Interferences flame atomic absorption spectroscopy

Eaithfull, N.T. (1971b) Flame interference in atomic absorption spectroscopy with a.c. modulated systems. Laboratory Practice 20(8), 641-643. 

For analytical purposes, bismuth can be determined without interference by use of air-acetylene flame atomic absorption spectroscopy (FAAS) (Welz and Sperling 1998, Ju 2002). The characteristic concentration at the 222.8 nm resonance line is 0.2mgL various other analytical lines are compiled in Table 5.1. An improved signal-to-noise (S/N) ratio can be obtained in the air-hydrogen flame with a limit of detection (LOD) of 0.015 mgL . ... 

Atomic absorption spectrometry (AAS) has been used to determine cationic and anionic surfactants indirectly. Two methods have been put forward based on the formation of the ion pair between surfactant and hexanitrocobaltate (for cationic compounds) or bis(benzoyl)pyridine thiosemicarbazone cobalt (III) (for anionic compounds). In the former case, the complex is extracted with 1,3-dicloroethane and in the latter with an isopentylacetate and isopentyl alcohol mixture. Concentration of cobalt is determined in the organic phase using electrothermal atomic absorption spectroscopy (ETAAS), while for anionic surfactants, flame atomic absorption spectroscopy (FAAS) can also be used. Interferences like metal ions, anions and organic compounds do not have a great relevance. The two methods were applied to determine dodecyltrimethylammonium bromide in shampoos (Chattaraj and Das, 1992) and sodium lauryl sulfate (SDS) in toothpastes (Chattaraj and Das, 1994). 

Two colorimetric methods are recommended for boron analysis. One is the curcumin method, where the sample is acidified and evaporated after addition of curcumin reagent. A red product called rosocyanine remains it is dissolved in 95 wt % ethanol and measured photometrically. Nitrate concentrations >20 mg/L interfere with this method. Another colorimetric method is based upon the reaction between boron and carminic acid in concentrated sulfuric acid to form a bluish-red or blue product. Boron concentrations can also be deterrnined by atomic absorption spectroscopy with a nitrous oxide—acetjiene flame or graphite furnace. Atomic emission with an argon plasma source can also be used for boron measurement. 

With flame emission spectroscopy, there is greater likelihood of spectral interferences when the line emission of the element to be determined and those due to interfering substances are of similar wavelength, than with atomic absorption spectroscopy. Obviously some of such interferences may be eliminated by improved resolution of the instrument, e.g. by use of a prism rather than a filter, but in certain cases it may be necessary to select other, non-interfering, lines for the determination. In some cases it may even be necessary to separate the element to be determined from interfering elements by a separation process such as ion exchange or solvent extraction (see Chapters 6, 7). 

McCracken et al. 164) compared atomic absorption with the tetraphenyl-boron method for determining potassium in 1190 fertilizers, and very close agreement was found between the two methods. Hoover and Reagor 16S) also found good agreement between the two methods, and atomic absorption was far more rapid. They reported that the 7665 A potassium line was more subject to interference than the less sensitive 4044 A line. Temperli and Misteli 166> reported far better results for low concentrations of potassium in soil extracts by atomic absorption spectroscopy than by flame emission spectroscopy. 

Atomic absorption spectroscopy is highly specific and there are very few cases of interference due to the similar emission lines from different elements. General interference effects, such as anionic and matrix effects, are very similar to those described under flame emission photometry and generally result in reduced absorbance values being recorded. Similarly, the use of high temperature flames may result in reduced absorbance values due to ionization effects. However, ionization of a test element can often be minimized by incorporating an excess of an ionizable metal, e.g. potassium or caesium, in both the standards and samples. This will suppress the ionization of the test element and in effect increase the number of test atoms in the flame. 

