Calibration reference materials

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. 

Now, with the advent of reference materials, the term standard is too general to be used alone and it is, therefore, normal to specify the type of standard, e.g. analytical, calibration, reference material, etc. Use of the term standard(s) is, therefore, only appropriate in the most general of contexts. 

Several uptake kits are commercially available, Tg uptake values for individual subjects depend on specific assay conditions such as incubation time and temperature, quantity and avidity of the binding material, and specific activity and choice of the label. To compensate for variations in uptake values, the percent uptake for each unknown should be determined by comparison with a calibrator, reference material, or euthyroid reference serum of Imown percent uptake. Almost aU commercial assays determine the percent uptake... 

Complex environmental monitoring measurements are generally based on successive analytical steps, such as extraction, derivatization, separation and detection. This succession multiplies the risk that the traceability chain is broken because of a lack of appropriate references (e.g. pure calibrants, reference materials). Reference methods usually refer to methods with high metrological qualities (they may actually be standardized methods), and examples are provided by primary methods that are directly traceable to SI units without the need to use an external calibration. 

Where available, reference materials need to be used for calibration or quality control. The National Institute of Standards and Technology (NIST) and many commercial sources supply such materials. Many high performing technology laboratories are capable of preparing their own reference materials using the same approach used by NIST in certifying their Standard Reference Materials (SRM). The calibration reference materials (RM) should be traceable to national standards, and the traceability must be preserved in the laboratory s documentation system. 

As described in Section 5.5, the sample heat capacity is generally estimated in conventional DSC from the difference in heat flow between the sample and an empty sample vessel, using sapphire as a calibrated reference material. Alternatively, C can be determined from the difference in heat flow... 

Standards Instrument Calibration Reference Materials Traceability Laboratory Information Management Systems. 

Basically, there are three approaches to peak area measurement (a) the cut and weigh method, which is accurate to within 5% (b) the planimeter method, which is accurate to within 1% (c) the integrator method, which is more accurate than method (b). It should be noted that when computer-equipped apparatus is used, enthalpy calculations are carried out automatically if the apparatus is correctly calibrated using calibration reference materials. 

Table 1.5 Melting points and enthalpies enthalpy calibration reference materials [53]... 

Melting temperatures of different materials are identified as different portions of the thermal curve. During calibration the melting temperature of the calibration reference materials (usually a pure metal such as indium) is identified as the onset temperature of the melting peak (Fig. 4). 

The next thing to do is to select two calibration reference materials that melt within the experimental temperature range. It is best to use the purest materials available. Materials that may be are commonly used for this temperature range (—10 to 200°Q are distilled water and indium. (Note that there are many other subambient calibration materials. Water was chosen to explain how to run a liquid sample.) The melting of these materials is well documented and reliable. At room temperature, water is a liquid and indium is a solid. Each sample will have to be prepared slightly differently. 

With most non-isothemial calorimeters, it is necessary to relate the temperature rise to the quantity of energy released in the process by determining the calorimeter constant, which is the amount of energy required to increase the temperature of the calorimeter by one degree. This value can be detemiined by electrical calibration using a resistance heater or by measurements on well-defined reference materials [1], For example, in bomb calorimetry, the calorimeter constant is often detemiined from the temperature rise that occurs when a known mass of a highly pure standard sample of, for example, benzoic acid is burnt in oxygen. 

Analysis of Standards The analysis of a standard containing a known concentration of analyte also can be used to monitor a system s state of statistical control. Ideally, a standard reference material (SRM) should be used, provided that the matrix of the SRM is similar to that of the samples being analyzed. A variety of appropriate SRMs are available from the National Institute of Standards and Technology (NIST). If a suitable SRM is not available, then an independently prepared synthetic sample can be used if it is prepared from reagents of known purity. At a minimum, a standardization of the method is verified by periodically analyzing one of the calibration standards. In all cases, the analyte s experimentally determined concentration in the standard must fall within predetermined limits if the system is to be considered under statistical control. 

Because of the complex nature of the discharge conditions, GD-OES is a comparative analytical method and standard reference materials must be used to establish a unique relationship between the measured line intensities and the elemental concentration. In quantitative bulk analysis, which has been developed to very high standards, calibration is performed with a set of calibration samples of composition similar to the unknown samples. Normally, a major element is used as reference and the internal standard method is applied. This approach is not generally applicable in depth-profile analysis, because the different layers encountered in a depth profile of ten comprise widely different types of material which means that a common reference element is not available. 

Throughout this book the use of a number of standard analytical samples is recommended in order that practical experience may be gained on substances of known composition. In addition, standard reference materials of environmental samples for trace analysis are used for calibration standards, and pure organic compounds are employed as standard materials for elemental analysis. 

In the positive ion mode, calibration is most frequently performed using perfluorokerosine, but different instrument manufacturers recommend different reference materials. Consult your instrument manual for the recorn-... 

