Calibration Primary standards

Primary calibration standard A calibration standard based on direct measure of a reference value. 

The NIST material SRM 1866a consists of a set of three common bulk mine-grade asbestos materials chrysotile, amosite and crocidolite, and one glass filter sample. SRM 1867 consists of a set of three imcommon mine-grade asbestos materials antophyllite, tremolite and actinohte. The optical properties of SRMs 1866a and 1867 have been characterized so that they may serve as primary calibration standards for the identification of asbestos types in building materials. 

M=2879, is recommended as a primary calibration standard in preference to pure liquids (too sensitive to impurities) or uniform-particle-size latexes (too sensitive to residual polydispersity). 

The problems with various primary calibration standards are still being resolved. The earlier discussion of DeMore et al, indicates the work undertaken by the carb in comparing oxidant calibration procedures. It was recommended that all oxidant analyzers in the California network be calibrated by a secondary standard consisting of an ultraviolet ozone analyzer. The primary standard recommended is ultraviolet photometry. These recommendations have been adopted by the cars. Potassium iodide, indicated as a second choice, requires the application of a correction factor of 0.78. 

A common primary calibration standard should be established for all monitoring networks. 

Quinine was introduced as a "...primary calibration standard for light testing." In this draft, a "... 5% solution of quinine monochloride dihydrate in water..." Approximately 20 mL of this solution, hermetically sealed in two colorless ampoules (Japanese Industrial Standards No. 5) colorless ampoules or equivalent were to be used. One of these ampoules was to be protected from exposure by being wrapped in aluminum foil, the other exposed at a distance of about 30 cm for three hours. It was specified that "the transmittance of the exposed solution in 1cm cells should lie between 55% and 60%, and that the control should show no significant change (about 70%)." Radiometers were permitted as "...secondary standard UV measuring devices."... 

Further problems arise with the technique by the lack of availability of suitable primary calibration standards and the high viscosity of the synthetic polymers in water, resulting in much lower sample concentrations being used for injection (as low as 0.01%). 

Following the selection procedure, tour gas mixtures designated as A, B, C and D were commercially prepared to IGT specifications, then analyzed by NBS for certification as primary calibration standards in this program and returned to IGT. 

In addition to primary calibration standards like indium, usable for calibration in heating, three secondary liquid crystalline substances [30,43,44], M24, HP-53... 

It has been noted [11] that when unique vibrational bands can be associated with each phase, it is not necessary in principle to calibrate against other standards, although it is often desirable to do this. All that is required is a set of samples of widely varying composition for which the relative band intensities can be determined and extrapolated to the 100% and 0% crystallinity levels. Raman measurement of crystallinity in polyethylene illustrates this approach [229], as does the IR measurement of crystallinity of nylon [224]. Extrapolation of the spectroscopic calibration has sometimes been used to determine the amorphous and crystalline densities of a polymer [224, 225], thereby adding to the information available from the primary calibration standard. However, one must note that vibrational spectroscopy, DSC, X-ray and density actually measure different physical parameters and so a crystallinity determined solely by vibrational spectroscopy may well differ from that obtained by other techniques [230]. Also, with anisotropic materials, molecular orientation can alter band intensities and invalidate calibrations developed using isotropic standards [48, 227, 231]. 

Direct Weighing. The direct weighing method of density determination measures the volume and mass of the mixture to give density. Although direct weighing is not considered a field-type instrument, it could serve as a primary calibration standard. Some of the advantages of this method are the simplicity of the equipment, repeatability, good frequency response, and the lack of... 

Water, redistilled, fi%shly boiled and cooled reagent water for use as a primary calibration standard. 

A limited number of pure substances are available from NIST, primarily clini-cally-relevant compounds such as cholesterol, urea, uric acid, creatinine, glucose, cortisol, tripalmitin, and bilirubin (NIST SRM website). These compounds are certified for purity (greater than 99 %) and are used as primary calibrants in definitive methods for these clinical analytes (see below). Several additional pure substances are available for specific applications such as microchemistry, i.e. elemental composition (acetanilide, anisic acid, cystine nicotinic acid, o-bromobenzoic acid, p-fluoro-benzoic acid, m-chlorobenzoic acid), polarimetric standards (sucrose and dextrose), acidimetric standard (benzoic acid and boric acid). Only three pure substance NIST RMs are available for environmental contaminants, namely the chlorinated pesticides, lindane, 4,4 -DDT, and 4,4 -DDE. 

