Question

Which of the following Atomic Absorption Spectrometers is used for determination of mercury?

• A. Graphite furnace atomic absorption
• B. Cold vapour atomic absorption
• C. Hydride generation atomic absorption
• D. Flame atomic absorption

Hint:

Mercury (\(\text{Hg}\)) is unique among metals for being volatile at room temperature.
Thus, it is analyzed using the Cold Vapour technique without any thermal flame or furnace.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Atomic Absorption Spectroscopy (AAS) is a sensitive analytical technique used to determine the concentration of specific metallic elements in soil, water, and plant samples.
The sample must be atomized so that ground-state atoms can absorb characteristic wavelengths of light emitted by a hollow cathode lamp.
Different atomization techniques are employed depending on the physical and chemical properties of the target element.
These include flame atomization, electrothermal (graphite furnace) atomization, hydride generation, and cold vapor techniques.

Step 2: Detailed Explanation:
Mercury (\(\text{Hg}\)) is a unique transition metal that exists as a liquid at room temperature and has an exceptionally high vapor pressure.
Because of its high volatility and tendency to exist in elemental form as a monoatomic gas at room temperature, it does not require thermal energy (such as a flame or furnace) for atomization.
The standard, highly sensitive technique used for mercury determination is Cold Vapour Atomic Absorption Spectrometry (CVAAS).
In CVAAS:
1. The mercury in the digested sample (which is in the ionic form, \(\text{Hg}^{2+}\)) is chemically reduced to elemental mercury (\(\text{Hg}^0\)) in a reaction vessel.
2. This reduction is carried out using a reducing agent, typically stannous chloride (\(\text{SnCl}_2\)) or sodium borohydride (\(\text{NaBH}_4\)).
3. The volatile elemental mercury is then swept out of the solution by a carrier gas (like nitrogen or argon) and routed into a quartz absorption cell held at room temperature (hence “cold vapor”).
4. The mercury atoms in the cell absorb light at \(253.7\text{ nm}\), and the absorbance is measured.
Let us review the other techniques:
Flame AAS and Graphite Furnace AAS are used for non-volatile elements that require high temperatures (\(2000^\circ\text{C}\text{ to }3000^\circ\text{C}\)) to be atomized.
Hydride Generation AAS is specifically used for metalloids that form volatile covalent hydrides (such as Arsenic, Selenium, Antimony, and Bismuth) using sodium borohydride in an acidic medium, which are then thermally decomposed in a heated quartz tube.

Step 3: Final Answer:
Therefore, the specialized Atomic Absorption Spectrometer technique used for the determination of mercury is Cold vapour atomic absorption.