Multielement accuracy

The fc0-NAA method has been developed to overcome the labour-intensive and time-consuming work of preparing multi-element standards when routine multielement or panoramic analyses are required [447]. It is intended to be an absolute technique in which uncertain nuclear data are replaced by a composite nuclear constant, the T 0-factor, which has been determined experimentally for each radionuclide with high accuracy. This k0 is given by ... 

Chapters 7 and 8 describe two major techniques for the monitoring of trace elements in environmental samples atomic absorption (AA) and inductively coupled plasma-atomic emission spectroscopy (ICP). AA is most ideally suited for analyses where a limited number of trace metal concentrations are needed with high accuracy and precision. ICP has the advantage of multielement analysis with high speed. 

An alternative to quantitative analysis by ICP-MS is semiquantitative analysis, which is generally considered as a rapid multielement survey tool with accuracies in the range 30-50%. Semiquantitative analysis is based on the use of a predefined response table for all the elements and a computer program that can interpret the mass spectrum and correct spectral Interferences. This approach has been successfully applied to different types of samples. The software developed to perform semiquantitative analysis has evolved in parallel with the instrumentation and, today, accuracy values better than 10% have been reported by several authors, even competing with typical ones obtained by quantitative analysis. The development of a semiquantitative procedure for multielemental analysis with ICP-MS requires the evaluation of the molar response curve in the ICP-MS system (variation of sensitivity as a function of the mass of the measured isotope) [17]. Additionally, in the development of a reliable semiquantitative method, some mathematical approaches should be employed in order to estimate the ionisation conditions in the plasma, its use to correct for ionisation degrees and the correction of mass-dependent matrix interferences. 

Another factor which influences the speed in performing an analysis is calibration of the instrument. Calibration is especially time-consuming in cases where different elements are run on every analysis but even in cases where the same elements are determined time after time, the frequency of instrument calibration required to maintain a desired level of accuracy is an important consideration. Since manual data collection is not feasible in multielement determinations, the ideal system would undoubtedly be computerized. The computer would handle all data collection steps, the construction of calibration curves by mathematical curve-fitting methods, and the calculation of concentrations from these curves. 

Since the introduction of the first commercial instrument in 1983, inductively coupled plasma mass spectrometry (ICP-MS) has become widely accepted as a powerful technique for elemental analysis. Two excellent books on ICP-MS have been published [1,2]. ICP-MS provides rapid, multielement analysis with detection limits at single parts part trillion or below for about 40 to 60 elements in solution and a dynamic range of 104 to 108. These are the main reasons most ICP-MS instruments have been purchased. Two additional, unique capabilities of ICP-MS have also contributed to its commercial success elemental isotope ratio measurements and convenient semiquantitative analysis. The relative sensitivities from element to element are predictable enough that semiquantitative analysis (with accuracy within a factor of 2 to 5) for up to 80 elements can be obtained using a single calibration solution containing a few elements and a blank solution. 

Before the 1960 s, the analysis of toxic elements in airborne materials employed separations and colorimetric determination for single-element problems, or spectrographic methods for multielement, multisample studies. Variable matrices in most aerosols sampled had prevented sensitive, but interference-prone, flame-emission methods from attaining much usage. The increased concern over the environmental effects of toxic elements in the late 1960 s resulted in a need for greater sensitivity and ease of operation in measurements of these elements. The many laboratories with increased responsibilities found AAS most useful because of its accuracy, sensitivity, and relative lack of matrix effects, plus the low cost of the equipment. 

EUTRON ACTIVATION ANALYSIS IS A VERY SENSITIVE TECHNIQUE for trace element determinations in various samples. If there are no elements that mutually interfere, the purely instrumental version of this method is often chosen for its established advantages such as accuracy, speed, sensitivity, simultaneous multielement determination, and sample preservation (1). For these reasons, instrumental neutron activation analysis (INAA) was applied to samples taken from a series of metal-working residues excavated at Tel Dan, Israel, from 1985 to 1986. 

