Lecture 1: Fundamentals of gas chromotography Flashcards
what is extraction?
- Isolation of analyte from a matrix
* Sample clean-up
what type of extraction is used for a gas and a liquid?
headspace analysis
when is liquid extraction used?
when your extracting from two liquids
what type of extraction is used when you have a solid and a liquid ?
chromatography
whats chromotography ?
A technique which allows the separation of a mixture of analyte molecules through their interaction with a stationary phase and a mobile phase.
whats a stationary phase?
The chromatographic phase (solid or liquid) that is held and through which the mobile phase moves.
whats a mobile phase?
The chromatographic phase that moves through the stationary phase, either a liquid or gas.
whats Eluent mean?
solvent entering a column or moving up a plate, becomes an Eluate when it leaves.
define elution?
process of passing a liquid/gas through a column
whats the Analyte/Solute?
the molecule undergoing separation.
what are the type of columns?
packed or open tubular
How are compounds separated in chromatography?
by means of their different affinity for a mobile
and stationary phase.
Analyte molecules _____ move between the ___ and
the ___ and interact with them.
continually
SP
MP
what is partition ( in C)?
the rate of movement of the analyte through the column is dependant upon the relative solubility of the analyte in the stationary phase and the mobile phase.
what is Absorption?
the relative polarities of the analyte and the stationary phase determines the rate at which the analyte moves.
why is gas chromatography sometimes sued?
to test the purity of a particular substance, or to separate the components of a mixture to determine the relative amounts of each.
GC is used on ____ analytes.
volatile
whats the mobile phase in GC?
moving gas
whats the stationary phase in GC?
a piece of glass or metal tubing called a column
what kin of samples can GC be used on?
sufficient volatility and thermal stability. should be volatile at around 400°C or below and do not decompose at these temperatures.
what happens first to the sample in GC?
instrument vaporizes a sample of the compound and transports it via a carrier gas into a column.
The components of the sample travel through the column at ___ _____ depending on their ____ ______.
varying rates
physical properties.
what happens second to the sample in GC?
eluted components enter a heated detector that generates an electronic signal based on its interaction with the component. A data system records the size of the signal and plots it against elapsed time to produce a chromatogram.
Compounds are separated by their different ____ to the column during the stationary phase.
affinities
Compounds with less affinity will elute from the column _____. compounds with greater affinity will elute ____.
sooner
later
what does GC seperate?
GC is used to separate polar and nonpolar compounds that are volatile.
where is GC applies?
Food and flavor analysis Environmental analysis Industrial chemical analysis Petroleum industry analysis Forensic samples
what separation technique is better for non volatile compounds ?
liquid chromatography
Name some non volatile compounds?
proteins , salts and polymers
what does A gas chromatograph consists of (explain)?
- A regulated and purified carrier gas source, which moves the sample through the instrument
- An inlet, which also acts as a vaporizer for liquid samples
• A column, in which the time separation occurs
• A detector, which responds to the components as they elute from the column by changing its electrical output
• Output: Data interpretation of some sort
what does A gas chromatograph consists of
gas source, inlet (sampler), column, detector and output
a carrier gas must be ?
pure
whats an example of carrier gases?
helium, nitrogen, hydrogen, or a mixture of argon and methane
what would happen if the carrier gas isn’t pure?
Contaminants may react with the sample and the column, create spurious peaks, load the detector and raise the baseline, and so on.
what does a carrier gas do?
transport the sample through the system.
what do detector gases do?
support certain detectors
what supplies the gas?
Compressed gas cylinders or gas generators
whats the most common inlets?
injection ports and sampling valves.
what does an inlet do?
The inlet introduces the vaporized sample into the carrier gas stream.
whats used to insert the sample through a septum into the carrier gas stream?
Liquid or gas syringes
what allows for the amount of material injected to be regulated?
split outlet
why are small sample sizes used in GC?
because Capillary columns have low sample capacities.
what does a split mod injector allow you to do?
inject a larger sample, vaporize it, and then transfer only a part of it to the column.
