GC-MS Flashcards
What is the difference between HPLC and GC-MS?
HPLC
- relies on retention time to identify compounds
GC-MS
- uses two separate analytical procedures to
1 - indicate the presence of a compound
2 - indicate the identity of a compound
What is the purpose of gas chromatography and mass spectrometry in GC-MS?
is useful for the identification of unknown compounds where retention time is unknown
GC
- separates the components of a mixture
MS
- produces a mass spectrum for identification
What are the problems with HPLC?
it does not guarantee the presence of a compound
- position of the peak depends on the compounds partition coefficient
but it is possible that another compound could share similar properties and therefore elute at the same time
What are the common solvents used in partition coefficient studies?
chloroform
octanol
cyclohexane
What are the different components in GC-MS?
injector column heated oven interface ionisation quadrople detector
How does injection of the sample occur? What are the factors required?
analytes must be volatile
- are often injected as a solution in a volatile solvent
= must be volatile as the sample must be in gas phase to enter the column
the injector port/inlet is a self-sealing/heated body that allows for the vaporisation of the sample then transfer to the capillary column
What are the types of injectors? What is their purpose?
they control the amount of analyse entering the column
split injector mode
- allows for the expansion of the volatilised sample
- prevents overloading of the column
splitless injector mode
- used for trace analysis and SPME fibres where the analyses are pre-concentrated onto the fibres
- overloading is unusual
What is the split ratio?
ratio of how much analyse enters the column and how much is not
small peaks/bad sensitivity can be changed by changing the split ratio to allow more analyte into the column
What is the column made up of?
column contains a liquid stationary phase which is adsorbed onto the surface of a thin fused silica capillary tube
How does the analyte move through the column?
samples transported through the column by the flow of an inert gaseous mobile phase
- carrier gas = nitrogen, helium, hydrogen (has some explosion risk)
as the analyte moves down the column, they interact with the walls
How can the heated oven work?
oven temperatures can be linear or ramped
linear
- temperature is held isothermally = at a single temperature
ramped
- temperature is increased
- useful for compounds with different volatilities present = allows separation at different rates
What is the purpose of a interface?
links the GC and MS together and allows for their functioning
- GC cannot work under a vacuum as it gives out gas at a high atmospheric pressure whereas MS must work under a vacuum
an ideal interface
- quantitatively transfers all the analyte
- reduces pressure flow from the GC to a pressure level the MS can handle
What is the most common interface?
jet (molecular) separator interface
is based on relative rates of diffusion
- smaller molecules diffuse more rapidly and miss the jet entry
- larger molecules diffuse less rapidly and are more likely to enter the jet
How does the sample undergo ionisation and at what stage?
ionisation occurs after passing the interface before entering the mass spectrometer
electron impact ionisation
- sample is bombarded with a beam of electrons
- when electrons pass through or near a particle/molecule they cause ionisation by knocking off electrons
- forms positive ions
ions are accelerated towards the mass analyser
How does fragmentation occur?
electron impact ionisation can also cause fragmentation as well as ionisation
fragments help in the identification of compounds
- without fragments you would only get one peak = molecular ion
