Multielement standards

Experience gained in the ZAF analysis of major and minor constituents in multielement standards analyzed against pure element standards has produced detailed error distribution histograms for quantitative EPMA. The error distribution is a normal distribution centered about 0%, with a standard deviation of approximately 2% relative. Errors as high as 10% relative are rarely encountered. There are several important caveats that must be observed to achieve errors that can be expected to lie within this distribution ... 

Rossbach M, Ostapczuk P, Emons H (1998) Microhomogeneity of candidate reference materials Comparison of solid sampling Zeeman-AAS with INAA. Fresenius J Anal Chem 360 380-383. Rossbach M, Stoeppler M (1987) Use of CRMs as mutual calibration materials and control of synthetic multielement standards as used in INAA. J Radioanal Nud Chem Artides 113 217-223. Sargent M (1995) Development and application of a protocol for quality assurance of trace analysis. Anal Proc 32 71-76. 

In the analysis of solid samples (e.g., LA-ICP-MS, SEM), synthetic standards cannot easily be prepared to the required concentrations, and accurate calibration of such techniques is often challenging. In some cases (e.g., SEM) pure element or single mineral standards are used, ideally with an appropriate standard for each element to be quantified. (It is possible in SEM, within limits, to use fewer standards than the number of elements to be determined, with the calibration for other elements being predicted from the response of the nearest element.) More often, however, multielement primary standards are used as the means of calibrating the instrument, e.g., for LA-ICP-MS of glasses, volcanics, and ceramics, two glass standards, NIST 610 and 612 (Pearce et al. 1996), are often used. It is always advisable to use more than one multielement standard in order to cover as wide a range of concentrations as possible, and to use at least one additional independent reference material as an unknown, for quality assurance purposes (see below). 

Step 5. Calibrate the instrument for mass response by feeding a multielement standard solution into the system. This is necessary to calibrate the mass response of the detector and introduction system (magnetic sector or quadrupole) to insure that the appropriate mass-window reading is being recorded. Record data in Data Table 19.1 and examine especially the region for masses 234, 235, and 238. 

In general, activation analysis relies on the use of standards that are irradiated under the same conditions and in the same position, and are also measured under the same conditions. Monoelement standards contain a known amount of one element. If they are applied to the evaluation of other elements the ratio of the cross sections o x/o s under the special conditions of irradiation and the ratio H /Hs of the relative abundances of the decay processes that are measured must be known (subscript x is for the sample and subscript s for the standard). Knowledge of the ratio o x/o s may cause problems, because the cross sections may vary drastically with the energy of the projectiles, for instance in the energy range of epithermal neutrons. These problems are not encountered with multielement standards that contain all the elements to be determined. However, the preparation of such multielement standards may be time-consuming. 

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... 

Fig. 1 (on p. 152). HP-Ge spectrum of a multielement standard (serum doped with chromium, iron, cobalt, zinc, selenium, rubidium, antimony, and cesium) irradiated 7 times for approximately 7 h at a neutron flux of 1.77-10 n-cm -s and counted for 12 h, 18 days after the end of the irradiation. The figure illustrates the excellent resolution of modern radioactivity counting equipment. As soon as their energies differ by some 5 keV, photopeaks are sufficientiy resolved to allow the calculation of their number of counts as in the case of the 563.2 and 569.3 keV and the 795.8 and 801.9 keV gamma ray photopeaks of Xs (indicated by i ). Only when their energies differ only by 2 or 3 keV, they largely overlap as exemplified by the 602.7 keV and 604.7 keV photopeaks, respectively of Sb and Xs (indicated by s ). 

After the measurement stage has been completed the concentrations of determined elements are calculated using calibration curves, multielement standards, standard reference materials, standard addition methods and single comparator approaches. 

