Atomic standard addition technique

Fuller, C. W. "A Simple Standards Additions Technique Using the Model 306 Atomic Absorption Spectrophotometer". 

The data and arguments presented indicate that the presence of sea salts alters the atomic absorption signal of these four elements from their response in de-ionized water-acid standards in a consistent manner for each particular element and sea salt concentration. This observation can be used to develop a modified standard addition technique. If a series of curves can be prepared that contain the range of metal and sea salt concentration expected in the samples, correction factors between actual and observed concentrations based on pure water-acid standards can be determined. This modified standard addition technique is illustrated for two of the elements discussed previously. For lead the actual concentration is plotted vs. the calculated concentration for a sea salt range of 1.0-5.0 parts per thousand sodium in Figure 4. This plot was prepared from solutions of known concentrations in a sea water medium. For a sample of unknown lead concentration, within the specified range. 

Indirect FAA methods, in which the hydrocarbon matrix is eliminated and the analyte is concentrated, could have been applied to all elements studied by the Project if a large enough sample were used. However, since indirect techniques still may encounter chemical (interelement) interferences, a minimum of 5 ml of solution must be available so that standard additions techniques can be applied. Some trace metals have been determined in petroleum by indirect FAA after a 100-g sample had been ashed (17), However, ashing such large samples, particularly crudes or residual fractions, is difficult and time consuming. Sulfated ash procedures were used in the Project to prepare various petroleum matrices for determining Cd, Co, Mo, Ni, and V by FAA. However, procedures were developed only for the first three elements, and cross-check data were collected only for cadmium. Since alternate techniques had greater sensitivity and allowed smaller samples to be ashed, flame atomic absorption was not widely used. 

Summary of the Method. The sample is diluted approximately 1 1 with tetrahydrofuran and analyzed directly using a standard additions technique and a Varian-Techtron model 63 carbon rod atomizer. 

Scope. This method is intended for the determination of cadmium in petroleum and petroleum products at levels down to 10 ng/g. The presence of other metals affects the atomic absorption signal, but the effect is overcome by using a standard additions technique. 

In the case of moss samples exposed in the Mansfelder Land area, inductively coupled plasma atomic emission spectrometry (ICP-AES) was used for the determination of As, Cu, Fe, Mn, Ni, Pb, Zn in both aqueous and acidic solutions by standard addition technique. Atomic absorption techniques were applied for the determination of arsenic (hydride generation coupled with graphite furnace atomization), cadmium and lead (both by Zeeman-corrected graphite furnace AAS) (Krauss et al., 1998, 2000). Ion chromatography was used for the determination of sulphate. 

The standard addition technique also is applicable to emission and atomic absorption spectroscopy. The technique involves the use of the unknown plus the addition of known amounts of standard. A signal intensity is obtained for the unknown X, then a series of solutions containing the unknown plus varying amounts of the standard are prepared and their signals obtained. The data are treated as shown in Figure 9-19. A known concentration of the element A is added to the unknown to produce solutions having concentrations of X, X + A, X 2A, etc. These data are... 

The isotopic abundances of the tracee or the tracer can be obtained from measured isotope ratios using Eqs. (16.2) and (16.3), while atomic masses can be taken from the literature (10]. Based on IDMS principles, measured isotope ratios can be converted into tracer to tracee ratios in the sample analyzed. The amount of tracee can then be calculated if the amount of tracer in the sample is known. The latter can be determined either by IDMS using an additional tracer or by non-IDMS techniques for quantitative analysis employing external calibration or standard addition techniques. 

The "method of standard additions" has been employed as a technique for standardization of atomic absorption analyses of metals In biological fluids (13,21) In this procedure, several concentrations of standard analyte are added to samples of the biological fluid to be analyzed The calibration curve which Is obtained after additions of the standard analyte to the biological fluid should parallel that obtained when aqueous standards are analyzed Extrapolation of the standard additions curve back to a negative Intercept on the abscissa furnishes an estimate of the concentration of the analyte In the original sample (21) This technique Is helpful In assessing the validity of methods of trace metal analysis (11,13,58) However, In the author s opinion, the "method of standard additions" Is neither practical nor reliable as a routine method for standardization... 

Table 8.76 shows the main characteristics of voltammetry. Trace-element analysis by electrochemical methods is attractive due to the low limits of detection that can be achieved at relatively low cost. The advantage of using standard addition as a means of calibration and quantification is that matrix effects in the sample are taken into consideration. Analytical responses in voltammetry sometimes lack the predictability of techniques such as optical spectrometry, mostly because interactions at electrode/solution interfaces can be extremely complex. The role of the electrolyte and additional solutions in voltammetry are crucial. Many determinations are pH dependent, and the electrolyte can increase both the conductivity and selectivity of the solution. Voltammetry offers some advantages over atomic absorption. It allows the determination of an element under different oxidation states (e.g. Fe2+/Fe3+). 

In atomic absorption spectroscopy (AAS) the technique using calibration curves and the standard addition method are both equally suitable for the quantitative determinations of elements. 

