Introduction:
Gas chromatography is possible to separate the volatile components of a very small
sample and to determine the amount of each component present (1). In this experiment,
the BTEX mixture was separated by gas chromatography at different isothermal
temperature and temperature programming. The temperature effect was determined in
this experiment.
Observation:
1) Because the organic solvents which were used in this experiment such as Hexane
and toluene are toxic, they were well stored and disposed in appropriate
containers.
Data:
BTEX: benzene, toluene, ethylbenzene, and xylenes (o-xylene, m-xylene, p-xylene) in
methanol.
Column length: 25m
Stationary phase:
dimethylpolysiloxane
Compound
Hexane
benzene toluene
ethylbenzene
p-xylene
m-xylene
o-xylene
bromobenzene
Boiling
69
80.1
136
138
139
144
156
point (°C)
110.6
Discussion:
The temperature of the column is directly proportional to the rate of a sample passes
through the column. The higher the column temperature, the faster the sample moves
through the column. However, the faster the sample moves through the column, the
less it interacts with the stationary phase, and the less the analytes are separated.
In the isothermal column temperature, the column resolution decreases as the column
temperature increases. And so does the retention factor. As the column resolution
represents the ability to separate two analytes.(2) The retention factor is widely used to
describe the migration rates of solutes on columns and adjust the separations.(2) The
resolution can be improved by optimizing retention factor k’. The bigger the column
resolution the more clearly separation.(1) Therefore, the separation can also be
improved by increasing the resolution factor k’.(1) Moreover, the bigger resolution
factor k’ the longer the elution time.(1) In sum, both the separation and elution time
decreases as the isothermal column temperature increases.
In the temperature program, the elution time is short, and the peaks are well and
uniform separated. The ideal gas chromatogram is short elution time and well
separated peaks. The temperature program has this advantage.
Question:
Q1) Plot and overlay the chromatograms at the 3 temperatures
Q2） Select 2 closely eluted compounds, calculate and tabulate the Rs and k’
values to evaluate the effect of column temperature on separation, resolution,
and analysis time.
o-xylene and p-xylene were compared.
Sample calculation:
At 55 °C 30cm/s
Rs= 2*[(tR)o-xylene- (tR)p-xylene]/(Wo-xylene+Wp-xylene)
= 2*(4.439-3.772)/(0.0383+0.0319) = 19.00
k’o-xylene =( tR-tm)/tm =(4.439-2.239)/2.239 = 0.98
k’p-xylene = ( tR-tm)/tm = (3.772-2.239)/2.239 = 0.68
k’= (k’o-xylene + k’p-xylene)/2 = 0.83
35 °C
55 °C
75 °C
Rs
36.67
19.00
11.69
k’
0.98
0.83
0.51
The resolution Rs of a column provides a quantitative measure of its ability to separate
analysis. The bigger Rs value the more clear separated peaks. In this experiment, by
comparing the Rs of o-xylene and p-xylene at different temperature, the lowest
temperature 35 °C creates the most clearly separation. The retention factor k’ is an
important parameter that is widely used to describe the migration rates of solutes on
column. The larger retention factor k’ value the longer the elution time. By the
comparison of retention factor k’ at 35 °C ,55°C and 75°C, high column temperature
(75°C) crates long elution time. In sum, high temperature creates low resolution and
short elution time, but the peaks may not be well separated; low temperature creates
large resolution and well separated peaks, but long elution time.
Q3) Sketch the separation of 2 hypothetical peaks with Rs of 1.0 and 1.5.
Q4)Plot the chromatogram.
Q5) Select 2 closely eluted compounds, calculate and tabulate the Rs and
k’values to evaluate the effect of temperature programming on separation,
resolution, and analysis time.
o-xylene and p-xylene were compared.
Sample calculation:
At 35-75 °C 30cm/s
Rs= 2*[(tR)o-xylene- (tR)p-xylene]/(Wo-xylene+Wp-xylene)
= 2*(5.57-4.867)/(0.0223+0.0292) = 24.97
k’o-xylene =( tR-tm)/tm =(5.57-2.039)/2.039 = 0.66
k’p-xylene =( tR-tm)/tm = (5.146-2.039)/2.039 = 0.46
k’= (k’o-xylene+ k’p-xylene)/2 = 0.56
35 °C
55 °C
75 °C
35-75 °C
Rs
36.67
19.00
11.69
24.97
k’bromobenzene
0.98
0.83
0.51
0.56
In the temperature programming, the temperature of a column is raised during the
separation to increase solute vapor pressure and decrease retention times of the late
eluting components.(2) Comparing the constant temperature and programmed
temperature chromatography, the programmed temperature chromatography has large
resolution , more uniform separated peak and short elution time.
Conclusion:
In this experiment, the temperature effect on the resolution Rs and retention factor k’
were determined. In a high isothermal column, more compounds are in the gas phase.
It interacts less with the stationary phase, hence the retention time is short, and the
quality of the separation deteriorates. In addition, beside the isothermal column, there
is another column temperature environment, temperature program. In the temperature
program, the retention time is short and the separation is more uniform than the
isothermal’s. In sum, as the isothermal column temperature increase, the quality of the
separation and the retention time decrease.
Therefore, in order to control the
characteristics of the gas chromatography, the temperature effect factor should be
considered or try to keep them in constant.
References:
1) SKOOG. HOLLER. NIEMAN. Principles of Instrumental Analysis, 5th ed. ;
Brookes Cole; United States of America,1997;Vol.6, p 355-365.
2) Introductory Instrumental Analysis: Chemistry 316 Laboratory Manual; Simon
Fraser University; Burnaby, 2010; p 13-21.
3) Wendy Strouse Watt, O.D. Fluorescein Angiogram, July, 2002.
Gas Chromatography:
Effect on column temperature and temperature programming
MiaoMiao Gu
301100545
Chem 316
Monday, Nov. 1st,2010