Fractional Distillation and Gas Chromatography
Background
Distillation
The previous lab used distillation to separate a mixture of hexane and toluene
based on a difference in boiling points. Hexane boils at 69 °C and toluene boils at 110
°C. In an idealized situation, you could imagine heating a mixture of hexane and toluene
to 69 °C and all of the hexane would start boiling, leaving the toluene behind. You could
then collect the vapor, condense it, and obtain pure hexane. Unfortunately the real
situation is not so simple. Just as with melting points, when you start mixing solvents
together, the mixed solution has a boiling point that is different from the pure
components. Figure 4.1 shows an approximate boiling curve, showing that a mixture of
hexane and toluene boils somewhere between pure hexane and pure toluene.
Figure 4.1 – Boiling points for mixtures of hexane and toluene
110 °C
temperature
69 °C
pure
hexane
50:50
mixture
mixture composition
fysiske egenskaber af zink sulfat
pure
toluene
When a mixture of hexane and toluene begins to boil, hexane is not the only
component to go into the gas phase. Some toluene molecules will also go into the gas
phase. However, since hexane is more volatile, more molecules of hexane will go into
the gas phase. So if a 50:50 mixture of hexane and toluene boils, the gas coming off will
not be pure hexane, but it will also not be a 50:50 mixture. In figure 4.2, the red curve
shows the boiling points for different mixtures of hexane and toluene. The blue curve
shows the composition of vapor at any given temperature. That is, if you follow the
vertical dotted line up for the 50:50 solvent mixture, you will find the boiling point for
that mixture on the red curve. Then follow the horizontal dotted line to the left, the point
that it intersects the blue curve gives the composition of the gas phase mixture at that
temperature. That is, the composition of the gas vapor boiling off will not be pure
hexane, but it will be “enriched” in hexane—having more hexane than the initial liquid
mixture.
Figure 4.2 – Composition of the gas phase mixture at various temperatures
110 °C
temperature
69 °C
pure
hexane
50:50
mixture
mixture composition
pure
toluene
In a simple distillation, you condense the vapor that boils off and collect it as a
liquid. However, given the discussion above, this will not be pure hexane, but will
instead be a mixture of hexane and toluene with somewhere between 50% and 100%
hexane (Figure 4.3).
Figure 4.3 – Simple distillation
110 °C
condense
temperature
boil
69 °C
collect
pure
hexane
initial mixture
50:50
mixture
pure
toluene
mixture composition
If you collected this enriched mixture of hexane and toluene and re-boiled it, you
would again get a gas phase mixture that had more hexane than you started with.
Collection of the condensate would provide a liquid that was even more enriched in
hexane (Figure 4.4).
Figure 4.4 – A second simple distillation
110 °C
temperature
69 °C
collect
pure
hexane
start
50:50
mixture
pure
toluene
mixture composition
If this process were repeated several times, every time you would collect a liquid
that was closer and closer to pure hexane. Eventually you would have one pure solvent.
However, this would be an extremely tedious process. The solution to this problem is
instead to perform a fractional distillation. In a fractional distillation setup (Figure 4.5),
the vapor that comes from the boiling mixture is not immediately collected, as it was in a
simple distillation. Instead there is a long fractionating column between the boiling
liquid and the condenser.
Figure 4.5 – Fractional distillation setup
As the vapor from the boiling solution rises up the fractionating column, it will
begin to cool and collect as a very thin film of liquid on the inside of the fractionating
column. This thin film will be enriched in hexane, as this is essentially the first simple
distillation described in Figure 4.3. There is enough heat energy present, though, to boil
this thin film of liquid, and it will re-enter the gas phase and continue to rise up the
fractionating column. Again it will cool and condense on the sides, forming a liquid that
is even more enriched in hexane. This is essentially the second simple distillation
described in Figure 4.4. By the time the vapor reaches the top of the column it will have
condensed and boiled many times. As shown in Figure 4.6, as you go back and forth
from the liquid to the gas phase, you become more and more enriched in hexane, and the
result of this fractional distillation will be a pure sample of hexane.
Figure 4.6 – Fractional distillation resulting in pure hexane
110 °C
temperature
69 °C
pure
hexane
50:50
mixture
pure
toluene
mixture composition
The number of times a solvent boils and condenses by the time it reaches the top
is called the number of “theoretical plates” of the column. The number of theoretical
plates can be increased if the column is longer, or if it has a greater surface area for the
vapor to condense onto. A greater surface area can be achieved with different columns
with bumpy inner surfaces, or by packing the column with something like glass beads.
Greater numbers of theoretical plates lead to more efficient distillations resulting in
extremely pure solvents.
Gas Chromatography
Like thin layer chromatography (TLC), gas chromatography (GC) involves
organic compounds carried through a stationary phase, pushed along by a mobile phase.
In TLC the stationary phase was silica gel powder coated onto a solid surface and the
mobile phase was an organic solvent that soaked up the TLC plate through the powder.
