GC INSTRUMENTATION Flashcards
Separation of volatile components in a mixture by GC is dependant on ______________________-
temperature.
Facts about the column oven in GC
The column is housed within an oven that can be either be maintained at a single temperature through the separation or heated up gradually throughout the separation.
The ability to increase the temperature can allow you to reduce the time it take to run the chromatogram and reduce band broadening in the later eluting peaks.
State the two types of GC column
- Packed Column
- Capillary columns – which are much longer (up to 500M) and thinner in diameter (approx. 0.1mm). They have a thin layer of stationary phase coated onto the inside. Despite the very long length, they are very efficient due to the small sample size usually used and the very thin stationary phase layer. So you tend to get very sharp peaks.
List the types of stationary phase used in GC column
- Methyl silicate– good for non-polar separation eg n-alkanes
- Cyanopropyl phenyl methyl silicate– Good for moderately polar compounds eg Aromatic compounds
- Trifluorosilicone- good for polar separations eg alcohols
Facts about GC detectors
Detectors must respond rapidly to the composition of the gas stream leaving the column. The response must be a linear one, preferably with a steep slope (i.e. a small change in component amount give a large detector response).
The sensitivity of a detector is important since it will ————–
influence the sensitivity of your analysis
State the two ways to measure a detectors sensitivity
- Measure the slope on a detector response versus amount of sample graph.
- Look at the limit of detection, i.e. the smallest peak one can actually say with confidence is a peak and not just background noise.
For a 95% confidence of a peak being an actual component the limit of detection (L.O.D) is equal to the mean background + 2 standard deviations from the mean.
For 99% confidence is the mean background + 3 standard deviations.
List examples of detectors used in GC
- Thermal conductivity
- Flames ionization
- Electron capture
- Phosphorus/Nitrogen
- Flame photometric for P and S compounds
An Ideal GC detector will————————-
- Be sensitive
- Have a linear response to solutes that extends over orders of magnitude
- Has a temperature range to at least 400oC
- Has a quick response time
- Has high reliability
- Should respond similarly to all solutes, or have a highly predictable response
- Should not destroy the sample
N/B: Not all detectors have all of these characteristics, but deciding what is important in your analysis helps to deciding which detector to use.
A Common detector used in GC is called?
Flame ionisation detector
Explain how flame ionization detector works
The column gas is mixed with air and H2(g) then burnt using a plasma to get to very high temperatures (>3000oC). This causes some compounds to be ionised (+ve ions) which flow to the cathode, and electrons flow to the anode. This gives a voltage difference that will show up as a peak on a current versus time graph
Advantages of using a flame ionization detector
- Sensitive
- Quick response time (<1 millisecond)
- Peaks are proportional to amount of solute
List the disadvantages of flame ionization Detector
- Can only detect combustible materials (mainly organic – carbon containing)
- Destroys the sample
The signals from the detectors are recorded by—-
The signals from the detectors (electrical) are recorded by a data recorder. This can be in the form of a chart plotter, or a computer.
This results in the formation of a chromatogram (just like in HPLC)
why Derivatising molecules for GC?
Some molecules are not volatile enough to be run on a GC, or they have a strong affinity for the stationary phase and therefore cannot be eluted from the column. One way to overcome these problems is by derivatising molecules before they are run on the GC.
