Reference material

Certified reference materials offer an ideal means of estimating bias within a laboratory. In addition, other reference or in-house materials, if assessed or evaluated against certified reference materials, offer an alternative means of assessing a laboratory s capability and establishing a means whereby the analysis is judged to be within acceptable limits. Unfortunately, there are insufficient certified reference materials available to match the wide-ranging variety of matrices and determinands analysed routinely within the environmental sector. Depending on how the reference materials are used, the capability of individual analysts or laboratories can be determined. 

Usually, certified reference materials are reported with a certified value of a determinand, together with narrow tolerance limits within which the result of the analysis should lie. The certified value may be obtained as a mean of values reported by a variety of methods, or it may be a method-based value determined using a specific method. For example, this may involve extracting the determinand with a known solvent composition. 

Certified reference materials should be used in the validation process to establish the capability of the method and then as part of on-going quality control procedures to ensure that the capability of the method is maintained in routine operation. Alternatively, reference or in-house materials (which are often less expensive) can be used in on-going quality control procedures, provided the materials have been evaluated against the certified reference material. 

BS EN ISO/IEC 17025 (2000) General requirements for the competence of testing and calibration laboratories. 

Burgess, C. (2000) Valid analytical methods and procedures. The Royal Society of Chemistry. ISBN 0-85404-482-5. 

In the development of new analytical methods, the use of well-characterized reference materials can be invaluable in comparing measurement technologies, assessing potential sources of bias, and in method validation. Reference materials are also useful as controls for quality assurance. Reference materials produced by national metrology institutes are usually classified as certified reference materials (CRMs)  

Primary reference materialP are highly purified chemicals that are directly weighed or measured to produce a solution whose concentration is exactly known. The lUPAC has proposed a degree of 99.98% purity for primary reference materials. 

These highly purified chemicals may be weighed out directly for the preparation of solutions of selected concentration or for the calibration of solutions of unknown strength. They are supplied with a certificate of analysis for each lot. These chemicals must be stable substances of definite composition that can be dried, preferably at 104 to 110 C, without a change in composition. They must not be hygroscopic, so that water is not absorbed during weighing. 

Secondary reference materials are solutions whose concentrations cannot be prepared by weighing the solute and dissolving a loiown amount into a volume of solution. The concentration of secondary reference materials is usually determined by analysis of an aliquot of the solution by an acceptable reference method, using a primary reference material to calibrate the method. 

Certified Reference Standards (Standard Reference Materials, SRMs) for clinical laboratories are available ffom the NIST and the IRMM. Cholesterol, the first SRM developed by the NIST, was issued in 1967. Today, the lists from the NIST and IRMM are extensive (Table 1-10 and Table 1-11, respectively). Not aU standard reference materials have the properties and the degree of purity specified for a primary standard, but each has been well characterized for certain chemical or physical properties and is issued with a certificate that gives the results of the characterization. These may then be used to characterize other materials. 

For both consistency over each day, and to check day-to-day reproducibility, it is very useful to analyse at regular intervals either synthetic standard solutions, prepared in an appropriate matrix, or, better still, standard or certified reference materials, which contain precisely known amounts of the elements or species of interest. Routine long-term time-plots of the results of such regularly repeated analyses are very useful in showing up errors arising as a consequence of hitherto unrealized procedural changes. 

Certified reference materials are now available for many environmental materials, and are often used in setting up new methods or for confirming that modifications to existing procedures are acceptable. If the results for a selection of four to five such materials covering a range of determinant and potential interferent concentrations are very close to certified values, this is strong circumstantial evidence that the method is working well. 

Certified reference materials are expensive to produce and correspondingly expensive to purchase. They should therefore be used sparingly and looked after carefully to avoid contamination. Many analysts prefer to produce their own standard reference materials in bulk for quality control purposes. Care is needed to make sure that such a standard material is homogenized as well as possible, which usually involves producing a material with a very fine particle size. 

