How to tell which peak is ethanol on a chromatogram with gasoline and increasing levels of additional ethanol
The correct answer and explanation is:
To identify the ethanol peak on a chromatogram, especially in the presence of gasoline and increasing ethanol levels, follow these steps:
- Retention Time Comparison: Each compound in a mixture has a characteristic retention time, which is the time it takes for the compound to pass through the chromatographic column. For ethanol, this retention time should be consistent across different runs, but it may shift slightly based on the column temperature, flow rate, and concentration of ethanol. When gasoline and ethanol are mixed, the ethanol peak typically appears earlier than heavier hydrocarbons in gasoline. So, as you increase ethanol levels, you will notice that the ethanol peak retains its position but might become more pronounced.
- Monitor Peak Area or Height: As ethanol concentration increases, the peak corresponding to ethanol will increase in size. This is because the area under the peak is proportional to the amount of ethanol present in the sample. If ethanol is present in varying amounts, the ethanol peak will have a larger area or height compared to the background or baseline, depending on the ethanol concentration.
- Use Known Standards: The best way to confirm the identity of the ethanol peak is by running a sample of pure ethanol (standard) under the same chromatographic conditions. The retention time of this ethanol standard will match the retention time of ethanol in the unknown samples.
- Compare with Gasoline Peaks: Gasoline contains a mixture of hydrocarbons, each with different retention times. These peaks will generally be farther along the chromatogram than the ethanol peak because gasoline consists of heavier molecules. The ethanol peak, being a lighter compound, will appear earlier. Gasoline peaks should remain relatively constant even as ethanol concentration increases.
By following these steps, the ethanol peak can be identified and differentiated from the gasoline peaks, and its concentration can be determined based on the peak area or height.