The 30-mm sediment slices of the segmented cylindrical cores obtained from box coring at the seven stations were dried, pulverized, and thoroughly mixed to yield a uniform sample for analysis. Sediment from each of these slices was analyzed by two independent methods. The first method used a Perkin-Elmer model 5000 atomic absorption spectrophotometer (AA) for the elements Fe, Mn, Ti, Pb, Zn, Cu, Cr, Ni, Co, Hg, and Cd (9). The second method utilized a Philips PW 1410 X-ray fluorescence spectrometer for the analysis of elements Fe, Mn, Ti, Ca, K, P, Si, Al, Mg, Na, Pb, Zn, Cu, Cr, V, and Ba (10). The AA analysis was chosen because of the known accuracy and sensitivity to a wide spectrum of elements. The XRF analysis was chosen for its accuracy and similar nondestructive mode of analysis equivalent to the shipboard XRF analysis. Good agreement between the AA and the XRF values was felt to be imperative because the Philips XRF equipment was to be used in the land-based multielement analysis of the CS -collected sediment samples. 

Inductively coupled plasma-atomic emission spectrometry was investigated for simultaneous multielement determinations in human urine. Emission intensities of constant, added amounts of internal reference elements were used to compensate for variations in nebulization efficiency. Spectral background and stray-light contributions were measured, and their effects were eliminated with a minicomputer-con-trolled background correction scheme. Analyte concentrations were determined by the method of additions and by reference to analytical calibration curves. Internal reference and background correction techniques provided significant improvements in accuracy. However, with the simple sample preparation procedure that was used, lack of sufficient detecting power prevented quantitative determination of normal levels of many trace elements in urine. 

The water sample is obtained from the Environmental Laboratory Approval Program. We have used ICP-AES multi-element Reference Standard supplied by E. Merck with Lot. No 0C030033 for standard calibration. These standards have certified data obtained from NIST as third-generation traceability, assuring reasonable accuracy in the estimations (Certificate of Analysis, Certipur - Reference Material 11355 ICP multielement standad IV made from NIST standards reference materials, Lot No. 0C030033. Dr. Harald Untenecker, Central Analytical Laboratory, Merck). An echello-gram showing all the elements is attached. 

At present elements in coal can be determined with acceptable accuracy and precision with proper choice of analytical procedure and sample pretreatment technique. Multielement standards and numerous consensus samples are now readily... 

Plasma emission spectrometry, especially ICP- and DCP-sources, has a fixed place in modern trace element analysis. In spite of the relatively small number of relevant elements detectable by these techniques for the biomedical and environmental fields of application, plasma emission spectrometry can deliver a lot of possibly important information. The main advantages are the multielement character of the technique (sequentially or simultaneously), nearly chemical interference free measurements, control of physical interferences, a relatively high level of accuracy and precision, high specificity, fast multielement determinations (especially in case of a simultaneous device), low sample consumption and in general a wide range of detectable elements (Table 13). 

X-ray fluorescence offers a number of impressive advantages. The spectra are relatively simple, so. spectral line interference is minimal. Generally, the X-ray mcthixl is nondestructive and can be used for the analysis of paintings, archaeological specimens, jewelry, coins, and other valuable objects without harm to the sample. Furthermore, analyses can be performed on samples ranging from a barely visible speck to a massive object. Other advantages include the speed and convenience of the procedure, which permit multielement analyses to be completed in a few minutes. Finally, the accuracy and precision of X-ray Huorcscence methods often equal or exceed those of other methods. ... 

The position of ICP-AES in respect to sensitivity is between FAAS and ETAAS. The multielement determination of about 15 elements is relatively easy to perform in all types of environmental matrices. However the direct determination of some ecotoxicologically important elements (Pb, As, Hg, Cd, Sb etc.) is not possible in natural waters, plants and aerosol samples. Therefore a preconcentration step is required. After preconcentration the determination of lanthanides in plants has been achieved (Markert, 1996) with excellent accuracy. 

