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P S Williams

Publications and source records attributed to P S Williams.

At least 19 recordsLinked to original sources
PubMed2001

A data analysis algorithm for programmed field-flow fractionation.

An algorithm that employs numerical integration for analysis of field-flow fractionation (FFF) data is presented. The algorithm utilizes detector response, field strength, and channel flow rate data, monitored at discrete time intervals during sample elution to generate a distribution of sample components according to particle size or molecular weight. The field strength and channel flow rate may either be held constant or programmed as functions of time, and it is not necessary for these programs to follow specific mathematical functions. If experimental conditions are monitored during a run, the algorithm can account for any deviation from nominal set conditions. The algorithm also allows calculation of fractionating power for the actual conditions as monitored during the run. The method provides greatly increased flexibility in the application of the FFF family of techniques. It removes the limitations on experimental conditions incurred by adherence to analytically available solutions to FFF theory, allowing ad hoc variation of field strength and other experimental parameters as necessary to increase sensitivity and specificity of the method. An implementation of the algorithm is described that is independent of the FFF technique (i.e., independent of field type) and mode of operation. To reduce computation time, it uses mathematical techniques to reduce the required number of numerical integrations. This is of particular importance when the perturbations to ideal FFF theory, such as those due to the effects of hydrodynamic lift forces, particle-wall or particle-particle interactions, and secondary relaxation, necessitate relatively lengthy numerical calculations.

Algorithms↗
PubMed2001

New insights in the development of Numerov-type methods with minimal phase-lag for the numerical solution of the Schrödinger equation.

Explicit Numerov-type methods with minimal phase-lag are developed in this paper. These methods are of algebraic order five and have phase-lag order eight and ten. The methods have new features; namely that they are dissipative, i.e. they are not symmetric and they have no interval of periodicity. Numerical illustrations using (i) the radial Schrödinger equation and (ii) coupled differential equations arising from the Schrödinger equation, indicate that these new methods are more efficient than older ones. It is seen that the property of the phase-lag is more important than the non-empty interval of periodicity in constructing numerical methods for the solution of the Schrödinger equations and related problems.

Journal Article↗
PubMed2001

Dissipative exponentially-fitted methods for the numerical solution of the Schrödinger equation.

The first dissipative exponentially fitted method for the numerical integration of the Schrödinger equation is developed in this paper. The technique presented is a nonsymmetric multistep (dissipative) method. An application to the bound-states problem and the resonance problem of the radial Schrödinger equation indicates that the new method is more efficient than the classical dissipative method and other well-known methods. Based on the new method and the method of Raptis and Allison (Comput. Phys. Commun. 14 (1978) 1-5) a new variable-step method is obtained. The application of the new variable-step method to the coupled differential equations arising from the Schrödinger equation indicates the power of the new approach.

Journal Article↗
PubMed2000

Aspirin for vascular dementia.

BACKGROUND: For patients with a diagnosis of vascular dementia there is evidence that aspirin is widely prescribed - in one study, completed by geriatricians and psychiatrists in the UK, 80% of patients with cognitive impairment (with vascular risk factors) were prescribed aspirin. However, a number of queries remain unanswered: Is there convincing evidence that aspirin benefits patients with vascular dementia? Does aspirin affect cognition or improve prognosis? In addition, does the risk of cerebral or gastric haemorrhage outweigh any benefit? The aim of this review is to assess the evidence of effectiveness of aspirin in those with a diagnosis of vascular dementia. OBJECTIVES: To assess the evidence of effectiveness of the use of aspirin for vascular dementia. SEARCH STRATEGY: Computerised databases were searched independently by two reviewers. In addition, relevant websites were searched and some journals were handsearched. Specialists in the field were approached for unpublished material and also any publications found were searched for additional references. SELECTION CRITERIA: All randomised controlled trials investigating the effect of aspirin for vascular dementia are included. Inclusion/exclusion of studies comprised systematic assessment of the quality of study design and the risk of bias. DATA COLLECTION AND ANALYSIS: Data were extracted independently by both reviewers, using a previously tested data extraction form and, where required, authors were contacted for data not provided in the papers. The aim was to evaluate data recorded via tools assessing cognitive and behavioural changes along with mortality, morbidity and institutionalisation data. MAIN RESULTS: One randomised controlled trial ( approximately approximately Meyer 1989 approximately approximately ) was included, and yielded data for analysis on a total of 70 patients. The only relevant outcome assessed in this trial was cognition. Change in cognitive outcome was towards being in favour of treatment. REVIEWER'S CONCLUSIONS: There is very limited evidence that aspirin is effective in treating patients with a diagnosis of vascular dementia. Further research is needed to assess the effect of aspirin on cognition, and also on additional outcomes such as behaviour, and quality of life. At present it is not possible to provide evidence for other queries regarding the use of aspirin for dementia (these are described in the Background section of this review).