In GC the stationary phase is a viscous liquid that coats the interior surface of a very long
thin tube (the “column”). The mobile phase is an inert gas such as helium that is
pressurized and pushed through the column. A gas chromatograph is shown in Figure 4.7
(reference: http://elchem.kaist.ac.kr/vt/chem-ed/sep/gc/gc.htm).
Figure 4.7 – Typical GC
In GC, the sample is in the gas phase, which means that it must be kept hot
enough so it does not condense into a liquid. To accomplish this, the column is kept in an
oven that maintains a set temperature. Figure 4.8 shows a schematic for a typical GC.
On the left hand side is a cylinder filled with the carrier gas. This is under pressure so it
runs through the coiled column and out through the detector. When a sample is injected,
it is carried along through the column (stationary phase) by the carrier gas (mobile
phase). Due to interactions between the organic compounds in the sample and the
stationary phase, different compounds move through the column at different rates. As
they reach the detector they are measured and a signal is recorded as a series of peaks,
called a chromatogram.
Figure 4.8 – GC schematic
inject sample here
recorder
flow
x
detector
He
gas
oven
Compounds are characterized by their retention time—the length of time between
the injection of the sample and the detection of the individual component. This is similar
to the Rf values in TLC. Compounds with lower boiling points move through the column
more quickly, and so have shorter retention times. Compounds with higher boiling points
have longer retention times. In a chromatogram, the different signals correspond to
different organic compounds.
Figure 4.9 shows a gas chromatogram of a car exhaust (reference:
http://www.mindfully.org/Air/2002/VOCs-New-Auto-Smell23dec99.htm). Each of the
different peaks represents a separate organic component. This is an example of how GC
can be used to monitor environmental pollutants. For example, peak 7 is toluene.
Figure 4.9 – Organic components of automobile exhaust
Figure
4.10
shows
a
medical
application
of
GC
(reference:
http://www.clinchem.org/cgi/content/full/43/6/1003). Part a) shows a portion of the
chromatogram of a urine sample. Part b) shows the same sample with ethanol (the
intoxicant in alcoholic drinks) added. The peak marked with an arrow represents the
ethanol. Note that in part a) there is no peak at the retention time marked with an arrow,
meaning that there was no ethanol in the urine. Part c) shows a urine sample taken a few
hours after the patient drank a glass of wine. The signal marked with an arrow shows that
ethanol can be found in the urine. You could easily imagine how such tests might be
used for legal or medical reasons.
Figure 4.10 – Ethanol in urine samples
In addition to acting as a qualitative tool, telling what different components are
present, GC can also act as a quantitative tool, telling how much of each component is
present. The relative sizes of the peaks, measured as the area under each peak,
corresponds to their relative concentrations in the sample.
To measure the relative sizes of the peaks, we treat the signals as if they were
triangles. Figure 4.11 a) shows a typical signal in black and a triangle in red. If we can
measure the area of the red triangle, the area of the peak is about the same. The area of a
triangle is calculated as one half of the height times the width at the base. Figure 4.11 a),
though, shows that at the bottom of the peak, the chromatogram signal deviates from the
red triangle quite a bit, so measuring the width of the base is difficult. In Figure 4.11 b),
though, it is shown that the area of a triangle can also be defined as the height times the
width of the triangle half-way up. In 4.11 a), about half-way up the red triangle and the
black peak signal match very closely, and this used to measure the area of the peak.
Figure 4.11 – Areas of peaks and areas of triangles
a)
b)
height
height
width at half
the height
base
To measure the area of the peak, follow the procedure shown in Figure 4.12. For
a given signal, use a ruler to draw a straight line connecting the baseline on the left side
to the baseline on the right side (shown in green). Then draw a vertical line from this
baseline to the top of the peak (shown in blue). Using your ruler, find the midpoint of
this vertical line and draw a horizontal line from one side of the peak to the other (shown
in red). Measure the lengths of the blue and red lines and multiply these. This is the
height times the width at half the height, or the area of the peak. Do this for each of the
signals. Add these all up to get the total area, and divide each peak by the whole and
multiply by 100 to get the percent areas.
Figure 4.12 – Approximating peak area
For example, in Figure 4.12, I get the following measurements (your
measurements will vary depending on how large you print this file, but the percent areas
should end up roughly the same). For the left-hand peak, the height is 67 mm and the
width at the halfway point is 49 mm. This gives an area of 3283 square mm. For the
right-hand peak, the height is 57 mm and the width at half the height is 16 mm, for a peak
area of 912 square mm. The total area of the two peaks is 3283 + 912 = 4195. The
percent area of the left-hand peak = (3283 / 4195) x 100 = 78%. The percent area of the
right-hand peak = (912 / 4195) x 100 = 22%. Note that the percent areas should always
add up to 100%.
Using GC, you can identify the components in a mixture by their retention times
and calculate the relative amounts of each component using the peak areas. This
analytical technique has a wide variety of applications in the laboratory, in medicine, in
environmental science, and many other fields.