The atomic absorption characteristics of technetium have been investigated with a technetium hollow-cathode lamp as a spectral line source. The sensitivity for technetium in aqueous solution is 3.0 /ig/ml in a fuel-rich acetylene-air flame for the unresolved 2614.23-2615.87 A doublet under the optimum operating conditions. Only calcium, strontium, and barium cause severe technetium absorption suppression. Cationic interferences are eliminated by adding aluminum to the test solutions. The atomic absorption spectroscopy can be applied to the determination of technetium in uranium and its alloys and also successfully to the analysis of multicomponent samples. 

The inductively coupled plasma13 shown at the beginning of the chapter is twice as hot as a combustion flame (Figure 21-11). The high temperature, stability, and relatively inert Ar environment in the plasma eliminate much of the interference encountered with flames. Simultaneous multielement analysis, described in Section 21 1. is routine for inductively coupled plasma atomic emission spectroscopy, which has replaced flame atomic absorption. The plasma instrument costs more to purchase and operate than a flame instrument. 

In atomic absorption spectroscopy (AAS) both ionization and chemical interferences may occur. These interferences are caused by other ions in the sample and result in a reduction of the number of neutral atoms in the flame. Ionization interference is avoided by adding a relatively high amount of an easily ionized element to the samples and calibration solutions. For the determination of sodium and potassium, cesium is added. To eliminate chemical interferences from, for example, aluminum and phosphate, lanthanum can be added to the samples and calibration solutions. 

Until now we have used the database for a very simple purpose, namely to extract information from a single file. However, it is also possible to connect several files. Let us suppose that we want to use dBASE for the following problem. In atomic absorption spectroscopy (AAS), one has to choose between the flame and the (flameless) graphite tube methods. The flame methods does not have such a low detection limit as the graphite tube, but it is easier to handle, less prone to interferences and more robust. For that reason the user s strategy will often be to apply the flame method above a certain concentration limit and the flameless method below it. The flame method has its own experimental characteristics and we suppose that we have another database file in which the characteristics for flame methods are given per element. In that case, we would like the consultation to go like this ... 

The development of fast and accurate procedures for the determination of calcium in biological materials represents one of the important early achievements of atomic absorption spectroscopy. The diflBculties encountered with calcium in emission flame photometry are well known (Dll, L6, S6, SIO), but spectral interferences and extreme dependency on flame temperature, serious obstacles in emission, are either nonexistent or of lower importance in absorption. Chemical interferences, however. 

Bismuth added to urine was recovered by Willis (W14) with solvent extraction and determined by atomic absorption spectroscopy. An absorption interference rarely encountered in atomic absorption spectroscopy was seen from the absorption of the 3068-A line of bismuth by the OH radical in the air-coal gas flame. 

Jensen and Padley (1 ) determined an equilibrium constant for the reaction Cs(g) + H20(g) CsOH(g) + H(g) at 2475 K by atomic absorption spectroscopy in a hydrogen-oxygen-nitrogen flame. These workers pointed out that interference from ionization of the metal had introduced some uncertainty in their equilibrium data. Using all JANAF functions (2), 3rd law analysis of the equilibrium constant gives A H (298.15 K) = 27.4 kcal mol". This leads to an enthalpy of formation,... 

Sodium and potassium levels are difficult to analyze by titrimetric or colorimetric techniques but are among the elements most easily determined by atomic spectroscopy (2,38) (Table 2). Their analysis is important for the control of infusion and dialysis solutions, which must be carefully monitored to maintain proper electrolyte balance. Flame emission spectroscopy is the simplest and least expensive technique for this purpose, although the precision of the measurement may be improved by employing atomic absorption spectroscopy. Both methods are approved by the U.S. (39), British (40), and European (41) Pharmacopeias and are commonly utilized. Sensitivity is of no concern, due to the high concentrations in these solutions furthermore, dilution of the sample is often necessary in order to reduce the metal concentrations to the range where linear instmmental response can be achieved. Fortunately, the analysis may be carried but without additional sample preparation because other components, such as dextrose, do not interfere. 

