Certified range

The sampling variance of the material determined at a certain mass and the number of repetitive analyses can be used for the calculation of a sampling constant, K, a homogeneity factor, Hg or a statistical tolerance interval (m A) which will cover at least a 95 % probability at a probability level of r - a = 0.95 to obtain the expected result in the certified range (Pauwels et al. 1994). The value of A is computed as A = k 2R-s, a multiple of Rj, where is the standard deviation of the homogeneity determination,. The value of fe 2 depends on the number of measurements, n, the proportion, P, of the total population to be covered (95 %) and the probability level i - a (0.95). These factors for two-sided tolerance limits for normal distribution fe 2 can be found in various statistical textbooks (Owen 1962). The overall standard deviation S = (s/s/n) as determined from a series of replicate samples of approximately equal masses is composed of the analytical error, R , and an error due to sample inhomogeneity, Rj. As the variances are additive, one can write (Equation 4.2) ... 

Figure 1.1 Results from an international intercomparison, IMEP 9, wl expanded uncertainty and ue the combined standard uncertainty [1] (see Chapter 6, Section 6.3). Reproduced by permission of EC-JRC-IRMM (Philip Taylor) from IMEP 9, Trace elements in water III, Cd, certified range 81.0-85.4nmoll-1 .

IMEP 9, Trace Elements in Water III, Cd, Certified Range 81.0-85.4 nmol l-1 , European Commission, Joint Research Centre, Institute for Reference Materials and Measurements (EC-JRC-IRMM), Geel, Belgium (1998). [www.irmm.jrc.be/html/interlaboratory comparisons/imep/ index.htm] (accessed 30 November, 2006). 

IMEP- 6 Trace elements in water synthetic water (sample " 1") Certified range (= i2uc) 1.55 1.77 pmol kg 1... 

Appreciable precision and low bias can be obtained by use of electrochemical analytical methods. Adeloju (1985) determined relatively low levels of copper In muscle, urine and NBS SRM Orchard leaves using this method and higher concentrations in NBS SRM Bovine liver using differential pulse polarography (DPP). All results were within the certified ranges and the precision ranged from 1.9% to 3.1%. The main limitations of this method are the time consumption if several elements have to be determined and the need for a... 

Due to their wide range of analytical challenges centralized analytical laboratories are required to adopt a series of QM systems simultaneously. For example, the Competence Center Analytics of BASF AG in Ludwigshafen is certified and accredited to operate under four different QM systems. Undoubtedly, QM systems play a vital role in a modern industrial analytical laboratory. The sale of many products of the chemical industry is not possible without a GLP-certified analytical laboratory. However, in practical tenus the different QM systems can potentially reduce the efficiency of the analytical process and lead to increased costs. 

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. 

Benchtop X-ray energy dispersive analyzer BRA-17-02 based on a gas-filled electroluminescent detector with an x-ray tube excitation and range of the elements to be determined from K (Z=19) to U (Z=92) an electroluminescent detector ensures two times better resolution compared with traditional proportional counters and possesses 20 times greater x-ray efficiency compared with semiconductor detectors. The device is used usually for grits concentration determination when analysing of aviation oils (certified analysis procedures are available) and in mining industry. 

Portable x-ray energy dispersive sulphur in oil analyser ASE-1 with measurement range 0.015 - 5% and a detection limit near 0.001%. SPARK-1-2M, BRA-17-02 and ASE-1 have been certified as measuring... 

Where a.c. supplies exist, transformer-rectifiers are the most economical source of d.c. for cathodic protection systems. In the case of pipelines, standard transformer-rectifiers, either oil or air cooled, can be employed. They range in size from 5A, 5V for small systems to 100 A, 48 V for major pipeline schemes. A typical output for a well-coated cross-country pipeline in the UK would be 5 A, 48 V. In the case of sea-water jetties where the voltage required is usually low because of the lower sea-water resistivity, a typical rectifier size for a major installation would be 500 A, 18 V. For offshore pipelines and loading platforms where a fire hazard exists, it is usual to employ certified flameproof or intrinsically safe rectifiers to overcome any possibility of fire hazard should faults develop in the unit. 

The National Institute of Standards and Technology (NIST) released certified standard materials of a baby food composite (SRM 2383) and an infant formula (SRM 1846) containing carotenoids however, the relative uncertainties of certified values are considerably high, ranging Irom 20% for P-carotene (cis + trans) to 28% for lutein (including esters) and to a 47% for free lutein reference value in SRM 2383. ... 

Valuable contributions were made by two Canadian agencies, particularly by the National Research Council Canada (NRCC) who, from about 1976, provided marine and marine biological CRMs certified for metals, metal species and organic constituents (Berman 1984 Willie 1997). More recently their Halifax laboratories have issued a highly respected range of CRMs for the determination of shellfish toxins. Another Canadian producer, the National Water Research Institute (NWRI) specialized in marine (water and sedimentary) CRMs, and from the late 1980 s their matrix materials certified also for organic compounds (Chau et al. 1979 Lee and Chau 1987). 

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... 

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. 

The methods It is advised that, prior to the certification measurements, the participants discuss their methods so that all participants have confidence in each others methods and there is a good level of agreement between laboratories. As it is preferred to certify on the basis of the agreement between different methods applied in different laboratories, a proposal should indude, where relevant and possible, a group of laboratories offering a range of widely different measurement methods. Each laboratory should use well established method(s), with which it can demonstrate adequate performance in terms of trueness and in terms of reproducibility. 

The BCR has also produced a large number of environmental matrix CRMs for PAHs, PCBs, pesticides, and PCDDs/PCDFs as shown in Table 3.12. These matrices include both natural contaminant level matrices as well as natural matrices spiked with low and high levels of contaminants. When viewed together the NIST and BCR CRMs provide a wide range of environmental matrices in which a considerable number of analytes have been assigned certified and reference values. 

Validation work indudes the determination of within-batch and between-batch imprecision, linearity, sensitivity and limit of detection - which can be carried off with any suitable spedmen. However, to demonstrate the accuracy of the procedure, to investigate possible interferences and to establish reference ranges, it is necessary to analyze RMs with known concentrations, ideally using certified materials. Where none are available, accuracy may be assessed using other approaches, but these are less reliable and are more complicated to perform (Taylor 1995). 

Figure 4.4 shows an example for the use of several CRMs from NIST and BCR with certified or indicative values for chromium for the construction of a calibration curve. Increasing intakes of the CRMs were used covering the range 3-23 ng Cr. 

This Section describes one of the first successful attempts, by a multi national project team funded by the European Commission s DC XII, to produce and certify a microbiological CRM that is both fit for purpose and meets the requirements of the ISO Guides. The first results are two bacterial strains, Enterococcus faecium (CRM 506) and Salmonella typhimurium (CRM 507) (fanning et al. 1995). Both are available as part of the EU BCR range of CRMs. 

Quartz on filter media in a clay matrix is also available from NIST. The SRM 2679a is certified for quartz at three levels 30.8, 80.2 and 202.7 [xg/filter respectively. Respirable silica in powder form is also issued by NIST SRMs 1878a and 1879a are crystalline silica materials with particles in the respirable range and they are intended for use in X-ray diffraction and infrared spectroscopy. 

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