Atomic absorption detection limit

Atomic Absorption Detection Limits for Selected Elements... 

Table 7.7. Atomic absorption detection limits and sensitivities with a conventional flame atomizer...

Table I. Atomic Absorption Detection Limits (Continued) ...

A comparison of the detection limits for the rare earth elements in flame atomic emission and absorption spectrometry (table 37D.3 in section 2.2.5) allows certain conclusions to be made. The fuel-rich oxyacetylene and nitrous oxide-acetylene flames are very effective in producing free atoms of these elements and are the flames of choice for both atomic emission and absorption analysis. The emission detection limits are equal to or better than those obtained by absorption techniques, and thus flame atomic emission methods are generally superior. Future improvements in hollow cathode discharge tubes (or development of other primary sources) may lower the atomic absorption detection limits and thereby make the two techniques more complementary. However, Kinnunen and Lindsjo (1967) have emphasized that locating the proper rare earth ab-... 

Cranston and Murray [35,36] took samples in polyethylene bottles that had been pre-cleaned at 20 °C for four days with 1% distilled hydrochloric acid. Total chromium Cr(VI) + Cr(III) + Crp (Crp particulate chromium) was coprecipitated with iron (II) hydroxide, and reduced chromium Cr(III) + Crp was co-precipitated with iron (III) hydroxide. These co-precipitation steps were completed within minutes of the sample collection to minimise storage problems. The iron hydroxide precipitates were filtered through 0.4 pm Nu-cleopore filters and stored in polyethylene vials for later analysis in the laboratory. Particulate chromium was also obtained by filtering unaltered samples through 0.4 pm filters. In the laboratory the iron hydroxide co-precipitates were dissolved in 6 N distilled hydrochloric acid and analysed by flameless atomic absorption. The limit of detection of this method is about 0.1 to 0.2 nM. Precision is about 5%. 

Thermally-released Hg is determined by means of a simple flameless atomic absorption detection system (Fig. 13-2). This has a sensitivity of 0.1 ng Hg and a limit of detection of 0.01 ng Hg. In the temperature range 120-150"C adsorbed Hg (subsequently termed Hg J, comprising Hg, HgCh, Hg CF and any Hg , is released and measured. When the temperature is raised to 800"C, all forms of Hg (Hg,) are released. The determination of Hg as these two groups of compounds aids the understanding of Hg dispersion and hence exploration. 

Sensitivity, in atomic absorption, is a measure of the amount of absorption produced by a given sample concentration and is generally given as p.p.m./l% absorption. Detection limit is defined in various ways to represent the minimum sample concentration which it is possible to distinguish from zero. The most common misconception is the belief that an increase in sensitivity—Le., higher absorption for the same concentration—automatically produces an improved detection limit. This is by no means true since the detection limit depends also on the stability and freedom from fluctuation of the signal produced. 

It is worth stating that, after a certain limit has been reached, further increase in lamp brightness is not useful for atomic absorption. Thus, when a lamp is bright enough to permit, say, lOOX scale expansion, a further brightness increase will not lead to better results. (However, in the new technique of atomic fluorescence, detection limits are directly proportional to lamp brightness.)... 

Furnace atomizers have conversion efficiencies much higher than do flame atomizers absolute detection limits are typically 100 to 1000 times improved over flame-aspiration methods. Our discussion will center on atomizers heated by electrical resistance. Although these are not generally useful for emission measurements, they are well suited for atomic-absorption and atomic-fluorescence measurements [3]. 

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. 

The detection limits in the table correspond generally to the concentration of an element required to give a net signal equal to three times the standard deviation of the noise (background) in accordance with lUPAC recommendations. Detection limits can be confusing when steady-state techniques such as flame atomic emission or absorption, and plasma atomic emission or fluorescence, which... 

The section on Spectroscopy has been retained but with some revisions and expansion. The section includes ultraviolet-visible spectroscopy, fluorescence, infrared and Raman spectroscopy, and X-ray spectrometry. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon induction coupled plasma, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-19, and phosphoms-31. 

Highly sensitive iastmmental techniques, such as x-ray fluorescence, atomic absorption spectrometry, and iaductively coupled plasma optical emission spectrometry, have wide appHcation for the analysis of silver ia a multitude of materials. In order to minimize the effects of various matrices ia which silver may exist, samples are treated with perchloric or nitric acid. Direct-aspiration atomic absorption (25) and iaductively coupled plasma (26) have silver detection limits of 10 and 7 l-lg/L, respectively. The use of a graphic furnace ia an atomic absorption spectrograph lowers the silver detection limit to 0.2 l-ig/L. 

Atomic Absorption/Emission Spectrometry. Atomic absorption or emission spectrometric methods are commonly used for inorganic elements in a variety of matrices. The general principles and appHcations have been reviewed (43). Flame-emission spectrometry allows detection at low levels (10 g). It has been claimed that flame methods give better reproducibiHty than electrical excitation methods, owing to better control of several variables involved in flame excitation. Detection limits for selected elements by flame-emission spectrometry given in Table 4. Inductively coupled plasma emission spectrometry may also be employed. 

Since 1970, new analytical techniques, eg, ion chromatography, have been developed, and others, eg, atomic absorption and emission, have been improved (1—5). Detection limits for many chemicals have been dramatically lowered. Many wet chemical methods have been automated and are controlled by microprocessors which allow greater data output in a shorter time. Perhaps the best known continuous-flow analy2er for water analysis is the Autoanaly2er system manufactured by Technicon Instmments Corp. (Tarrytown, N.Y.) (6). Isolation of samples is maintained by pumping air bubbles into the flow line. Recently, flow-injection analysis has also become popular, and a theoretical comparison of it with the segmented flow analy2er has been made (7—9). 

Atomic absorption spectroscopy is more suited to samples where the number of metals is small, because it is essentially a single-element technique. The conventional air—acetylene flame is used for most metals however, elements that form refractory compounds, eg, Al, Si, V, etc, require the hotter nitrous oxide—acetylene flame. The use of a graphite furnace provides detection limits much lower than either of the flames. A cold-vapor-generation technique combined with atomic absorption is considered the most suitable method for mercury analysis (34). 

Atomic absorption spectroscopy is an alternative to the colorimetric method. Arsine is stiU generated but is purged into a heated open-end tube furnace or an argon—hydrogen flame for atomi2ation of the arsenic and measurement. Arsenic can also be measured by direct sample injection into the graphite furnace. The detection limit with the air—acetylene flame is too high to be useful for most water analysis. 

Although the most sensitive line for cadmium in the arc or spark spectmm is at 228.8 nm, the line at 326.1 nm is more convenient to use for spectroscopic detection. The limit of detection at this wavelength amounts to 0.001% cadmium with ordinary techniques and 0.00001% using specialized methods. Determination in concentrations up to 10% is accompHshed by solubilization of the sample followed by atomic absorption measurement. The range can be extended to still higher cadmium levels provided that a relative error of 0.5% is acceptable. Another quantitative analysis method is by titration at pH 10 with a standard solution of ethylenediarninetetraacetic acid (EDTA) and Eriochrome Black T indicator. Zinc interferes and therefore must first be removed. 

Because of the increasing emphasis on monitoring of environmental cadmium the detemiination of extremely low concentrations of cadmium ion has been developed. Table 2 Hsts the most prevalent analytical techniques and the detection limits. In general, for soluble cadmium species, atomic absorption is the method of choice for detection of very low concentrations. Mobile prompt gamma in vivo activation analysis has been developed for the nondestmctive sampling of cadmium in biological samples (18). 

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