In: Chemistry
What are two factors that a GC column uses to separate compounds?
Hi,
Very good question. Having worked on GC instrument for several years, some of the factors that affect seperations in Gas chromatography include boiling point of a compound, polarity of components to be seperated,column temperature, carrier gas flow rate, column length and amount of material to be separated. These factors are briefly summarized below:
1. Boiling
point
The boiling point of a compound is often related to its polarity
(see also polarity chapter). The lower the boiling point is, the
shorter retention time usually is because the compound will spent
more time in the gas phase. That is one of the main reasons why low
boiling solvents (i.e., diethyl ether, dichloromethane) are used as
solvents to dissolve the sample. The temperature of the column does
not have to be above the boiling point because every compound has a
non-zero vapor pressure at any given temperature, even solids. That
is the reason why we can smell compounds like camphor (0.065
mmHg/25 oC), isoborneol (0.0035 mmHg/25 oC),
naphthalene (0.084 mmHg/25 oC), etc. However, their
vapor pressures are fairly low compared to liquids (i.e., water (24
mmHg/25 oC), ethyl acetate (95 mmHg/25 oC),
diethyl ether (520 mmHg/25 oC)).
2. The polarity of
components versus the polarity of stationary phase on
column
If the polarity of the stationary phase and compound are similar,
the retention time increases because the compound interacts
stronger with the stationary phase. As a result, polar compounds
have long retention times on polar stationary phases and shorter
retention times on non-polar columns using the same temperature.
Chiral stationary phases that are based on amino acid derivatives,
cyclodextrins and chiral silanes are capable of separating
enantiomers because one enantiomer interacts slightly stronger than
the other one with the stationary phase, often due to steric
effects or other very specific interactions. For instance, a
cyclodextrin column is used in the determination of the
enantiomeric excess in the chiral epoxidation experiment (Chem
30CL).
3. Column
temperature
A excessively high column temperature results in very short
retention time but also in a very poor separation because all
components mainly stay in the gas phase. However, in order for the
separation to occur the components need to be able to interact with
the stationary phase. If the compound does not interact with the
stationary phase, the retention time will decrease. At the same
time, the quality of the separation deteriorates, because the
differences in retention times are not as pronounced anymore. The
best separations are usually observed for temperature gradients,
because the differences in polarity and in boiling points are used
here (for examples see the end of the chapter)
4. Carrier gas flow
rate
A high flow rate reduces retention times, but a poor separation
would be observed as well. Like above, the components have very
little time to interact with the stationary phase and are just
being pushed through the column.
5. Column
length
A longer column generally improves the separation. The trade-off is
that the retention time increases proportionally to the column
length and a significant peak broadening will be observed as well
because of increased longitudinal diffusion inside the column. One
has to keep in mind that the gas molecules are not only traveling
in one direction but also sideways and backwards. This broadening
is inversely proportional to the flow rate. Broadening is also
observed because of the finite rate of mass transfer between the
phases and because the molecules are taking different paths through
the column.
6. Amount of material
injected
Ideally, the peaks in the chromatogram display a symmetric shape
(Gaussian curve). If too much of the sample is injected, the peaks
show a significant tailing, which causes a poorer separation. Most
detectors are relatively sensitive and do not need a lot of
material in order to produce a detectable signal. Strictly
speaking, under standard conditions only 1-2 % of the compound
injected into the injection port passes through the column because
most GC instruments are operated in split-mode to prevent
overloading of the column and the detector. The splitless mode will
only be used if the sample is extremely low in concentration in
terms of the analyte.
7. Conclusion
High temperatures and high flow rates decrease the retention time,
but also deteriorate the quality of the separation.
Hope your query is solved.