Secondary reference materials

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. 

Much of the early work with certified reference materials was linked to the derivation of reference methods and there was a period in which primary or definitive (i.e. very accurate but usually very complex) and secondary (or usable) methods were reported e.g. steroid hormones (Siekmann 1979), creatinine (Siekmann 1985), urea (Welch et al. 1984) and nickel (Brown et al. 1981). Although there are some application areas, such as checking the concentrations of preparations listed in a pharmacopoeia, where a prescribed, defined method has to be used, in practice such work is limited. However, this approach to chemical analysis is no longer widely used and will not be further discussed. The emphasis now is placed on using RMs to demonstrate that a method in use meets analytical criteria or targets deemed to be appropriate for the application and to develop figures of merit (Delves 1984). 

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). 

Secondary reference materials, i.e. manufacturers in-house calibrants and controls (including commercial reference materials)... 

In Tables 6.3 and 6.4 RMs of three major producers are mentioned, i.e. the World Health Organization (WHO, International Standards), BCR (European Union, CRMs) and the National Institute of Standards and Technology (NIST, USA, SRMs). Some important national producers of clinical reference materials are the Chemicals and Inspection Testing Institute (CITI, Japan), National Institute for Biological Standards and Control (NIBSC, UK), and Deutsche Gesellschaft fiir Klinische Chemie (DGKC). There are numerous commercial producers of secondary reference materials. 

One approach to providing secondary RMs is the NIST Traceable Reference Material (NTRM) program for gas standards (Jenks et. al. 1998 NIST 1997). The NTRM program is to be extended to metals, trace elements and pure substances. It may be possible to extend this concept to other types of RM, but it is difficult to see how it can easily be applied to the production of complex matrix CRMs. 

Secondary metals production may refer to secondary metal extraction or to secondary alloy production. Secondary metal extraction normally extracts one metal from the waste and sells any valuable by-products to other metal extractors or chemical processors. In secondary alloy production, the scrap is blended and refined so as to yield an alloy of the desired chemistry. In both cases it is necessary to deal with various sizes, shapes, and compositions of waste materials and the processing route adopted moreover, the plant where the process is implemented should have sufficient flexibility to handle such widely varying input material. 

The development and characterization of Certified Reference Materials is an expensive process. Because of this, emphasis on the use of Certified Reference Materials is usually directed more towards the initial validation of a method it is rarely economical to use a reference material for routine quality control although it can be used to calibrate other, cheaper, secondary materials which can be used for routine quality control. 

Although most assays perform well with regard to specificity and reproducibility, the major problem remains their standardization (A9, Dl, K30, L4). There is currently no internationally accepted standard, and the selection of a reference material raised many problems (A8, G5, K30, L4). A number of questions have not been solved Should the standard consist of several apo(a) isoforms Can the reference material be lyophilized Should results be expressed as mass or as moles of apoprotein or lipoprotein How should the protein mass of the primary standard be determined What are optimal storage conditions for the secondary standard Which method can be used as a reference method Can recombinant apo(a) represent an alternative for a primary standard These problems came to light in the course of the international surveys whose results were presented at the Lp(a) Workshop in New Orleans (1992) (L4). 

If reference materials are not available, the challenge study lives np to its name. Specificity may still be demonstrated by a comparison of test results containing the impurities of interest to a second, well-characterized procedure (e.g., USP method). If a secondary method is unavailable, peak purity evaluation may be used to further demonstrate specificity of the method. 

Other classes of reference materials now in existence include secondary reference materials. These are materials produced commercially for reference purposes, but whose guarantee rests soley with the producer. "Analyzed" materials such as geological materials obtained from the United Staes Geological Survey, represent test samples that complement the variety available from the previously mentioned sources. However, the "accepted" analyses reported for these materials are based upon consensus values obtained from large scale interlaboratory collaborative tests (round robins). Analysis of these materials can provide a useful means of comparing performance with other laboratories, but it does not ensure accuracy. In addi-... 

