Gas Chromatography
In gas chromatography, a liquid sample is injected into a port on the instrument that
immediately vaporizes your sample. A carrier gas (usually helium and/or nitrogen gas) flows
throughout the system in order to move your sample through the instrument. The sample quickly
reaches a long metal column that is coated either with glass or some polymeric material. This
column is housed within an oven whose temperature is usually 10-20 degrees less than the
injection port. Once your sample reaches the column, the various components of the sample will
interact with the material on the column (the stationary phase). The function of the column’s
stationary phase is to provide a surface upon which your vapor may condense. The general idea
here involves a solid-vapor or liquid-vapor phase separation of your material. That is, your
sample will undergo a series of vaporizations and condensations (just like distillation) until it
reaches the end of the column.
(a)
If your sample contains a component with a high boiling point, then it will more readily
condense into the liquid and adhere to the column and take a long time to get to the end.
We say that components that readily condense onto the column have a high affinity for
the column (stationary phase) and consequently, have a low affinity for the carrier gas
(mobile phase). As a result, these components will spend a relatively longer time on the
column and will have a long "retention time".
(b) If your sample has a component with a low boiling point, it is likely to remain in the
vapor phase a move along with the carrier gas and spend a relatively short amount of time
on the column. We say that components that readily travel through the column with the
carrier gas have a low affinity for the column (stationary phase) and consequently, have a
high affinity for the carrier gas (mobile phase). As a result, these components will spend
a relatively short time on the column and will have a short "retention time".
The bottom-line here is that high boiling components have higher affinities for the column and
longer retention times and low boiling components have lower affinities for the column and
shorter retention times. Therefore, GC allows us to separate components of a mixture based upon
their differences in boiling point.
NMR and IR Spectroscopy
The theory of these techniques will be described in class and during lab.
Vapor pressure equilibrium and boiling
Simple distillation and boiling point-composition diagrams
A representative fractional distillation set-up is shown below (Figure 2). The only
difference for the simple distillation set-up is the absence of the Vigreux column (or some
facsimile thereof).
Figure 2
If we were to distill a pure liquid, then we would expect that the liquid would boil at the
boiling point of that pure liquid and that the temperature of the vapor above the liquid would be
equal to the boiling point of the liquid. Furthermore, the temperature of the vapor should remain
constant throughout the process. However, things change considerably when distilling mixtures.
In this experiment, you will be distilling a mixture of two liquids in order to separate them based
upon a difference in their boiling points.
Let us consider a simple mixture of toluene and carbon tetrachloride. The vapor pressure
varies as a function of temperature similar to that as shown in Figure 1 (above). According to
this graph, the pure liquids will boil at 110 ˚C and 77 ˚C, respectively, at standard atmospheric
pressure (1 atm = 760 torr). However, if we have a mixture of the two liquids, the vapor pressure
above this binary mixture will have contributions from BOTH components. As a result, the
mixture will end up boiling at some intermediate temperature.
What is that intermediate temperature? In order to answer this question, chemists
routinely use a boiling point-composition diagram (Figure 3). A boiling point-composition
diagram enables us to interpret the behavior of a binary mixture as a function of composition and
temperature.
Figure 3
The boiling point of a mixture is defined as the temperature at which the TOTAL vapor
pressure of the mixture equals the atmospheric pressure on the solution. This total vapor pressure
is simply the sum of the partial vapor pressures of the individual components (Dalton's Law of
Partial Pressures):
Ptot = pa + pb + ........
How do we find the partial vapor pressure of each component in a mixture? The pressure and
composition of vapor above a mixture at a given temperature can be calculated if: (a) the
composition of the mixture and (b) the vapor pressures of the pure components are known.
According to Raoult's law:
pa = pa˚Xa
where pa is the vapor pressure of A above the mixture
pa˚ is the vapor pressure of pure A
Xa is the mole fraction of A in the mixture
Basically, Raoult's law states that the vapor pressure of any one component in a mixture is
proportional to the number of molecules of that component present in the liquid solution. If we
take Dalton's and Raoult's laws together, we can conclude that for an ideal mixture at any
temperature, the most volatile component has a greater mole fraction in the vapor than in
the solution.
If we return to the boiling point-composition diagram (Figure 3) for the mixture of
toluene and carbon tetrachloride, we can look at some practical applications of Raoult's and
Dalton's laws in terms of a distillation. If we start with a 50:50 mixture of the two liquids, the
observed boiling point should be at some intermediate value between the boiling points of the
pure substances. The boiling point is obtained by finding the appropriate binary composition on
the x-axis and drawing a vertical line upwards until it intersects the liquid curve. To find the
boiling point, simply draw a horizontal line until it intersects the y-axis. Therefore, from Figure
3 it can be shown that our 50:50 mixture will boil at 86 ˚C. This means that a thermometer
placed in the boiling LIQUID will read 86 ˚C. However, the observed temperature for the vapors
condensing in the distillation head will be different. In this case, the condensing vapors that are
enriched in the lower boiling (more volatile) component will have a boiling point of 78 ˚C
(Figure 3). The composition of this enriched vapor can be determined by drawing a vertical line
downwards toward the x-axis from this lower boiling point. From Figure 3, we can see that the
vapor generated from our original 50:50 mixture of carbon tetrachloride/toluene has been
enriched to 80:20 carbon tetrachloride/toluene. This demonstrates that the original mixture has
indeed been enriched in the lower boiling, more volatile component (carbon tetrachloride).