How does split mode work?
split valve remains open. The sample is injected into the liner, where it vaporizes. The vaporized sample divides between the column and the split vent.
splitless mode is suited for what type of samples?
low concentration samples.
what does a split less mode do to the sample?
traps the sample at the head of the column while venting residual solvent vapor.
steps to splices mode
Step 1: Split valve closed, sample injected. The solvent (the major component) creates a saturated zone at the head of the column, which traps the sample components.
Step 2: Once the sample is trapped on column, open the split valve. The residual vapor in the inlet, now mostly solvent, is swept out the vent.
what are the temps for splitless injection and split injection?
for splitless injection the temp is 220c , for split injection the temp is 350c
which type of split/splitless injection involves a split vent?
split injection
separations are ____ ___ dependent.
highly temperature
what does it mean when the oven temp is ramped?
temperature is raised during run to elute all compounds
when the oven temp is Isothermal it means?
temperature stays the same for run ( consistent)
what happens during Temperature programming?
the temperature of column is raised during separation.
whats a Typical temperature program?
– Initial oven temperature of 40-60°C,
– Hold 0 - 2 min,
– Raise temperature with 8-12°C/min to 220-280°C.
As flow decreases it raises…
inlet pressure
what is a capillary GC column composed of?
narrow tubing with a thin polymer coating
Selecting the right capillary column is critical and depends on what factors ?
such as selectivity, polarity, and phenyl content.
what does Column diameter influence?
efficiency, solute retention, and carrier gas flow rate.
what does Column length affect?
solute retention, and costs.
In gas solid chromatography what is the mobile phase and what is the stationary phase?
- Mobile phase: gas (H2, He, N2)
* Stationary phase: solid (Charcoal, Mol. Sieves)
In gas liquid chromatography what is the mobile phase and what is the stationary phase?
- Mobile phase: gas (H2, He, N2)
* Stationary phase: liquid (very thin film
after the column, the gas stream passes through what?
the detector
what does the output form the detector become?
r becomes the chromatogram.
what do detectors do?
- Produce a stable electronic signal (the baseline) when pure carrier gas (no components) is in the detector
- Produceadifferentsignalwhena component is passing through the detector.
In GC how many common detectors are there?
7
what are the GC detectors called?
Thermal conductivity detector
Flame ionization detector Electron capture detector Nitrogen-phosphorus detector
Flame photometric detector Atomic emission detector Mass selective detector
what does a Thermal conductivity detector do?
Detects compounds with thermal conductivity that differs from carrier gas
what does a flame ionisation detector detect?
Detects compounds that burn or ionize in a flame
what does a Electron capture detector do?
Detects electron-capturing compounds (for example, halogenated compounds)
what does a Nitrogen-phosphorus detector do?
Detects compounds that contain nitrogen and phosphorus
what does a Flame photometric detector do?
Detects compounds that contain sulfur and phosphorus
what does a Atomic emission detector do?
Tunable to many elements
what does a mass selective detector do?
Identifies components from mass spectra (when combined with GC, the most powerful identification tool available)
what does a Flame Ionisation Detector require?
requires Hydrogen and Air
In FID ion production is ________ to
number of carbon atoms
proportional
is FID destructive?
yes
FID is a ______ detector.
universal
FID can detect to as low as _____.
20pg
Electron capture Detector is sensitive to?
compounds with a high electron affinity
what does Electron capture Detector contain?
Contain radio-active 63Ni.
ECD detects the..
constant current being measured, changes result in peaks.
what is a chromatogram?
a graph
what does a chromatogram plot?
abundance against time.
what does the peak size on a chromatogram represent?
amount of compound in the sample
what causes a large repack in a chromatogram?
As the compound`s concentration increases, a larger peak is obtained.
what is retention time?
the time it takes of a compound to travel through the column.
whats the retention time rule?
If the column and all operating conditions are kept constant, a given compound will always have the same retention time.
what are the strengths of gas chromatography?
- Easy to use • Robust
- Many detectors
- Low cost
what are the limitations of GC?
- Lack of confirming data other than retention time, except for mass spectrometer detection
- Compounds must be thermally stable
