Reference materials contamination

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]. 

Comparative references are devices which are used to verify that an item has the same properties as the reference. They may take the form of materials such as chemicals which are used in spectrographic analyzers or those used in tests for the presence of certain compounds in a mixture or they could be materials with certain finishes, textures, etc. Certificates should be produced and retained for such reference materials so that their validity is known to those who will use them. Materials that degrade over time should be dated and given a use by date. Care should be taken to avoid cross contamination and any degradation due to sunlight. A specification for each reference material should be prepared so that its properties can be verified. 

Provide secure storage for reference materials and avoid cross contamination and degradation. 

In the preparation of many solid state reference materials, reduction of the grain size plays an important role. Usually this reduction is required because of the measurement methods to be used both in the projects and later by the users of the reference material, as well as to come to an acceptable minimum sample intake. The minimum sample intake can be defined as the minimum amount of material needed, so that the heterogeneity of the material does not affect the repeatability of the measurement method. The reduction of the grain size is usually implemented by crushing and/or grinding techniques. The techniques employed and the equipment used must be suitable for the purpose of processing the material. Potential problems of contamination, loss of volatile components, and/or other physical and... 

International Atomic Energy Agency and United Nations Environment Programme (1995) Survey of Reference Materials, Volume i Biological and Environmental Reference Materials for Trace Elements, Nudides and Micro-contaminants. IAEA-TECDOC-854, IAEA, Vienna. 

Wise SA (1993) Standard reference materials for the determination of trace organic constituents in environmental samples. In Barcelo D, ed. Environmental Analysis Techniques, Applications and Quality Assurance, pp 403-446. Elsevier Science Publishers, Amsterdam, The Netherlands. Wise SA, and Schantz MM (1997) Standard reference materials for the determination of trace organic contaminants in environmental samples. In Clement R and Siu M, eds. Reference Materials for Environmental Analysis Making and Using Them, pp 143-186. Lewis Publishers, Boca Raton, FL. 

Frozen reference materials have been produced by NIST (Wise et al. 1993). These materials do not have the disadvantages of the oils or freeze-dried materials, but are more difficult to transport. Obviously they have to be kept deep-frozen during transport, which makes their use rather expensive. Since the early 1990 s a new approach in this field has been introduced. This concerned the use of wet, sterilized fish and shellfish samples. These samples, packed in glass jars or in tins, were firstly used in the QUASIMEME program as reference materials for inter-laboratory studies (de Boer 1997). Later, when it appeared that the stability was maintained for longer periods, tests for organic contaminants based on this principle were also prepared. 

Reference materials for radioisotopes have mainly been used for purposes relating to nuclear and radiation safety. Historically, the development of such materials first arose from the need to assess the risk to human populations caused by worldwide contamination of food and the environment as a consequence of atomic bomb testing - particularly from bombs exploded in the atmosphere. Even now, although atmospheric testing ceased many years ago, the residues from these tests still remain the main source of radionuclides such as Cs and °Sr in the global environment (though locally, other sources may be more important in some countries). 

The nuclear power plant accident at Chernobyl in April 1986 (IAEA Technical Report 1991) proved to be a much more potent source of environmental contamination in many surrounding countries, over distances up to several thousands of kilometers, and was a cause of worldwide problems in international trade in food products contaminated (or possibly contaminated) with radionuclides. The resulting requirement by many countries to establish systems for monitoring radionuclides in foodstuffs and in the environment led to a large worldwide increase in the demand for suitable reference materials. 

Wise SA, Schantz MM, Koster BJ, Demiralp R, Mackey EA, Greenberg RR, Burow M, OsTApczuK P, Lhlestole TI (1993) Development of frozen whale blubber and liver reference materials for the measurement of organic and inorganic contaminants. Fresenius J Anal Chem 345 270-277. 

Initial efforts by workers at the Institute for Reference Materials and Measurement (IRMM), Geel, Belgium, to produce certified reference materials for GMOs have demonstrated that the provision of suitable reference materials is not easy and that together with the development of suitable analytical methods there are many challenges to be solved ahead. The first two examples produced jointly by the IRMM in Belgium and Fluka Chemie AG in Switzerland were based on Round-Up Ready Soya and BT 176 Maize. The reference materials are needed to validate EU and Swiss regulations which permit non-GMO products to be contaminated by up to 1 % GMO material and still be accepted. 

