Calibration of Measurement

The term calibration is often used when in fact what is meant is verification . Calibration of an instrument or a piece of equipment (e.g. glassware) involves making a comparison of a measured quantity against a reference value. For example, to calibrate a spectrophotometer response, the appropriate reference material is selected and the spectrophotometer response to it, under the specified conditions, is measured. Then, the measured value is compared to the value quoted in the literature. Either a correction is made to the results from subsequent measurements or an adjustment is made to the instrument. 

Solutions of pure chemicals of known concentration used for instrument calibration are frequently referred to as standard solutions . However, the term 

The fundamental unit in chemical measurement is the mole - amount of substance. A mole is the amount of a substance that contains as many atoms, molecules, ions or other elementary units as the number of atoms in 0.012 kg of carbon 12 (12C). It is the only dimensionless SI unit. In practical terms, it is almost impossible to isolate a mole of pure substance. Substances with a purity of better than 99.9% are rare one exception is silver, which can be obtained with a purity of 99.9995% which is referred to as five nines silver . Another problem is that it is not always possible to isolate all of the analyte from the sample matrix, and the performance of the chemical measurement may be matrix-dependent - a given response to a certain amount of a chemical in isolation may be different from the response to the same amount of the chemical when other chemicals are present. If it is possible to isolate quantitatively all of the analyte of interest from the accompanying sample matrix, then a pure chemical substance may be used for calibration. The extent to which the analyte can be recovered from the sample matrix will have been determined as part of the method validation process (see Chapter 4, Section 4.6.3). 

What is essential in establishing traceability is that the measurand is specified unambiguously. This may be, e.g. in terms of extractable cadmium from soil by using a named acid mix or the concentration of a metal in a particular oxidation state, e.g. Fe(n) or Fe(m). The units used to report the result should also be known and acceptable SI units are preferred. The method used will be validated and if used in accordance with the written procedures should produce results that are fit for purpose . The class of glassware to be used will be specified in the method procedure, e.g. Class A pipettes and volumetric flasks, as these are manufactured to a specified tolerance. Instruments will be regularly calibrated and their performance verified daily. In terms of the chemicals used, these will 

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From the ventilation point of view, the fixed points -38.83 °C (triple-point of mercury), 0.010 °C (triple-point of water), 29.76 °C (melting point of gallium), and 156.60 °C (freezing point of indium) are of relevance. The triple-point of water is relatively simple to achieve and maintain with a triple-point apparatus. Some freezing point cells are covered in standards. In practical temperature calibration of measuring instruments, the lTS-90 fixed points are not used directly. 

Prepare procedures for controlling the calibration of measuring devices. 

An additional requirement not noted in Table 1 is compliance with GLP7 These practices establish a paper trail for all procedures involved in the determination of residues. With regard to immunoassays, GLPs require calibration of measurement devices such as adjustable pipettors and dedicated spectrophotometers. Computer software output, as noted above, must be verified prior to use. This process can be simplified by limiting the application of specialized software to the operation of an instrument and carrying out the residue calculations in a broadly available spreadsheet such as Excel. On a positive note, in recent years, the software accompanying most microtiter plate readers has become generally easier to use and usually incorporates internal spreadsheets that are compatible with external systems. 

A plan is draw up detailing how checks will be made to ensure that the critical limits are not exceeded. It shows how often the checks are made, how and by whom, so that it is clear who is responsible that it is done correctly. It also specifies regular inspections and calibrations of measuring equipment such as thermometers. 

In addition, a system for making sure staff are appropriately qualified and trained for the work that they are doing must be in place. This will enable an auditor to see clearly the demonstrated competence of the staff and how this has been checked. The requirements for all major items of equipment must be listed, to ensure that the equipment in use is suitable for the task, is in working condition and, where necessary, is calibrated. For all of the instrumentation there needs to be a documented schedule for maintenance. Measurements must be traceable, that is, the laboratory must be able to show how the calibration of measurement instruments is traceable to National or International Standards. Where this presents practical problems, as in some chemical measurements for example, interlaboratory comparison and the use of reference materials (and preferably Certified Reference Materials) will be required. 

Does the system include the retention of calibration certificates or data used in support of all calibration of measuring equipment Does the record system allow for calling forward, at the appropriate interval, equipment requiring calibration ... 

Problems in the packaging of oral liquids have included potency (fill) of unit dose products and accurate calibration of measuring devices such as droppers, which are often provided. For unit dose solution products the label... 

Refining the standardized measurement referred to above to provide a calibrated scale is a more difficult task, although calibration beads of various types now available facilitate the process. Beads with manufacturer s reported equivalent fluorescein content have been available for some time (Flow Cytometry Standards Corporation, San Juan, Puerto Rico), and these are useful for calibration of measurements using fluorescein-labeled antibodies, although... 

