Atom Trapping Techniques

The sensitivity difference between direct flame and furnace atomisation has been bridged via the general method of atom trapping proposed by Watling [1]. A silica tube is suspended in the air - acetylene flame. This increases the residence time of the atoms within the tube and therefore within the measurement system. Further devices such as water-cooled systems that trap the atom population on cool surfaces and then subsequently release them by temporarily halting the coolant flow are sometimes employed. The application of atom-trapping AAS to the determination of lead and cadmium has been discussed by Hallam and Thompson [2]. 

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Zhang D-Q, Li C-U, Yang L-L, Sun H-W (1998) Determination of cadmium in vegetables by derivative flame atomic absorption spectrometry with atom trapping technique. J Anal At Spectrom 13 1155-1158. 

With the main resonance line for cadmium at 228.8 nm, it is hardly surprising that this element is not determined usefully by flame AES. However cadmium is a very easily atomized element, and the determination by flame AAS is sensitive, with detection limits sometimes as low as 1 ng ml-1 often being cited for the air-acetylene flame.1 Determination by flame AFS may result in detection limits two orders of magnitude lower than this, if a suitable excitation source is available.12 The determinations in acetylene flames are virtually free from chemical interference. Because of the ease of atomization, the element may be readily determined using atom-trapping techniques or boat or cup techniques, as discussed in Chapter 6. Recently a cold vapour sample introduction technique has also been suggested for cadmium determination.13,14... 

Figure 15-12 is a schematic illustration of a technique known as acid volatile sulfides/ simultaneously extracted metals analysis (AVS/SEM). Briefly, a strong acid is added to a sediment sample to release the sediment-associated sulfides, acid volatile sulfides, which are analyzed by a cold-acid purge-and-trap technique (e.g., Allen et ai, 1993). The assumption shown in Fig. 15-12 is that the sulfides are present in the sediments in the form of either FeS or MeS (a metal sulfide). In a parallel analysis, metals simultaneously released with the sulfides (the simultaneously extracted metals) are also quantified, for example, by graphite furnace atomic absorption spectrometry. Metals released during the acid attack are considered to be associated with the phases operationally defined as "exchangeable," "carbonate," "Fe and Mn oxides," "FeS," and "MeS."... 

Spin-adduct spectra often reveal splittings to substituent atoms other than hydrogen. Indeed, since the spin-trapping technique provides a convenient route to many nitroxides containing structural features likely to be of spectroscopic interest, it has frequently been used to this end. Chlorine and bromine splittings, as well as those from fluorine, have been encountered and many nitroxide spectra have been reported in which there is splitting from a second nitrogen, from phosphorus, or even from a metal atom. 

To extend the levels of detection for mercury stiU lower, several workers, especially in this area of atomic absorption techniques, have chosen to collect the mercury on gold or other noble metal trapping systems prior to revaporizing the mercury into the measurement technique. Figure 7.14 shows the configuration of a specific system to concentrate mercury onto an amalgam preconcentrator prior to analysis. 

A special technique which has yielded highly unstable and simple dinitrogen complexes trapped in low-temperature solids (N2 or Ar) is the metal atom cocondensation technique in low-temperature matrices. By the cocondensation of nickel atoms and N2 at 4.2-10 K the binary compounds Ni(N2) (n = 1-4) result, whose structure has been inferred from IR and... 

In the late twentieth eentury, a number of new instrumental techniques w erc developed lor determining atomic properties with increased precision and reliability. Of marked importance is the Increased facility for measuring minute dimensions and units of time at the respective nanometer and nanosecond levels. Laboratory techniques include laser atom probes, cold neutron research, .canning-tunneling microscopy, and atom trapping, among others. 

Matusiewicz, H. 1997. Atom trapping and in situ preconcentration technique for flame atomic absorption spectrometry. Spectrochim. Acta B 52 1711-1136. 

For direct detection by the atomic absorption technique, the stripping gas stream can be used to carry the sample into the burner. The arsines are condensed by immersing the gas trap in liquid nitrogen. By slowly warming the traps to room temperature, the arsines are released in the sequence of their boiling points. 

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