A number of instrumental methods have been used to determine ppb levels of cobalt in water (4,5,6), biological tissues (7,8), and air particulates (9, 10). Kinetic methods are capable of measuring sub-parts-per-billion (11,12). Potentially any of these techniques could be used in the analysis of petroleum, but only neutron activation analysis (I, 3) and atomic absorption spectroscopy (13,14) have been applied to any appreciable extent. Flame and heated vaporization atomic absorption techniques were selected for more detailed study by the Project because atomic absorption is sensitive, subject to relatively few interferences, and is rather generally available. 

The Sampling Boat. The author has had some success with a different approach. Here, the sample is loaded into a narrow boat-shaped vessel, made out of tantalum or a similar material, and dried. The boat is then placed into the middle of a standard air-acetylene flame. For lead, selenium, cadmium, silver, and zinc, encouraging results have been obtained. The detection limit for lead, for example, is about one-fifteenth that of conventional atomic absorption spectroscopy. However, at least at the present stage of development, drawbacks exist here also. Interferences are multiplied, because of the low temperatures at which atoipiza-... 

Since the foundations of atomic absorption spectroscopy were laid by Walsh a number of improvements in instrumentation and techniques have been made. Russell, Shelton, and Walsh modulated the hollow cathode signal and used an amplifier tuned to the modulating frequency so measurements could be made without interference from flame emission. Sullivan and Walsh developed very high-intensity hollow cathode lamps that led to lower detection limits. Willis proposed the use of nitrous oxide-acetylene flame as a means of overcoming certain interferences and produce a higher population of free atoms in the flame. 

For analytical purposes it is essential that interference effects in atomic absorption spectroscopy be eliminated or minimized. If they cannot be eliminated, it is necessary to compensate adequately for their presence through use of proper standards or other compensating techniques. This section deals with this problem. Reference also should be made to Chapter 9, as many techniques used for flame emission also apply to atomic absorption. 

Other physical interferences are similar to those observed in flame emission and atomic absorption spectroscopy. They include effects due to viscosity and temperature of the sample solution. Any factor that can alter the rate of uptake of the sample solution requires control. The best method to use to control these effects is to prepare a blank with physical properties similar to those of the test sample. 

Most of the processes in the flame that interfere with fluorescence intensity are similar to those that occur in flame emission and atomic absorption spectroscopy. The reader is referred to sections dealing with this topic in Chapters 9 and 10. 

The potential/time profile for anodic stripping voltammetry and a typical experimental curve for the determination of a mixture of heavy metal ions is shown in Fig. 11.14. The method is clearly limited to the determination of metals which form simple amalgams (inter-metallic compounds must also be avoided). This limitation, however, introduces some desirable selectivity and most organic compounds will not interfere with the determination of the metals. Using acceptable deposition times, analysis of very low concentrations is possible. Certainly for heavy metal ions, the sensitivity of anodic stripping analyses compares well with that of atomic absorption spectroscopy even with non-flame atomization (see Table 11.4). Moreover, these data do not represent the ultimate detection limit since the plating time can be extended. 

Early in the development of atomic absorption spectroscopy it was recognized that enhanced absorbances could be obtained if the solutions contained low-molecular-weight alcohols, esters, or ketones. The effect of organic solvents is largely attributable to increased nebulizer efficiency the lower surface tension of sueh solutions results in smaller drop sizes and a resulting increase in the amount of sample that reaches the flame. In addition, more rapid solvent evaporation may also contribute to the effect. Leaner fuel-oxidant ratios must be used with organic solvents to offset the presence of the added organic material. Unfortunately, however, the leaner mixture produces lower flame temperatures and an increased potential for chemical interferences. 

In atomic absorption spectroscopy the source radiation is modulated to create an ac signal at the detector. The detector is made to reject the dc signal from the flame and measure the modulated signal from the source. In this way, background emission from the flame and atomic emission from the analyte is discriminated against and prevented from causing an interference effect. 

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