes a gradient chamber. Quantification of triiodide, including reaction with thiosulfate, is used as a model system. A thin-layer electrochemical detector consisting of a platinum working electrode and glassy carbon counter electrode with a 200-mV difference imposed between them yields linear current response for triiodide concentrations between 0.1 and 1.2 mM. Equations based on the kinetic model are evaluated for situations in which neither the flow stream nor gradient chamber contain reactant (thiosulfate) initially, only the flow stream containes reactant, and both the flow stream and gradient chamber contain reactant. Both calibration data and response curves exhibit very good agreement between theory and experiment except for the situation in which only the flow stream contains reactant. In this case, the theoretical treatment accurately predicts the general nature of responses but predicts slightly longer time intervals for completion of the process than are observed experimentally.

Kinetic treatment of unsegmented flow systems

Abstract A variable-time kinetic model is used to derive a comprehensive set of equations for an unsegmented flow system with a gradient chamber for the situations in which the analyte, reactant or product is monitored. Equations are presented for single-channel systems with excess analyte and excess reactant. These response equations are used to develop relationships that permit one to calculate whether analyte or reactant is in excess, to understand the merits and limitations of dispersion coefficients and to relate measured time intervals to initial analyte concentration for both single- and dual-channel systems. Relationships between time intervals ( Δt ) and the logarithm of initial analyte concentration ( C ) are inherently non-linear and include terms for the reference-point concentrations between which time intervals are measured. Special cases for which Δt vs. In C relationships are linear and do not include the reference-point concentrations are discussed and the assumptions that lead to these simplified equations are identified. Computed response curves are included to illustrate behavior in different phases of the processes.

Regression data processing methods for parallel zero- and first-order processes applied to lactate dehydrogenase subunit determinations

Abstract Multipoint data-processing methods developed for parallel zero-order and first-order processes have been evaluated for the simultaneous determination of the heart and muscle subunits of lactate dehydrogenase. Conditions are adjusted such that pyruvate inhibition of heart and muscle subunits of the enzyme follows pseudo-first-order kinetics with apparent rate constants of k H ⋍ 0.7 s -1 and k M ⋍ 1.4 s -1 for the heart and muscle subunits, respectively. The influence of these first-order inhibition processes on the zero-order catalytic reaction is used to determine the two subunits in single- and two-component samples. Stopped-flow mixing is used and 250 data points collected during the early part of the reaction are fitted to four different mathematical models for the parallel process in order to determine initial velocity components resulting from each subunit present, and these velocity components are related to enzyme concentration. Enzymes in synthetic samples are determined in the 0–80 nmol l -1 range with uncertainties of about 1 nmol l -1 for single- and two-component samples prepared from purified preparations for the enzymes.

Systematic comparison of data-processing options for kinetic-based single-component determinations of non-catalysts: Part 1. Review, systematic classification, mathematical descriptions, performance characteristics and perspectives

Abstract This paper addresses several aspects of the applications of kinetic-based methods for single-component determinations of non-catalysts. One goal is to review many of the different approaches to these kinetic-based determinations. A second goal is to organize many of these different approaches into a systematic classification scheme. A third goal is to present mathematical descriptions that help to differentiate among the different approaches. A fourth goal is to differentiate among important features of the different approaches with emphasis on the ruggedness of the different options. In this context, ruggedness is defined as the ability of a method to resist changes in experimental variables. It is concluded that available options can be grouped into one or more of four general categories in this regard, namely those with no ability to resist changes in experimental variables, those that resist such changes via temporal optimization, those that can be used in modes that compensate for changing variables extrinsically and those that have built-in (intrinsic) variable compensation. A fifth goal is to place the many different options to kinetic-based determinations into a somewhat broader and more consistent perspective than may have been done previously. In particular, an aim is to identify similarities among measurement and data-processing options applied to transient (kinetic) aspects of chemical, physico-chemical and physical processes.