ORGANIC I LABORATORY
KELLY
SEPARATION OF LIQUIDS BY GAS CHROMATOGRAPHY
READ:
Landgrebe pp 99-106
I. Purpose:
To Learn the techniques of gas-liquid chromatography by analysis of the
distillation fractions collected in the last experiment
II. Samples to Analyze:
A. A 50:50 mixture of cyclohexane and toluene (provided at GC station)
B. An unknown mixture of cyclohexane and toluene (provided at GC station)
C. Fraction 1 --- the first 2 mL of distillate from previous experiment
D. Fraction 2 --- the residue left in the distillation pot
III. Procedure
A. Check the gas chromatograph and recorder settings to see that they match
those recommended on the next page under Gas Chromatograph and
Recorder Settings . You will inject into the right (nonpolar) column. The
stationary phase is a silicone oil which allows compounds to separate
primarily by their volatility. You will determine the identity of the peaks
on the basis of their retention times and the boiling points of the
cyclohexane and toluene.
B. Turn on the recorder chart drive.
C. Flush the syringe twenty times with the sample you will inject. Never
force the syringe plunger. If it sticks, ask the instructor for assistance.
Inject 0.5 µL sample sizes. While injecting, insert the needle through the
center of the septum, push in the plunger, and withdraw the needle - - all in
one smooth action. Do not force the needle, if you encounter resistance.
Withdraw it and try again.
D. In each chromatogram, the largest peak should be at least half way up the
chart, but not off the top of the chart. If you do not obtain this result on
the first injection, reinject a second sample, adjusting the sample size or
attenuation, as necessary. Increasing sample size or lowering the
attenuation will increase peak size.
E. When you have completed your injections, turn off the chart drive. If you
are working in pairs, you must give your chromatograms to your
instructor or lab assistant, to be photocopied so that each of you have the
chromatogram to turn in with your reports.
Separation of Liquids by Gas Chromatography
V. Gas Chromatograph and Recorder Settings
Page 2
GAS CHROMATOGRAPH SETTINGS
Injection size:
0.75 mL
Attenuation:
256 (25 in the old units)
Polarity:
right
Right (Recorder plug cord on the
on the old units)
Column Temperature:
Detector Temperature:
D.
E.
F.
G.
low (Setting "C" on old units)
Bridge Current:
units)
none (plug in banana plug on old
Zero Control:
Adjust until recorder pen is on the "1"
RECORDER SETTINGS
Volts:
Speed (On/Off):
÷ Switch:
CHT:
VI.
A.
B.
C.
80° (Setting "C" on old units)
0.001
1
1
On (when you are ready to inject)
Report
Cover Sheet with Experiment Title and your name
Short Introduction, describe the purpose of the experiment.
Chromatograms (on each of which the following must be shown)
1. The injection point (mark and label according to sample injected)
2. The identity of each peak (you may have a small air peak)
3. The type of column used (polar or nonpolar)
4. The actual temperature of the column. (Use a thermometer)
5. Your name
For each calculation below, show your method of finding the solution on
at least one example for each type of calculation.
Calculate the areas of each peak by multiplying the width at half height
times the full height of each peak. Show, by each peak on the
chromatograms, the numbers you get.
The detector will not be equally sensitive to both compounds. You can see
that by looking at the relative areas for toluene and cyclohexane in the
50:50 mixture. Determine the relative molar response (RMR) of the
toluene peak relative to the cyclohexane peak using the chromatogram of
the 50:50 mixture. (The RMR for toluene relative to cyclohexane is just
the ratio of the toluene peak area to the cyclohexane peak area.) Then all
cyclohexane peaks must be corrected by multiplying their areas by the
toluene RMR value.
Calculate the percent cyclohexane in each sample using the formula below.
area corrected CH peak
% cyclohexane (CH) = [area corrected CH peak] + [area of Tol peak] x 100
Separation of Liquids by Gas Chromatography
Page 3
H. Summarize your calculations in a table which shows the following
information for each chromatographic analysis.
Sample Identity
Area for
Cyclohexane
Area for
Corrected
Cyclohexane
Area for
Toluene % Cyclohexane
I. Calculate the number of theoretical plates per foot of GC column using the
formula on page 101 of your lab manual. Use the peak for cyclohexane in
the 1st fraction from your distillation experiment. The GC column is 5
feet long.
VII. Study Questions for the exam.
1. As a mixture of an alcohol and a hydrocarbon passes down a polar gas
chromatography column, one of the compounds will move out ahead of
the other. Discuss the physical changes which are occuring that cause
the one compound to move through the column faster. This will
probably take a half a page to cover adequately.
2. Why does a nonpolar substrate on a gas chromatography column always
separate compounds on the basis of boiling point only?
3. Explain why it was necessary to correct the peak area for the
cyclohexane before it was used in your calculations for the %
cyclohexane.
4. It would have been possible to have corrected the peak area for the
toluene rather than the cyclohexane and still obtain the same answer for
the % cyclohexane. Do the calculation this way for your unknown.
Show your method of calculation for each calculation used.
5. How does the gas chromatograph detect compounds when they exit the
column.
6. To what type of compounds is gas chromatography limited?
7. Be able to show a schematic of a gas chromatograph and explain the
function of each part. (see Figure 6.1, page 82, in lab text)
8. Understand the technique of mixed injections and how it can be used to
provide evidence for the identity of a peak in a chromatogram