Today advanced sophisticated analysis techniques are capable of providing trace metal analysis in ppm, ppb or even ppt levels. The most popular techniques are Atomic Absorption Spectroscopy, ICP – MS and ICP – OES. Determinations at low levels can be reliable only if adequate precautions are taken at different stages of analysis such as sampling, storage, and sample handling under controlled environmental conditions. Practices that should be adopted for minimization of contamination at various stages of analysis are briefly covered in the article.
Proper sampling practices and precautions should be taken to ensure reliability of analysis in the first stage of testing itself:
- Samples should be representative of the whole. The sample should be homogeneous in nature. In most cases the original sample is found to be inhomogeneous. Such samples abound in nature and range from minerals and rocks, water bodies likes lakes, rivers, oceans, soils, etc. In such cases representative lots are obtained by drawing larger sample quantities on the basis of established sampling procedures and after thorough homogenization sample should be drawn for analysis.
- The sampling vessels should be cleaned and rinsed with the sample at least 2 to 3 times before sample collection
- The sample container should be sealed airtight and packed securely in case it is to be transported to the analysis site.
- Analysis samples requiring storage should be stored under recommended conditions of storage such as temperature, humidity, exposure to light,etc
- The time between sample collection and analysis should be kept to the minimum to avoid any degradation or decay
Sample preparation and treatment
Trace metal analysis in samples such as water does not require much sample preparation. In case of solid samples or biological specimens additional steps such as fusion with flux materials or ashing or acid digestion would be necessary for extraction of trace metals from complex matrices. In such cases high purity grade reagents and acids should be used so that they do not contribute metallic impurities to the sample.
- Clean volumetric glassware should be used always
- The analyst should use clean graduated cylinders, spatulas and other items for transferring of reagents and sampling media.
- Do not keep sample longer than required in the fume hood chamber as this increases risk of contamination. It is preferable to use microwave digestion systems over open-air acid digestions.
- The analyst should avoid sample contamination from self by making use of disposable gloves, head covers and face masks while handling samples and carrying out dilutions
Environmental controls in the laboratory
- The sample preparation area should be isolated from the sophisticated instrument facilities
- Entry to the sample preparation and analysis areas should be restricted. All authorized visitors should adhere to the required dress code to prevent ingress of external contaminants
- The test area should be isolated and provision made for cleaning the air using HEPA filters
- Flooring of trace metal analysis laboratory should be vinyl coated and mopped periodically during the day
- Analyst should be allowed inside test area after changing to laboratory footwear so that outside contamination does not enter the laboratory.
The greater the precautions taken the greater will be reliability of trace metal analysis. The required environmental controls are even more stringent when ultra trace level determinations are required to be made.