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?

Answer 1

First you need to calculate your Response factor:

Where Ax is the Area of the analyte and Ais is the Area of the internal standard.

Cx is the concentration of the analyte and Cis is the concentration of the internal standard.

Now using the data from the analysis you can calculate the ratio of Cx/Cis

Now you should note that when the sample was prepared the internal standard was diluted the final concentration is:

If multiply the ratio Cx/Cis times Cis you obtain the concentration in your sample: