2.3  Specialized Sample Introduction Techniques and Analysis

2.3.3 Hydride Generation

Another external attachment to a FAAS instrument is the hydride generation system that is used to analyze for arsenic (As), bismuth (Bi), mercury (Hg), antimony (Sb), selenium (Se), and tellurium (Te); some of these are notable toxins. This technique works in a similar manner to the cold vapor technique but sodium borohydride is used as the reducing agent to generate a volatile metal hydride complex. In addition, a flame is used to decompose the metal hydride. Again water, soil, and tissue samples must be digested to free the metal from organic and inorganic complexes and place it in its cationic state. The generated metal hydride is passed through the flame as a pulse input where it is degraded by heat to it gaseous elemental state. In this state the metal will absorb the source radiation (again from a HCL) and the absorbance reading is directly proportional to the concentration of metal in the sample. Instrumental calibration and data processing is identical to the cold vapor mercury technique.

An example reaction that occurs in the reaction vessel for As is shown below

3BH₄^-_(aq) + 3H⁺_(aq) + 4H₃AsO_3(aq) ⇒ 3H₃BO_3(aq) + 4AsH_3(g) + 3H₂O_(l)

A video illustration of this technique can be viewed in Animation 2.5. As with the cold vapor technique the advantage is an improved detection limit, with the disadvantage of high labor costs associated with digestion and individual and manual instrumental analysis.

Animation 2.5 Illustration of a Hydride Generation Sample Introduction System.

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Section 2.3.4

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