Interferences, AAS

Any difference in the behaviour of the analyte atoms in the sample and in the standard implies an interference. AAS using a line source for excitation suffers little spectral interference. Background interference in AAS is more important. This nonspecific absorption is caused by ... 

This section gives information on the technical limitations of the standard method caused by, for example, sample matrix effects (coloured samples can interfere with the photometric detection or element specific spectral interferences (AAS, ICP-OES) or chemical interferences (precipitation reactions, formation of reaction byproducts). These details are validated experimentally in laboratories participating actively in the standardization work. The documentation of the interferences m help potential users of a specific standard to decide whether the standard method could be applicable for the requirements of their analytical businesses. 

A simple electrochemical flow-through cell with powder carbon as cathodic material was used and optimized. The influence of the generation current, concentration of the catholyte, carrier stream, flow rate of the sample and interferences by other metals on the generation of hydrogen arsenide were studied. This system requires only a small sample volume and is very easily automatized. The electrochemical HG technique combined with AAS is a well-established method for achieving the required high sensitivity and low detection limits. 

Spectral interferences in AAS arise mainly from overlap between the frequencies of a selected resonance line with lines emitted by some other element this arises because in practice a chosen line has in fact a finite bandwidth . Since in fact the line width of an absorption line is about 0.005 nm, only a few cases of spectral overlap between the emitted lines of a hollow cathode lamp and the absorption lines of metal atoms in flames have been reported. Table 21.3 includes some typical examples of spectral interferences which have been observed.47-50 However, most of these data relate to relatively minor resonance lines and the only interferences which occur with preferred resonance lines are with copper where europium at a concentration of about 150mgL 1 would interfere, and mercury where concentrations of cobalt higher than 200 mg L 1 would cause interference. 

Apart from the interferences which may arise from other elements present in the substance to be analysed, some interference may arise from the emission band spectra produced by molecules or molecular fragments present in the flame gases in particular, band spectra due to hydroxyl and cyanogen radicals arise in many flames. Although in AAS these flame signals are modulated (Section 21.9), in practice care should be taken to select an absorption line which does not correspond with the wavelengths due to any molecular bands because of the excessive noise produced by the latter this leads to decreased sensitivity and to poor precision of analysis. 

Key L = fuel-lean R = fuel-rich AA= air/acetylene AP = air/ propane NA= nitrous oxide/acetylene AH = air/hydrogen Notes (1) If there are many interferences then NA is to be preferred. 

Note that the interfacing of LC techniques with MS puts significant constraints on the solvents that can be used i.e., they must be volatile, with a low salt concentration, for MS compatibility. Narrow-bore columns, which use much smaller amounts of salt and organic modifier, appear to have potential for facilitating IEC-MS applications.40 Despite the excellent sensitivity of MS detection for most elements, however, there are cases where matrix effects can interfere. In this situation, combination of IEC with atomic emission spectrometry (AES) or atomic absorption spectrometry (AAS) may be preferable, and can also provide better precision.21 32 4142 Other types of... 

The logical approach to problem solving for rubber analysis at Polysar Ltd was described by Chu [73] (cf. Schemes 2.4 and 2.5). Systematic analysis involves sampling, elimination of interference and measurement. Methods employed include chromatography (GC, HS-GC, HPLC, SEC, IC), spectroscopy (AAS, UV/VIS, IR, NMR), MS, microscopy and thermal analysis. The specific role of each of these techniques for the analysis of rubber compounds with or without... 

Today, ICP-AES is an indispensable inorganic analytical tool. However, because of the high plasma temperature, ICP-AES suffers from some severe spectral interferences caused by line-rich spectra of concomitant matrix elements such as Fe, Al, Ca, Ni, V, Mo and the rare-earth elements. This is at variance with AAS. The spectral interference can of course be minimised by using a (costly) high-resolution spectrometer. On the other hand, the high temperature of the ICP has the advantage of reducing chemical interferences, which can be a problem in AAS. 

In the above derivation we may assume that aA-(S) = aA- and aB (S) = aB, because by analogy with the build-up of an electrode potential (see pp. 26-27) the build-up of the ion-exchange potential will not significantly alter the original concentrations of A- and B in the solution under test. Hence in eqn. 2.80 the ratio aB-(n)/aA- n), which reflects the exchange competition of B versus A, still depicts the interference ratio of B in more straightforward manner than does the so-called selectivity constant (k), usually mentioned by ISE suppliers. 

UA, AA, and DOPAC, with the concentrations approximating their extracellular fluid levels were investigated at +300 and -lOOmV at the SOD-based biosensors at +300mV, the interferences from AA and UA were considerable, for instance 15% and 23% current responses were obtained for 500 pM AA relative to 13 nM 02 with Fe-SOD/MPA-modified and Mn-SOD/MPA-modified Au electrodes, respectively. In addition, 10% current response was obtained for 50 pM UA relative to 13 nM 02 at both electrodes. Fortunately, such interferences were well suppressed when the electrodes were polarized at -lOOmV. Besides, the interferences of H202, 5-HIAA, HVA, and DOPAC were negligible at both +300 and -lOOmV at both electrodes [138],... 

Maximum power heating, the L vov platform, gas stop, the smallest possible temperature step between thermal pretreatment and atomisation, peak area integration, and matrix modification have been applied in order to eliminate or at least reduce interferences in graphite furnace AAS. With Zeeman effect background correction, much better correction is achieved, making method development and trace metal determinations in samples containing high salt concentrations much simpler or even possible at all. 

Crisp et al. [212] has described a method for the determination of non-ionic detergent concentrations between 0.05 and 2 mg/1 in fresh, estuarine, and seawater based on solvent extraction of the detergent-potassium tetrathiocyana-tozincate (II) complex followed by determination of extracted zinc by atomic AAS. A method is described for the determination of non-ionic surfactants in the concentration range 0.05-2 mg/1. Surfactant molecules are extracted into 1,2-dichlorobenzene as a neutral adduct with potassium tetrathiocyanatozin-cate (II), and the determination is completed by AAS. With a 150 ml water sample the limit of detection is 0.03 mg/1 (as Triton X-100). The method is relatively free from interference by anionic surfactants the presence of up to 5 mg/1 of anionic surfactant introduces an error of no more than 0.07 mg/1 (as Triton X-100) in the apparent non-ionic surfactant concentration. The performance of this method in the presence of anionic surfactants is of special importance, since most natural samples which contain non-ionic surfactants also contain anionic surfactants. Soaps, such as sodium stearate, do not interfere with the recovery of Triton X-100 (1 mg/1) when present at the same concentration (i.e., mg/1). Cationic surfactants, however, form extractable nonassociation compounds with the tetrathiocyanatozincate ion and interfere with the method. 

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