A sample for which the true response is already known or is established is called a standard. A standard can be a primary standard, which is a standard through which other substances or solutions are made to be standards. It can also be a secondary standard, a solution whose concentration is known accurately either because it was prepared using a primary standard or because it was compared to another standard. All standards must ultimately be traced to a standard reference material (SRM). Standard reference materials are available from the National Institute of Standards and Technology (NIST) and should not be used for any other purpose in the laboratory (Section 5.4). Standardization is an experiment in which a solution is compared to a standard in order for itself to be a standard. The solutions used to establish a standard curve are often called reference standards and these must also be traceable to an SRM. 

Reference materials are often real samples that have been carefully prepared and analyzed by many laboratories by many different methods. In this way, their known value and accompanying confidence limits are determined. The regular use of reference materials not only provides for calibration and standardization, but it also can demonstrate an analyst s proficiency with a method. It should be noted that SRMs are expensive, however, and are not often used for routine calibration and standardization work. Usually, primary and secondary standards are used for that. Another important fact about RMs is that they are considered to have a finite shelf life and cannot be confidently used as RMs after a certain period of time. 

Isotope ratios are given as deviations, in relation to a defined primary standard (zero point). The polyethylene foils CH 7 and NBS 22-oil are commercially available secondary standards, certificated and managed by the International Atomic Energy Agency. However, GC-lRMS systems cannot be calibrated without the aid of alternative peripheries like an elemental analyser (EA) or a dual inlet, owing to the lack of commonly accepted reference materials applicable in GC-IRMS techniques (Fig. 17.11). 

Regular use of certified reference materials and/or internal quality control secondary reference materials... 

Standard reference material (SRM) for wavelength accuracy, stray light, resolution check, and photometric accuracy can be purchased from NIST. Certified reference materials (CRMs) which are traceable to NIST and recertification services can be purchased from instrument manufacturers and commercial vendors [12]. The cost of neutral-density filters and prefabricated standard solutions in sealed cuvettes can be substantial. When purchasing performance verification standards from a secondary supplier other than a national standard organizations such as NIST in the United States and National Physical Laboratory (NPL) in the United Kingdom, make sure that the traceability of the standards are available in the certificates. The traceability establishes the relationship of individual results to the national standard through an unbroken chain of comparisons. 

A standard reference material for infant formula has also been certified for vitamin C, riboflavin, niacin, and pyridoxine (197). Another recent study (198) indicated that off-the-shelf wheat germ is suitable for use as a secondary reference material for thiamine and niacin riboflavin was also evaluated but found to be somewhat unstable over time. Powdered orange drink was deemed suitable as a secondary reference material for vitamin C. 

PA Marshall, JM Vandepeer, I Pant, VC Trenerry, P Scheelings, DR Buick. The development and evaluation of secondary food reference materials for the determination of cholesterol, fatty acids and selected water-soluble vitamins in foods. Food Chem 58 269-276, 1997. 

It consists of three levels. At the top of the structure a network of national laboratories provides the primary chemical measurement standards and ensures that these are linked up with the international reference framework for chemical measurements. Via primary reference materials and reference measurements, a secondary level consisting of accredited chemical calibration laboratories, including verification authorities in the regulated area, is connected to the national standards level. 

This secondary or intermediate level has an important multiplier function. It is firmly linked to the national standards and provides traceable calibration means (mainly certified reference materials) and test samples to the workshop level, which consists essentially of chemical testing laboratories (including medical laboratories) which are required to give evidence to their customers that their measurement results are traceable to recognized references. In the case of medical laboratories, the traceability requirement also has a legal background. 

Meeting these stringent requirements enables the chemical calibration laboratories to act as providers of calibration means at the secondary level. This task requires that the value assignment to the reference materials and other calibration means provided to the user is more accurate than the measurement results at the user level need to be. Chemical calibration laboratories also exist in other countries, for example in the Netherlands and in the U.K. 