A Perkin Elmer Optima 3000 Spectrometer was used to determine the cation content of solutions. Samples and multielement standards (0,1,10 and 100 mgL" ) were diluted with 5% nitric acid. All vials used were cleaned with 1 M sulphuric acid. Detection limits are 3, 5, 0.1, 5, and 70 igL for Fe, Al, Ca, Na, and K, respectively. 

The multielement standard in oil Conostan S21 (Conoco, Ponca City, OK) was employed for calibration and recovery studies. N-heptane was used as the solvent (Merck, Darmstadt, Germany). The standard 10 mg/kg content of each element was diluted with n-heptane to obtain the required concentration of element. 

Determine the elements present in powdered infant formula by weighing a known amount of formula, dissolving the formula in deionized water and aspirating it into an ICP. Aqueous multielement standards can be used for calibration. Compare your results to the label. Compare your results from this dissolution procedure to a sample of formula you prepare by wet-ashing or dry-ashing. 

Standards are prepared as solids or in solution from pure elements or compounds. Standard solutions sold for inductively coupled plasma (ICP) spectrometric techniques are ideal. These are prepared as multielement standards for use as liquids or dried on a suitable solid substrate. The standard should have a similar geometry to that of the sample. The composition is not so critical, however, and standards can be used over several orders of magnitude. If irradiation and counting conditions are kept constant during analysis, the standard data may be stored in a database for use on future occasions, with inclusion of a monitor to check the irradiation and counting conditions. In variable conditions of irradiation, decay and y-ray spectrometry standards are included with the samples during analysis, and this is the common... 

Glass fragments are cleaned, and then dissolved in a mixture of hydrofluoric and hydrochloric acids. A suitable element is added as an internal standard. The solution is nebulized and the resultant aerosol is introduced into the plasma by argon carrier gas. The intensities of the emission lines of the chosen elements are measured sequentially. Quantitative results are achieved by comparing intensities against those obtained from analysis of multielement standard solutions. 

Determine the elements present in powdered infant formula by weighing a known amount of formula, dissolving the formula in DI water, and aspirating it into an ICP. Aqueous multielement standards can be used for calibration. Compare your results to the label. Compare your results from this dissolution procedure to a sample of formula you prepared by wet ashing or dry ashing. Determine the elements present in a multivitamin tablet prepared by digestion in concentrated nitric acid and dilution to a suitable volume in DI water. You may need to filter out a white insoluble powder (or allow it to settle) after digestion. Pharmaceutical tablets often contain silica or titania, which will not dissolve in nitric acid. Aqueous multielement standards in the same concentration of nitric acid should be used for calibration. 

Standards for comparison have been prepared in all conceivable forms, using solids or liquids containing a single element of interest or complex multielement mixtures. Solid standards are commonly made from high purity metals and compounds, certified reference materials, or other materials with known content of the element(s) of interest. Frequently solutions of high purity elements or compounds are prepared as single- or multielement standards, or commercial products such as liquid standards for spectrometry techniques are used for INAA. Care is usually taken that specifically prepared (multielement) standards minimize interferences and allow approaching the unity condition for comparison of the unknown with the standard. 

The method is based on the simultaneous irradiation of the sample with standards of known quantities of the elements in question in identical positions, followed by measuring the induced intensities of both the standard and the sample in a well-known geometrical position. A relative standardization can be performed by means of individual monoelement standards, or by using synthetic or natural multielement standards. In the comparison method, one needs to correct the difference in decay between the miknown sample and the comparator standard. One usually decay corrects the measured counts (or activity) for both samples back to the end of irradiation using the half-life of the measured isotope. The equation used to calculate the mass of an element in the unknown sample relative to the comparator standard is... 

There are commercial multielement Standards Reference Materials (SRM) available, however, the use of home-made multielement standards can be an answer to these problems. 

