Chromatography
1.
An extract from a plant was analyzed using thin-layer chromatography with a non-
polar solvent. The chromatogram obtained is shown in Figure 6.4.
Yellow
Orange
Dark green
Light green
Figure 6.4
Chromatogram.
Table 6.2 gives the Rf values of some chemicals commonly found in plants.
Table 6.2 Rf values of some plant materials.
Chemical
Xanthophyll
β-Carotene
Chlorophyll a
Chlorophyll b
Leutin
Neoxanthin
Rf
0.67
0.82
0.48
0.35
0.39
0.27
Make A Table For All Your Answers
A) Measure and record the distance from the origin to the center of each band, andthe distance of the solvent front from the origin.
B) Calculate the Rf value of each band.
C) Compare Rf values for the bands to the Rf values in Table 6.2 and name thechemicals present in the extract.
D) If water had been used as the solvent, would the chromatogram be likely to havebeen similar in appearance?
Explain.
2. There are several types of chromatography, including thin-layer and paper, gas and high performance liquid chromatography. What features are common to all kinds of chromatography?
3.
Australian wines are routinely tested for ethanol content. A quick and reliable method
is by gas–liquid chromatography. The peak areas produced by a sample of wine and a
number of standard solutions of ethanol are shown in Table 6.3.
Table 6.3 Peak areas from GLC analysis of a wine sample and standards.
Wine
Standard 1
Standard 2
Standard 3
Standard 4
% ethanol
?
4.00
8.00
12.0
16.0
Relative ethanol peak area
82 400
31 200
62 900
94 200
125 700
A)Plot a calibration curve of concentration of ethanol against peak area.
B)Determine the percentage of ethanol in the wine sample (from your graph).
C)Why is it necessary to measure the peak areas produced by a number ofstandards?
4. A sample of brown dye from a lolly is placed at the origin on a strip of achromatography plate. The solvent front moves 9.0 cm from the origin. A bluecomponent of the dye moves 7.5 cm and a red component 5.2 cm in the same time.Calculate the Rf values of the two components.
5. Thin-layer chromatography showed that the black dye used in a brand of writing ink
contained colored components. The Rf values of these substances
using ethanol as the solvent arefor blue, red, orange and yellow 0.59, 0.32, 0.80 and 0.19, respectively.
a How far apart would the blue and yellow components be after the solvent front
had moved 8.0 cm from the origin?
b When the red component had travelled 6.0 cm from the origin, how far would theorange part travel?
6.GAS Chromatogram of four hydrocarbons.
A mixture of four alkanes, decane, heptane, hexane and octane, were separated by gas chromatography. The solubility of the hydrocarbons in the liquid phase on the column was directly proportional to the relative molecular mass of the alkane. Time is the x axis
a Identity the hydrocarbons A, B, C and D.
b Why does decreased solubility of a chemical in the stationary phase decrease the retention time?
c Why do the peaks become broader with increased retention time?
d List the factors that can increase the retention time of a particular chemical in GLC.
7.In gas–liquid chromatography:
a What is the usual mobile phase?b What is the stationary phase?
c Why is the column packed with very fine particles?d Why is the injection port heated?
8. The organic compounds dimethyl ether (CH3OCH3) and ethanol (CH3CH2OH) havethe same molecular formula, C2H6O, and the same molar mass; however they havedifferent structural formulas.
A sample containing both ethanol and dimethyl ether is analysed by GLC using aflame ionisation detector.
a.Will the two compounds produce peaks with the same retention time? Explainyour answer.
b. If the sample has the same concentration of both chemicals, will the peaksproduced have the same area?
Explain your answer.