Lab_5 - Fall 2007 Chemistry 300 Lab 5 Cornell University...

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Fall 2007 Chemistry 300 Cornell University Lab 5 Elucidation of the Fraction of Benzaldehyde in Imitation Almond Extract by Gas Chromatography I. INTRODUCTION T he goal of this one-week lab is to determine the amount of benzaldehyde present in imitation almond extract using gas chromatography. Benzaldehyde is the component of almond extract that gives it its character- istic almond-like Favor and smell. II. MATERIALS AND METHODS You will need Equipment Gas Chromatography Glassware Pasteur pipettes Scintillation vials Syringe: 10 μ L Chemicals Benzaldehyde (freshly distilled) Ethanol Methyl Benzoate Materials Imitation Almond Extract B efore coming to lab, you should read the following sections in Harvey’s book covering internal standards and chromatography: 5B.5, 12A, 12B intro, 12B.1, 12D intro, and 12D.1 to 12D.7. The goal of this lab is to quantify the amount of ben- zaldehyde in imitation almond extract. Calibration samples are prepared containing known amounts of benzaldehyde, methyl benzoate (as an internal standard), and ethanol (as a solvent). An internal standard is required because the volume of injected solution is only reproducible to 10 - 20%; we wish to determine the amount of benzaldehyde in imita- tion almond extract to much higher precision than this. These standard solutions are also run through the gas chromatograph and peak heights measured. A calibra- tion curve is constructed by plotting the mass ratio of benzaldehyde to methyl benzoate on the x -axis and the peak ratio on the y -axis. A solution is then prepared of the commercial material, also with an internal standard. The mass of benzalde- hyde present in the commercial sample is determined by converting the observed peak ratio to a mass ratio using the calibration curve. Since the mass of methyl benzoate added to the commercial material is known, the mass of the unknown benzaldehyde may be inferred. In this experiment you will use gas chromatograph employing a polyethylene glycol stationary phase, a helium carrier gas, and a thermal-conductivity detector. The gas chromatograph is a model GC-8A from Shi- madzu with a Carbowax 20M column, an injection tem- perature of 200 C, a column temperature of 160 C, a detector current of 60 mA, and a Fow rate of 12 mL/min.
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Lab_5 - Fall 2007 Chemistry 300 Lab 5 Cornell University...

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