Conventional XRF analysis uses calibration by regression, which is quite feasible for known matrices. Both single and multi-element standards are in use, prepared for example by vacuum evaporation of elements or compounds on a thin Mylar film. Comparing the X-ray intensities of the sample with those of a standard, allows quantitative analysis. Depending on the degree of similarity between sample and standard, a small or large correction for matrix effects is required. Calibration standards and samples must be carefully prepared standards must be checked frequently because of polymer degradation from continued exposure to X-rays. For trace-element determination, a standard very close in composition to the sample is required. This may be a certified reference material or a sample analysed by a primary technique (e.g. NAA). Standard reference material for rubber samples is not commercially available. Use can also be made of an internal standard,... 

The application of PSA measurements for clinical monitoring of prostatic carcinoma requires fine tuning of PSA assays. One important aspect of this tuning is to have well-defined standards (primary calibrators). Calibrators or primary reference materials consisting of PSA complexed with ACT have been prepared and are available to sponsors of commercial immunoassays. As a result of this, some sponsors have studied calibration stability and have shown that calibration did not change within 14 to 90 days. Primary references of 90 percent PSA-ACT and 10 percent f-PS A have been shown to minimize differences in PSA measurements between different assays [NCCLS Document—Primary Reference Preparations Used to Standardize Calibration of Immunochemical Assays for Serum Prostate... 

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. 

Because of the cost of primary standards, it is normal to use them very sparingly (certainly not as calibration standards), but to include them as unknowns in the analytical run as a measure of the quality of the analysis (see QA, below). Even so, this becomes prohibitively expensive, and it is common to use materials which can be included in each analysis, which may not be fully certified, but whose values can be related to a primary standard. These are often referred to as in-house standards, although they may be more generally available. They can be home-made, providing sufficient attention has been paid to homogenizing the material thoroughly. A simple example is... 

It is necessary, however, to correct for volume if the calibration tool is not a true primary calibration source. Our wet test meter is supposed to be accurate within 4 0.5%, and the volume has been calibrated by personnel at the University of N.C. at Chapel Hill against their primary standard. There are also small differences in flow rates if there are large temperature changes between the calibration site and the sampling site (3). 

Most currently used oxidant and ozone monitors need to be calibrated with a predetermined concentration of ozone in air. Regardless of the principle used to measure ambient ozone or oxidant concentrations, the primary reference standard for calibrating each monitoring device or system should be identical everywhere. This requirement remains to be achieved in practice. Up to June 1975, at least seven calibration procedures were practiced in the United States. These are listed in Table 6-5... 

TABLE 6-5 Primary Reference Standards for Calibration of Ozone or Oxidant Analysis... 

From the primary calibration curve based on polystyrene standards and the Mark-Houwink constants for polystyrene (K,a) a universal calibration curve (Z vs. v), based on hydrodynamic volume is constructed. Z is calculated from... 

The international prototype kilogram is an example of a primary measurement standard its value and uncertainty have been established without relation to another standard of the same kind. When the primary measurement standard is used for calibration, it becomes a primary calibrator. 

Primary calibrators, indeed all calibrators, must be commutable that is, they must behave during measurement in an identical manner to the native analyte material being measured. Matrix reference materials made by mixing a pure reference material with the components of the matrix are unlikely to be entirely commutable, and for this reason some authorities (EURACHEM for one [EURACHEM and CITAC 2002]) advise against using matrix-matched CRMs for calibration, recommending instead their use to establish recoveries, after calibration by a pure reference standard. 

A primary measurement standard is expected to have a known quantity value with minimum uncertainty, although this is not specified as such in the definitions. Because the measurement uncertainty is propagated down the calibration hierarchy, you should start with as small an uncertainty as possible and choose methods to establish purity of primary reference materials with this in mind. 

Calibration is based on the principle of traceability from a primary standard through intermediate standards to the test equipment, with estimates of the uncertainty which increases at each step in the chain. Wherever possible, bought in calibrations should be carried out by an accredited laboratory. It is perfectly acceptable for the test laboratory to do its own calibration but then they must maintain appropriate calibration standards and operate a measurement management system in accordance with IS01001213. One factor which has hindered full appreciation of the detailed needs of... 

Primary calibration with increasing weights of oil is the most accessible, because new calibration standards can easily be reproduced as long as representative seed oil is available however, this calibration does not take into consideration the effect of the seed matrix on the NMR signal of the oil, and therefore may not be entirely accurate. Also, the stability of extracted oil may be somewhat more poor than in the intact oilseed. Freshly extracted oil should be used to ensure a representative sample. 

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