Quality of Measurement

Precision is the degree of agreement from one measurement to another. According to the condition of comparison, there are three terms related to 

1) Standard deviation is defined as the degree of agreement in the results from repeated measurements on the same instrument by the same operator  

The uncertainty of optical radiation metrology is high compared to electrical measurements, which are accurate to many decimal places. But with electrical quantities, there is no distribution of signal, except perhaps for time. Because either voltage 

Optical radiation is distributed over time, position, direction, wavelength, and polarization, so field measurement accuracies within 10% are normal. There are over 30 potential sources of error in light measurement (D. Ryer, personal communication, 1983-1999), six of these sources involving optical error are considered foremost. 

Luminance and radiance are a function of the source, independent of distance 

Diffusers are made from machined quartz, Teflon, or opal. The type of material is chosen based on the wavelengths of interest. Other applications, like radiance and luminance, call for a narrow field of view so an occluding hood or lensed barrel is used on the photodetector to limit the spatial response to a small, well-defined area. 

Graphing the two responses on a polar plot provides an easy way to show how well a photodetector s spatial response matches the cosine response function. 

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Those instructions outlining how to properly use equipment and instrumentation to ensure the quality of measurements. 

During this preparation stage, analysts will frequently find that there is insufficient quantity or quality of measurements to close the material balance. Analysts should make eveiy effort to measure aU stream flows and compositions for the actual test. They should not rely upon closing material balances by back-calculating missing streams. Tne material balance closure will provide a check on the vahdity of the measurements. This preparatoiy material balance will help to identify additional measurements and schedule the installation of the additional instruments. 

The quality of the determination of risk levels set by regulation depends on the quality of measurements of lethal concentrations and doses (LC and LD50). From safety data in Part Three it is seen that this quality is far from being reached. 

Accuracy is often used to describe the overall doubt about a measurement result. It is made up of contributions from both bias and precision. There are a number of definitions in the Standards dealing with quality of measurements [3-5]. They are only different in the detail. The definition of accuracy in ISO 5725-1 1994, is The closeness of agreement between a test result and the accepted reference value . This means it is only appropriate to use this term when discussing a single result. The term accuracy , when applied to a set of observed values, describes the consequence of a combination of random variations and a common systematic error or bias component. It is preferable to express the quality of a result as its uncertainty, which is an estimate of the range of values within which, with a specified degree of confidence, the true value is estimated to lie. For example, the concentration of cadmium in river water is quoted as 83.2 2.2 nmol l-1 this indicates the interval bracketing the best estimate of the true value. Measurement uncertainty is discussed in detail in Chapter 6. 

The laboratory environment can affect the quality of measurements in ways other than direct contamination. 

In most field analysis in which separation techniques are the main difficulties, the traceability chain could not be accomplished easily by the use of calibration standards of a simple matrix. Consequently, either the validation of analytical methods or calibration by complex matrix reference materials is required. However, unless the process is clearly described with corresponding uncertainty, the validation process becomes a bottleneck for establishing a traceable measurement. Then, in most applications, the role of CRMs of a similar matrix becomes crucial in the quality of measurements. 

Present-day analytical laboratories are increasingly under pressure to supply objective evidence of their technical competence, of the reliability of their results and performance, and to seek formal certification or accreditation. This pressure may come from the laboratory s customers (e.g., industry and national bodies) but may also be due to scientific considerations. A QM system in place, validation of methods, uncertainty evaluation, the use of primary standards and CRMs, participation in ILCs, and PT, all serve to assure and demonstrate the quality of measurements. Compared to, say, 30 years ago, the stability of the equipment now available is much improved, and a greater range of RMs for method validation and calibration is accessible. Nevertheless, to achieve mutual (international) acceptance of various bodies of evidence for QA activities, a number of protocols have been developed. The most widely recognized protocols used in chemical measurements and testing are the ISO Guide 9000 2000, ISO/IEC 17025 2005, and OECD Guidelines for GLP, as well as its national and sector equivalents. 

In addition to keeping the controllers tuned, other methods are available to improve the quality and reliability of process measurements. Overall process balance calculations and the use of predictor/estimator filters (e.g., Kalman filters) can help to improve the quality of measurements. These better-quality measurements are contributing to better control of performance, which will be discussed in more detail in the following subsections. 

The most frequent, reliable approach for element speciation in real samples today uses combined - or hyphenated - techniques, where species are selectively separated (e.g., by a chromatographic technique) and then the elements in the various chemical forms are selectively detected. To enhance the quality of measurements, molecule-selective detection is also coupled to separation devices. These combinations provide extended flexibility and broad applicability. Disadvantages are that, as complexity increases, the risk increases of system failure. Besides, species equilibrium can be drastically altered during separation due to dilution, some components can be removed, and so on. Under such conditions, species transformation and destruction is a likely consequence. Therefore, the total separation time should be shorter than the transformation rate of species. Figure 17.1 gives an overview of the strategy and various speciation methods mostly used for element speciation in human milk. 