Sulphur gases in permeation devices are the usual means for calibrating the response of a gas chromatographic detector. Liquefied gases are sealed in Teflon tubes, or in glass or stainless steel tubes with Teflon windows on the ends. Gases permeate through the Teflon at a constant rate until the enclosed supply is exhausted. The rate of permeation is 

A soil-gas sample collected by hollow probe needs no further preparation for injection into a GC system. However, the results obtained from sulphur analysis of a soil gas may be inaccurate representatives of their original state in the field. The time elapsed between collection of the sample in the field and analysis in the laboratory permits many changes in the sample. Some sulphur components may adsorb on the walls of the glass, plastic, or stainless-steel syringes or sample carriers used for sample transportation. Non-adsorbed components may interact with each other. The most reliable soil-gas analyses are those made almost immediately after collection. Even these analyses may not be completely accurate. 

Gas chromatography Several sulphur compounds determined simultaneously Difficult and, in some cases, impossible to use in the field 

Detector tubes Easy to use in field Inexpensive Collects and analyses gases Limits of detection too high for geochemical exploration 

Wet chemical (solutions and filters) Widely used for H2S and SO2 air pollution studies Tedious 

A general conservative guideline is that the accuracy of the mass in a TGA experiment while heating or at elevated temperature is an order of magnitude less than that at static room temperature conditions. Both precision and accuracy can be improved markedly through appropriate use of tares and careful baseline subtraction techniques. 

The preceding section has indicated the uncertainties in determining the change in mass. Establishing the exact temperature to associate with the mass. 

Because of the thermal transport conditions, the heating rate has a strong influence on the apparent temperature of events. The inability to instantly heat or cool the sample means that there is inevitably a lag between the sample s response and the temperature program. This lag obviously increases with increasing scan rate. 

Two other factors give rise to the departure of the sample s actual temperature from the desired programmed temperature. One is associated with the enthalpy of the event. An endothermic reaction will consume the heat supplied rather than raise the temperature of the sample. Conversely, an exothermic reaction can lead to the temperature getting ahead of the intended temperature. If the sample combusts, the temperature can get far ahead. The 

In summary, no general values can be listed for the precision or accuracy to be expected in a TGA experiment, even when well calibrated. As with the 

Aspen Plus does not permit the use of activities in the reaction rate expressions. User subroutines are used to incorporate this feature when necessary. 

Aspen Dynamics is used to study dynamics and control of the real systems. The type of reactions that can be used are limited (they must be kinetic and of power law form). These restrictions make the use of Aspen products somewhat less convenient than we would like. 

There are many reactive distillation systems and many recent publications and patents. Doherty and Malone give 61 chemical systems (see their table 10.5) and cite 134 references in their chapter on reactive distillation. An updated literature survey shows that there were 1105 publications and 814 US patents between 1971 and 2007. 

A literature search using Compendex showed some interesting chronological features. The search was limited to only journal articles in English. From 1969 to 1994 there were only 35 citations in reactive distillation design and a mere six in reactive distillation control. From 1995 to 2007 there were 435 citations in reactive distillation design and 106 in reactive distillation control. This clearly indicates the recent level of interest, particularly in control. 

There are four books that deal with reactive distillation, among other subjects  

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A motor fuel has an octane number X if it behaves under tightly defined experimental conditions the same as a mixture of X volume % of isooctane and (100 - X)% of n-heptane. The isooctane-heptane binary mixtures are called primary reference fuels. Octane numbers higher than 100 can also be defined the reference material is isooctane with small quantities of tetraethyl lead added the way in which this additive acts will be discussed later. 

Sing (see Ref. 207 and earlier papers) developed a modification of the de Boer r-plot idea. The latter rests on the observation of a characteristic isotherm (Section XVII-9), that is, on the conclusion that the adsorption isotherm is independent of the adsorbent in the multilayer region. Sing recognized that there were differences for different adsorbents, and used an appropriate standard isotherm for each system, the standard isotherm being for a nonporous adsorbent of composition similar to that of the porous one being studied. He then defined a quantity = n/nx)s where nx is the amount adsorbed by the nonporous reference material at the selected P/P. The values are used to correct pore radii for multilayer adsorption in much the same manner as with de Boer. Lecloux and Pirard [208] have discussed further the use of standard isotherms. 

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. 

Head A J and Sabbah R 1987 Enthalpy Recommended Reference Materials for the Realization of Physicochemical Properties ed K N Marsh (Oxford Blackwell)... 

Approximate Acidities of Some Hydrocarbons and Reference Materials... 

Thus, if the specific surface of the reference material is known (e.g. from the nitrogen isotherm) the specific surface of the test sample can be calculated from the ratio of the slopes of the a -plots. The specific surfaces of all the... 

