Primary calibrant

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

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 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. 

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. 

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... 

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). 

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). 

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. 

Much care must be exercised in comparing atmospheric concentrations between one place and another, because of differences in primary calibration techniques or in instrumentation. Chapter 6 summarizes these problems in detail. 

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

Evaluation of primary calibration procedures applicable nationwide for ozone measurement. 

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... 

Our primary calibration curve obtained with narrow MWD linear polystyrene samples could now be used to determine the molecular weight distribution of branched PVAc (B ) by means of eqs. (39), (40) and (17). The calibration curve for branched PVAc was not linear but showed a slight upturn at the low elution volume range as shown in Fig. 9. The calibration curve in Fig. 9 could be represented by... 

An unbroken chain of comparisons/calibrations means that there must be an identified path of assignments of value to calibrators from the calibrator used in the final measurement back to the primary calibrator that is the embodiment (also known as the realization) of the unit. 

At the top of the traceability chain is the stated metrological reference, which for our purposes is the definition of a unit. It might not be obvious how a piece of paper from Paris giving the interesting, but rather esoteric, definitions of the SI base units can be used as a reference. The metrological reference allows the creation of a primary calibrator that embodies the value of the unit (or some multiple or submultiple of it). The international prototype... 

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. 

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. 

It is relevant to ask how often the routine measurement procedures currently used in laboratory medicine provide results that are traceable to high-level calibrators and reference measurement procedures (Lequin personal communication). It turns out that primary reference measurement procedures and primary calibrators are only available for about 30 types of quantity such as blood plasma concentration of bilirubins, cholesterols and sodium ion. International reference measurement procedures from the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) and corresponding certified reference material from BCR are available for the catalytic activity concentration of a few enzymes such as alkaline phosphatase and creatine kinase in plasma. For another 25 types of quantity, such... 

The CRMLN model for cholesterol fits into the International Organization for Standardization s traceability scheme [30]. Table 2 shows how the CDC and CRMLN define the various components of the hierarchy. The model for TG is similar to that for cholesterol, using SRM 1595 (tripalmitin) as the primary calibrator. The models for HDLC and LDLC are different because these heterogeneous analytes are not traceable to the SI unit, and primary reference methods do not exist. The reference methods at CDC are the highest order that can be obtained for HDLC and LDLC. 

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%). 

The test strips or slides cannot be calibrated with aqueous solutions (primary calibrator). This is because the matrix of the sample and of the primary calibrator is not the same. Calibration with so-called secondary calibrators is an alternative. The concentration or activity of the intended analyte is determined by chemical, enzymatic, physical or immunological analysis in any case it should be obtained by the best available method. 

In general, to standardize a particular protein, a purified form of that protein is used as a primary calibrator (see Chapter 20). The purified preparation must, however, express the same immunoreactivity as the native protein. Unfortunately, once removed from its natural milieu, apo B-lOO is insoluble in aqueous buffers. This phenomenon is attributed to the very hydrophobic nature of apo B-100. An LDL preparation with density of 1,030 to 1.050 g/mL, often referred to as narrow-cut LDL, is generaUy used as the primary standard for apo B-100. The protein concentration of the purified preparation is determined by amino acid analysis. In contrast, freshly purified apo A-I is soluble in aqueous buffers and is suitable as a primary standard. 

When the operator has determined the totality of the uncertainty budget within the possibilities offered by his laboratory, the uncertainty due to the presence of (a) possible systematic error(s) or bias remains. Only one possibility exists to detect such a bias. It lies in external help. Comparing the results of the test method to another method developed in-house involves the risk of having an unknown laboratory bias, e g. biased primary calibrants etc. Therefore, it is more appropriate to look for external help. This can come from the comparison of results obtained on a reference sample with those obtained on the same sample by another laboratory or by analysing a certified reference material. Both possibilities will be dealt with in the next two chapters of this book. 

BCR studies that substances on the market had purity figures of sometimes less than 70 M) (even 50%) where they were stated to be pure at 98% This demonstrates that in organic and organo-metallic trace analysis impurity, and in general uncertainty due the quality of primary calibrants, cannot be neglected and are sources of important bias in measurements. Therefore, it is of importance for analytical chemists to identify reliable suppliers. If necessary, it may be necessary to purify purchased substances. Collaboration in such studies and exchange of substances is one of the most useful outcomes one can get from colleagues. BCR projects have often initiated such relations. 

Discrepancies in INAA sets of data were noticed for Zn. Discussions focused on the irradiation position of the samples in the rotating rig, the primary calibrants used, possible shelf shielding in the calibrant, the resolution of the gamma-energies in the detector and the counting geometry none could explain the discrepancies. 

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