Topol, L.E. "Plan for Controlling the Quality of Measurements and Data Base in the Utility Acid Precipitation Study Program (UAPSP)" Environmental Monitoring Services, Inc., January 1982. 

Quality assurance and quality control (QA/QC) A system of procedures, checks, and audits to judge and control the quality of measurements and reduce the uncertainty of data. Some quality control procedures include having more than one person review the findings and analyzing a sample at different times or using different laboratories to see if the findings are similar Quantitative risk assessment (QRA) A process that relies on mathematical modeling and estimations usually derived from animal test results and the probability of risk for a chemical substance at the low dose to which the human population is normally exposed Radionuclide A nuclide with radioactive properties... 

The current and past operation should be compared so that the timing of the observed problems is established. The possible causes (hypotheses) can be compared against the measurements found on the log sheets. The number of possible causes can then be reduced. When the quantity or quality of measurements is insufficient to further reduce the set of causes, additional measurements are required. These may require special instruments (e.g., gamma-ray scanning) not routinely used in the plant. Alternative operating conditions may also be required to further reduce the number of causes. As part of the problem identification, it is always important to look for measurements that are inconsistent with the proposed explanation. They will be more informative than the ones justifying the hypothesized cause. Ultimately, with appropriate additional measurements, the cause can be identified. This is not an exact science and, as stated above, relies heavily upon the communication, technical, and investigative skills of analysts. 

Outcomes 6 Cieariv defined primary and secondary outcome measures and, when aopiicabte, anv methods used to enhance the Quality of measurements (e.a., muitipie observations, training of assessors). ... 

Volatile and semivolatile organic chemicals placed on surface soils have very similar chemical process behavior patterns as those shown above for Hg. Numerous laboratory and a few field flux measurements have been performed on this class of chemicals as documented in a recent review. The review was performed to assess the quantity and quality of measured flux data and the availability of process-based emission models. The basic difference between the flux chamber model and the field-scale model is that the former involves a steady-state flux process and the latter is a transient one. The EC models needed for the large time-scale involved with most field applications must consider the so-called weathering effect which characterizes the depletion of the available-for-volatilization chemical mass in the... 

Deviations from Beer s law may also arise from insufficient quality of measuring instruments, mainly from the use of non-ideal monochromatic light, improper width of the spectral band, or scattering of radiation. The detector signal should be proportional, over a wide range, to the intensity of the radiation recorded. 

Clay materials used in the brick industries are currently monitored to control air and groundwater pollution. Some areas suffer from severe pollution by emission of fluorine to the atmosphere and its subsequent transport into the groundwater since clay may contain 200 to 1200 mg kg of F. As an example, the brick and ceramic industry is responsible for 50 /o of the total fluoride emission in The Netherlands. The emissions are calculated from the difference in the F content of the clay and the final product (brick). In some cases, raw materials are rejected if the fluoride contents are too high which cause an economic concern. As a consequence, many European countries have implemented regulations for the maximum authorised amount in gaseous emission of F from brick industries, which strongly stresses the need for the control of the quality of measurements using representative CRMs. 

In the past, molecular luminescence spectrometry was always conducted with single channel systems involving a photomultiplier tube (PMT) as the detector. The availability of multichannel detectors with internal gain has provided a new powerful tool for luminescence measurements, and several types of applications have been reported (1-15). This paper is concerned with the application of an intensified diode array dynamic molecular fluorescence and chemiluminescence measurements. In this paper the types of measurements and analytical systems for which multichannel detectors are used in our laboratory are introduced. Next the specific IDA system used is presented along with important hardware and software considerations. Third, the characteristics of the IDA detector are reviewed to give some perspective about its influence on the quality of measurements. Finally, some typical applications to chemical systems are presented to illustrate the advantages of multichannel detection. 

As Experiments 12 and 13, this experiment supports a classroom discussion concerning significant figures, the metric system, and the measurement of physical properties. In addition, it introduces the concept of control charting, a popular and important means of monitoring the quality of measurements and products in quality assurance laboratories. It, thus, utilizes a familiar consumer product to demonstrate an activity of the chemist s real world of work and to tie in the classroom discussion. 

Uncertainty is a key topic of metrology in chemistry.296 The term chemical metrology is discouraged since metrology operates on the same principles, almost independently of the field of application. The other terms used in describing quality of measurement, e.g., comparability, traceability, validation, can be defined in terms of uncertainty as the basic characteristic. 

The Robens Institute of the University of Surrey, United Kingdom, conducts an international project to assess the quality of measurements of trace elements in biological fluids (Taylor et al. 1994). Two water specimens and two dialysis specimens are distributed monthly to participants. The Centre de Toxicologic du Quebec s Quebec... 

The quality of an experimental design can and should be assessed before the experiments are started, and not after. This does not mean that the quality of the results does not depend on the precision and accuracy of the experimental measurements. These are just as important as one would suppose, but the quality of measurement can be separated from the question of quality of design. 

The variance of the experimental response o, which itself depends on the quality of measurement in the individual experiments (that is, the experimental method, the quality of the equipment, and the skill of the operator). 

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