Detection limit in AAS

If the burner head Is rotated to reduce sensitivity, we find that the limiting noise Is no longer flame transmission flicker, but source shot noise, since the absorption path has been reduced by a factor of 20. Although the sensitivity Is decreased by a factor of 20, the detection limit Is decreased by only a factor of 10, since the flame transmission noise Is no longer limiting. Thus, referring back to a statement made earlier, sensitivity, or more correctly characteristic concentration [18] cannot be used as an accurate measure of detection limit in AAS. Unlike the case of SBR in emission, because of the complexity of noises In atomic absorption, a general and simple relationship cannot be derived to relate characteristic concentration and detection limit. 

More recently, Newton and Davis (779) used 10-mil tungsten-rhenium wire (3% Re) and preelectrodeposition times of 200 sec and were able to obtain absolute detection limits in AAS for 19 metals (Ag, As, Au, Be, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Ni, Pb, Sb, Se, Sn, V, and Zn, without regard to specia-tion), which were lower than flame atomization and comparable to the carbon-rod atomizer. Spontaneous preconcentration was reported when the wire loop was immersed in the solution in the absence of an applied potential. The... 

The catalysis was performed batch-wise (Figure 4.36). After reaching ca. 90% conversion, the bulk phase was replaced and similar turnover frequencies (TOF) of about 25 h"1 were obtained in the following three runs 2, 3 and 4. When the catalyst capsule was removed, no further activity was detected. Furthermore, the Ru content in the bulk phase was always below the detection limit of AAS, which shows good catalyst retention by the membranes used. 

Flame AAS can be used to measure about 70 elements, with detection limits (in solution) ranging from several ppm down to a few ppb (and these can be enhanced for some elements by using a flameless source). Both sensitivity and detection limits (as defined fully in Section 13.4) are a function of flame temperature and alignment, etc. The precision of measurements (precision meaning reproducibility between repeat measurements) is of the order of 1-2% for flame AA, although it can be reduced to <0.5% with care. The accuracy is a complicated function of flame condition, calibration procedure, matching of standards to sample, etc. 

Elemental composition Cu 64.18%, Cl 35.82%. Copper(I) chloride is dissolved in nitric acid, diluted appropriately and analyzed for copper by AA or ICP techniques or determined nondestructively by X-ray techniques (see Copper). For chloride analysis, a small amount of powdered material is dissolved in water and the aqueous solution titrated against a standard solution of silver nitrate using potassium chromate indicator. Alternatively, chloride ion in aqueous solution may be analyzed by ion chromatography or chloride ion-selective electrode. Although the compound is only sparingly soluble in water, detection limits in these analyses are in low ppm levels, and, therefore, dissolving 100 mg in a liter of water should be adequate to carry out aU analyses. 

Manganese in aqueous solution may be analyzed by several instrumental techniques including flame and furnace AA, ICP, ICP-MS, x-ray fluorescence and neutron activation. For atomic absorption and emission spectrometric determination the measurement may be done at the wavelengths 279.5, 257.61 or 294.92 nm respectively. The metal or its insoluble compounds must be digested with nitric acid alone or in combination with another acid. Soluble salts may be dissolved in water and the aqueous solution analyzed. X-ray methods may be applied for non-destructive determination of the metal. The detection limits in these methods are higher than those obtained by the AA or ICP methods. ICP-MS is the most sensitive technique. Several colorimetric methods also are known, but such measurements require that the manganese salts be aqueous. These methods are susceptible to interference. 

A flame AAS (FAAS) detector can monitor the GC effluent continuously to provide on-line analysis. However, as the gas flow rates for the flame are quite high, the residence time in the flame is short, and this can adversely affect the detection limits. Detection limits in the microgram range are usually achieved. Improved detection limits can be obtained if the additional techniques of hydride generation or cold vapour mercury detection are used as described in Section 4.6. 

As might be anticipated, the reduction in flame temperature has a deleterious effect upon the incidence and extent of matrix interferences when such boat techniques are used. As a consequence, precise matrix matching is necessary for accurate results, or the standard additions method may be employed.6 If the user is in any doubt as to whether matrix matching alone is sufficient, the adequacy of this approach may be confirmed by the analysis of certified reference materials and/or by applying the standard additions technique as well to a selection of samples to make sure that both techniques give the same results. For bismuth, cadmium, lead, silver, and thallium, detection limits by AAS are a few ng ml -1 or better.6 For arsenic, selenium, and tellurium they tend to lie in the range 10-30 ng ml-1, depending upon the source used. 

Electrodeposition is a unique concentration technique, because the separation of trace metals from interfering matrix species can be easily carried out at the same time. Thus, the detection limits of AAS can be improved remarkably without further chemical preconcentration steps and spectral and chemical interferences due to major components in seawater can be also eliminated easily. Thus, many applications of the technique to seawater analysis by AAS have been made, especially in flameless AAS [69—78]. The... 

