Investigation Part II:
Separation of the Maltenes using Column Chromatography
Chromatography is a general term that represents several different lab
techniques, including: column, thin layer, and gas chromatography, to name a few.
All types of chromatography consist of a solute (mixture) moving in a mobile phase
along a stationary phase. This process is known as elution. The mobile phase, called
the eluant, is either a gas or a liquid. The stationary phase can be a liquid coated
onto a solid, such as silica gel, or alumina, packed into a column. The solute that has
a greater affinity for the stationary phase will move through the column more
slowly.
Column chromatography is used to separate components in a liquid mixture.
In this case it can be used to separate the saturated hydrocarbons (branched
chains and cyclic rings), from aromatics, and resins (polar aromatic rings containing
nitrogen, oxygen or sulphur). Due to intermolecular attractions, the individual
components are retained by the stationary phase (alumina) differently. They
separate from each other, and move at different speeds with the eluant(s) through
the column.
Purpose
The purpose of this investigation is to separate the maltene samples of light,
medium and heavy oils into their Saturated hydrocarbon, Aromatic and Resin
fractions using micro column chromatography in a Pasteur pipette. The SARA
fractions will then be compared in light, medium and heavy oils. The Asphaltene
fraction was separated and collected in Part I of this investigation.
Problem
What are the percent abundances of saturated hydrocarbons, aromatics, and
resins in samples of light oil, medium oil and heavy oil?
Design
Micro-scale column chromatography will be used to separate the oil samples into
fractions of the saturated hydrocarbons, aromatics, and resins. Students will
measure the mass, and calculate the percent abundance of each fraction in their
sample of oil. They will then compare the percent abundance of SARAs in the
samples of light oil, medium oil, and heavy oil.
Safety Precautions
Wear safety goggles at all times. Hexane and dichloromethane are volatile, so only
use in a well-ventilated area. Wear gloves at all times when handling the oil samples
and organic solvents.
Materials
-
1.5 mL glass Pasteur pipette
- ring stand
clamp to secure pipette
- wisp of cotton or glass wool
approx 1.5 g of alumina
- 4 glass sample vials with caps
2 mL of dichloromethane (DCM)
- aluminum foil
20-30 mg of oil sample (light, medium, heavy)
12 mL of hexane
- 2 Pasteur pipettes
6 mL of methanol
- 10 mL graduated cylinder
fume hood
- labels for vials
Procedure
Day 1:
1. Set up a ring stand with a small “claw” clamp that can support a 1.5 mL glass
Pasteur pipette.
2. Using the tip of a 2 mL Pasteur pipette, carefully push a small wisp from a
cotton ball, or a small wisp of glass wool, down the neck of a 1.5 mL Pasteur
pipette and plug the bottom. Make sure the glass wool is pushed down snugly
at the base. Fill the pipette with alumina until it is approximately two thirds
full by dipping the top of the pipette into the alumina, and scooping it in. Tap
the column to settle the alumina and remove any bubbles. Do not overfill the
pipette, as you will need room for the sample and the eluant.
3. Clamp the pipette securely to the ring stand.
4. Label four clean, and dry, glass sample vials as follows: Maltene (Light,
Medium, or Heavy); Sat HC; Aromatic; and Resin. Weigh the empty Sat HC,
Aromatic, and vials and record the masses. Place the vials into a secure
holder.
5. Tare the empty Maltene sample vial on an analytical balance and add
approximately 0.020 g – 0.030 g (20 – 30 mg) of the maltene sample from
Part I. Use the transfer method discussed in Part I, Day 1 of the procedure.
6. Measure 8 mL of hexane into a clean, and dry, graduated cylinder. Using a
Pasteur pipette, add approximately 2 mL of hexane to the top of the column
and allow the eluant to drain into a waste vial. Remember to dispose of
organic solvents into the waste container, located in the fume hood, and
NOT down the sink.
7. Place the pre-weighed vial labeled Sat HC under the tip of the Pasteur
pipette column.
