Flames background absorption

Devoto 115)has described an indirect procedure for the determination of 0.1 ppm arsenic in urine. The arsenomolybdic acid complex is formed and extracted from 1 ml of urine at pH 2 into 10 ml of cyclohexanone. The molybdenum in the complex is then measured. Before extracting the arsenic, phosphate in the urine is separated by extracting the phosphomolybdic acid complex at pH 1 into isobutyl acetate. The direct determination of arsenic in biological material and blood and urine is best done using a nitrous oxide-acetylene flame 116>. The background absorption by this flame is low at 1937 A, and interferences are minimized due to the high temperature of the flame. 

Practical system for flame atomic absorption spectrometry including a deuterium background corrector. 

Application of this principle is used in two types of background absorption correction set-up. Single beam atomic absorption instruments have an electromagnet at the level of the graphite furnace (or flame) and a polariser in the optical path (Fig. 14.14). However, this accessory is quite expensive. 

Under normal conditions (e.g. 10 mA) with the sample in the flame, a global measurement of background absorption and absorption by the element is obtained. However, under strained lamp conditions (e.g. 500 mA), only background absorption is measured. Comparison of both measurements allows the calculation of absorption due only to the analyte. 

Beam chopping corrects for flame emission but not for scattering. Most spectrometers provide an additional means to correct for scattering and broad background absorption. Deuterium lamps and Zeeman correction systems are most common. 

Commercial standard solutions for flame atomic absorption are not necessarily suitable for plasma and furnace analyses. The latter methods require purer grades of water and acids for standard solutions and, especially, for dilutions. For the most sensitive analyses, solutions are prepared in a dust-free environment (a clean room with a filtered air supply) to reduce background contamination that will be detected by your instruments. 

Flame AFS combines features of both AAS and FES. The excitation of atoms is by the absorption of light. When individual element spectral line sources are used, the spectral selectivity should be as high as that in AAS, although scatter may be more of a problem in AFS. Quantification is by comparison of the intensity of fluorescence emitted by samples with that emitted by standards of known concentration. At low determinant concentrations, it is necessary to discriminate between small fluorescence emission signals and the background light levels associated with thermally excited emission from the flame. Therefore in AFS, as in FES, it is desirable to have low flame background emission. This is discussed further in Chapter 2, where instrumental aspects of flame spectrometric techniques are discussed. 

H. Becker-Ross, M. Okruss, S. Florek, U. Heitmann, M.D. Huang, Echelle-spectrograph as a tool for studies of structured background in flame atomic absorption spectrometry, Spectrochim. Acta, 57B (2002), 1493. 

Spectral interferences are uncommon in AAS owing to the selectivity of the technique. However, some interferences may occur, e.g. the resonance line of Cu occurs at 324.754 nm and has a line coincidence from Eu at 324.753 nm. Unless the Eu is 1000 times in excess, however, it is unlikely to cause any problems for Cu determination. In addition to atomic spectral overlap, molecular band absorption can cause problems, e.g. calcium hydroxide has an absorption band on the Ba wavelength of 553.55 nm while Pb at 217.0nm has molecular absorption from NaCl. Molecular band absorption can be corrected for using background correction techniques (see p. 174). The operation of a flame atomic absorption spectrometer is described in Box 27.6. 

In the laboratory, aerosol samples were individually cut from the filter-tape roll and extracted on a 47-mm filter holder (Millipore) with 200 mL of distilled deionized water maintained at 50-60 °C. The leachates were analyzed for sodium content by flame atomic absorption with a Perkin-Elmer Model 373 spectrophotometer. The soluble sodium was assumed to be derived only from sea salt. Estimates of total sea salt in each sample were obtained by multiplying the sodium value by 3.25, which is the salinity-to-sodium ratio in bulk seawater (12). The sodium content of the 5-cm diameter, glass-fiber filter blanks averaged 18 /xg 12% (20 samples). Based on an average sample volume of 27 m, the background salt level produced by the sodium blank in each filter would be equivalent to 2.2 /xg/m. ... 

