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H L Pardue

Publications and source records attributed to H L Pardue.

At least 19 recordsLinked to original sources
PubMed1997

Improved ruggedness for membrane-based amperometric sensors using a pulsed amperometric method.

This paper introduces a new approach to the use of membrane-based amperometric sensors which is expected to improve the ruggedness of these sensors significantly relative to the steady-state method in common use. In this new method, the fixed-voltage source used with the conventional steady-state method is replaced by a pulsed-voltage source. Unlike the fixed-source approach, which yields steady-state currents corresponding to large differences between analyte concentrations inside and outside the isolating membrane, the pulsed-source approach permits measurement of currents corresponding to near-equilibrium conditions between solutions inside and outside the membrane. Because the measured currents correspond to near-equilibrium conditions, results are expected to be virtually independent of variables that affect rates of mass transport to and across the membrane. The new approach is evaluated using the "oxygen electrode" as a model system. Results obtained using the new method are compared with results obtained using the conventional steady-state option as well as a coulometric approach described recently. The reproducibility of the pulsed amperometric approach and the scatter of data about least-squares calibration lines are an order of magnitude or more better than for the conventional steady-state option. As expected, the pulsed amperometric method is 40-100-fold less dependent on changes in membrane thickness, stirring rate, and temperature than the conventional steady-state option.

Biosensing Techniques↗
PubMed1997

The inseparable triad: analytical sensitivity, measurement uncertainty, and quantitative resolution.

The formal definition of sensitivity associates the term with the change in the response of a system for a small change of the stimulus causing the response, i.e., the ratio of the response of a system to the stimulus causing it. One interpretation of sensitivity associates the rate of change of the response for a small change of the stimulus as the slope of a calibration plot of response vs stimulus. An alternative interpretation associates sensitivity with the smallest value of the stimulus that can be resolved with a given degree of confidence, i.e., the detection limit. Applications of the first usage to analytical chemistry date at least to the beginning of this century; applications of the second interpretation are of more recent origin. The accompanying paper argues in favor of the second interpretation on the basis that, among other things, the "slope" interpretation conflicts with the formal definition of sensitivity and is meaningless as a descriptor of the performance of a measuring system. In this paper I offer arguments to support my belief that the slope definition of sensitivity is consistent with both formal definitions and accepted usage in analytical chemistry and, more importantly, that it is an invaluable descriptor of one of the most important characteristics of any analytical method. I include information to support my belief that proper use of the slope definition yields much more information than is available in the "detection limit" interpretation.

Clinical Chemistry Tests↗
PubMed1996

Unified view of kinetic-based analytical methods with emphasis on ruggedness. A review.

All analytical determinations can be grouped into two general categories, namely equilibrium-based and transient-based methods. This is an important grouping because most conventional approaches to transient-based methods are much less rugged than their equilibrium-based counterparts. As a result it is necessary to control variables within much narrower tolerances for transient-based methods than equilibrium-based methods to obtain similar degrees of reliability. This paper reviews measurement and data-processing methods developed to reduce effects of variables on transient-based methods, with emphasis on a general approach that is applicable to a wide variety of methodologies. The approach emphasized is identified as a pseudo-equilibrium method. In this method, transient data are used to compute signals that would be measured if all processes that affect the measurement could be monitored to equilibrium. Results included show that the pseudo-equilibrium method is applicable to the three most common types of responses from the transient phases of chemical and physico-chemical processes. Data included show 10- to 100-fold improvements in ruggedness relative to conventional measurement and data-processing options.

Kinetics↗
PubMed1994

Predictive steady-state chromatography. 1. Algorithms for leading and trailing edges of resolved and unresolved peaks in liquid chromatography.

This paper describes mathematical models and curve-fitting procedures that permit steady-state saturation signals to be computed accurately from data along leading and trailing edges of liquid chromatograms. This new approach to quantitative chromatography is called predictive steady-state chromatography (PSSC). It is shown that the computed saturation signals are virtually the same when determined from data along leading and trailing edges and they vary linearly with analyte concentration. Most importantly, the computed saturation signals for a given analyte concentration are virtually independent of experimental variables such as sample volume and flow rate. For example, for sample volumes between 25 and 45 microL, the average computed saturation signal for a 0.025 mM solution of theophylline was 0.11 V with a standard deviation of 0.00097 V (RSD = 0.8%); similar results were found for other concentrations and for changes in flow rate. Dependencies of the PSSC method on sample volume and flow rate were compared with dependencies for peak-height and peak-area methods by using relative error coefficients. Dependencies on sample volume were 0.04%/microL for the PSSC method and 3 and 4%/microL for peak-height and peak-area methods, respectively. Dependencies on flow rate were 2%/mL/min for the PSSC method and 17 and 120%/mL/min for the peak-height and peak-area methods, respectively. Thus, the predictive steady-state method is 10-100-fold more rugged than peak-height and peak-area methods.

