Quality control improvement schemes

The variety of examples presented here can be seen as good evidence that simulations have become a valuable, partly indispensi-able tool for the study of chemicals in solution. The inclusion of ab initio QM procedures for the calculation of forces in every step of the simulation ensures the necessary accuracy of the simulation results to predict both structural and dynamical data, thus also providing a correct picture of the molecular and supermolecular species formed simultaneously in solution. The recently developed QMCF MD methodology has overcome several of the previous problems of MD simulations, mostly not only because of the possibility to renounce any kind of empirical or fitted solute-solvent potentials, but also because of an improved embedding scheme and the use of actual atom populations for the calculation of Coulombic forces. Besides its universality of application to various chemical compounds it also offers a straightforward way of further improvement and method-inherent quality control by the employment of correlated ab initio methods, although at a price which is not yet affordable with present computational facilities, but should become feasible within a few years. 

Quality assessment is performed by including pooled plasma as an internal control in every run. External quality control is achieved using samples obtained from the European Research Network for evaluation and improvement of screening, Diagnosis and treatment of Inherited disorders of Metabolism (ERNDIM) Special Assays Scheme, run according the scheme schedule. 

Precision within a laboratory can be tightly controlled by the operation of internal quality control schemes. This can be achieved by undertaking replicated analysis of samples and/or possibly by analysing replicate samples. The analysis of certified reference materials offers a means whereby bias can be estimated. Another means of estimating a laboratory s performance is via participation in inter-laboratory proficiency-testing schemes. Originally, external quality control or inter-laboratory proficiency-testing schemes had been made available to assist laboratories to identify those areas where remedial action was considered necessary, in order to improve the quality of data... 

Although many process variables are easily measured, lack of on-line sensors for key polymer properties renders quality control of polymer plants difficult. Process control schemes based on process variables p, T, flow-rate and feedstock compositions) alone are no longer sufficient, because these cannot reveal all material property variations. Significant efforts are being spent on improvements to process control systems, as exemplified by the numerous attempts to monitor polymer properties during processing, such as composition, density, viscosity and dispersion of a minor phase, etc., all of which are somehow difficult to measure. The development of an on-line inferential system for polymer property is a very active research area of polymerisation reactor control [1]. A schematic of inferential systems is illustrated in Fig. 7.1. For highest quality... 

PT schemes often are not only a help for the laboratories to improve their quality but also a control tool for accreditation bodies, customers and authorities... 

The most powerful tool to minimise the component of variance due to error in the laboratory is the discipline which recognised accreditation schemes bring. They encompass all the likely areas which produce mistakes, documented procedures, training, checking procedures, control of samples, monitoring conditions, formal audits and perhaps above all calibration. The general quality movement has produced pressures to make laboratory accreditation commonplace and as more laboratories reach this status it must be expected that reproducibility will improve. In the current economic climate, a problem is finding sufficient laboratories able to devote sufficient time to precision trials. 

Williams et al. (Wl), describe the results of studies of the automatic control of continuous fractional distillation. These studies were made on an analog computer which could simulate a five-plate tower. The effects of column design, varying feed rate, imperfect sampling, and quality of feed and reflux on controllability were evaluated. An earlier article by Rose and Williams (R2) on the same system compares various schemes for controlling fractionation columns. One interesting conclusion is that derivative control action cannot improve the control for any of the various combinations of measurement and regulation that were studied. 

Advanced control systems are becoming a necessity because of the ever increasing pressures toward product quality and reliability. All material processing operations can benefit from improved process control. However, increased control on the more complex systems, such as composites, compounds and blends, and on the more critical applications will yield the highest benefits. In addition, process control schemes based on process variables alone are not sufficient anymore... 

---