Calibration curves atomic absorption

The choice between X-ray fluorescence and the two other methods will be guided by the concentration levels and by the duration of the analytical procedure X-ray fluorescence is usually less sensitive than atomic absorption, but, at least for petroleum products, it requires less preparation after obtaining the calibration curve. Table 2.4 shows the detectable limits and accuracies of the three methods given above for the most commonly analyzed metals in petroleum products. For atomic absorption and plasma, the figures are given for analysis in an organic medium without mineralization. 

Provide an SOP for the determination of cadmium in lake sediments by atomic absorption spectrophotometry using a normal calibration curve. 

A double-beam atomic absorption spectrophotometer should be used. Set up a vanadium hollow cathode lamp selecting the resonance line of wavelength 318.5 nm, and adjust the gas controls to give a fuel-rich acetylene-nitrous oxide flame in accordance with the instruction manual. Aspirate successively into the flame the solvent blank, the standard solutions, and finally the test solution, in each case recording the absorbance reading. Plot the calibration curve and ascertain the vanadium content of the oil. 

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... 

Electrothermal atomic absorption spectrophotometry with Zeeman background correction was used by Zhang et al. [141] for the determination of cadmium in seawater. Citric acid was used as an organic matrix modifier and was found to be more effective than EDTA or ascorbic acid. The organic matrix modifier reduced the interferences from salts and other trace metals and gave a linear calibration curve for cadmium at concentrations < 1.6 pg/1. The method has a limit of detection of 0.019 pg/1 of cadmium and recoveries of 95-105% at the 0.2 pg of cadmium level. 

A continuous source can be used for atomic absorption, but since only the center part of the band of wavelengths passed by the slit will be absorbed (due to the sharp line nature of atomic absorption), sensitivity will be sacrificed, and the calibration curve will not be linear. This curvature is because even at high concentrations, only a portion of the radiation passing through the slit will be absorbed, and the limiting absorbance will approach a finite value rather than infinity. With a sharp line source, the entire width of the source radiation is absorbed and so the absorption follows Beer s law. A continuous source works best with the alkali metals because their absorption lines are broader than for most other elements. Specificity is not as great with a continuous source because nearby absorbing lines or molecular absorption bands will absorb part of the source. 

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. 

The mathematical model may not closely fit the data. For example. Figure 1 shows calibration data for the determination of iron in water by atomic absorption spectrometry (AAS). At low concentrations the curve is first- order, at high concentrations it is approximately second- order. Neither model adequately fits the whole range. Figure 2 shows the effects of blindly fitting inappropriate mathematical models to such data. In this case, a manually plotted curve would be better than either a first- or second-order model. 

Cadmium in acidified aqueous solution may be analyzed at trace levels by various instrumental techniques such as flame and furnace atomic absorption, and ICP emission spectrophotometry. Cadmium in solid matrices is extracted into aqueous phase by digestion with nitric acid prior to analysis. A much lower detection level may be obtained by ICP-mass spectrometry. Other instrumental techniques to analyze this metal include neutron activation analysis and anodic stripping voltammetry. Cadmium also may be measured in aqueous matrices by colorimetry. Cadmium ions react with dithizone to form a pink-red color that can be extracted with chloroform. The absorbance of the solution is measured by a spectrophotometer and the concentration is determined from a standard calibration curve (APHA, AWWA and WEF. 1999. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington, DC American Public Health Association). The metal in the solid phase may be determined nondestructively by x-ray fluorescence or diffraction techniques. 

Metal Analysis The concentration of Pd in the effluent was determined by ICP or AA and the concentration of Ru was extrapolated from a UV-Vis calibration curve. A Feeman Tabs, Inc. Direct Reading Eschelle ICP-AES was used for Pd concentration determination in the Deloxan MP studies while a Varian Spectra AA 55B flame atomic absorption spectrophotometer was used to determine the Pd concentration in the Deloxan THP II pilot studies. Once the organic solvent was... 

The limit of detection is a useful figure which takes into account the stability of the total instrumental system. It may vary from instrument to instrument and even from day to day as, for example, mains-borne noise varies. Thus, for atomic absorption techniques, spectroscopists often also talk about the characteristic concentration (often erroneously referred to as the sensitivity—erroneously as it is the reciprocal of the sensitivity) for 1% absorption, i.e. that concentration of the element which gives rise to 0.0044 absorbance nnits. This can easily be read off the calibration curve. The characteristic concentration is dependent on such factors as the atomization efficiency and flame system, and is independent of noise. Both this figure and the limit of detection give different, but useful, information about instrumental performance. 

In a flame, as the concentration of atoms increases, the possibility increases that photons emitted by excited atoms in the hot region in the centre will collide with atoms in the cooler outer region of the flame, and thus be absorbed. This self-absorption effect contributes to the characteristic curvature of atomic emission calibration curves towards the concentration axis, as illustrated in Fig. 4.4. The inductively coupled plasma (ICP) tends to be hotter in the outer regions compared with the centre—a property known as optical thinness—so very little self-absorption occurs, even at high atom concentrations. For this reason, curvature of the calibration curve does not occur until very high atom concentrations are reached, which results in a much greater linear dynamic range. 

Is the use of the standard calibration curve justified for atomic absorption ... 

