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Sampling the Effluent
The sampling of the effluent is vital in the determination of trace metals. An improper sampling technique can leave you with a sample of the effluent that's not representative of the entire stream. Your strategy must remove a sample from the bulk of the stream and not from the sides or from places where eddies can occur. Manufacturing facilities usually have access ports in the pipe that carries the effluent out of the plant for waste disposal where you can obtain your sample. When you open the valve, allow the stream to flow for a few minutes to clean out the effluent that remained in the port since the last sampling.
Decide on a Preconcentration Method
Preconcentration is essential when performing trace metal analysis. Analytical instrumentation is usually limited in its ability to detect analytes in concentrations higher than those found in samples that contain only trace amounts. An analyte is the chemical you're monitoring --- in this case, metals. By preconcentrating a sample, you raise the amount of metal present in the sample to the level that analytical instrumentation can detect it. Solvent extraction, solvent evaporation and dissolution, and sorbent extraction are all techniques that aid in the preconcentration of analytes.
Remove Interferences and Other Components
Effluents from process streams are composed of a variety of chemicals and waste products other than those of interest. Separating the analyte from these other constituents makes the determination of the amount of the analyte much easier. Adding a chelating agent to the sample and following it by solvent extraction allows you to remove the analyte from all of the other components of the effluent. Depending on the volumes used in the solvent extraction, you may achieve a level of preconcentration with this process as well.
Method of Detection
The method chosen to detect the analyte depends on the instrumentation you possess. For the analysis of multiple metals in the effluent, atomic emission techniques provide simultaneous detection of multiple metals at one time. Atomic absorption spectroscopy and inductively coupled plasma instruments are the most common instruments for this task. After the sample preconcentration and cleanup, you may introduce the samples to the instrument and analyze the results. Both of these types of instruments require calibration before analysis of your sample.