Mercury in Water by AFS

Atomic fluorescence spectrometry (AFS) is a highly sensitive technique widely used to determine mercury in water. Because mercury can be converted to elemental vapor at room temperature, AFS is often combined with cold vapor generation (CV-AFS), making the method simple, selective, and suitable for trace analysis.

The principle of AFS is based on the absorption and re-emission of light by free atoms. In the case of mercury, ground-state Hg atoms are generated from the sample. When these atoms are exposed to radiation from a mercury hollow cathode lamp or another suitable source at 253.7 nm, they absorb energy and become excited. As they return to lower energy states, they emit fluorescence. 

The intensity of this fluorescence is proportional to the number of mercury atoms in the atomizer, and therefore to the mercury concentration in the original sample. Because the measurement is made against a dark background, AFS can achieve very low detection limits, often at the nanogram-per-liter level.

The analytical method usually begins with sampling and preservation. Water samples are collected in clean containers, acidified with nitric acid, and kept cool and dark to prevent changes in mercury speciation or loss through adsorption and volatilization. For total mercury determination, digestion is required to break down organic mercury compounds and convert all forms of mercury to divalent mercury (Hg²⁺). Common digestion approaches use oxidizing agents such as bromine chloride, potassium permanganate, or persulfate under acidic conditions. After digestion, excess oxidant must be reduced before the cold vapor step.

In the cold vapor generation stage, a reducing agent such as stannous chloride or sodium borohydride is added. This converts Hg²⁺ to elemental mercury (Hg⁰). Because Hg⁰ is volatile, it is easily purged from the solution by a carrier gas, usually argon. The gas-liquid separator transfers the mercury vapor into a drying tube or membrane to remove moisture, and then into a quartz fluorescence cell. There, the mercury atoms are excited by the light source and the resulting fluorescence is detected by a photomultiplier tube.

Quantification is normally performed by external calibration with mercury standard solutions or by standard addition when matrix effects are significant. A blank is carried through the entire procedure to correct for reagents and contamination. Quality control measures include duplicate samples, spiked samples, certified reference materials, and regular calibration checks. Interferences may arise from volatile organic compounds, moisture, chlorine, or light scattering, but these can usually be controlled by proper digestion, gas drying, and blank correction.