Aspirin↗
PubMed1999

Quantification of cellular properties from external fields and resulting induced velocity: cellular hydrodynamic diameter.

An experimental technique is discussed in which the size distribution of a population of cells is determined by calculating each cell's settling velocity. The settling velocity is determined from microscopically obtained images which were recorded on SVHS tape. These images are then computer imaged and processed, and the cell's location and velocity are determined using a computer algorithm referred to as cell tracking velocimetry (CTV). Experimental data is presented comparing the distribution of human lymphocytes and a human breast cancer cell line, MCF-7, determined using a Coulter counter and the CTV approach.

Algorithms↗
PubMed1999

Quantification of cellular properties from external fields and resulting induced velocity: magnetic susceptibility.

An experimental technique is discussed in which the magnetic susceptibility of immunomagnetically labeled cells can be determined on a cell-by-cell basis. This technique is based on determining the magnetically induced velocity that an immunomagnetically labeled cell has in a well-defined magnetic energy gradient. This velocity is determined through the use of video recordings of microscopic images of cells moving in the magnetic energy gradient. These video images are then computer digitized and processed using a computer algorithm, cell tracking velocimetry, which allows larger numbers (>10(3)) of cells to be analyzed.

Cells, Cultured↗
PubMed1999

Flow rate optimization for the quadrupole magnetic cell sorter.

The quadrupole magnetic cell sorter is a form of split-flow thin-channel (SPLITT) separation device. It employs a quadrupole magnetic field and annular channel geometry. Immunomagnetic labels are used to bind to specific receptors on the surface of the cells of interest. It is the interaction of these labels with the magnetic field that brings about the selective isolation of these cells. The SPLITT separation devices have generally been based on parallel-plate geometry, usually with effectively constant field strength applied across the channel thickness. The nonconstant field strength and annular channel geometry of the magnetic cell sorter require that a new strategy be developed for optimization of inlet and outlet flow rates. We present such a strategy here based on a consideration of certain specific cell trajectories within the system.

Cell Separation↗
PubMed1998

Numerical simulation of band-broadening during hydrodynamic relaxation in frit-inlet field-flow fractionation channels.

The frit-inlet technique is a promising implementation of hydrodynamic relaxation of samples in field-flow fractionation (FFF). The optimization of the process is of great importance in order to maximize overall system efficiency. The mechanism of band-broadening that takes place during hydrodynamic relaxation has been examined using a three-dimensional simulation of the flow inside the triangular end-piece of the channel. This is the first time this contribution to band-broadening has been considered and studied. Particle trajectories in the absence of a transverse field were numerically calculated, thereby isolating this effect from the familiar field-driven relaxation effect. As a first step towards an optimization of the system, the influence of the length of frit element was examined. Band-broadening was examined by determining the number of particles passing through the triangular end-piece as a function of transit time for a uniform particle distribution at the injection point. Due to the complexity of the flow patterns within the system, it is concluded that such numerical simulations are necessary for the optimization of the design and operation of this type of channel.

Algorithms↗
PubMed1997

'Progressive' versus 'indolent' idiopathic membranous glomerulonephritis.

There is still controversy about safe and effective therapy for idiopathic membranous glomerulonephritis (MGN). Over 20 years, we have simply observed our patients clinically after diagnosis, and only used aggressive therapy with steroids in high dosage and azathioprine for 21 patients with progressive renal failure. The other 42 were thus classified as 'indolent' MGN. Those with 'progressive' MGN had heavier proteinuria and worse renal function on presentation, but the overlap was considerable. Patients with progressive MGN were treated after 1-4 years. All responded promptly, and 5 years after presentation all were alive, and only one was on dialysis. By 10 years, most were still alive, and of these most were off dialysis. In five patients, dialysis was delayed by several years. There were two deaths on dialysis, and three other deaths, mostly in older patients. All but one patient with indolent MGN remained stable on symptomatic treatment only, for at least 5 years after presentation. In many, proteinuria fell to insignificant levels over 4 years. In these remitting patients, there was a prevalence of thyroid disease (7), rheumatoid disease (3) and nephrotic presentation in pregnancy (4). After 6-10 years three patients developed worsening proteinuria and renal failure. Five older patients died from unrelated causes.

