Calibration traceability

Many of the technical requirements of the Standard are covered in Chapters 4 to 7. The analytical requirements, including choosing a method and method validation, are covered in Chapter 4. The other measurement requirements, such as calibration, traceability and equipment qualification, are dealt with in Chapter 5. Some of the general issues not covered elsewhere are mentioned in the following sections. It has already been mentioned that staff should be trained and proven to be competent to carry out the testing. This applies to permanent and contracted staff. The laboratory should have a job description for all members of staff. There are more stringent requirements on staff who are also able to provide customers with opinions or interpretation of the results. 

Potency, yield, physical parameters Method, instrumentation, calibration, traceability, precision, accuracy... 

Instruments and Methods of Measurements. A Leeds and Northrup Type K-3 universal potentiometer, in conjunction with a General Electric Model 29 galvanometer, was used to measure electromotive force. The potentiometer was calibrated by means of a Weston Standard Cell which had been calibrated against a National Bureau of Standards (NBS) certified standard cell. Galvanic cells which were maintained at constant temperatures of 25°, 35°, and 45°C d= 0.01° by being immersed in a water bath at the desired temperature. The temperatures of the baths were set using a Fisher Scientific calibrated standard thermometer, with calibration traceable to the NBS. An adaptation of the cell sketched by Ives and Janz (II) was used. The modification of the cell was that described by Mclntrye and Amis (10). 

At present, no calibration procedure for black and white standard thermometer is available that includes all stress factors (air temperature, air velocity, and humidity). Today, calibration traceability is guaranteed by a contact thermometric procedure. It would be preferable to measure the temperature at the surface of the coated sensor because this is the temperature of interest. A contactless surface temperature measurement requires knowing the emission ratio of the material and a minimization of the reflected and scattered radiation. For a minor error contact surface temperature measurement, a known method is the multiprobe measurement with extrapolation to the surface temperature. 

Instrumental and technical factors equipment within specification, working correctly, properly calibrated, procedures established for operational control and calibration, traceability of measurement to the criteria for test validity. 

Al.2.1 Calibrate working standard viscometers against master viscometers having a certificate of calibration traceable to a national standard. Viscometers used for analysis shall be calibrated in comparison with working standard... 

Calibration Cahbration 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 cahbration of measurement devices. Furthermore, compliance with the ISO 9000 standards requires that the working standard used to cahbrate a measurement device must be traceable to an internationally recognized standard such as those maintained by the National Institute of Standards and Technology (NIST). 

The so-called standard instrument is used for interpolation between the fixed points and for the calibration of other thermometers lower in the metrological hierarchy. The standard instrument in the moderate temperature range is a special platinum resistance probe, as it has to fulfill set requirements. It is important in all calibration that traceability to a primary normal, here the fixed-point ITS-90 scale, exists. 

J. V. N. White. Traceable Temperatures An Introduction to Temperature Measurement and Calibration. New York John Wiley ic Sons, 1994. 

A note in clause 4.11.2 also accepts a serial number traceable to the device calibration record as meeting the intent of the requirement on calibration status indicator. 

In the upper diagram each supplier individually interprets the customer s requirements and either imposes additional requirements or neglects to pass on requirements. The net result at the end of the chain is that the external customer (the one who buys from the organization) does not get satisfaction from the transaction. In the lower diagram, each supplier refers back to the external customer s requirements to calibrate the internal customer s demands. This ensures that the net result matches exactly what the customer ordered. In reality, such calibration should not be necessary if the internal customers demonstrate traceability to external customer requirements. This can be achieved through process reviews performed in each process before instructions are transmitted to subsequent processes. 

Traceability is also important to control processes. You may need to know which products have been through which processes and on what date, if a problem is found some time later. The same is true of test and measuring equipment. If on being calibrated a... 

Knowing what checks have been made using the instrument since it was last checked, so that you can repeat them should the instrument be subsequently found out of calibration - this is only necessary for instruments whose accuracy drifts over time, i.e. electronic equipment. (It is not normally necessary for mechanical devices. You will need a traceability system for this purpose.)... 

This requirement is similar to that stated in clause 4.11.2 of the standard and addressed later in this chapter. The checks and rechecks required to prove that the software is capable of verifying the acceptability of product are a means of calibrating test software. However, test software does not wear or drift with age or use and so cannot be calibrated against a standard traceable to national standards. To control test software you need to consider what it is that you need to control. As a minimum you should control its use, modification, location (in terms of where it is installed), replication, and disposal. Requirements for other controls are covered in clause 4.11.2 of the standard, where they can be applied to test software. 

Sec. 820.72 Inspection, measuring and test equipment - Use traceable calibration standards and maintain caiibration records for measuring equipment... 

Another aspect of traceability of the results is the linkage of data from the homogeneity study, the stability study, and the characterization study of the reference material. In order to establish this link, the coordinator must be in the position to demonstrate that the results of these three studies have a common reference. Such a reference can be a calibrant, reference material, or possibly some realization by means of a suitable method. If such a common reference is not available, it is impossible to link the data sets, and therefore it is impossible to translate the results from the homogeneity and stability studies to the characterization of the material. 

