Gas Chromatography

Organic Chemistry Lab 315
Fall, 2014
• Today
– At beginning of lab – Isolation of Caffeine
– At end of lab -- copy of laboratory
notebook pages for today's experiment
• Next Week
– No lab. Monday classes meet
• Two Weeks
– Gas Chromatography Report (see
instructions on my lab web page.)
– Spectroscopy Problem II. #3
Report Notes – Last Week
• Reports
• You need to follow instructions.
– Comprehensive instructions for the
Unknown lab report were on the web site.
– Such things as page order; drawing the
structure on the IR and marking the
absorptions; how to approach the written
analysis, etc.
– These kinds of details count and will count
more in future.
Report Notes – Last Week
– And most importantly, making sure your
experimental and literature IR match. Not
an approximate match, but a “fingerprint”
– The output from the IR spectrometer is a
spectrum (plural: spectra).
– In succeeding weeks, there will be
guidelines for writing different kinds of
reports. Please make sure you read the
appropriate guideline, as well as the
“general” reports guideline.
Report Notes – Last Week
• It is not allowable to cite “experimental
error” to explain away discrepancies.
– You’ve been told many times to check your thermometer for
gaps and to confirm it is reading a reasonable room
temperature. Do not say “thermometer error” to explain a that doesn’t correspond to literature.
• B.pts. are not as accurate indicators of purity as m.pts. are.
– Never again should you say there is something wrong with the
refractometer. If you think there is, tell me during lab. They
are highly precise and accurate instruments. It is (almost)
better to say you still have not learned how to use the
Spectroscopy Problem Notes
• Each problem in Part II will count 20 points.
• Half of the points are for consistency and confirming
identity of unknown by looking it up on a spectra
– If you say it is a particular compound, then it is easily
confirmed with an exact match to the literature.
– I know the answers are “out there”, so if you are actually
doing these problems by yourself, then just check the
answer in the database.
– There is very little excuse for giving a structural formula
that does not have the same M.W. as the mass spectrum M+
• If you made a major mistake on #1 (they will be
returned during lab) that you repeated for #2, then
you can turn #2 in by Thursday, 12:00 noon, with no
Gas Chromatography
- 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
Gas Chromatography
Carrier gas
Gas Chromatography
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
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
phases we are using.
Gas Chromatography
The components of the sample, now
separated, pass over the detector
(thermal conductivity detector, TCD).
The output is recorded as a
Gas Chromatography
Determination of
Retention Time
Gas Chromatography
Factors Affecting Separation
Flow Rate of Carrier Gas
Temperature of Column
Choice of Liquid Phase
Molecular weights, functional groups, and polarities of
component molecules are factors in selecting liquid
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.
– 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.
– See Pavia for method of triangulation to calculate
peak areas. Do your calculations exactly as shown.
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).
R= ethyl, propyl, butyl, pentyl
• Determine retention times of the known
Gas Chromatography Experiment
• Inject Unknown mixture (two or more of the
compounds in the known mixture).
• Determine retention times of unknown
• Match to retention times of known
compounds to identify components.
• Calculate peak areas and approximate mole
percentages of components of unknown
• There are 2 GC’s.
• Work in groups of 2-3
(alphabetically by 1st
name – see last slide).
– Each group injects
standard sample
– Each student injects an
unknown sample.
– No student may leave
the lab until the last
group of students
Abbas Ashely Ben
Khoa Ly Megan
Muhammad Nafisa Ngoc
Olivia Richard Shepket
Stephan Stephanie Tatiana
Tyler Veronika
• 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 web page for
writing a GC Report.
Notebook Keeping
• Record all the chromatographic
– Temperature of injection port
– Temperature of detector
– Flow rate of carrier gas
– Identity of liquid phase column
– Recorder chart speed
Lab Clean-up
• Idris