Adult↗
PubMed1994

Theory of field-programmed field-flow fractionation with corrections for steric effects.

This paper deals with the principal perturbation to ideal normal-mode elution of particles in field-flow fractionation (FFF). This perturbation is due to the finite size of particles undergoing migration in the FFF channel. The effects of a first-order correction for particle size are examined. Equations are derived for retention time, fractionating power, and steric inversion diameter for operation at constant field strength, as well as under conditions of both exponential and power programmed field decay. Useful limiting equations for fractionating power are derived and their validity is confirmed for typical experimental conditions. The derived equations are necessary for the future development of a systematic optimization strategy for the selection of operating conditions for particle size analysis by FFF. Calculations confirm our previous conclusion that the fractionating power for exponential field programming varies strongly with particle size; this variation is only slightly reduced by steric perturbations. The uniform fractionating power of power programming is slightly disturbed by steric effects although fractionating power remains much more uniform than for exponential programming. It is shown that a higher uniformity in fractionating power can be gained by manipulating the parameters of power programming but that no improvement is possible with exponential programming. Phenomena giving rise to higher order perturbations and to secondary relaxation are discussed and the conditions identified under which these effects are minimized.

Chemical Fractionation↗
PubMed1992

Rapid breakthrough measurement of void volume for field-flow fractionation channels.

A peak breakthrough technique is described and evaluated for measuring the void volume of field-flow fractionation (FFF) channels, particularly those used for flow FFF. This technique uses a high-molecular-mass macromolecular or particulate probe that can be displaced rapidly by flow through the FFF channel with minimal transverse diffusion. The particles that emerge first are those carried through the entire length near the channel centerline at the apex of the parabolic flow profile. These particles generate a sharp breakthrough profile. The measured breakthrough time is two thirds of the void time, thus making it possible to calculate both the void time and the associated void volume. This method, although applicable to all FFF channels (and capable of extension to open tubes), is particularly useful for flow FFF because conventional low-molecular-mass void probes can diffuse into the permeable walls and thus distort void measurements. The theoretical basis of the breakthrough technique and an explanation for the sharpness of the breakthrough front are given. A method for compensating for deviations from perfect sharpness is developed in which the breakthrough time is identified with the time needed to reach 85-88% of the breakthrough peak maximum. Preliminary experimental results are shown using various protein probes in four different FFF channel systems.

Chemical Fractionation↗
PubMed1991

Hydrodynamic relaxation in flow field-flow fractionation using both split and frit inlets.

Two means are described for achieving hydrodynamic relaxation and thus avoiding the stopflow injection procedure in field-flow fractionation (FFF): split flow injection and frit inlet injection. The advantages, disadvantages, and the theoretical basis of these procedures are discussed. Incremental band broadening due to the final relaxation step is examined theoretically and shown to be negligible when the flow rate of the sample inlet substream is small compared to the total channel flow rate. The optimization of the sample inlet flow rate is discussed. Experimental results for both injection procedures are reported for flow/steric (or hyperlayer) FFF applied to latex standards, confirming the expected trends. However, closer examination shows that the observed incremental band broadening associated with hydrodynamic relaxation is somewhat larger than the value predicted.

Chemical Phenomena↗
PubMed1991

Comparison of power and exponential field programming in field-flow fractionation.

Field programming in field-flow fractionation has the purpose of expanding the molecular weight or particle diameter range subject to a single analytical run. The two most widely used field programs are those in which the field strength decays with time according to an exponential function and a power function, respectively. The performances of these two programming functions are compared by obtaining limiting equations showing how retention time tr, standard deviation in retention sigma t, and fractionating power Fd vary with particle diameter d. It is shown that uniform fractionating power (Fd independent of d) can be obtained with power programming but that in exponential programming Fd is always non-uniform, varying as d-1/2. In exponential programming a linear relationship arises between tr and log d. This particular relationship is impossible to realize in power programming but an alternative linear relationship can be obtained by plotting tr versus dt/3. These results are made more concrete by plotting and comparing field strength, relative field strength, Fd and tr for specific programming cases.