Background Caused by Filters. Since all of the particles were collected on membrane filters it was necessary to determine the blank metal concentrations in the filter. This enabled an estimation of how many particles must be collected in order that the levels of the metals were significantly greater than the blank filter. For this study, both neutron and flame atomic absorption spectrometric analyses were used and the results are shown in Table I. The analyses by neutron activation were made on the filter directly whereas those by atomic absorption spectrometry were obtained by extracting the filter with nitric acid (16M Ultrex). There are apparent differences between the two sizes of membrane filters which are probably related to the fact that these filter sets were obtained at different times. Also, while the metal blanks within a particular batch of filters vary by negligible amoimts, the variations between batches are considerable. These determinations are near the detection limits for both techniques, and therefore there are considerable uncertainties associated with the results. However, these blanks did indicate the minimum level of metals which must be collected if the analyses are to be significant. 

Apparatus. A nonflame atomic absorption spectrometer (Varian-Techtron AA-5, Model 63 Carbon Rod Atomizer) with background correction was used for all of the analyses with the exception of calcium. Calcium was determined by flame atomic absorption spectrometry (Varian-Techtron Model 1000). 

For the analytical determination of metals (Cd, Cu, Fe, Mn, Pb and Zn) in surface sediments, suspended particulate matter and biological matrices, digestion with a 3 1 HNO3-HCIO4 mixture under controlled temperature was used (36). Analysis of sediments and suspended particulate matter were made by Flame Atomic Absorption Spectrometry (FAAS) with air-acetylene flame and deuterium background correction. The analysis of metals in lichens and molluscs were performed by ICP-AES. The operating conditions for FAAS and Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) analysis are shown in Tables 6.1 and 6.2, respectively. 

Flame atomic absorption is subject to many of the same chemical and physical interferences as flame atomic emission (see Section 28C-2). Spectral interferences by elements that absorb at the analyte wavelength are rare in AA. Molecular constituents and radiation scattering can cause interferences, however. These are often corrected by the background correction. schemes discussed in Seetion 28D-2. In some cases, if the source of interference is known, an excess of the interferent can be added to both the sample and the standards. The added substance is sometimes called a radiation buffer. 

In most instruments, the radiant flux is modulated periodically. This can be achieved by modulating the current of the primary source or with the aid of a rotating sector (g) in the radiation beam. Accordingly, it is easy to differentiate between the radiant density emitted by the primary source and that emitted by the flame. Both single beam and dual-beam instruments (see also Fig. 77) are used. In the latter the first part of the radiation of the primary source is led directly into the monochromator, whereas the second part initially passes through the flame. In this way fluctuations and drift can be compensated for insofar as they originate from the primary radiation source or the measurement electronics. Furthermore, the spectrometer can be provided with equipment for a quasi-simultaneous measurement of the line and background absorption [253]. 

For the homogeneity and stability studies, the trace element contents (Cd, Cr, Cu, Ni, Pb and Zn) were determined by flame atomic absorption spectrometry (FAAS) or electrothermal atomic absorption spectrometry with Zeeman background correction (ZETAAS), strictly following the sequential extraction procedure. Differences between the within-bottle and between-bottle CVs observed for the step 2 were considered to be rather an analytical artefact than an indication of inhomogeneity which would have been reflected in the spread of results submitted in the certification. The material is then considered to be homogeneous for the stated level of intake (1 g). 

For the homogeneity studies, the extractants (0.05 mol L EDTA, 0.43 mol L" acetic acid and 0.005 mol L DTPA) were prepared as laid out in the certification reports [15, 17], The trace element contents (Cd, Cr, Cu, Ni, Pb and Zn) in the extracts were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) for the CRMs 483/484, flame atomic absorption spectrometry (FAAS) or electrothermal atomic absorption spectrometry with Zeeman background correction (ZETAAS) for the CRM 600. In the case of the CRM 483, little analytical difficulty was experienced as illustrated by the good agreement obtained between the within-bottle and between-bottle CVs for the CRM 484, lower extractable contents, closer to the detection limits and consequent poorer analytical precision was observed in particular for Cr (EDTA extractable contents), Cd and Pb (acetic acid extractable contents). No particular difficulties were experienced for the CRM 600. On the basis of these results, the materials were considered to be homogeneous at a level of 5 g for EDTA- and acetic acid-extractable contents and 10 g for DTPA-extractable contents (as specified in the extraction protocols). 

The application of the Zeeman effect in AAS requires the application of an electromagnet at the graphite furnace (or at the flame). Two assemblies exist for correcting the background absorption. 

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