FAAS is the oldest version of AAS. It works with liquid samples which after nebu-lization are mixed with acetylene and introduced in a flame atomizer burner with air-acetylene or N20-acetylene flame. A single measurement can be completed within 10 s. Theoretically the method is applicable to 60-70 elements and due to its low cost, selectivity and simple operation is preferred whenever the concentration of the determined elements is within its possibilities. The sensitivity of FAAS is of the same order of magnitude as of ICP-AES and for some elements even worse which in recent years has lead to the replacement of FAAS with the multielement ICP-AES. To increase the sensitivity of FAAS usually preconcentration procedures are applied before measurement. Otherwise FAAS permits direct measurements in the low mg/kg range of a number of metals in soils and sediments, six to ten elements in plants and in natural waters. It is sensitive enough for the direct determination of Al., Ba, Ca, Cu, Fe, K, Na, Mg, Mn and Zn in different environmental materials (alkaline metals are determined by flame atomic emission spectrometry). Due to the narrow dynamic range problems with the accuracy appear (e.g. Djingova et al., 1991) and very often dilutions are necessary which decreases relative sensitivity and increases the possibilities for errors. 

There exist of the order of 50,000 analyses of archaeological ceramics, most all of which were obtained by neutron activation analysis. The recent development of inductively-coupled plasma emission (ICP) methods, which are frequently multielement, relatively inexpensive and generally of quite acceptible sensitivity, precision and accuracy, promises that in the future that method will also make a significant contribution to archaeological analytical study. 

Methods of quality control (QC) of convenience reagents vary according to type. Simple solutions are controlled by analysis and for standard solutions an accuracy of 0.1% is required wherever attainable. Multielement metal standards are prepared from high-purity starting materials that have been well analyzed, and are tested by a multielement method to ensure that no extraneous contamination has occurred. Prepared reagent mixtures for specific applications and test kits are subject to both QC analysis of the individual components and then, after preparation, have to meet performance criteria. 

Research into new analytical techniques for foodstuffs continues, striving for greater accuracy, sensitivity or simplicity, for more rapid methods, for simultaneous multielement analysis, etc. Chromatographic techniques, e.g., LC, GLC, GC-MS, have led to great improvements in the levels of accuracy, sensitivity, and detection that can be achieved for many analytes including carbohydrates, certain vitamins, chemical residues, and additives. Work is still required, for instance, in the area of vitamin analysis in order to provide standard techniques that are applicable to all food types and that would enable concurrent multi-vitamin analysis to take place. Many of the microbiological assays currently used for vitamin determination involve long incubation times and more rapid techniques are needed. 

Detection limit mg per kg or mg l ) Multielement Selectivity Matrix interference Precision %) Accuracy... 

The primary objective of RAA is to increase the sensitivity and/or the accuracy of the analysis, while preserving the main advantage of the instrumental techniques, their multielement character. The instrumental modes of activation analysis are widely used analytical techniques for the nondestructive and simultaneous determination of trace element mass fractions or concentrations. As a result of chemical separation, interferences caused by other elements/radionuclides are removed thus additional elements of low concentration become detectable and/or the accuracy of the determination is improved. In this sense, RAA complements instrumental AA methods. 

Certified reference materials (CRMs), described in the Appendix such as those available from NIST (http //ts.nist.gov/), IAEA (http //www.iaea.org), BCR (http //www.irmm.jrc.be), or other national or international standards bodies, are occasionally used as multielement calibration standards. The chief drawbacks of these materials are the difficulty of selecting an appropriate material for all the elements of interest and the inferior accuracy of certified element content compared with in-house preparations. The most important use of CRMs is to validate the results of a measurement if the certified value is obtained when a sample is analyzed in the same manner as the unknown, then the measurement is less likely to be in... 

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