Step 1: Understanding the Concept:
Mercury (\(\text{Hg}\)) is a highly toxic heavy metal that can contaminate drinking water through industrial runoff or natural deposits.
Because it can cause severe neurological damage even at trace concentrations, food safety standards (such as FSSAI and BIS) require highly sensitive and precise analytical methods to detect low levels of mercury in packaged drinking water.
Step 2: Detailed Explanation:
Let us evaluate the analytical methods used to detect heavy metals:
- Flame Atomic Absorption Spectrometry (Flame AAS) (A): A standard method used to detect metals, but it lacks the sensitivity required to detect trace levels of mercury because mercury volatilizes easily and has a low absorption signal in a hot flame.
- Electrothermal AAS (Graphite Furnace AAS) (B): Offers better sensitivity than Flame AAS, but still suffers from mercury volatility losses during the drying and ash stages.
- High-Performance Liquid Chromatography (HPLC) (D): Used primarily to separate and detect non-volatile organic compounds, not trace inorganic metals.
- Cold Vapor Atomic Absorption Spectrometry (CV-AAS) (C): The most common, sensitive, and standardized method used to detect trace levels of mercury.
Mercury is unique because it is the only metallic element that exists as a liquid at room temperature and has a significant vapor pressure.
In CV-AAS:
1. A reducing agent (such as stannous chloride or sodium borohydride) is added to the water sample to convert ionic mercury (\(\text{Hg}^{2+}\)) into elemental mercury vapor (\(\text{Hg}^0\)).
2. This volatile elemental mercury is swept by a carrier gas into an absorption cell at room temperature (cold vapor).
3. The absorption of light at 253.7 nm by the cold mercury vapor is measured, allowing for highly sensitive detection down to parts-per-billion (ppb) levels.
Therefore, Cold Vapor AAS is the correct method.
Step 3: Final Answer:
The standard method used to detect trace levels of mercury in packaged drinking water is Cold Vapor AAS.