Algorithms↗
PubMed1994

Kinetic method for the quantitative resolution of structural isomers based on the catalytic properties of beta-cyclodextrin.

This paper describes a new approach for the quantitative resolution of mixtures of structural isomers. The method is based on the observation that rate constants for the cyclodextrin-catalyzed hydrolysis of selected structural isomers are significantly different. By using cure-fitting methods, it is possible to use these differences in rate constants to resolve kinetic responses for mixtures into the responses for the individual components. The new approach is evaluated for the ortho-, meta-, and para-isomers of nitrophenyl acetate. At pH 10, with beta-cyclodextrin as catalyst, ratios of rate constants for the three isomers differ by ratios of 1:6.7:1.6 in the order mentioned above. Results are reported for both two- and three-component mixtures. For two-component mixtures of the ortho- and para-isomers which have rate constants differing by only 1.6-fold, linear least-squares slope and intercept of determined vs prepared concentrations for the ortho-isomer were 1.00 +/- 0.02 and 2 +/- 2.2 mumol/L for three runs on each of five samples in the concentration range from 22 to 176 mumol/L. The pooled standard deviation for these 15 runs was 3.7 mumol/L, corresponding to a relative standard deviation of 3.7% for the average concentration. Similar results were obtained for other two- and three-component mixtures.

Catalysis↗
PubMed1994

Systematic top-down approach to clinical chemistry.

This paper introduces a systematic approach to organizing the discipline of clinical chemistry. The approach is called a top-down systems approach because it starts at the top with the most general concepts and works down through less general concepts to the most specific details and techniques. The hypothesis is that the discipline can be organized into hierarchical levels of functional processes and operational approaches to those processes. The functional processes represent what clinical scientists do; the operational approaches represent how they do it. Because functional processes change little, if at all, with time, they are used to develop a stable infrastructure or framework for the discipline. That infrastructure is then used to organize and understand operational approaches that tend to change rapidly with time in response to technological advances. This paper begins with the most general functional processes and then uses selected examples of the more general functions to illustrate lower hierarchical levels of functional processes and operational approaches.

Chemistry, Clinical↗
PubMed1994

International Federation of Clinical Chemistry (IFCC): systematic top-down approach to clinical chemistry.

This paper introduces a systematic approach to organizing the discipline of clinical chemistry. The approach is called a top-down, systems approach because it starts at the top with the most general concepts and works down through less general concepts to the most specific details and techniques. The hypothesis is that the discipline can be organized into hierarchical levels of functional processes and operational approaches to those processes. The functional processes represent what clinical scientists do; the operational approaches represent how they do it. Because functional processes change little, if at all, with time, they are use to develop a stable infrastructure or framework for the discipline. That infrastructure is then used to organize and understand operational approaches that tend to change rapidly with time in response to technological advances. This paper begins with the most general functional processes and then uses selected examples of the more general functions to illustrate lower hierarchical levels of functional processes and operational approaches.

Animals↗
PubMed1993

Analytical applications of catalytic properties of modified cyclodextrins.

This paper describes the evaluation of the catalytic properties of modified cyclodextrins for analytical applications. The beta-dimethylcyclodextrin was modified by adding one and two imidazolyl groups at carbon three positions. The modifications produced enhancements of catalytic activity for the hydrolysis of p-nitrophenyl acetate at neutral pH by factors of 1000 or more relative to the unmodified cyclodextrins. The catalytic properties of the monosubstituted cyclodextrin were evaluated for the quantification of p-nitrophenyl acetate in the concentration range of 10-90 mumol/L. Results obtained by equilibrium, initial-rate, and error-compensating predictive kinetic methods were compared. The equilibrium and predictive kinetic options yielded virtually identical results, with linear changes with concentration throughout the range studied and severalfold larger than the initial-rate option and dependencies on temperature, pH, and catalyst concentration that are 5-10-fold smaller than the initial-rate option.