Figure 21 -27 Atomic absorption calibration curve for Sr added to distilled wafer and standard addition of Sr to aquarium water. All solutions are made up to a constant volume, so the ordinate is the concentration Of added Sr. [Data from L D. Gilles de PeKchy,...

A series of silver standards gave a linear calibration curve in flame atomic absorption with a slope of 807 meter units per ppm Ag in the standard. (The meter units are arbitrary numbers proportional to absorbance, and ppm refers to ig Ag/mL.) An unknown cyanide solution passed through the silver membrane gave a meter reading of 198 units. Find the molarity of CN in the unknown-... 

Copper, Chromium, Manganese, and Nickel. The analytical method for determining copper, chromium, manganese, and nickel involves digesting the coal with nitric and perchloric acids, fusing the residue with lithium metaborate, and determining the combined digestion and leach solutions by atomic absorption spectrophotometry. Since there is no standard material to analyze for the construction of calibration curves, the standard additions method is used for the assay. While this method increases the time required for analysis, it helps to eliminate the effect of the matrix. 

Ross et al. [6] analysed samples of soil leachates from laboratory columns and of soil pore water from field porous cup lysimeters for aluminium by atomic absorption spectrometry under two sets of instrumental conditions. Method 1 employed uncoated graphite tubes and wall atomisation method 2 employed a graphite furnace with a pyrolytically coated platform and tubes. Aluminium standards were prepared and calibration curves used for the colorimetric quantification of aluminium. Method 1 gave results which compared favourably with method 2 in terms of both sensitivity and interference reduction for samples containing 1-15 uM aluminium. 

In analytical spectrometry there are many types of calibration curves which are set up by measuring spectrometric reference solutions. The measurements yield a curve of absorbance versus concentration, and the points between the data of the reference solutions are interpolated by fitting a suitable curve, which normally follows the Beer-Lambert law and which gives rise to a straight line through the origin of the coordinate system. The measurement conditions and the results of the calibration curve evaluations in the case of chromium and lead measurements by electrothermal atomic absorption spectrometry are presented in Table 1. 

Fig. 2 The calibration curves for chromium measurement by atomic absorption spectrometry...

The most common calibration method is to prepare standards of known concentrations, covering the concentration range expected in the sample. The matrix of the standard should be as close to the samples as possible. For instance, if the sample is to be extracted into a certain organic solvent, the standards should be prepared in the same solvent. The calibration curve is a plot of detector response as a function of concentration. A typical calibration curve is shown in Figure 1.3. It is used to determine the amount of analyte in the unknown samples. The calibration can be done in two ways, best illustrated by an example. Let us say that the amount of lead in soil is being measured. The analytical method includes sample preparation by acid extraction followed by analysis using atomic absorption (AA). The stan-... 

Belal et al [40] reported on the use of flame atomic absorption spectroscopy (FAAS), coupled with ion-exchange, to determine EDTA in dosage forms. EDTA is complexed with either Ca(II) or Mg(II) at pH 10, and the excess cations retained on an ion-exchange resin. At the same time, the Ca(II) or Mg(III) EDTA complexes are eluted and determined by AAS. Calibration curves were found to be linear over the range of 4-160 and 2-32 pg/mL EDTA when using Ca(II) or Mg(II), respectively. The method could be applied to eye drops and ampoules containing pharmaceuticals. Another combined AAS flow injection system was proposed for the determination of EDTA based on its reaction with Cu(II). The calibration curve was linear over the range of 5-50 pg/mL, with a limit of detection of 0.1 pg/mL [41]. 

A major detraction for LS AAS has always been the relatively short linear region of the calibration curves, typically not more than two orders of magnitude in concentration. The limits of the linear working range arise from stray radiation and the finite width of the emission lines of the radiation source, which is not monochromatic and just three to five times narrower than the absorption profile. With HR-CS AAS, there is no theoretical limit to the calibration range, only the practical limits imposed by the size of the array detector, the increasing possibility of spectral interferences, and the ability to clean the atomizer after extremely high analyte concentrations have been introduced. 

An example of a calibration curve for Cd with a Zeeman electrothermal atomization atomic absorption spectrometry (ET-AAS) is presented in Figure 6.1. A simple linear regression model is fitted through the data points. The response of the ET-AAS is placed on the ordinate and the concentration of the injected standard solutions on the abscissa. The concentration of the unknown samples can be calculated back as X = (Yt — a)/b. 

W. Xiaoning, J. Smeyers-Verbeke, D. L. Massart, Linearization of atomic absorption calibration curves, Analusis, 20 (1992), 209-215. 

All the solid phases were identified and characterized for crystallinity by X-ray powder diffraction (Philips PW 1730/10 diffractometer, Cu Kq radiation equipped with a PW 1030/70 vertical goniometer and connected to a P.C. computer for quantitative analyses). Crystallinities for Nu-10 and cristobalite were computed by comparing the intensity of the most characteristic diffraction peaks of each sample to that of the corresponding pure 100% crystalline phases used as standards. In some cases calibration curves derived from Nu-10/cristobalite mechanical mixtures were used. Si, Al, and alkali contents were determined either on precursors or calcined samples (900 C, air flow, 4h) by atomic absorption, using a Perkin-Elmer 380 AA instrument after digestion and dissolution of the samples in H,S04/HF solutions and further elimination of HF by gentle heating at 60 C for 12 n. 

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