Transcript Gas Chromatography & Gas

Gas Chromatography &
Gas-Liquid
Chromatography
Both gas chromatography and gas-liquid
chromatography work in a very similar way
Components
• Carrier gas: moves through the column, carrying the sample
along (mobile phase)
• Sample Injection: A small amount of sample is injected into
the column:
• Oven: Sample is heated to a sufficient temperature to
instantly vaporise the sample, which is then swept into the
column by the gas.
• Column: series of loops of inert solid, in gas liquid
chromatography the column is coated with porous solid
coated with liquid hydrocarbon with a high boiling point, the
liquid acts as the stationary phase.
• Detector: Flame ionisation
How does it work?
o Gas chromatography separates chemicals based on the ease
with which they evaporate into a gas.
o The chemical mixture being analysed is injected and carried
through the instrument by a carrier gas (commonly nitrogen).
o There is a column in the oven that makes it possible to
separate the chemicals based on how quickly they travel
through it.
Injection of the sample
o A very small quantity of the sample being analysed is injected
into the machine via the injection port using a syringe.
o The injector is contained in an oven in which the temperature
can be controlled. It is hot enough so that all the sample boils
and is carried into the column with the carrier gas.
The column in the oven
o The column is normally made of stainless steel and is between
1 and 4 meters long. It has an internal diameter of just 4mm.
o The column is coiled up so that it will fit inside the
temperature controlled oven.
o Inside the column, it is packed with a very porous rock (solid
which contains many minute channels or open spaces). Which
is coated the a high boiling liquid, normally a waxy polymer.
o The temperature of the oven gets up to 250°C, it is cooler
than the injector oven, so that some components of the
mixture may condense at the beginning of the column.
The separation.
• A few things may happen to a particular molecule once
injected into the column:
o A compound with a boiling point higher than the temperature
of the column will condense at the start of the column.
o Some molecules may dissolve in the liquid stationary phase.
Some compounds will be more soluble in the liquid than
others. The more soluble ones will spend more of their time
absorbed into the stationary phase; the less soluble ones will
spend more of their time in the gas.
Retention Time
• Retention time is the time taken for a substance to emerge
from the column (when a peak is seen on the recorder)
o This time is measured from the time at which the sample is
injected to the point at which the display shows a maximum
peak height for that compound.
o Different compounds have different retention times
Factors that effect retention
time:
o Boling Point – a compound that boils at a higher temperature than
that of the column temperature is going to spend most of its time
condensed as a liquid at the beginning.
High boiling point means long retention time.
o Solubility in the liquid phase – The more soluble, the less time it will
spend being carried along by the gas.
High solubility means high retention time
o Temperature of the column – A very high temperature will excite
molecules into the gas phase, because they evaporate more readily
or because they are so energetic that the attractions of the liquid no
longer hold them. A high column temperature shortens retention
times for everything in the column.
o Low temperatures in the column means better separation, but it
could take a very long time.
o High temperatures in the column means everything will pass
through very quickly although less will be separated out.
Detector
Detector
• There are many types of detectors that can be used, although flame
ionisation detectors are often used.
o The whole detector is enclosed in its own oven which is hotter than
the column temperature. That stops anything condensing in the
detector.
o As the sample emerges from the column, the sample mixture is
burnt in a hydrogen-oxygen flame. This causes the sample to
become ionised and a small electric current is produced. This
current is amplified and results as a peak on the chart.
o During the process of burning organic compounds, small amounts of
ions and electrons are produced in the flame. The presence of these
can be detected.
o The more of the organic compound there is in the flame, the more
ions will be produced, and so the higher the current will be.
o Disadvantage: It destroys everything coming out of the column as it
detects it.
Interpreting the data.
o The output will be recorded as a series of peaks - each one
representing a compound in the mixture passing through the
detector.
o The areas under the peaks are proportional to the amount of
each compound which has passed the detector.
Calibration
o A gas chromatography test can be calibrated by running at
least 3 standards of different know concentrations.
o Record the retention time and area of each peak. Plot area vs.
concentration.
o Run an unknown if it has the same retention time as your
standards use it's area to calculate concentration.
Advantages & Disadvantages of Gas
chromatography/ Gas-liquid
chromatography
• Advantages:
o It is the most sensitive of the chromatographic techniques
o Capable of being able to detect 10^-12g of a compound
o Suitable for volatile organics such as petrol.
• Disadvantages:
o Limited to compounds that can be readily vaporised without
decomposing.
o The sample is destroyed when it goes through the detector,
and so cannot be analysed any other way.
What gas chromatography can
analyse.
o Drug analysis
o toxicology
o organic compounds
o analysis of body fluids for the presence of illegal substances
o testing of fibber and blood from a crime scene
o detect residue from explosives
o Urine samples that are routinely taken from athletes competing in
major events to ensure they are not benefitting from the use of
illegal, performance-enhancing drugs.
oQualitative analysis: Gas chromatography can be used
for qualitative analysis by using the retention time. Molecules
of a substance should always take the same time to move
through the column at the same conditions.
oQuantitative analysis: Gas chromatography can be used
for quantitative analysis because the higher the concentration,
the more the flame will flare up. The area under the peak
reflects the concentration of the substance. It can be
compared to a standard solution to determine concentration.
How does it compare to other
techniques?
• Gas chromatography is the most sensitive of all the
chromatographic techniques.
• Its limited to such compounds that have a relative molecular
masses less than 300.
• High performance liquid chromatography, can separate
compounds with a relative molecular masses of over 1000.
Links
• http://www.chem.agilent.com/cag/7890A_5975C_Videos/Agil
entPlayer.html
• http://www.youtube.com/watch?v=08YWhLTjlfo