Sampling essentials for Water analysis using Atomic Absorption Spectroscopy

River water sampling

Analysis of trace metals in water is an important application of Atomic Absorption Spectroscopy. Trace metals in water play a crucial role as majority of metals act as essential nutrients at low concentrations but excess levels can have harmful consequences. It is imperative to test for concentration levels of trace metals in water before it can be certified fit for human consumption or industrial applications.

The contribution of sampling in analysis cannot be overruled and the quality of analysis will depend to a great extent on procedures adopted and precautions taken at the sampling stage.

Sampling requirements for analysis of trace metals by Atomic Absorption Spectroscopy can be briefly covered under three main headings:

• Sample Collection
• Sample Storage
• Sample Transportation for Analysis

Several errors can creep in at each of the three stages. The objective of this article is to suggest ways that will help reduce errors with particular reference to sample collection from natural water bodies such as rivers, lakes, underground reservoirs, etc. It also covers essential details that need to be recorded during sample collection and storage.

Water from natural resources can show variations in composition due to weather fluctuations, rainfall, water flow changes, distance from bank and depth of sample collection point. A representative sample would be one collected from the midpoint and mid-depth of the water body. At times it may become necessary to collect samples periodically to monitor real-time variations in composition of trace elements.

Sample Collection

• Sample collection containers should be free of any contamination prior to use
• Leave some space between ceiling of sample container and sample to permit expansion during transportation and storage.
• Samples requiring acidification or matrix modification should be treated with ultra pure grade acids or reagents and preferably same lot of samples should be subjected to reagents and acids from same stock
• Important details on sample collection should include location of collection point with help of GPS or other landmarks, if any, date and time of sample collection, water temperature, pH, water flow rate and name of collector
• Samples should be labeled properly in case there are any associated hazards such as harmful bacteria or radioactivity in the collection area.
• Relatively stable water bodies such as lakes may require sampling only at selected location and depth whereas flowing bodies like rivers and streams would require sampling over specified intervals to get representative compositional variations. Instead of a single sample several samples from same point may be mixed to get a composite homogeneous sample.
• Remember variation in composition can result from changes in levels of dissolved oxygen, carbon dioxide, pH or temperature.

Sample Storage

After sample collection its storage also plays a crucial role.

• Metal containers are not recommended for estimation of metals as they can result in sample contamination over storage period
• Glass is also not an ideal choice as silicon, sodium or other metallic impurities from glass containers can leach and contaminate the sample. On the other hand some trace metals can get deposited on container walls and their concentrations can drop over the storage period.
• PTFE or plastic containers are ideal options for most analysis

Sample Transportation

It is advised to conduct the analysis immediately after collection of samples. However, this may not be possible every time and transportation to testing laboratory becomes necessary .It is essential to ensure that the samples are properly sealed in their containers and are not exposed to direct sunlight or extremes of temperature during transportation and storage.

Atomic Absorption Spectroscopy is a well established and proven technique which helps analysis of water samples at required concentration levels. However, with the growing safety and health concerns some elements may be required to be determined at even lower levels which could necessitate adoption of more sophisticated techniques such as ICP – MS. However always keep in mind that such sophisticated techniques can generate highest reliability data only if proper care is taken in sample collection, handling and treatment