You wish to measure the concentration of methyl benzoate in a plant stream by ga

Question

You wish to measure the concentration of methyl benzoate in a plant stream by gas chromatography. You prepare a sample of butyl benzoate to use as an internal standard. A preliminary run, shown at the right involved a solution containing 1.79 mg/mL of methyl benzoate (peak A) and 1.97 mg/mL of butyl benzoate (peak B). The area of peak A was determined to be 319 and of peak B to be 373 (measured in arbitrary units by the computer). To measure the sample, 1.00 mL of a standard sample of butyl benzoate containing 2.37 mg/mL was mixed with 1.00 mL of the plant stream material, and analysis of the mixture gave a peak area of 441 for peak A and 411 for peak B. What is the concentration of the methyl benzoate in the plant stream? Make sure to use the dilution factor for the standard butyl benzoate.

I got 1.35 but it says I took into account the dilution factor for the analyte methyl benzoate, but not the dilution factor for the standard butyl benzoate.

Explanation / Answer

concentration of methyl benzoate is given by =Rf . A

where A is the area under the peak

Rf is the response factor obtained from calibration standards and = ratio of concentration with that of area of calibration values

therefore Rf of methyl benzoate=1.79/319=0.0056

and

concentration of methyl benzoate in sample =0.0056X441=2.474mg/ml

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