The participants The range of participants should, whenever possible, be chosen in such a manner that widely different methods (based on different physical or chemical principles) can be used. The number of participants (recommended 15) should be sufficient to allow meaningful statistical processing of the results. When the laboratories feel the need for a CRM, either because the available calibrants are not comparable and a primary calibrant appears necessary for traceability, or because a reliable certified control material is needed but not available, then it is recommended that these laboratories do not plan a certification project entirely on their own, but that they involve laboratories having a background in traceability. 

A further distinction is made between pure solutions and matrix-based RMs. The former are available for many organic and inorganic analytes, with certified concentrations, but their role in quality control and assessment is limited. They may be used for the preparation of calibration solutions for a particular measurement but more usually these materials represent the base for traceability, through secondary... 

Some of the intended categories of use of radioisotopic reference material have been reviewed recently by Fajgelj et al. (1999). They include assignment of property values, establishing the traceability of a measurement result, determining the uncertainty of a measurement result, calibration of an apparatus, assessment of a measurement method, use for recovery studies and use for quality control purposes. It should be noted however that, in general, natural matrix reference materials are not recommended for calibration purposes. This should preferably be done with pure chemical forms of the element labelled with the isotope of interest. Calibrated isotopic sources of this kind are available from a number of commercial suppliers and are not the subject of this review. 

The concept of preparing a reference material with property values traceable to SI units is a relatively recent one which has not yet been applied in practice to natural matrix radioisotopic reference materials. This is one of the main reasons why such materials are generally not recommended for calibration purposes. 

A second reason for using reference materials in clinical chemistry is to ensure values obtained are traceable to those in a recognized, authoritative reference material (Johnson et al. 1996). As a result, the assignment of values of secondary and tertiary reference materials, calibrants, controls, and proficiency samples shordd be performed as precisely as possible (Johnson et al. 1996). Surprisingly there is still debate on this topic, and on the need for clinical chemistry to incorporate the principals of analytical quality assurance (Dybkaer et al. 1999). 

Only a direct matrix match of sample and CRM, and the CRM s use as a direct calibrant will allow the user to demonstrate accuracy and subsequently traceability close to the uncertainties established during the CRM certification ( note matrixmatching may not be necessary with matrix-independent techniques). This reality places a significant burden on the CRM producers, since large uncertainties in the certified values may degrade the perceived value of the CRM. 

In a different example, traceability in the amount-of-substance analysis of natural potassium, thorium, and uranium by the method of passive gamma-ray spectrometry was demonstrated by Nir-El (1997). For an absolute quantitative determination, accurate values of two parameters were required (i) the emission probability of a gamma-ray in the decay of the respective indicator radionuclides, and (2) the detection efficiency of that gamma-ray. This work employed a number of CRMs in the critical calibration of the detection efficiency of the gamma-ray spectrometer and the establishment of precise emission probabilities. The latter results compared well with literature values and provided smaller uncertainties for several gamma-rays that were critical for the traceabUity claim. The amount-of-substance analytical results of the long lived naturally occurring radionucHdes K, Th, and... 

As part of maintenance, some equipment may need to be calibrated. SOPs must include calibration methods, and a report format needs to be available at the time of calibration. Calibration SOPs must include pass/fail specifications as well as corrective actions to be taken in the event of calibration failure. For quality calibration standards use National Institute of Standards and Technology (NIST) traceable standards or other intrinsic standards of known purity, quality, and stability. These standards should have certificates attesting to their performance properties. After calibration, the metrologist or other responsible person should review the calibration reports to identify any issues with the equipment that may need further attention. 

Conventional calibration MDRD equation [used only with those creatinine methods that have not been recalibrated to be traceable to isotope dilution mass spectrometry (IDMS)]... 

Hasselbarth W (1995) Traceability of measurement and calibration in chemical analysis. Fresenius J Anal Chem 352 400... 

It is important that a measurement made in one laboratory by a particular analyst can be repeated by other analysts in the same laboratory or in another laboratory, even where the other laboratory may be in a different country. We aim to ensure that measurements made in different laboratories are comparable. We are all confident that if we measure the length of a piece of wire, mass of a chemical or the time in any laboratory, we will get, very nearly, the same answer, no matter where we are. The reason for this is that there are international standards of length, mass and time. In order to obtain comparable results, the measuring devices need to be calibrated. For instance, balances are calibrated by using a standard mass, which can be traced to the primary mass standard (see also Chapter 5). The primary standard in chemistry is the amount of substance, i.e. the mole. It is not usually possible to trace all of our measurements back to the mole. We generally trace measurements to other SI units, e.g. mass as in 40 mg kg-1 or trace back to reference materials which are themselves traceable to SI units. 

Identify the critical parts of the analytical process. These will have to be controlled by careful operation and calibration. Cross-reference to the relevant sections above. Include calibration of equipment - what needs to be calibrated, how, with what and how often Consider the appropriate traceability of calibrants and highlight any limitations associated with available calibrants. 

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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