Catalysis↗
PubMed1993

Initial studies of a new approach to the design and use of enzyme-based reactor/sensor systems: amperometric system for glucose.

This paper describes the development and evaluation of a new approach to the design and use of enzyme-based reactor/sensor systems (so-called "enzyme electrodes"). In the new approach, the reactor/sensor design is such that the measured response corresponds to reaction of all substrate in a fixed volume of solution. The result is that equilibrium-based measurements can be made, which in turn should result in advantages such as extended linear ranges and reduced dependencies on experimental variables such as enzyme activity, temperature, activators, inhibitors, etc. The concept was implemented with a glucose oxidase/electron-mediator reactor system immobilized on a glassy-carbon electrode operated in an amperometric mode. The reactor/sensor system was used in a thin-layer (14 microns) cell such that the mean diffusion time of substrate (glucose) across the cell was very short (< 1 s) and the rate-limiting process was the chemical reaction at the reactor surface. In this way, it was possible to quantify the electrical charge corresponding to reaction of all the substrate in a fixed volume of solution perpendicular to the plane of the reactor system. Because the determined charge is dependent only on the total amount of substrate in the fixed volume, results exhibit linear range up to at least 2-fold the Michaelis constant and reduced dependency on pH relative to results obtained with steady-state responses from the same experimental system. A mathematical treatment is presented which yields equations that are consistent with time-dependent responses for current and charge and which provide a rational basis for several data-processing options evaluated.

Biosensing Techniques↗
PubMed1993

Development and evaluation of an error-compensating predictive data-processing method for liquid chromatography.

This paper describes an alternative data-processing approach for liquid chromatographic responses. Transient data from the leading edges of chromatographic peaks for nonsaturating amounts of sample are used with a suitable mathematical model and curve-fitting program to predict the steady-state response that would be measured if sufficient sample were used to saturate the system. Results obtained by this approach are compared with peak-height and peak-area options by using aspirin as an analyte. For aspirin concentrations from 0.6 to 5.0mmol/L, each data-processing option yields linear calibration plots for each of several sample volumes from 50 to 100 microL and flow rates from 1.0 to 3.0 mL/min. As expected, the predictive option yielded lower dependency on sample volume (20-30-fold improvement) and flow rate (10-fold improvement) than the peak-height and peak-area options. However, the peak-height option provided slightly better (approximately 2-fold) calibration statistics.

Chromatography, Liquid↗
PubMed1993

Improved data-processing method for atomic absorption spectroscopy with electrothermal atomization.

A new approach is described for processing transient data from electrothermal atomizers used in atomic absorption spectroscopy. The transient responses are first integrated and then a pseudo-first-order model is fit to the time-dependent integrals in order to predict the response that would be measured if the atomization process were monitored to completion. The principal advantage expected and observed for the new approach is its ability to reduce effects of variables such as atomization temperature. For all elements studied (Cr, Mn, K, Yb, Fe), the new predictive approach is shown to be virtually independent of temperature in the range from 2200 to 2600 degrees C. The predictive approach exhibited lower temperature coefficients than either the peak-height or peak-area options for all elements examined. For the more volatile elements (Mn, K, Yb, Fe), the improvement ratio at 2400 degrees C of the predictive approach relative to the others ranged from 1.4 to 8.2. For chromium at 2400 degrees C, the temperature coefficient of the predictive method was approximately 10- and 30-fold lower than those for the peak-area and peak-height options, respectively.

Electrochemistry↗
PubMed1993

Kinetic study of the reaction of acetoacetate with glycine and sodium nitroprusside.

This paper describes an extensive kinetic study of the reactions involved in the determination of acetoacetate in body fluids. It is concluded that acetoacetate reacts with glycine to produce an imine intermediate that tautomerizes to an enamine. It is also concluded that nitroprusside reacts with the imine intermediate to produce an unstable product with an absorption maximum near 540 nm. This product decays slowly to produce a stable product with an absorption maximum near 393 nm. A proposed reaction pathway is used to develop kinetic equations, rate constants, equilibrium constants, and molar absorptivity of the unstable product that permit quantitative prediction of the kinetic behavior for a wide range of reactant concentrations.

Acetoacetates↗
PubMed1993

Error-compensating kinetic method for enzymatic determination of DNAs.

