Atomization cell cold vapour

Elements such as As, Se and Te can be determined by AFS with hydride sample introduction into a flame or heated cell followed by atomization of the hydride. Mercury has been determined by cold-vapour AFS. A non-dispersive system for the determination of Hg in liquid and gas samples using AFS has been developed commercially (Fig. 6.4). Mercury ions in an aqueous solution are reduced to mercury using tin(II) chloride solution. The mercury vapour is continuously swept out of the solution by a carrier gas and fed to the fluorescence detector, where the fluorescence radiation is measured at 253.7 nm after excitation of the mercury vapour with a high-intensity mercury lamp (detection limit 0.9 ng I l). Gaseous mercury in gas samples (e.g. air) can be measured directly or after preconcentration on an absorber consisting of, for example, gold-coated sand. By heating the absorber, mercury is desorbed and transferred to the fluorescence detector. 

Flame atomisation is not necessary for the atomic absorption spectrophotomehy of mercury. The cold vapour technique described here employs a reduction vessel (which may be purchased) to produce mercury vapour the vapour is led to a quartz absorption cell within the atomic absorption inshument. The method is applicable to inorganic and organic mercurial compounds in urine. 

Several types of atomization cell are available flame, graphite furnace, hydride generation and cold vapour. Flame is the most common. In the premixed laminar flame, the fuel and oxidant gases are mixed before they enter the burner (the ignition site) in an expansion chamber. The more commonly used flame in FAAS is the air-acetylene flame (temperature, 2500 K), while the nitrous oxide-acetylene flame (temperature, 3150K) is used for refractory elements, e.g. Al. Both are formed in a slot burner positioned in the light path of the HCL (Fig. 27.4). 

A modified reduction-aeration method to analyse mercury in environmental water samples is described. After aeration of the sample, the mercury is pre-concentrated on gold-coated sand which is then analysed by thermal desorption and cold vapour atomic absorption spectrometry (CVAAS). When applying reduction-cells of volumes up to 1 1, limits of detection as low as 1 ng.r are obtained. The method avoids matrix-effects caused by complexing agents such as Cl and 1, which interfere in the direct measurement of mercury by common CVAAS-techniques. The method was applied to dty drinking-water. Mercury levels ranging from less than 1 ng.l" to 10 ng.T were found. 

In principle, the devices used for hydride removal can also be employed with cold vapour — which in addition requires a non-heated quartz-cell atomizer. In any case, research interests have focused on systems for specific use with mercury vapour. 

J. C. Andrade, C. Pasquini, N. Baccan, and J. C. van Loon, Cold Vapour Atomic Absorption Determination of Mercury by Flow Injection Analysis Using a Teflon Membrane Phase Separator Coupled to the Absorption Cell. Spectrochim. Acta, 38 (1983) 1329. 

Ebdon, L. and Wilkinson, J.R. (1981) Determination of sub-nanogram amounts of mercury by cold-vapour atomic fluorescence spectrometry with an improved gas-sheated atom cell. Anal. Chim. Acta, 128,45-55. 

Lau, O.. Hon, P. Cheung, C., and Wong, M. (1984) Uses of silica cells for the static cold vapour atomic-absorption determination of mercury without background correction. Analyst, 109, 1175-1178. 

Hydride generation AAS (HGAAS) and cold vapour AAS (CVAAS) are special combinations of chemical separation and enrichment with AAS. In HGAAS the analyte is transformed to a volatile hydride, stripped off by an inert gas and atomized in a quartz tube, flame-in tube etc. About ten elements (As, Se, Bi, Sb etc.) can be determined by this technique. The accuracy and detection limits depend on the proper isolation of the hydride. CVAAS is the universally acknowledged most sensitive method for determination of Hg. The generation of elemental mercury vapour is similar to the hydride generation however the quartz cell may not be heated and this gives the name of the method. 

Early experiments witli MOT-trapped atoms were carried out by initially slowing an atomic beam to load tire trap [20, 21]. Later, a continuous uncooled source was used for tliat purjDose, suggesting tliat tire trap could be loaded witli tire slow atoms of a room-temperature vapour [22]. The next advance in tire development of magneto-optical trapping was tire introduction of tire vapour-cell magneto-optical trap (VCMOT). This variation captures cold atoms directly from the low-velocity edge of tire Maxwell-Boltzmann distribution always present in a cell... 

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