Because an increase in resolution causes a decrease in sensitivity, it is best to operate at the lowest resolution commensurate with good results. Some instrument data systems will allow calibration with an external reference material such as perfluorokerosene and then use of a secondary reference material for the internal mass reference. Tetraiodothiophene, vaporized using the solids probe inlet, is recommended as the secondary reference. The accurate masses are 79.9721, 127.9045, 162.9045, 206.8765, 253.8090, 293.7950, 333.7810, 460.6855, and 587.5900. For a higher mass standard, use hexaiodobenzene. Because the mass defect for these internal reference ions are so large, a resolution of 2000 is ample to separate these ions from almost any sample ions encountered in GC/MS. 

Requirements for standards used In macro- and microspectrofluorometry differ, depending on whether they are used for Instrument calibration, standardization, or assessment of method accuracy. Specific examples are given of standards for quantum yield, number of quanta, and decay time, and for calibration of Instrument parameters. Including wavelength, spectral responslvlty (determining correction factors for luminescence spectra), stability, and linearity. Differences In requirements for macro- and micro-standards are considered, and specific materials used for each are compared. Pure compounds and matrix-matched standards are listed for standardization and assessment of method accuracy, and existing Standard Reference Materials are discussed. 

Procedures for determining the spectral responslvlty or correction factors In equation 2 are based on radiance or Irradlance standards, calibrated source-monochromator combinations, and an accepted standard. The easiest measurement procedure for determining corrected emission spectra Is to use a well-characterized standard and obtain an Instrumental response function, as described by equation 3 (17). In this case, quinine sulfate dlhydrate has been extensively studied and Issued as a National Bureau of Standards (NBS) Standard Reference Material (SRM). 

The following table, given on next page, shows some materials suitable for such calibration, along with the temperature at which the solid state change occurs. Note that the reference material, a-Al203, is thermally inert whereas the... 

Mills has concluded in his review article on molten slags that (1) most viscosity measurements were subject to experimental imcertainties of 25% (2) in some cases experimental uncertainties could be > 50% and (3) experimental uncertainties as low as 10% could be achieved by careful calibration of viscometers with high and low temperature reference materials. 

Nevertheless, this method was successfully applied by Gulyaeva et al. for the log P and log D determination of 15 P-sympatholytic drugs [56]. Another study by Welerowicz and Buszewski compared the HpophiHcity values of P-blockers obtained with a column made of a monoHthic-silica Cjg with a conventional porous silica particles Cjg as reference material [27]. A modified method was used for evaluating logP with two main differences (i) logfeg was considered rather than retention times, and (ii) benzene and butyl-benzene were used as calibration compounds. 

ISO Guide 32 (1997) Calibration in analytical chemistry and use of certified reference materials. International Organization for Standardization, Geneva. 

For relatively simple matrices, such as pure metallic CRMs synthetic reference materials for direct calibration were prepared and used, for example Bi, Cd, Hg, Pb and T1 in high purity gallium (HUtenkamp and Jackwerth 1988), Ag in copper (Pau-wels et al. 1990) and Au and Pd in silver (Hinds 1993). Direct calibration by solid biological materials with added analyte belongs also to these quite successfully applied techniques (Hofmarm et al. 1992). 

Within collaborative work on element concentrations in a number of biological reference materials using solid sampling and other analytical methods, calibration of Cd, Cu, Pb and Zn in BCR CRM 185 Bovine liver with solid CRMs was performed for each element with a reference material of the same matrix, NIST SRM 1577... 

During production and characterization of various internal animal tissue reference materials for a number of metals, a comparative study was performed for Pb in six bovine teeth and two bovine bone materials using calibration with a solid RM and two versions of wet chemical analysis with GF-AAS and electrochemical (DPASV) detection. There was good agreement in the range of approx. 1.3-3 tng/kg dry weight for all techniques used (Liicker et al. 1992). 

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. 

These reference materials are also suitable for calibration of instruments for isotopic measurements, calibration and evaluation of isotopic measurement procedures, development of isotopic measurement methods, and nuclear material accountability measurements. Many of these IRMM materials are, in fact, concerned with the last of these applications, i.e. in the area of nuclear material accountability. Some of them have been examined by a nuclear experts committee and certified as EC-NRMs (European Community - Nuclear Reference Materials). 

Quality assurance of radiopharmaceutical preparation and use is obviously a very important topic because of its direct impact on patient diagnosis, treatment and health (see, e.g. Abreu 1996). Reference materials play only a small - but nevertheless important -role in this process, mainly in the area of calibration of radioactivity-measuring instruments. The materials of interest are all pure chemical containing calibrated activities of selected radionuclides used commonly in nuclear medicine (e.g. Co, Ga, I,... 

Rossbach M, Ostapczuk P, Emons H (1998) Microhomogeneity of candidate reference materials Comparison of solid sampling Zeeman-AAS with INAA. Fresenius J Anal Chem 360 380-383. Rossbach M, Stoeppler M (1987) Use of CRMs as mutual calibration materials and control of synthetic multielement standards as used in INAA. J Radioanal Nud Chem Artides 113 217-223. Sargent M (1995) Development and application of a protocol for quality assurance of trace analysis. Anal Proc 32 71-76. 

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