Surprisingly, few certified reference materials or quality control materials for use in the measurement of airborne contaminants are commercially available from world-wide producers. The main reason for the scarcity of such materials is related to great difficulties in producing realistic samples and the lack of interest from... 

Filter samples can be prepared to airborne workplace concentrations by spiking each filter with aqueous solution containing elements with concentrations gravimetrically traceable to ultrapure metals or stoidiiometricaUy well defined oxides. The amormts correspond for some of the materials to current threshold limit values of contaminants in workroom atmospheres provided that the simulated filter has been exposed to one cubic meter of air. The certified values are based on a gravimetric procedure, i.e. weight per volume composition of the primary reference material dissolved in high purity sub-dis-tiUed acids. The National Institute of Occupational Health in Oslo, Norway, has produced several batches of such materials certified for 20 elements. Additionally, information values are reported for four other elements see Table 6.2. 

There are no special problems with the reconstitution or handling of these kinds of materials for electrolytes. The concentration of electrolytes is high, and no contamination problems are to be expected. Many commercial suppliers deliver reference materials for electrolytes in serum and urine. 

TXRF is frequently used for contamination control and ultrasensitive chemical analysis, in particular in relation to materials used in semiconductor manufacturing [278,279], and metallic impurities on resin surfaces, as in PFA sheets [279,280], TXRF has been used by Simmross et al. [281] for the quantitative determination of cadmium in the four IRMM polyethylene reference materials (VDA-001 to 004). Microsamples (20-100 ig) from each reference material were transferred by hot pressing at 130 °C as 3 xm thin films straight on to quartz glass discs commonly used for TXRF analysis. The results obtained were quite satisfactory (Table 8.50). Other reports of the forensic application to plastic materials by TXRF have appeared [282], including a study of PE films by elemental analysis [283],... 

Contaminated Land Proficiency Testing Scheme Control of Substances Hazardous to Health Certified Reference Material cumulative sum... 

Lee and Chau [66] have discussed the development and certification of a sediment reference material for total polychlorobiphenyls. Alford Stevens et al. [49] in an inter-laboratory study on the determination of polychlorobiphenyls in environmentally contaminated sediments showed the mean relative standard deviation of measured polychlorobiphenyl concentrations was 34%, despite efforts to eliminate procedural variations. Eganhouse and Gosset [67] have discussed the sources and magnitude of bias associated with the determination of polychlorobiphenyls in environmental sediments. Heilman [30] studied the adsorption and desorption of polychlorobiphenyl on sediments. 

A QC assessment was done for all samples. De-ionized water field blanks were collected on seven different days to evaluate process contamination. Site duplicates were taken at eight sites to evaluate repeatability and site variation. Instrumental precision was constrained by analysis of laboratory duplicate solutions, and is typically less than 5%. Finally, standard reference material (SRM) water standards were analyzed with sample batches, to assess instrumental accuracy. 

The laboratory shall have procedures for safe handling, transport, storage and ttse of reference standards and reference materials in order to prevent contamination or deterioration and in order to protect their integrity. 

As far as the use of the certified reference material is concerned, in an ideal situation the supplier or producer should give advice on the appropriate use to the user. In addition it is essential to obtain information relevant to storage temperature, influence of moisture and influence of contamination. The possibility of dividing the certified reference material into different portions after opening in order to maintain its original properties should always be considered. 

The certified reference material (CRM 450), used for the validation of the method, is real contaminated powdered milk with a certified content in PCB-52, PCB-101, PCB-118, PCB-156, and PCB-180. This material contains approximately 3.9% water and 25% fat. It is used after reconstituting and was supplied by the EC Community Bureau of Reference (BCR). 

Lead was extracted from nine replicates of NIST standard reference material 2709 (a contaminated soil) using various methods. The efficiencies of the various extraction methods were compared. 

Bias and Accuracy If there is a significant bias associated with a given method, it must be possible to compensate for it or the net results will not be accurate. Thus, while one may choose the ICP method mentioned above because of its lower detection limit, it may still not be a good choice because of an uncompensatable bias at the concentration level in question. It is appropriate to always check the method using a reference material, or by using an alternative method, to determine bias. Also, a bias can be caused by some correctable component of the analysis scheme, such as sampling problems or contamination, and not the method itself. 

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