Water-cooled atom traps

The development of new, highly efficient nebulizers, described in detail in the section on ICP-OES (Section 12.2.4.4.1), has meant that a more concentrated aerosol and a more sensitive FAAS determination is achievable. Similarly, the use of slotted tube atom traps (STATs) and water-cooled atom traps (WCAT)10 11—the latter have undergone modification in recent years12—enhances sensitivity with regard to volatile elements like Cd and Pb because of the long residence time of these atoms in the tube. 

West and co-workers11 achieved far greater sensitivity enhancement by trapping readily atomized elements, which had been nebulized conventionally for periods of up to a few minutes, onto the outer surface of a small bore, water-cooled quartz tube. If the water was then drained rapidly, the tube temperature quickly rose and the element was atomized off the surface. For cadmium in calcium chloride extracts of soil, for example, a detection limit of 4 ng (g soil) -1 was reported. A water-cooled double silica tube atom trap similarly has been very successfully employed for the determination of cadmium and lead in natural waters by flame AAS.12 Some further examples of applications of atom trapping are included in Chapter 7. 

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]. 

Andreae described a method for the sequential determination of arsenate, arsenite, mono-, di- and trimethyl arsine, MMAA, DMAA and trimethylarsine oxide in natural waters with detection limits of several ng/1. The arsines are volatilized from the sample by gas stripping the other species are then selectively reduced to the corresponding arsines and volatilized. The arsines are collected in a cold trap cooled with liquid nitrogen. They are then separated by slow warming of the trap or by gas chromatography, and measured with atomic absorption, electron capture and/or flame ionization detectors. He found that these four arsenic species all occurred in natural water samples. 

An intimate mixture of 150 g. (0.64 mole) of dry e-benzoyl-aminocaproic add (p. 20) and 26.4 g. (0.85 gram atom) of dry red phosphorus is placed in a i-l. three-necked flask provided with a separatory funnel, an air-cooled condenser connected through a calcium chloride tube to a water trap, and a mechanical stirrer (Note i). The reaction flask is surrounded by an ice-salt mixture, the stirrer started, and 408 g. (131 cc. 2.55 moles) of dry bromine added dropwise from the separatory funnel. When all the bromine has been added, the cooling bath is removed. The mixture is warmed slowly at first and finally heated on a steam bath, with stirring, until the bromine vapors have practically disappeared. The hot mixture is poured slowly into 400 cc. of water in a i-l. beaker with hand stirring. The viscous acid bromide reacts with the water with the evolution of heat, and the solid acid is formed. The lumps are pulverized, the mixture is replaced in the original reaction flask, and the whole treated with a slow stream of sulfur dioxide to remove excess bromine. The solid product is filtered on a Buchner funnel, washed with... 

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