Chromatography
in the gas phase is one of the cornerstones of instrumental analysis. After
rapid development in the Seventies and Eighties of the last century, this technique
has now, with the help of digital electronic and data processing, established
itself on a highly advanced level. Suitable for all mixtures which can be vaporised without resulting in chemical changes, it even
allows the separation of complex mixtures of substances in very small portions.
In many cases, the retention times can be used to identify individual
components; the peak areas allow quantitative determination after calibration.
The operating principle of a gas chromatograph (Wikipedia)
Principle of operation
An inert
carrier gas, usually hydrogen or helium, flows in succession through a pressure
control unit , injector, separation column and detector . The mixture to be separated is
introduced into the heated sample injector with a microlitre
syringe and vaporised there. The gaseous components
of the mixture are transported by the carrier gas into the column and
subsequently into the detector. The column separates the gaseous components
spatially by retaining them for differing periods. As a result, these
components emerge from the column at different points in time. When a component
of the mixture emerges from the column, the detector issues an electrical
signal which is recorded as a function of time. Earlier, this was done using a
laboratory recorder; nowadays, the signal is converted into digital form and
stored on standard media (e.g. hard disks or flash memory).
The chromatographic separation
principle
Distribution
Distribution between two phases
The process of
distribution is best elucidated by means of a small experiment. A few crystals
of iodine are added to a potassium iodide solution having a concentration c =
1mol/l. When shaken, the iodine crystals dissolve, turning the solution brown.
If the solution is overcoated with a layer of hexane,
the iodine particles diffuse through the phase limit into the hexane, turning
it violet. After that, the experiment is performed in reverse order. A small
quantity of iodine is dissolved in hexane and undercoated with a potassium
iodide solution. This time, the iodine diffuses from the alkyl phase into the
aqueous phase. Both these initial situations classically result in a dynamic
equilibrium. If one waits long enough in both cases, the number of particles
crossing the phase boundary from "top" to the "bottom" and
vice versa turns out to be identical per unit of time, i.e. the concentration
of particles remains unchanged over time in both phases.
According to
NERNST, the following equation applies to small values of c in this case:
c(iodine
in phase 1) / c(iodine in phase 2) = const
Consequently,
the ratio of concentrations is a constant. This value is also termed
distribution coefficient.
Distribution in the case of mobile phases
In an
imaginary experiment, several test tubes of the type shown above are now
arranged consecutively, but it is additionally assumed that one of the phases
is laterally movable (mobile phase) while the other one remains fixed
(stationary phase). In addition, the mobile phase is assumed to be slow in
comparison with the diffusion rate.
This imaginary
experiment provides several results:
·
The particles in such a system drift with the mobile
phase, but more slowly, because they are only moved by the phase when they are
actually in it.
·
The range over which the particles to be separated
drift through the column increases during the course of the experiment.
·
The speed of the particles relative to the mobile
phase is directly dependent on the distribution coefficient, because
statistically, a particle present more often in the stationary phase remains at
rest longer. For this reason, its average speed is less than that of a particle
which is present more often in the mobile phase. The relative speed of the
particles is thus dependent on the substances involved with the result that it
can be used for analytical separation.
Distribution in mobile phases
The
distribution coefficient is strongly dependent on the temperature. This can be
easily understood in mechanochemical terms, because
as the temperature increases, so does the drift rate of the particles and,
consequently, their tendency to remain in the gaseous state (where the entropy
is higher). This property is frequently made use of in chromatography.
TemperatureProgram
If the components
in a mixture to be separated have widely differing boiling points, it might not
be possible to separate all the individual substances given a constant column
temperature. Either the substances with higher boiling points take a very long
time to emerge from the column (elution) when the column has a low temperature,
or the substances with lower boiling points are not separated when the column
has a high temperature. This can be remedied by programming the temperature of
the column furnace.
In practice, measurements
are usually commenced at a column temperature matched with the most volatile
component. After a phase of constant temperature (t0-t1), the column
temperature is made to increase linearly over time (t1-t2) to allow elution of
the less volatile components from the column. After this linear heating phase,
the temperature is usually adjusted back to a constant level (t2-end). This
type of temperature programming for the column furnace is a widely used
technique nowadays.
The figure
below shows a standard temperature program. The heating rate T/t
is stated in degrees/minute; values here typically lie between 1 and 10
K/minute
Temperature program