8. Using the Pasteur pipette, add a small volume (less than 1 mL) of hexane
from the graduated cylinder to the maltene sample vial. Force the air from
the pipette into the maltene sample to help it mix. Carefully, and slowly, add
small volumes of the maltene/hexane mixture to the top of the column using
the Pasteur pipette. Wait, and allow the whole sample to sink into the
alumina and form a layer at the top before continuing.
9. Using the same pipette, add another 1 mL of the
hexane to the maltene sample vial and mix.
Without disturbing the maltene layer in the
column, transfer a small volume from the vial to
the top of the column. As the eluant sinks into the
alumina, carefully and slowly, transfer the
contents from the vial to the column. Repeat with
more hexane to quantitatively transfer the entire
maltene sample to the column.
10. As the fraction begins to drain into the sample
vial, continue to top off the column with the
remaining hexane.
11. When drops are no longer coming off the column, rinse the tip of the column
into the collection vial with a few drops of hexane. Remove the Sat HC vial
and place in the fume hood to evaporate overnight.
12. In the same 10 mL graduated cylinder, prepare 6 mL of a 1:2 ratio of DCM to
hexane by measuring 2 mL of DCM, and filling to the 6 mL mark with 4 mL of
hexane. Using the same Pasteur pipette, stir the solvent mixture gently by
forcing air into the graduated cylinder.
13. Place the pre-weighed vial labeled Aromatics under the tip of the
column.
14. Slowly add the DCM-hexane mixture to the top of the column with the
pipette. Continue adding until all of the aromatics have been removed and
the drops coming off the column are colorless. Rinse the tip of the column
with the last few drops of eluant and place the sample vial in the fume hood
to evaporate overnight.
15. Place the pre-weighed vial labeled Resin under the tip of the column.
16. Add 6 mL of methanol to the 10 mL graduated cylinder. Slowly add the
eluant to the top of the column. Continue adding until the Resin fraction has
come off. Rinse the tip of the column with a few drops of methanol and place
the sample vial in the fume hood to evaporate overnight.
17. Place the columns in the fume hood to dry overnight.
18. Cover the top of the unused maltene sample vial from Part I with aluminum
foil, screw on the cap, and return it to your teacher.
Day 2:
1. Weigh each of the SAR fraction sample vials and record the masses.
2. Record qualitative observations of the four SARA samples.
3. Dispose of the contents of the column by pouring the alumina into the waste,
and putting the column in the glass waste/recycling.
4. Dispose of the vials contents of the vials by dissolving in a small volume of
organic solvent (e.g. methanol) and pouring into a waste container. The glass
vials can then dry in the fume hood and be reused.
Complete the Evidence, Analysis and Evaluation.
Extension
The following chromatogram is a sample of a saturated hydrocarbon from
medium oil analyzed using a chromatography technique known as: gas
chromatography – mass spectroscopy (GC-MS).
In gas chromatography, a volatile liquid sample is injected and vaporized. The
gas travels with the mobile phase (usually an inert carrier gas such as helium or
hydrogen) through the column containing the stationary phase. As eluates emerge,
they are detected, and the time they remain on the column is measured.
Adjusted retention time = t = tr – tm
Where, Tm is the time needed for the mobile phase to travel the length of
the column, and the retention time, tr, of each component is the time needed after
injection of the mixture onto the column until that component reaches the
detector. A chromatogram is a graph showing detector response as a function of
elution time.
As the compounds are detected, they can then be sent directly to a mass
spectrometer for immediate analysis. This technique is commonly used to help
identify the molecular structure of compounds in samples from oil sands. And again,
this research will aid in understanding how to best extract and refine the oil sands
in situ.
Analyze the chromatogram and compare GC-MS to column chromatography.
Credits
Thank you to F.Q. Song and T. Oldenburg for all of their time, help and advice in
the development of this experiment. The Saturated hydrocarbon chromatogram
was prepared by F.Q. Song.
Thank you to the Petroleum Reservoir Group (PRG), the Alberta Ingenuity Centre
for In Situ Energy, and the University of Calgary for the donation of samples and
use of their lab facilities.
The light oil is courtesy of PRG, from the North Sea Project.
The SAGD medium oil sample and the bitumen are also courtesy of PRG, and are
from the Athabasca Oil Sands project.