Reference materials, analytical

Michalski G., Savarino, J., Bohlke J.K. Thiemens, M. (2002) Determination of the Total Oxygen Isotopic composition of nitrate and the calibration of a AnO nitrate reference material. Analytical Chemistry 74, 4989 4993. 

Keywords Reference materials Analytical chemistry Measurements in chemistry Validation Method development... 

IAEA (1989). Intercomparison runs reference materials. Analytical quality control services. International Atomic Energy Afgency, Vienna. 72 pp. 

High precision in stable isotope mass spectrometry is achieved through repeated measurements of a reference gas, and inter-laboratory compatibility is maintained through calibration against standard reference materials. Analytical precision for most modem mass spectrometers is about 0.1%o for both 6 C and... 

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. 

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. 

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. 

It is known that the reliability of analytical information obtained depends particularly on the range of reference materials (RM) used. The most of RMs developed by the Institute of Geochemistry, SB RAS are included in the State Register of certified types of National Certified Reference Materials of Russian Federation. The reference materials are routinely analyzed for the stability and their life durations are timely prolonged. Developed RMs (27 samples) characterize mainly mineral substances. 

Laser based mass spectrometric methods, such as laser ionization (LIMS) and laser ablation in combination with inductively coupled plasma mass spectrometry (LA-ICP-MS) are powerful analytical techniques for survey analysis of solid substances. To realize the analytical performances methods for the direct trace analysis of synthetic and natural crystals modification of a traditional analytical technique was necessary and suitable standard reference materials (SRM) were required. Recent developments allowed extending the range of analytical applications of LIMS and LA-ICP-MS will be presented and discussed. For example ... 

Vapor-phase decomposition and collection (Figs 4.16 to 4.18) is a standardized method of silicon wafer surface analysis [4.11]. The native oxide on wafer surfaces readily reacts with isothermally distilled HF vapor and forms small droplets on the hydrophobic wafer surface at room temperature [4.66]. These small droplets can be collected with a scanning droplet. The scanned, accumulated droplets finally contain all dissolved contamination in the scanning droplet. It must be dried on a concentrated spot (diameter approximately 150 pm) and measured against the blank droplet residue of the scanning solution [4.67-4.69]. VPD-TXRF has been carefully evaluated against standardized surface analytical methods. The user is advised to use reliable reference materials [4.70-4.72]. 

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. 

The amounts of the standard isotopic species and the tracer isotopic species are represented by X and X for the sample and the reference material. The reference substance is chosen arbitrarily, but is a substance that is homogeneous, available in reasonably large amounts, and measurable using standard analytical techniques for measuring isotopes (generally mass spectrometry). For instance, a sample of ocean water known as Standard Mean Ocean Water (SMOW) is used as a reference for and 0. Calcium carbonate from the Peedee sedimentary formation in North Carolina, USA (PDB) is used for C. More information about using carbon isotopes is presented in Chapter 11. 

In many analyses, fhe compound(s) of inferesf are found as par of a complex mixfure and fhe role of fhe chromatographic technique is to provide separation of fhe components of that mixture to allow their identification or quantitative determination. From a qualitative perspective, the main limitation of chromatography in isolation is its inability to provide an unequivocal identification of the components of a mixture even if they can be completely separated from each other. Identification is based on the comparison of the retention characteristics, simplistically the retention time, of an unknown with those of reference materials determined under identical experimental conditions. There are, however, so many compounds in existence that even if the retention characteristics of an unknown and a reference material are, within the limits of experimental error, identical, the analyst cannot say with absolute certainty that the two compounds are the same. Despite a range of chromatographic conditions being available to the analyst, it is not always possible to effect complete separation of all of the components of a mixture and this may prevent the precise and accurate quantitative determination of the analyte(s) of interest. 

As in other forms of chromatography, the identification of analytes is effected by the comparison of the retention characteristic of an unknown with those of reference materials determined under identical experimental conditions. 

Standardization. Standardization in analytical chemistry, in which standards are used to relate the instrument signal to compound concentration, is the critical function for determining the relative concentrations of species In a wide variety of matrices. Environmental Standard Reference Materials (SRM s) have been developed for various polynuclear aromatic hydrocarbons (PAH s). Information on SRM s can be obtained from the Office of Standard Reference Materials, National Bureau of Standards, Gaithersburg, MD 20899. Summarized in Table VII, these SRM s range from "pure compounds" in aqueous and organic solvents to "natural" matrices such as shale oil and urban and diesel particulate materials. 

The history of reference materials is closely linked with the development of analytical chemistry. In the 19th Century all chemicals were, in comparison with those of today, of poor purity. Thus, for volumetric analysis suitable purified materials as primary standards had to be specified. One of the first examples was the recommendation of As(III) oxide by Gay-Lussac (1824) for this purpose. Somewhat later, Sorensen (1887) proposed criteria for the selection of primary chemical standards. These were further elaborated by Wagner (1903) at the turn of the last century. It is worthwhile mentioning that their criteria were quite similar to those used today. 

NIST s first four certified reference materials were steel samples, and these were followed by many others. The program supplied analytically well characterized homogeneous materials. This program included, from the beginning, homogeneity... 

Traceability of measurement results is essential in the establishment of a certified reference material. As stipulated in ISO Guides 30 and 35, a certified reference material can only be certified if there is an uncertainty statement with a traceability statement. Basically, traceability means anchoring. In classical analytical chemistry, that SI system is often the best choice as a reference (= anchoring poinf). However, there is a wide range of parameters either defined by a method or defined by the... 

Chau ASY, Caeron J and Lee H-B (1979) Analytical reference materials. II. Preparation and sample integrity of homogeneous fortified wet sediment for polychlorinated hiphenyl quality control studies. J Assoc Off Anal Chem 62 1312-1314. 

Dybczynski R (1995) The contribution of various analytical techniques to the certification of reference materials. Fresenius J Anal Chem 352 120-124. 

Fajgelj a and Parkany M, eds.(i999) The Use of Matrix Reference Materials in Environmental Analytical Processes. The Royal Society of Chemistry. 

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

The use of matrix reference materials in environmental analytical processes, pp 188-195. The Royal Society of Chemistry, Cambridge. 

Lee H-B and Chau ASY (1987) Analytical reference materials Part VII. development and certification of a sediment reference material for total polychlorinated biphenyls. Analyst 112 37-40. Meinke WW (1971) Standard reference materials for clinical measurements. Anal Chem 43(6) 28A-47A. 

Muntau H (1980) Measurement quality improvements by application of reference materials. In Brattee P and Schramel P, eds. Trace Element Analytical Chemistry in Medicine and Biol-ogy, PP707-726. Walter de Gruyter Co Berlin New York. 

Okamoto K and Yoshinaga J (1999) Proper use of reference materials for elemental speciation studies. In Fajgelt A and Parkany M, eds. The use of matrix reference materials in environmental analytical processes, pp 46-56. Royal Sodety of Chemistry, Cambridge. 

In the meantime, SS-ZAAS has gained in popularity in numerous apphcations, and has become of increasing importance for analyte homogeneity determination in the production and use of reference materials. Examples are Pb, Cd, Hg, Zn, and Fe in codfish candidate RM, Hg in copper metal, Zn in mussel tissue, Cd, Pb, Hg,... 

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