ISO TR 14253-2 (1998) Geometrical product specifications (GPS) - Inspection by measurement of work-pieces and measuring equipment -Part 2 Guide to the estimation of uncertainty in GPS measurement, in calibration of measuring equipment and in product verification. ISO, Geneva Kadis R (1998) Accred Qual Assur 3 237-241... 

For calibration of measuring equipment by measurement standards or CRMs, a straight-line response function is usually postulated... 

Calibration Calibration entails the adjustment of a measurement device so that the value from the measurement device agrees with the value from a standard. The International Standards Organization (ISO) has developed a number of standards specifically directed to calibration of measurement devices. Furthermore, compliance with the ISO 9000 standards requires that the working standard used to calibrate a measurement device be traceable to an internationally recognized standard such as those maintained by the National Institute of Standards and Technology (NIST). 

The problems with obtaining suitable reference materials necessary for the validation of analytical methods and the calibration of measuring and control instruments... 

CRMs are also used for calibration of the measurement instmment. For example, pure metals or alloys are used for X-ray fluorescence (XRF) and other spectroscopic methods in industrial laboratories. Because the uncertainty associated with a certified value contributes to the total uncertainty of the measurement, Wref should be as small as possible. Therefore, wherever possible, CRMs used to calibrate instruments should be pure substances or solutions. Matrix CRMs are not recommended for calibration of measuring instruments, because the uncertainty of certified values are generally higher than those of pure substances or solutions. Moreover, using this approach, an analyst is deprived of independent verification of the accuracy of obtained results, which should be the primary function of CRMs. 

CRMs are an important tool for quality assurance and quality control in analytical laboratories. They are used for verification of accuracy and precision (i.e., reliability of the results of analysis, validation of analytical procedures, establishing measurement traceability, and calibration of measurement equipment). Use of CRMs is recommended by the ISO/IEC 17025 standard and therefore it is obligatory for those who wish to obtain and maintain accreditation of the laboratory. 

SOPs for use, maintenance, calibration, and cleaning of the plant must be developed during the OQ process, as well as schedules for maintenance and calibration. Each OQ document contains a list of required SOPs for the use of the plant. At this point, training of the technical staff of the plant should take place. The training must be documented and checked using prepared forms. Within the OQ process, the calibration of measuring and controlling devices must be checked. Critical parameters and circumstances identified in the risk analysis (RA) must be checked for conformity to the acceptance criteria. 

The first calibration of measurement and control devices must be checked. 

Calibration of measurement setup reduction of parasitic effects... 

However, for a fully developed gas-solid pipe flow, the particle diffusive mass flux is usually negligibly small compared with the particle mass flux (Zhu et al., 1991a). Hence, the isokinetic sampling of a gas-solid suspension flow in principle is able to yield the particle mass concentration provided that the particle velocity can be determined. This principle has served as the most primary method for the calibration of measuring systems on particle mass concentration. 

Electron diffraction patterns were observed with a Phillips EM 420 electron microscope, operated at 100-120 kV. Gold, deposited on representative samples, served as a standard for the calibration of measured d-spacings. 

Calibration is necessary in all quantitative NDA measurements to relate measured responses (e.g., neutron coincidence rate or specific gamma intensity) to nuclear material characteristics. An accurate measurement depends crucially on the effective calibration of measurement instrumentation. This calibration is based on similar items whose nuclear material content is very accurately known. The resulting calibration functions including all necessary correction factors (such as those relating to neutron multiplication or gamma-ray... 

Control and calibration of measuring and test equipment Maintenance tools and equipment control Station material condition inspection Management involvement Maintenance history Anafyses of maintenance problems... 

However, calibration of LA-ICP-MS measurements for accurate determination of trace elements is still a problem, which is mainly due to fractionation processes and lack of certified standard reference materials with a matrix composition that is sufficiently similar to that of the various sample types that need to be analyzed [39]. A great variety of different calibration methods have therefore been developed, including matrix-matched and non-matrix-matched calibration techniques [39-42], but also techniques relying on the use of standard solutions, with corresponding aerosol introduction or direct ablation of the liquid, have been suggested [43, 44]. As a result of the different difficulties and limitations of these calibration methods, LA-ICP-IDMS is a promising alternative for calibration of measurements in powdered and liquid samples. 

Since working standards are used for the calibration of measuring instruments, the choice of tolerance depends upon the requirements of the instrument. The weights are usually used at the assumed nominal values and appropriate tolerances should be chosen. 

Provision for plant performance testing requires careful liaison between the appropriate personnel of both parties. Broad requirements of such tests will usually have been stipulated in contract documents but it remains for site teams to schedule the test arrangements in detail. These ate likely to include the selection and calibration of measuring devices together with the application of correction factors and tolerances, sampling and analyses, datalogging and effects of interruptions. It is important that procedures are drawn up and agreed upon well in advance of proposed test periods. 

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