We describe the adaptation and evaluation of an error-compensating method for kinetic determinations of deoxyribonucleic acids (DNAs). The DNA is first reacted with ethidium bromide to produce a fluorescent intercalation complex. Subsequent treatment of the complex with DNase catalyzes hydrolysis of the DNA, causing a time-dependent decrease in fluorescence, which is monitored. A model for two-component parallel first-order processes is fit to the decay curve to predict the total change in fluorescence expected if the process were monitored to equilibrium. The predicted change in fluorescence response varies linearly with DNA concentration with an intercept corresponding to 0.13 mg/L DNA. Results by the predictive method are 47-, 58-, and 250-fold less dependent on DNase activity, temperature, and ethidium bromide concentration, respectively, than are results for an initial-rate method utilizing the same data. Moreover, the predictive method yields a significantly wider linear range than the initial-rate method, and is much less affected by blank fluorescence and RNA interference than is an equilibrium method based on the reaction of DNA with ethidium bromide alone.

Calibration↗
PubMed1992

Data-processing method to reduce error coefficients for membrane-based analytical systems. 1. Amperometric-based sensor evaluated for quantification of oxygen.

This paper describes the use of a predictive, curve-fitting method to reduce the effects of experimental variables on results obtained with membrane-based devices. Multipoint data from the transient regions of responses are used with suitable models and curve-fitting methods to predict the signal that would be measured for the system at equilibrium. The resulting equilibrium response usually is much less dependent on experimental variables than the transient responses used to predict it. The approach is evaluated for the membrane-based amperometric electrode for oxygen. Current vs time data are used to predict the equilibrium current expected when oxygen concentrations are the same on both sides of the membrane. Predicted equilibrium currents vary linearly with oxygen concentration. Relative to the more common steady-state method, the sensitivity of the predictive method is about 5-fold higher, the measurement time is about 17-fold shorter and the dependencies on membrane thickness and stirring rate are 125- and 8-fold lower, respectively. Pooled standard deviations (n = 40) correspond to uncertainties in oxygen concentration of about 0.009 mmol L-1.

Chemistry Techniques, Analytical↗
PubMed1992

Evaluation of transient responses of ammonia-selective potentiometric electrodes for quantitative applications.

An error-compensating, predictive kinetic method is adapted and evaluated for quantitative applications based on transient responses from an ammonia-selective electrode. Transient data collected during the early part of the electrode response are used with a curve-fitting method and appropriate mathematical models to predict the signal that would be measured if the response were monitored to equilibrium. Several different theoretical and empirical models were evaluated, and all but one of the models tested permitted reliable prediction of equilibrium potentials for most responses. Predicted values of equilibrium signal were close to measured values and exhibited the expected logarithmic dependence on ammonia concentration in the range from 0.1 to 100 mmol/L. Slopes of calibration plots E infinity vs log C, varied from about 50.5 to 65.0 mV per decade with an average value of 56.9 +/- 4.2 mV per decade. Quantitative data are used to rank the different models in terms of their utility for kinetic-based determinations of ammonia by using the ammonia-selective electrode.

Ammonia↗
PubMed1992

Instrumentation for the breath-by-breath determination of oxygen and carbon dioxide based on nondispersive absorption measurements.

This paper describes the development and evaluation of instrumentation for the breath-by-breath determination of oxygen and carbon dioxide in respiratory gases. The method is based on nondispersive absorption and uses the 145-nm absorption band for detection of oxygen and the 4.3-micron band for detection of carbon dioxide. A xenon discharge lamp with a sharp band at 147 nm was chosen as the source for the determination of oxygen, and a carbon dioxide discharge lamp with a sharp band at 4.3 micron was chosen for determination of carbon dioxide. A vacuum photodiode was used as the detector for oxygen, and a photoconductive cell with a built-in interference filter was used for detection of carbon dioxide. Plots of absorbance (A) vs concentration (C, %) were linear for oxygen and were nonlinear for carbon dioxide. Typical least-squares calibration equations were A = 0.020C + 0.02 for oxygen (0-100%) and A = 0.0012C2 + 0.050C + 0.008 for carbon dioxide (0-8%). Comparisons of computed (y) vs prepared (x) values for the concentrations given above were linear for both gases, yielding y = (1.00 +/- 0.01)x - 0.13 +/- 0.73 for oxygen and y = (1.07 +/- 0.02)x - 0.04 +/- 0.06 for carbon dioxide. The standard deviations were 1.2% at 50% oxygen and 1.5% at 4% carbon dioxide. Records are presented to illustrate breath-by-breath monitoring of these gases in a healthy subject.

Absorption↗