Consequently, the remaining liquid gradually shifts toward a purer solution of toluene.
Therefore, we expect that the observed boiling point of the remaining liquid will also rise
gradually.
Fractional distillation
The methodology introduced above is for the process of simple distillation. You would
start with a mixture that has each component present in a certain proportion (e.g. 50:50). You
would distill the liquid and collect a fraction containing the enriched mixture (80:20). However,
what would you do if you wanted to purify the 50:50 mixture to say 95:5? Well, there are
actually two ways to accomplish this. To separate the mixture, a series of fractions could be
collected and each of these can be distilled. Essentially, you would be performing a series of
simple distillations. However, there is a better way. The series of distillations can be
accomplished "automatically" in a fractional distillation. A common method of purification by
fractional distillation is the refining of crude oil. Crude oil passes through a series of "bubble
plates" that are placed one on top of another to heights of 150 ft. Their capacity can run up to
200,00 barrels of oil per day!! An example of a bubble plate column is shown in Figure 4. The
entire set-up consists of:
(a) a series of plates that support a layer of distillate
(b) capped risers that allow for vapors to ascend
(c) overflow pipes to return excess distillate to the lower plate.
Figure 4
The bubble plate process functions in the same manner as the fractionating column that
we will use in this experiment. Basically, the process involves a series of vaporizations and
condensations. Each plate in the set-up corresponds to a single simple distillation. Therefore, the
first plate on the column corresponds to the first fraction obtained in a simple distillation and is,
thus, enriched in the lower boiling component. In an idealized situation, each plate in the column
will achieve an incremental improvement in separation equivalent to one simple distillation. The
separation process as achieved by fractional distillation is shown in Figure 3. If we start with our
original 50:50 mixture of carbon tetrachloride/toluene and subject it to a separation by fractional
distillation, we should achieve an incremental improvement in separation with each plate. Let's
follow the dashed line in the diagram. If we vaporize mixture A and subsequently condense the
distillate (vapor), we end up with mixture B. Mixture B is enriched in the lower boiling
component (carbon tetrachloride) 80:20. Repetition of this process starting with B results in the
enriched mixture C (90:10). This process continues for each successive plate. Table 1 shows the
improvement in separation for three different experiments. The first (simple distillation) achieves
a modest separation. The last experiment (fractional--3 plates) shows a significant improvement
over the first.
TABLE 1
Experiment
Composition of liquid
Composition of vapor
Pre-distillation
50:50 CT/T
0:0 CT/T
#1 Simple distillation
20:80 CT/T
80:20 CT/T
#2 Fractional distillation 10:90 CT/T
90:10 CT/T
(2 plates)
#3 Fractional distillation 5:95 CT/T
95:5 CT/T
(3 plates)
Separation efficiency
Separation efficiency in fractional distillation is measured by the theoretical plate. A
theoretical plate is defined as the unit of separation corresponding to the composition ratio, , for
the liquid mixture and its corresponding vapor.
For example, for our original 50:50 mixture, a distillation utilizing a single theoretical plate will
result in an enrichment to 71:29. Plugging into the equation, we get the following result:
This means that the first fraction of distillate is 2.5 times richer in the lower boiling component.
The length of a packed fractionating column needed to obtain this degree of separation is the
HETP (Height Equivalent to a Theoretical Plate). This number is used to determine the
efficiency of the column. The general rule is the smaller the value of HETP, the more efficient
the column. The primary variable in determining HETP is the type of packing material used for a
particular liquid mixture. Representative packing materials include carborundum chips, glass
beads, copper mesh, and glass helices. As an alternative, there are special columns that are
available that achieve the series of vaporizations/condensations due to their construction. An
example is the Vigreux column that will be on display during your lab period.
We want to have some idea of the number of theoretical plates required to efficiently
separate mixtures so that we may design successful experiments. Essentially, if we know the
number of theoretical plates, then we can choose appropriate packing materials, columns, etc. that
will optimize the separation. For instance, an equation has been derived for 50:50 mixtures,
which determines the number of theoretical plates require to achieve an enrichment in the lower
boiling component to 95% for the first 40% of the mixture distilled (Form A). The usefulness is
enhanced if we are able to modify the equation so that it is expressed in terms of the difference in
boiling points for the two components (Form B).
Reflux ratio and holdup
The primary technique factors that influence separation efficiency are the reflux ratio and
holdup. Reflux is the process of vaporizing the liquid to the top of the column and recondensing
it onto the column. Takeoff is the process of vaporizing the liquid and condensing it out of the
column as distillate. At the extremes we may experience total reflux (where no distillate is
received) or total takeoff (where all of the vapor is immediately removed as distillate). Neither of
these will produce good separation efficiency. Obviously, the best-case scenario is to have some
type of compromise between these two extremes.
The other confounding factor is the total amount of vapor present in the column at an
instant in time (holdup). The general rule of thumb is if there is more than 10% of our liquid
mixture on the column at any time, then the separation efficiency is greatly reduced. Taken
altogether, it appears that a moderately slow distillation (2-3 drops of distillate per min) will
produce the best results.
A note on azeotropes