Water Sampling: Collection, Preservation, and Pretreatment

Accurate water quality analysis fundamentally depends on proper sample collection, preservation, and pretreatment. Errors introduced during these initial stages are often irreversible and can compromise all subsequent laboratory results. This article outlines the key principles for these critical procedures.

1. Sample Collection
The primary goal is to obtain a representative sample that reflects the water body's true condition at the point and time of sampling.

• Planning: Define the objectives (e.g., regulatory compliance, trend monitoring) to determine sampling locations, frequency, parameters, and methods.

• Equipment: Use clean, appropriate containers (often glass or polyethylene, pre-washed with acid or sample). For trace metal analysis, acid-washed containers are essential. Different samples (grab, composite, integrated) serve different purposes.

• Technique: Follow specific protocols. For dissolved components, samples are often filtered in situ at the time of collection. Avoid aeration, contamination from the sampler, or disturbing sediments unnecessarily. Collect samples from flowing water mid-channel and at mid-depth when possible.

• Documentation: Record all relevant metadata immediately: location (GPS), date/time, weather, water temperature, pH, conductivity, and any visual observations.

2. Sample Preservation
Preservation aims to slow down physical, chemical, and biological changes between collection and analysis. No single method can completely stabilize a sample; therefore, analysis should begin as soon as possible.

• Cooling (4°C): The most common method, used to inhibit microbial activity and chemical reactions.

• Chemical Addition:

• Acidification (with HNO₃ or H₂SO₄): Used for metal cation preservation (prevents adsorption and precipitation) and for alkalinity/CN⁻ stability.

• Basification (with NaOH): Used for preserving cyanide, amines, and phenols.

• Bactericides (e.g., HgCl₂, CHCl₃): Used for samples like nutrients (nitrogen, phosphorus forms) to halt biological activity.

3. Sample Pretreatment
Pretreatment prepares the sample for a specific analytical technique and is often performed in the lab.

• Filtration: Separates dissolved (passes through a 0.45 μm filter) and particulate fractions. Crucial for analyzing "dissolved" metals or nutrients.

• Homogenization: Ensures a uniform sample, especially for suspended solids or biological materials.

• Extraction/Digestion: For non-volatile organic compounds or total metal analysis, samples may require solid-phase extraction or strong acid digestion to transfer the analyte into a measurable form.

• Derivatization: Chemically modifying a compound (e.g., some pesticides, organic acids) to make it detectable by the chosen instrument (e.g., GC).

Adherence to standardized protocols—such as those from the US EPA, APHA, or ISO—for collection, preservation, and pretreatment is non-negotiable for data quality. These steps form the foundation of reliable water quality assessment, ensuring that analytical results truly represent field conditions and support sound environmental and public health decisions.