Determining Zinc in Water by Atomic Absorption Spectrometry

Atomic absorption spectrometry (AAS) is a widely used technique for trace metal analysis. For zinc in water samples, flame AAS offers high sensitivity, simplicity, and reliability. Below is a step‑by‑step outline of the procedure.

Principle

A hollow cathode lamp specific for zinc emits light at 213.9 nm. The sample is aspirated into an air‑acetylene flame where zinc atoms are vaporized and atomized. These ground‑state atoms absorb light from the lamp. The absorbance is proportional to the zinc concentration, following the Beer‑Lambert law.

Sample Collection and Preservation

Collect water samples in acid‑washed polyethylene or glass bottles. Immediately acidify with concentrated nitric acid to pH < 2 (about 2 mL per liter). This prevents adsorption of zinc onto container walls and keeps it in solution. Store at 4 °C until analysis.

Sample Preparation

Most fresh or clean water samples require only filtration (0.45 µm membrane) to remove suspended solids. If dissolved zinc is of interest, acidify further to 0.5–1% (v/v) HNO₃. For wastewater or samples with high organic content, a hot acid digestion (e.g., HNO₃/H₂O₂) is recommended to release bound zinc. After digestion, filter and dilute to an appropriate volume with deionized water.

Instrument Setup

Lamp current: as recommended by the manufacturer (typically 3–10 mA)

Wavelength: 213.9 nm with a slit width of 0.5–1.0 nm

Flame: air‑acetylene, fuel‑lean (oxidizing) to minimize interferences

Burner height: adjusted for maximum absorbance

Calibration

Prepare a set of standard zinc solutions (e.g., 0.1, 0.2, 0.5, 1.0, 2.0 mg/L) from a certified stock solution, using the same acid matrix (e.g., 0.5% HNO₃) as the samples. Run the standards to construct a calibration curve. Always include a reagent blank (acidified deionized water).

Measurement

Aspirate the blank first, then standards, and finally samples. Between samples, aspirate deionized water to avoid carryover. Record the absorbance; the instrument’s software automatically converts it to concentration using the calibration curve. For samples above the linear range, dilute them and re‑measure.