Chemical Fractionation↗
PubMed1991

Particle size distribution by sedimentation/steric field-flow fractionation: development of a calibration procedure based on density compensation.

Because of the important but mathematically complex role played by hydrodynamic lift forces in sedimentation/steric FFF, applied generally to particles greater than 1 micron in diameter, retention cannot readily be related to particle diameter on the basis of simple theory. Consequently, empirical calibration is needed. Unfortunately, retention is based on particle density as well as size so that a purely size-based calibration (e.g., with polystyrene latex standards) is not generally valid. By examining the balance between driving and lift forces, it is concluded that equal retention will be observed for equal size particles subject to equal driving forces irrespective of particle density. Therefore by adjusting the rotation rate to exactly compensate for density, retention can be brought in line with that of standards, a conclusion verified by microscopy. Linear calibration plots of log (retention time) versus log (diameter) can then be used. This approach is applied to two glass bead samples (5-30 and 5-50 microns) using both a conventional and a pinched inlet channel. The resulting size distribution curves are self consistent and in good agreement with results obtained independently.

Calibration↗
PubMed1991

Failure of dietary protein and phosphate restriction to retard the rate of progression of chronic renal failure: a prospective, randomized, controlled trial.

Ninety-five patients (63 male, 32 female), age 45 +/- 2 years (mean +/- SEM) with chronic renal failure of varied aetiology were randomized to receive either a conventional low protein diet (0.6 g/kg/day protein, 800 mg phosphate; n = 33), a low phosphate diet (providing approximately 1000 mg phosphate plus an orally administered phosphate binder, minimum protein intake 0.8 g/kg/day; n = 30) or to control (minimum protein intake 0.8 g/kg/day, no phosphate restriction; n = 32). Patients were reviewed for a minimum of 6 months before randomization and were withdrawn from the study if plasma creatinine exceeded 900 mumol/l, plasma phosphate was greater than 2.0 mmol/l or at the onset of uraemic symptoms. Following randomization patients were studied for an average of 19 +/- 3 months. Mean plasma creatinine rose from 398 +/- 33 to 600 +/- 50 mumol/l. Dietary protein intake was estimated at 0.69 +/- 0.02 g/kg/day in the low protein group, 1.02 +/- 0.05 in the low phosphate and 1.14 +/- 0.05 in the controls, phosphate intake was 815 +/- 43, 1000 +/- 47, and 1315 +/- 57 mg/day, respectively. Urinary urea excretion and protein catabolic rates were significantly reduced (p less than 0.01) only in those on protein restriction, at 213 +/- 9 mmol/24 hours and 0.71 g/kg/day, respectively. Phosphate excretion was significantly lower (p less than 0.05) in both the low protein group (17.9 +/- 0.8 mmol/24 hours) and the low phosphate group (18.6 +/- 1.0 mmol/24 hours) compared to controls. Changes in body weight, muscle mass and serum transferrin, albumin and immunoglobulins were comparable between the groups. Mean blood pressure following randomization was 150/89 +/- 3/1 (low protein), 148/87 +/- 3/1 (low phosphate) and 146/87 +/- 3/1 (controls). Progression of renal failure was analysed by rate of all of creatinine clearance (ml/min/1.73 m2/month), by rate of deterioration derived from reciprocal plasma creatinine against time plots (1/mmol/year) and to assess individual patient's response to treatment by two phase linear regression ('breakpoint') analysis of reciprocal plasma creatinine/time plots. Progression was analysed only in patients seen for at least 3 months following randomization. The rate of fall of creatinine clearance was not significantly different between the groups (ANOVA): 0.56 +/- 0.08 ml/min/1.73 m2/month (low protein, n = 28), 0.44 +/- 0.07 (low phosphate, n = 23) and 0.69 +/- 0.11 (control, n = 27).(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1990

Multiple organ failure--a role for plasma exchange?

Two patients with multiple organ failure, septicaemia and a deteriorating clinical course were treated by plasma exchange in addition to standard supportive measures. Dramatic improvements were seen in cardio-respiratory (patient 1), neurological and renal parameters (patient 2) which were attributable to the exchanges. Plasma exchange might be of value as adjunctive therapy where overwhelming septicaemia occurs with multiple organ failure.

Adult↗