The detection limits of flame AAS are particularly low for fairly volatile elements, which do not form thermally stable oxides or carbides and have high excitation energies, such as Cd and Zn. Apart from these and some other elements such as Na and Li the detection limits in flame AAS are higher than in ICP-AES (see Table 20 in Section 10). 

The use of furnaces as atomizers for quantitative AAS goes back to the work of L vov and led to the breakthrough of atomic absorption spectrometry towards very low absolute detection limits. In electrothermal AAS graphite or metallic tube or cup furnaces are used, and through resistive heating temperatures are achieved at which samples can be completely atomized. For volatile elements this can be accomplished at temperatures of 1000 K whereas for more refractory elements the temperatures should be up to 3000 K. 

The detection limits in Zeeman AAS could be expected to be lower than in the case of the background correction with a D2 lamp. Indeed, here the system uses only one source. Accordingly, it can be operated at high intensity, through which detector noise limitations are avoided. This advantage will certainly be most pronounced when one component is measured in an alternating field. 

Coherent forward scattering (CFS) atomic spectrometry is a multielement method. The instrumentation required is simple and consists of the same components as a Zeeman AAS system. As the spectra contain only some resonance lines, a spectrometer with just a low spectral resolution is required. The detection limits depend considerably on the primary source and on the atom reservoir used. When using a xenon lamp as the primary source, multielement determinations can be performed but the power of detection will be low as the spectral radiances are low as compared with those of a hollow cathode lamp. By using high-intensity laser sources the intensities of the signals and accordingly the power of detection can be considerably improved. Indeed, both Ip(k) and Iy(k) are proportional to Io(k). When furnaces are used as the atomizers typical detection limits in the case of a xenon arc are Cd 4, Pb 0.9, T11.5, Fe 2.5 and Zn 50 ng [309]. They are considerably higher than in furnace AAS. 

Owing to space limitations, all methods for metals cannot be presented here. Standard Methods presents the following methods for metals of interest in textiles. Numbers in parentheses are typical detection limits in pg 1 for AA and ICP. (Note there are several AA methods and the detection limits presented are for the direct aspirational method. Other methods vary slightly.)... 

For air sampling in industrial settings, personnel mercury vapor samplers rely on a hopcalite filter absorber, followed by chemical treatment and atomic absorption spectrometry (AAS) analysis. This method requires field and reagent blanks, but has a reported detection limit in the pg m range (Turner and Boggle 1993). A passive diffusion sampler has also been developed as a useful technique for mercury vapor monitoring in the atmosphere and as a personal mercury dosimeter (Kvietkus and Sakalys 1994). 

The most important diagnostic we have for furnace AAS is the stable slope of the working curve. The characteristic mass, mo. has been defined to represent analyte sensitivity. The mo is measured in terms of the mass of analyte in pg that will produce an integrated A signal. Aj, equal to 0.0044 s. This measure of sensitivity is analogous to the flame AAS term for sensitivity which is the concentration in mg/L that will produce 1% absorption (0,0044 absorbance). The mo values are specific for each analyte and relatively independent of the matrix. Using analytical conditions summarized in Table 1. the characteristic masses are summarized in Table 2. Table 2 also includes detection limits in fig/L. The fig/L detection limits assume a sample aliquot of 100/characteristic mass of about 20% may reflect differences between individual instruments. For an individual instrument, the day-to-day variation should spread less than about 20%. This slope is matrix independent. 

For the practical analysis of Ni in biological materials, GF-AAS techniques are by far the most important, and for biological fluids demand only very simple steps for sample preparation. Voltammetrio methods can provide lower detection limits in specially prepared samples, while less sensitive methods such as inductively coupled plasma-atomic emission spectrometry (ICP-AES) may be useful in multielement protocols with tissues and other solid samples. 

Flame OES can be used to determine the concentrations of elements in samples. The sample usually must be in solution form. Generally, one element is determined at a time if using an AAS system in emission mode. Multichannel instruments are available for the simultaneous determination of two or more elements. Detection limits can be very low as seen in Appendix 7.1, Table Al. Detection limits for the alkali metals are in the ppt concentration range when ionization suppression is used. One part per trillion in an aqueous solution is 1 pg of analyte per mL of solution or 1 x g/mL. Most elements have detection limits in the high ppb to low ppm range. 

Note. This table presents a comparison of graphite furnace AAS with the use of a graphite furnace (electrothermal atomizer) as the sample vaporization step in conjunction with ICP-MS and ICP-OES. The solution detection limit, the sample volume, and the absolute detection limit in picograms are given for each technique. The isotope measured is specified for the ICP-MS technique the isotope number has no meaning for the AAS or ICP-OES results. 

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