Sampling oil in water involves several steps to ensure accurate and representative results. Here's a breakdown of a common method using solvent extraction and FTIR analysis:
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Obtain a Homogenized Sample: Use a syringe to extract a 10 ml sample. Ensure the water and oil are thoroughly mixed (homogenized) before extracting the sample to guarantee a representative proportion of oil. This is crucial because oil and water tend to separate.
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Solvent Extraction: Process the sample volume through an extractor. This typically involves adding a solvent (like a fluorocarbon) that selectively dissolves the oil from the water. The extractor separates the solvent/oil mixture from the water.
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Drying the Extractor: Dry the extractor or the extracted solvent/oil mixture to remove any residual water that might interfere with the measurement. This is typically done using a desiccant or by controlled evaporation.
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Measure Oil Concentration: Use an FTIR (Fourier Transform Infrared Spectroscopy) unit to measure the oil concentration in the solvent. FTIR measures the absorption of infrared light by the oil molecules, providing a quantitative analysis.
In more detail, here's a more comprehensive overview of different methods:
Sampling Methods for Oil in Water
The specific method used to sample oil in water depends on the source, type of oil, and the desired accuracy. Here are some common approaches:
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Grab Samples: These are single samples taken at a specific point in time. They are suitable for relatively homogeneous systems. Important considerations for grab samples include:
- Sample Location: Choose a location where the oil and water are well mixed.
- Sample Volume: Ensure the sample volume is sufficient for the analytical method.
- Sample Container: Use a clean, inert container (glass or Teflon) to prevent contamination.
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Composite Samples: These are samples created by combining multiple grab samples taken over a period of time. They provide a more representative average concentration.
- Time-Weighted Composites: Samples are collected at regular intervals and combined proportionally to the time elapsed.
- Flow-Weighted Composites: Samples are collected proportionally to the flow rate of the water.
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Continuous Sampling: A continuous stream of water is passed through a sampling device. This provides real-time monitoring of oil concentration.
Procedures after collection
Homogenization
The collected sample, no matter the original method, must be properly homogenized. The following tips are useful:
- Mixing: Aggressively mix the collected sample just before sampling.
- Avoid Over-Mixing: Over-mixing may cause separation of the oil and water.
Analysis
The concentration of oil can be determined by the following common techniques:
- Solvent Extraction with Gravimetric Analysis: Extract the oil with a solvent. Evaporate the solvent and weigh the remaining oil.
- Spectrophotometry: Measure the absorbance of the extract at a specific wavelength. The absorbance is proportional to the concentration of oil.
- Gas Chromatography (GC): Separate the oil components and measure their concentrations using a flame ionization detector (FID).
- FTIR (Fourier Transform Infrared Spectroscopy): As described above.
By following these steps, you can obtain a reliable and representative sample of oil in water for accurate analysis.
