Calibration and traceability

A calibration chain B sample treatment, C associated quantities. The numbers indicate the link between steps. TS transfer standard (masses, wavelength calibration devices, etc), RM laboratory or any non certified reference material, CRM certified RM, PCRM primary CRM e.g, pure substances or matrix CRMs certified with a primary method (balance, IDMS etc). RMs and CRMs can be used at various stages of the measurement process. For chain A PCRMs or CRMs do not exist for all substances in particular for organic and organo-metallic determinations. For step 10, 11 and 12, PCRMs or even CRMs rarely exist. For step 6 laboratories may prepare raw extracts to follow the purification stage. 

Such calibration materials can be obtained under various forms and quality. When certified they are often considered as Primary Reference Materials (PRMs). As they allow one to establish the traceability of measurements to the fundamental units (mole, kg) of the S.I., they are used for the calibration step itself but also in the validation procedure when spiking of material is necessary. 

Solutions or mixtures of one or more substances mainly for trace element analysis, more rarely for organic traces, but often for alloys and gases. If they are certified on the basis of primary methods, e.g. gravimetry, and do not present risk of degradation they can also be called PRMs. Sets of materials of mixtures of pure substances are used for WDXRF calibration and in general for calibration of comparative methods (see section 2,1.3) 

The use of the materials and the best chemical forms to be chosen have been discussed briefly in the section on validation. Details taken from Moody et al. and Wells are given in Tables 2.5a and b. The chemometric tools to be applied for the calibration of the signal and the calculation of the confidence interval of the calibration curve have been extensively detailed in several manuals and in particular the second book of Massart et al. [7] or in the text book of the FECS [48]. Modern automated spectrometers usually include adapted and properly validated calibration software. 

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Recovery tests or use of internal standards for pre-treatment and use of reference materials for analytical calibration and traceability [5]. 

Proper calibration and traceability to standards using reference dosimetry... 

The purpose of an analytical method is the deliverance of a qualitative and/or quantitative result with an acceptable uncertainty level. Therefore, theoretically, validation boils down to measuring uncertainty . In practice, method validation is done by evaluating a series of method performance characteristics, such as precision, trueness, selectivity/specificity, linearity, operating range, recovery, LOD, limit of quantification (LOQ), sensitivity, ruggedness/robustness, and applicability. Calibration and traceability have been mentioned also as performance characteristics of a method [2, 4]. To these performance parameters, MU can be added, although MU is a key indicator for both fitness for purpose of a method and constant reliability of analytical results achieved in a laboratory (IQC). MU is a comprehensive parameter covering all sources of error and thus more than method validation alone. 

Calibration curves must be made from chemicals with the highest purity as possible. To avoid dilution errors a multi-level calibration curve (six points) based on three stock solutions is recommended. One must also be aware that low concentrations of for example, PAHs (2 ppm) may be adsorbed by the vials up to -90% (Pinto, Jose and Cordero, 1994). A calibrated and traceable balance or a traceable pipette must be used for accurate preparation and dilution of the standards. The calibration curve must cover the concentration range that is needed for the analysis. Both the slope and the intercept must be used to calculate the concentration in the sample, especially if the intercept is different from zero. 

The measurement assurance concept in calibration and traceability at NBS/NIST 103... 

In our experiments crystalline bovine serum albumin from Fluka, cat.no. 05470, (FLUKA albumin) was used for calibration and traceability purposes. The Fluka product was tested for traceability against the National Institute of Standards and Technology (NIST), USA Standard Reference Material (SRM) 927c (Total Protein Standard) bovine serum albumin (NIST albumin). Two different statistical techniques were used to evaluate traceability of the FFUKA albumin to the NIST albumin. 

Our results indicate the importance of using a unified protein standard for calibration and traceability. Having no RM to make a comparison against, we used bovine serum albumin (Fluka, cat. No. 05470) which was traceable to the NIST SRM 927c (Total Protein Standard) bovine serum albumin. Both preparations can be used for traceability. 

Each country has its own accreditation body for laboratories. In the United States, NIST is administering the National Voluntary Laboratory Accreditation Program (NVLAP) [109]. The criteria for accreditation used by NVLAP is consistent with ISO Guide 25, which involves the following items (1) quality system, (2) staff competence and training, (3) facilities and equipment, (4) calibration and traceability, (5) test methods and procedures, (6) recordkeeping, and (7) test reports. 

Webb, K.S., VAM report on calibration and traceability for primary chemical methods application of isotope dilution mass spectrometry (IDMS) to organic ultra-trace analysis (1994). 

Competent calibration and traceability of measurements is essential for industrial manufacture and a major criterion in addressing product liability. It features prominently in both accreditation to ISO/ lEC 17025 and certification to ISO 9001. It also underpins all testing. 

A very complete account of traceability in the context of chemical measurements (Eurachem 2003) emphasizes that laboratories are under ever increasing pressure to demonstrate that their use of measurement and reference standards is appropriate and sufficient. Many of the physical quantities used in routine chemical measurement (e.g., mass and volume, Chapter 2) are underpinned by extensive and effective calibration and traceability systems, making the establishment of traceability for these quantities relatively straightforward. However, the values of chemical quantities... 

It is important that the all inspections be carried out with equipment that is calibrated and traceable to internationally recognized standards. Two of the documents that govern inspection equipment and facilities are ISO-17025 and IPC-QL-651. 

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