1. The student from class has an exam to study for and asks you to analyze her d
ID: 1060130 • Letter: 1
Question
1. The student from class has an exam to study for and asks you to analyze her data. Ethical problems aside, here are the data (the area of each peak – you should assume injection volume is always the same):
Sample #
Peak 1
Peak 2
Peak 3
Peak 4
Peak 5
S1
0.37
5.62
3.98
4.72
11.32
S2
0.34
2.78
1.92
3.78
17.16
S3
0.35
6.92
13.02
1.92
3.78
S4
0.35
1.74
10.72
8.82
4.36
A. Why are there 5 peak areas given for each sample, when the mixture contains only four alcohols?
B. You know that the column in the lab’s GC is packed with the nonpolar material PDMS. What is the most likely elution order for these alcohols (ethanol, 1-propanol, 1-butanol, 1-pentanol)?
C. Calculate the percentage of each peak’s area to the total area. Depending on the type of detector, this can be a pretty good approximation to the actual percent composition by mass of the mixture.
2. In normal-phase HPLC, one might use an alumina column (Al2O3) as the stationary phase, with a mixture of acetone and hexane as the mobile phase. For a gradient elution (i.e., weak mobile phase to strong), would you increase or decrease the amount of acetone in the mobile phase?
Sample #
Peak 1
Peak 2
Peak 3
Peak 4
Peak 5
S1
0.37
5.62
3.98
4.72
11.32
S2
0.34
2.78
1.92
3.78
17.16
S3
0.35
6.92
13.02
1.92
3.78
S4
0.35
1.74
10.72
8.82
4.36
Explanation / Answer
From the data
A. The first peak in each set is probably the solvent peak used to prepare the mixture of alcohols.
B. With the non-polar stainary phase, the polar compound would elute first and non-polar would adhere stringly on the staionary surface and elute out at the end.
The elution order from first to last would be : ethanol (Ist), 1-propanol (IInd), 1-butanol (IIIrd) and last 1-pentanol.
C. Percentage of each peak area by total area is calculated by averaging out all values of Peak 1/Peak all
Total area = 0.3525 + 4.265 + 7.41 + 4.81 + 9.155 = 28.9925
Peak 1 % = 0.3525/25.9925 = 1.35%
Peak 2 % = 14.71%
Peak 3 % = 25.56%
Peak 4 % = 16.59%
Peak 5 % = 41.79%
2. For Al2O3 stationary phase, we would increase the concentration of more polar phase in mobile solvent phase for elution, thus acetone concentration would be increased for gradient run.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.