Download Gas Chromatography

Organic Chemistry Lab 315
Fall, 2016
Spectroscopy II Problems
• You must analyze the Elemental Analysis
information, not simply rewrite what is given.
Show the calculation.
• You must state what the UV-vis absorption
value(s) indicate about the structure of the
unknown compound, just as you do for the IR
information.
DUE DATES
• Today
– At beginning of lab – TLC Report
– At end of lab -- copy of laboratory
notebook pages for today's experiment
– Spectroscopy Problem II #3
• Next Week
• Gas Chromatography Report (see
instructions on my lab web page.)
– Spectroscopy Problem II #4
Gas Chromatography
• Uses
- Separation of compounds in a mixture
- Determination of purity of a compound
- Quantitative determination of relative
amounts of components of a mixture
- Isolation of pure compounds
• The first two also apply to TLC
• The second two pertain mainly to GC
Gas Chromatography
Components of a Gas Chromatograph
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Carrier gas
Injector port (heated)
Column (containing stationary phase) in
oven
Detector
Recorder
Gas Chromatography
Practice
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Inject liquid sample with a microliter syringe
through a rubber septum.
Heated injection port vaporizes liquid sample
Carrier gas (He) mixes with sample and both move
through the column.
Column packed with the stationary liquid phase - a
non-volatile liquid, wax, or low melting solid. Liquid
phase is coated onto a support material, such as
crushed firebrick.
Gas Chromatography
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The sample dissolves in and interacts with the
molecules in the liquid phase as it is pushed
through the column by the carrier gas (elutes).
Different components of the sample will interact
to different extents with the liquid phase.
Some components of the mixture will interact
less and move faster; other will interact more
and move more slowly through the column.
The more volatile (low boiling point) compounds
travel faster through the column with the liquid
phase we are using.
Gas Chromatography
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The components of the sample, now
separated, pass over the detector
(thermal conductivity detector, TCD).
The output is recorded as a
chromatogram.
Gas Chromatography
The components of a sample
mixture are separated over time
and are recorded as peaks on the
chromatogram.
The time each component spends
on the column is the retention
time (t).
In this example, the component
peaks are well resolved
(separated). The peaks are
symmetrical and the leading and
trailing ends of each peak are on
the baseline.
Gas Chromatography
•
Factors Affecting Separation
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Flow Rate of Carrier Gas
Temperature of Column
Choice of Liquid Phase
•
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Molecular weights, functional groups, and polarities of
component molecules are factors in selecting liquid
phase.
Length of Column
•
Similar compounds require longer columns than
dissimilar compounds. Isomeric mixtures often require
quite long columns.
Gas Chromatography
• Quantitative Analysis
– The area under a peak is proportional to the
amount (moles) of compound eluted.
– See Pavia for method of triangulation to calculate
peak areas. Do your calculations exactly as shown
there.
– The mole percent composition of a mixture can be
approximated by comparing the relative areas of
the peaks in the chromatogram.
• This method assumes that the detector is equally
sensitive to all compounds and its response is linear.
Gas Chromatography Experiment
• Gas Chromatography of Alkyl Acetates
• Inject standard mixture of a known
equimolar mixture of four esters: (ethyl,
propyl, butyl, and pentyl acetate).
O
CH3C O R
R= ethyl, propyl, butyl, pentyl
• Inject Unknown (two or more of the esters).
• KEEP THE VIALS CAPPED AT ALL TIMES
–
Ethyl acetate is volatile and will evaporate
Gas Chromatography Experiment
• The retention times of the peaks will be
printed on the chromatogram. The esters
will elute in order of increasing boiling point.
• Match to retention times of known esters in
order to identify ester components in
unknown.
• Calculate peak areas and approximate mole
percentages of components of unknown
ester mixture.
Experiment Notes
• Clean the syringe before injecting a new
mixture.
–
Draw up acetone into the syringe. Expel the acetone into
the waste beaker. Repeat twice more.
–
Pull out and depress the syringe plunger several times to
air-dry the syringe.
• You may see a small acetone peak in your
next injection if the syringe is not dry.
Ignore the peak.
Experiment Notes
• There are 2 GC’s.
• Work in groups of 2-3
(alphabetically by 1st name).
– Each group injects
standard equimolar ester
mixture.
– Each student injects an
unknown ester mixture.
– No student may leave the
lab until the last group of
students finishes.
Alexandra Briana Djanabou
Elisabeth Eman Emily G.
Emily E.
Erin Gregory T.
Gregory L.
Hemani
Kevin
Lisa
Marlon
Nelia
Samuel
Teresa
Kelsey
Maritza
Rabia
Sara
Young
Sumaiyah
Notes
• Review significant figures.
• All calculations of peak area to be done
exactly as shown in Pavia text on the
chromatogram itself.
• Read the guidelines on the class web
page for writing a GC Report.
Notebook
• Record all the chromatographic
conditions
– Temperature of injection port
– Temperature of detector
– Flow rate of carrier gas
– Identity of liquid phase column
Lab Clean-up
• Lemek, Greg
• Malcolm, Briana