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HGAAS - Hydride Generation Atomic Absorption Spectroscopy...

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Hydride Generation Atomic Absorption Spectroscopy Introduction Atomic absorption absorption spectroscopy (AAS) is one of the commonest instrumental methods for analyzing for metals and some metalloids. But because of interferences, poor reproducibility, and poor detection limits an alternative method for some elements--mostly metalloids--has been devel- oped. Hydride generation atomic absorption spectroscopy (HGAAS) is available via an option for many modern AAS instruments. It "only" requires the hydride generation module. Metalloids like antimony, arsenic, selenium, and tellurium are now routinely analyzed by HGAAS (see www.shsu.edu/~chm_tgc/ sounds/sound.html). Inductively coupled plasma (ICP) is also a powerful analytical, instrumental method for these elements but at this point its much higher cost limits it widespread use as compared to AAS or HGAAS. As the animation on HGAAS here shows, many of the main parts of the HGAAS system are identical to that of AAS: a hollow cathode lamp, air/acetylene flame, and optical system but include (in most systems) an optical cell and the relatively complex hydride generation system. The nebulizer required in AAS is not used in HGAAS. The system described here is a continuous flow system, but batch flow systems have been used in the past. The job of each component is detailed below: Job of the hollow cathode lamp Provide the analytical light line for the element of interest Provide a constant yet intense beam of that analytical line Job of the hydride generation system Suck up (aspirate) liquid sample at a controlled rate Mix liquid sample with sodium borohydride and HCl Create a volatile hydride of the analyte metalloid from that reaction Flow that gaseous hydride into the optical cell Job of the optical cell and flame Decompose the hydride form of the metalloid from the hydride generation module Thereby create atoms (the elemental form) of the element of interest Se0, Sb0, Te0, etc. Job of the monochromator Isolate analytical lines' photons passing through the optical cell Remove scattered light of other wavelengths from the optical cell In doing this, only a narrow spectral line impinges on the PMT.
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Job of the photomultiplier tube (PMT) As the detector, the PMT determines the intensity of photons of the analytical line exiting the monochromator. The Hollow Cathode Lamp The hollow cathode lamp (HCL) uses a cathode made of the element of interest with a low internal pressure of an inert gas. A low electrical current (~ 10 mA) is imposed in such a way that the metal is excited and emits a few spectral lines characteristic of that element (for instance, Te 214.3 nm and a couple of other lines; Se 196 nm and other lines, etc.). The light is emitted directionally through the lamp's window, a window made of a glass transparent in the UV and visible wavelengths.
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