It should be emphasized that this graph represents a single-element calibration. However, because ICP-MS is usually used for multielement analysis, multielement standards are typically used to generate calibration data. For that reason, it is absolutely essential to use multielement standards that have been manufactured specifically for ICP-MS. Single-element AA standards are not suitable, because they usually have only been certified for the analyte element and not for any others. The purity of the standard cannot be guaranteed for any other element and as a result cannot be used to make up multielement standards for use with ICP-MS. For the same reason, ICP-OES multielement standards are not advisable either, because they are only certified for a group of elements and could contain other elements at higher levels, which will affect the ICP-MS multielement calibration. 

Another area of concern with regard to contamination is in the selection of calibration standards. Because ICP-MS is a technique capable of quantifying up to 75 different elements, it will be detrimental to the analysis to use calibration standards that are developed for a single-element technique such as atomic absorption. These single-element standards are usually certified only for the analyte element and not for any others, although they are often quoted on the certificate. It is therefore absolutely critical to use calibration standards that have been specifically made for a multielement technique such as ICP-MS. It does not matter whether they are single or multielement standards, as long as the certificate contains information on the suite of analyte elements you are interested in as well as any other potential interferents. 

It should be noted that the costs of calibration standards, reference materials, chemicals, solutions, and acids are also something you have to plan for, but will not be used in this evaluation as they are not considered instrument-running costs. However, they are also required to carry out a complete assessment of each of the four techniques. For example, in ICP-MS, multielement standards are generally less... 

It should be anphasized that this graph represents a single-element calibration. However, because ICP-MS is usually used for multielement analysis, multielement standards are typically used to generate calibration data. For that reason, it is absolutely essential to use multielement standards that have been manufactured... 

Initial calibration verification (IC V) A multielement standard of known concentrations prepared to verify instrument calibration. This solution must be an independent standard prepared near the midpoint of the... 

Continuing caiibration verification (CCV) A multielement standard of known concentrations prepared to monitor and verify the instrument s daily continuing performance. This is monitored after every 10 samples and at the end of an analytical sequence. 

Reporting iimit (RL) verification standard The minimum concentration that can be reported with a specified degree of confidence. The RL can be no lower than the concentration of the lowest initial calibration standard. Laboratory control sample (LCS) A multielement standard of known concentrations that is carried through the entire sample preparation and analysis procedure. This solution is used to verify method performance in an ideal sample matrix. 

Calibration standards of the 18 elanents and a calibration blank were prepared from multielement standards in 2% NaCl + 1% HNO3, using an external linear-through-zero calibration graph. A different blank (1% HNO3) was subtracted from all samples due to the fact that some contaminations were present in 2% NaCl. Concentrations of the six multielement standards were 25, 50,100, 500,1000, and 5000 ppt. 

It should be noted that the costs of calibration standards, reference materials, chemicals, solutions, and acids are also something you have to plan for, but will not be used in this evaluation as they are not considered instrument-running costs. However, they are also required to carry out a complete assessment of each of the four techniques. For example, in ICP-MS, multielement standards are generally less expensive than purchasing the same number of single-element standards. In flame AA, it is fairly common to use iouization buffers to minimize the effects of easily ionizable elements. In ETA, matrix modifiers are widely used to change the volatility of analyte or matrix elements. Whereas in ICP-OES and ICP-MS, internal standards are used in the majority of analyses, especially if the sample matrices are different from the calibration standards. 

Figure 3.2 Typical energy dispersive XRF spectrum for a multielement standard.

A single multielement calibration standard is used to establish a relative sensitivity factor (R.) for each analyte (i) to be determined in the multielement analysis. For solution analysis, this multielement standard is usually prepared from high-purity metal salts dissolved in deionized water, with sufficient nitric acid added to stabilize their concentrations (pH 2 or less). Because this is only a semiquantitative analysis, matrix matching of the calibration standard to the matrix of the sample is not required. When using solid analysis techniques (i.e., slurry nebulization, laser ablation, etc.), an appropriate solid phase multielement calibration standard is most desirable however, novel approaches for the use of a solution standard have been used with some methods. 

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