The most extensive effort involves quality assurance. CDC provides common quality control (QC) materials and all members use standardized QC procedures. In addition, CDC regularly surveys the CRMLN laboratories to ensure they meet the required criteria. Initially, surveys were conducted monthly however, as the laboratories gained experience and performance improved, the survey schedule was changed to bimonthly. These surveys use CDC frozen serum secondary reference materials that have been prepared using NCCLS C37-A protocol [28], TC surveys include three levels analyzed in duplicate in two runs. HDLC surveys include four levels analyzed in duplicate in four runs. As an additional check on the quantitative step of the HDLC method, a low-total cholesterol material with cholesterol concentration <100 mg/dl is added to the TC survey. LDLC surveys include four levels analyzed in quadruplicate in four runs. 

ACL Secondary reference materials CDC SRM 1951a, two reference materials... 

NCCLS (1999) Preparation and validation of commutable frozen human serum pools as secondary reference materials for cholesterol measurement procedures. NCCLS, Wayne, Pa. NCCLS document C37-A... 

Secondary pH reference materials can be derived from the PS buffer solutions by different measurement procedures, which provide results for pH(SS) of the same nominal composition as pH(PS) pH(SS) of different composition pH(SS) not compatible with platinum hydrogen electrodes. 

An example of such a secondary buffer is acetic acid for which a consistent chemical quality is hard to achieve. Calcium hydroxide and potassium tetraoxalate do not fulfil the criteria for a primary pH reference material because the contribution of hydroxyl or hydrogen... 

The quantity pH is used to characterise the acidity of a system, but also in speciation [22] because of the importance of the species H+ for controlling the chemical equilibrium. The required comparability of pH can be ensured only if the buffers used for the calibration are traceable to primary pH reference materials. pH(PS) and pH(SS) values of primary and secondary reference... 

The principal services include provision of high level calibration services (Fig. 3) and/or standards to suppliers offering secondary services or standards, preparation of standards and reference materials, and limited services direct to field... 

In the field of clinical chemistry a complete structure of methods and materials was proposed in the late 1970s for better accuracy and compatibility of laboratory results (Boutwell, 1977 Alvares et al., 1982). The system includes primary and secondary reference materials and control materials, as well as definitive, reference, and laboratory methods. Each of the materials or methods would guarantee the transfer of accuracy from the highest level to the final routine level of the laboratories. For this transfer the three types of materials would in principle be used in conjunction with the three levels of methods. A schematic overview of this system would be as follows ... 

In this scheme, the primary reference material is defined as a chemical substance of the highest (and known) purity, or a well-characterized substance in a matrix, This classification of materials is, however, fairly arbitrary. It is ideal when used in connection with standards characterized in terms of biological activity. Primary standards are thus the International Reference Preparations (IRP) produced by the World Health Organization (WHO). In this case the primary standard for a particular antibiotic is the WHO reference preparation which constitutes the unit of that antibiotic. When people wish to use it they have to prepare a large batch of samples calibrated to the primary. This is then called a secondary standard. However, for well-defined chemical parameters, the term certified reference material is preferred. 

An important part of the sampling methodology relates to the use of control samples in order that the data can be quality controlled and quality assured (see Johnson et al., 2008). While it is only duplicates that are created during the sampling process, these, along with replicates, blanks (for waters) and secondary reference materials need to be assigned sample numbers so they are included as part of the routine sample submission and are blind to the analyst. Duplicate samples are collected from a... 

The G-BASE project collects samples in random number order (Plant, 1973), as this helps identify any correctable systematic errors introduced during sample preparation and analysis, processes in which the samples are handled in numeric order. For every block of one hundred numbers, five numbers are reserved for control samples so when they are submitted within a batch of samples they are blind to the analyst. The control samples inserted are one duplicate sample, two replicate samples, two blanks, and two secondary reference materials (SRM) used to monitor accuracy and precision as well as to level data between different field campaigns (see Johnson et al, 2008). Along with the original sample ofthe duplicate pair, this means 8% of samples submitted are control samples, a point not to be overlooked in setting the budget for analyses. 

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