Gas Chromatography to Identify Orange Oil in a n-Hexane Unknown

Written by Juan

1. Purpose: The purpose of this experiment was to determine the percentage of orange oil in an n-hexane unknown solution by analyzing the wt% of d-limonene through gas chromatography, using anisole as an internal standard.
2. Sample Calculations

Weight of Anisole in Standard #1

Weight of Limonene in Standard #1

Limonene: Anisole weight ratio in Standard #1

Limonene: Anisole peak area ratio in Standard #1

LLS Slope, intercept, and equation for internal standard plot

GC measurement for vial #1 was thrown out due to inconsistent peak area ratio

Least Squares Calculation for the internal standard plot

1. Data and Results

Data for the Internal Standard Plot

GC Oven Temperatures:

Injector: 60^◦C

Column within oven: 250^◦C

Detector: 300^◦C

A retention time: 1.192

L retention time: 3.049

0.0650 g O added                          0.6305 g of O/g of A

Equation of Internal Standard Plot Line:

Wt % Orange oil in unknown =

1. Conclusion:

I found that the wt% Orange Oil in the original unknown was 1.92%. A source of error pertinent to my results is that the ratio of O: A I measured out fell below the range of the internal standard solutions I made, which added some uncertainty to my calculated values due to having to extrapolate to determine the final concentration. The peak area ratio for vial #1 was also not consistent with the rest of the values and so was not used in the final calculations.

a)     If the n-hexane used to prepare the L standard and the A standard for the Monday labs contained some n-pentane, but the n-hexane used to prepare the O solution was pure, then the weight ratio of anisole and limonene would be the same because the ratios of the standards are kept the same since the same bottle of contaminated n-hexane was used. Furthermore, since the wt% was calculated from weight rather than volume/density, the wt% of anisole and limonene in A and L, respectively, and so the plot and results would be the same (as well as the wt% orange oil). The only difference would be that in there would be an extra peak towards the beginning when injecting the solutions containing some n-pentane.

b)    If the staff person who prepared the L standard solution used an old bottle of limonene, which somebody had contaminated with a compound that co-elutes with limonene, then the calculated weight of limonene in each standard would be falsely high. This means that the Limonene: Anisole weight ratios would also be falsely high yet the peak area ratios would remain the same since the contaminant co-elutes with limonene so the peak areas of both would add together. This effectively shifts the right, since the perceived ratio is higher yet the area ratio remains the same. Then, when reading the Limonene: Anisole weight ratio for the unknown, the value would be falsely high which would ultimately lead to a falsely high wt% Orange Oil in the original unknown.

Some advantages of the Internal Standard Method are that it accounts for routine variation in the response of the chromatographic system, for the variations in the exact volume of sample or sample extract introduced into the chromatographic system and retention times of the target compound and the internal standard may be used to calculate the relative retention time of the target compound and can then be used to compensate for small retention time shifts.

Some disadvantages are that the internal standards must be compounds that are not found in the samples to be analyzed and they must produce an unambiguous response on the chromatographic detector system.