Analysis sheet for Simulated Rainwater (SRM 2694a). Adapted from NIST Special Publication 250 Standard Reference Materials Catalog 1995-96, p. 54 U.S. Department of Commerce, Technology Administration, National Institute of Standards and Technology. 

Consider the situation when the accuracy of a new analytical method is evaluated by analyzing a standard reference material with a known )J,. A sample of the standard is analyzed, and the sample s mean is determined. The null hypothesis is that the sample s mean is equal to p. 

Because of the risk of lead poisoning, the exposure of children to lead-based paint is a significant public health concern. The first step in the quantitative analysis of lead in dried paint chips is to dissolve the sample. Corl evaluated several dissolution techniques. " In this study, samples of paint were collected and pulverized with a Pyrex mortar and pestle. Replicate portions of the powdered paint were then taken for analysis. Results for an unknown paint sample and for a standard reference material, in which dissolution was accomplished by a 4-6-h digestion with HNO3 on a hot plate, are shown in the following table. 

The %w/w lead in a lead-based paint Standard Reference Material and in unknown paint chips is determined by atomic absorption using external standards. 

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. 

For example, if a carbonaceous sample (S) is examined mass spectrometrically, the ratio of abundances for the carbon isotopes C, in the sample is Rg. This ratio by itself is of little significance and needs to be related to a reference standard of some sort. The same isotope ratio measured for a reference sample is then R. The reference ratio also serves to check the performance of the mass spectrometer. If two ratios are measured, it is natural to assess them against each other as, for example, the sample versus the reference material. This assessment is defined by another ratio, a (the fractionation factor Figure 48.2). 

National Institute of Standards and Technology (NIST). The NIST is the source of many of the standards used in chemical and physical analyses in the United States and throughout the world. The standards prepared and distributed by the NIST are used to caUbrate measurement systems and to provide a central basis for uniformity and accuracy of measurement. At present, over 1200 Standard Reference Materials (SRMs) are available and are described by the NIST (15). Included are many steels, nonferrous alloys, high purity metals, primary standards for use in volumetric analysis, microchemical standards, clinical laboratory standards, biological material certified for trace elements, environmental standards, trace element standards, ion-activity standards (for pH and ion-selective electrodes), freezing and melting point standards, colorimetry standards, optical standards, radioactivity standards, particle-size standards, and density standards. Certificates are issued with the standard reference materials showing values for the parameters that have been determined. 

Physical or artifactual standards are used for comparison, caUbration, etc, eg, the national standards of mass, length, and time maintained by the National Institute of Standards and Technology (NIST) or the standard reference materials (SRMs) collected and distributed by NIST. Choice of the standard is determined by the property it is supposed to define, its ease of measurement, its stabiUty with time, and other factors (see Fine chemicals). 

Standard reference materials provide a necessary but insufficient means for achieving accuracy and measurement compatibiUty on a national or international scale. Good test methods, good laboratory practices, well-qualified personnel, and proper intralaboratory and intedaboratory quaUty assurance procedures ate equally important. A systems approach to measurement compatibiUty is ikustrated in Figure 2. The function of each level is to transfer accuracy to the level below and to help provide traceabiUty to the level above. Thus traversing the hierarchy from bottom to top increases accuracy at the expense of measurement efficiency. 

Analytical standards imply the existence of a reference material and a recommended test method. Analytical standards other than for fine chemicals and for the NIST series of SRMs have been reviewed (6). Another sphere of activity ia analytical standards is the geochemical reference standards maintained by the U.S. Geological Survey and by analogous groups ia France, Canada, Japan, South Africa, and Germany (7). 

The predorninant method for the analysis of alurninum-base alloys is spark source emission spectroscopy. SoHd metal samples are sparked direcdy, simultaneously eroding the metal surface, vaporizing the metal, and exciting the atomic vapor to emit light ia proportion to the amount of material present. Standard spark emission analytical techniques are described in ASTM ElOl, E607, E1251 and E716 (36). A wide variety of weU-characterized soHd reference materials are available from major aluminum producers for instmment caUbration. 

Vitreous siUca aimealed at 1100°C has been designated NIST Standard Reference Material 739 (LI and L2). Its expansion coefficient, a, may be calculated for 300—700 K from the following expression (144), where Tis the absolute temperature in Kelvin. 

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