Contamination of silicon wafers by heavy metals is a major cause of low yields in the manufacture of electronic devices. Concentrations in the order of 1011 cm-3 [Ha2] are sufficient to affect the device performance, because impurity atoms constitute recombination centers for minority carriers and thereby reduce their lifetime [Scl7]. In addition, precipitates caused by contaminants may affect gate oxide quality. Note that a contamination of 1011 cnT3 corresponds to a pinhead of iron (1 mm3) dissolved in a swimming pool of silicon (850 m3). Such minute contamination levels are far below the detection limit of the standard analytical techniques used in chemistry. The best way to detect such traces of contaminants is to measure the induced change in electronic properties itself, such as the oxide defect density or the minority carrier lifetime, respectively diffusion length. 

The standard addition method of calibration (see Chapter 1) is often used to combat the uncertainties of varying interference effects in electrothermal atomization. However, care should be taken with this approach, as errors from spurious blanks and background may go undetected. It must also be emphasized that the technique of standard additions does not correct for all types of interference. 

The contribution of flow analysis to improving the performance of atomic spectrometry is especially interesting in the field of standardisation. FIA can provide a faster and reliable method to relate the absorbance, emission or counts (at a specific mass number) to the concentration of the elements to be determined. In fact, flow analysis presents specific advantages to solving problems related to the sometimes short dynamic concentration ranges in atomic absorption spectrometry, by means of on-line dilution. The coupling of FI techniques to atomic spectrometric detectors also offers tremendous possibilities to carry out standard additions or internal standardisation. 

A useful aspect of the mercury(II) hydride method is that it can be directly coupled with the many standard techniques for heteromercuration of alkenes and cyclopropanes. The resulting overall transformation adds a heteroatom and a carbon atom across the carbon-carbon double bond of an alkene or the carbon-carbon single bond of a cyclopropane. This is a difficult transformation to conduct by standard ionic techniques. An alkene thus becomes an equivalent of synthon (12) and a cyclopropane of synthon (13 Scheme 34). Many equivalent transformations (like haloetherification and phenylselenolactoniza-tion) are available to make precursors for tin hydride mediated additions. 

In 140 water samples from the river Saale, sampled from 1986 to 1988 according to the technique described in Section 8.1.1.1, the heavy metals iron and zinc were determined using flame AAS and lead, cadmium, chromium, cobalt, copper, and nickel by AAS with electrothermal atomization in the soluble fraction (particle diameter <0.45 pm). The sampling points, located in Thuringia (Germany), are illustrated in Fig. 8-7. The method of standard addition, with three additions, was used to minimize matrix effects. The components ammonium, chloride, magnesium, nitrate, nitrite, phosphate, oxygen,... 

The technique may be subject to a number of positive and/or negative systematic errors, depending on the element to be determined, the instrumental technique used, the matrix composition, and still other factors. However, as shown in Table 2.2, there is a tendency towards the use of the standard additions method and CRMs to minimize some possible matrix effects and to ensure validity of results. Nevertheless, it appears from the survey of the literature that the solubilization sampling introduction technique compares favorably with other atomic spectrometric methods for the determination of trace elements in a variety of matrices. 

The ET-AAS technique is especially prone to nonspectral interferences, that is, effects on the formation of atoms. Such interferences cannot be eliminated by BC. Some of these effects can, however, be corrected by the use of the standard addition procedure. To investigate whether standard addition is necessary, the... 

Atomic absorption techniques can be used to analyse gases indirectly, as liquid samples. To prepare the liquid sample the metals are removed from the gas stream or atmospheric sample using a filter medium such as a milli-pore filter disc. This is then either dissolved or washed in nitric acid and the solution analysed by standard additions. These procedures are now extensively used by Health Safety Executive Inspectors to monitor (particularly) heavy metals in working environments. The reader is referred to Chapter 4C. 

In summary, a method for the analysis of molybdenum in biological fluids has been presented. The method requires the destruction of the organic materials in the sample by low-temperature ashing. Detection was accomplished by using a graphite furnace—atomic absorption technique and the standard additions method. The method is sufficiently sensitive to distinguish between molybdenum levels in the blood, serum, and urine from exposed and unexposed individuals. 

In the Trace Metals Project standard additions were used in all heated vaporization atomic absorption (HVAA) procedures. These procedures use sample aliquots of 10 /J or less consequently, a microstandard addition technique was developed. In this technique successive microliter aliquots of a /xg/ml standard are added to the sample solutions which have also been prepared on weight/volume basis. Since the volumes added or removed from the solution are negligible, the entire analysis is done on a single solution as detailed in the individual procedures. 

Two methods were developed independently, in separate laboratories. The sample preparation for the two methods is essentially the same. However, because of the differences in instrumentation, separate detailed methods are required. In both methods the sample is diluted with tetra-hydrofuran, and an aliquot is injected into the atomizer. The average signals for the sample and the sample plus three standard additions are obtained. After appropriate blank corrections have been made, the beryllium concentration is obtained by graphical or calculation techniques. 

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