Chromatography Flashcards
What is a phase?
A state of a substance, such as a solid liquid or gas
In chromatography, what is the mobile phase and stationary phase?
Mobile phase- the phase that moves
Stationary phase- the phase that does not move
How does the stationary phase affect the separation of components in chromatography?
The stationary phase can be solid or liquid
If the stationary phase is solid, separation depends on the balance between adsorption and retention
If the stationary phase is liquid, separation depends on the balance between solubility and retention
What can affect the solubility or retention of a molecule?
Polarity
What are the three types of chromatography?
Thin layer chromatography
Gas chromatography
Column chromatography
What are the uses opf thin layer chromatography?
To check the purity of and to identify compounds
What are the components of thin layer chromatography?
TLC plate (what the components move up)
TCL lid
Liquid solvent
Baseline drawn in pencil
Solvent front
What are the stationary and mobile phases in thin layer chromatography?
Stationary phase- solids ( Al2O3, Si)2) on a glass support
Mobile phases- liquid solvents that move up the TLC plate
Describe how to separate components with thin layer chromatography
Dissolve a small sample of the mixture in the solvent.
Draw the baseline in pencil, at the bottom of the TCL plate.
Add a small spot of the sample on the pencil line and allow it to dry
Place the TCL plate in a beaker containing the solvent- the solvent must be below the pencil line.
Seal the beaker with a lid, allow the solvent to rise up the TCL plate
Once the solvent is almost at the top, remove the plate and mark the solvent front immediately
Use locating agents to make the compounds visible
What are examples of locating agents in thin layer chromatography?
Iodine I2
Ninhydrin AND UV light
What is the equation for Rf values?
Distance moved by the spot/ distance moved by solvent
It is always between 0 and 1
What are limitations of thin layer chromatography?
Similar compounds have similar Rf values
If a compound is unknown, there is no data to find it
Solvents are difficult to set right
What are the uses of column chromatography?
To separate and collect large volumes or components
What are the stationary and mobile phases in column chromatography?
Stationary phase- solid Al2O3, SiO2 (powder)
Mobile phase- a liquid solvents called the eluent
What is an eluent?
A liquid mobile phase in column chromatography
Describe how to set up and carry out column chromatography
Fill a glass tube with the stationary phase, it is held in place by a filter/ glass wool plug. The tap at the bottom is closed
Cover the powder in the solvent (the mobile phase)
Dissolve the sample in the minimum amount of solvent, and place it on top of the stationary phase.
Open the tap so the mixture can run through the column- continue to add solvent at the top
The time taken for each component to reach the end of the column is recorded. This is known as the retention time.
Collect the different eluent fractions in different test tubes so they can be analysed and identified
What is the retention time in column chromatography?
The time taken for the components to each reach the end of the column
What is gas chromatography and what is it used for?
It is a very sensitive quantitative method
Used to separate volatile liquids
What are the stationary and mobile phases in gas chromatography?
Stationary- thin layer of liquid or solid coating the inside of the column
Mobile- an inert gas (He, N2) that moves through the column
What are the components of gas chromatography?
A carrier gas (the mobile phase)
An injector that injects the sample into the chromatograph
A long and coiled column that goes into an oven
The detector
Describe how to carry put gas chromatography?
The mixture is injected into the gas chromatograph, it is vaporised
The carrier gas (mobile phase) flushes the mixture through the long, coiled column
The components slow down as they interact with the stationary phase
Each leaves the column at a different time, and is detected by the detector
The retention time is recorded
What is the retention time in gas chromatography?
The time taken for a component to pass from the column inlet to the detector
What does a longer retention time indicate?
The component has a high retention by the stationary phase, and is less soluble in its moving phase
What does a shorter retention time indicate?
The component is retained less by the stationary phase, and is more soluble in the mobile phase
What are limitations of gas chromatography?
Many components have the same retention time
Larger peaks can hide smaller peaks
Unknowns have no reference values
On a gas chromatagram, what does the number of peaks indicate and why?
The minimum amount of components in the mixture
Minimum because larger peaks can hide smaller peaks with the same retention time
On a gas chromatagram, what does the area under the peaks indicate?
Area under the peaks is proportional to the amount of component in the mixture (the larger the peak the more there is of it0
What is the x axis on a gas chromatagram?
Retention time
After gas chromatography is used to separate components, what is used to identify them? What is this called
Mass spectrometry
GCMS- gas chromatography mass spectrometry
Describe the process of GCMS
Components are separated in gas chromatography, their abundance can be calculated using a gas’s chromatagram
The components are directed to the mass spectrometer, where a mass spectra is plotted (mass to charge ratio against relative abundance)
Mr of each components can be determined
What is GCMS used for?
Airport security
Environmental analysis
Forensic and drug analysis
Space probes
What is mass spectrometry?
A modern analytical technique used to determine the relative molecular mass of a compound
Describe the stages of electrospray mass spectrometry
Sample dissolved in a volatile solvent
Injected through a fine hypodermic needle to give a fine mist
Needle is Attached to the positive end of a high voltage supply
Molecules gain a proton to become positively charged
A negative electric field is applied to the sample, the molecules are attracted and accelerate
Molecules reach the detector at different time depending on their charge and mass, they gain electrons and create a current that is read on an ammeter. The size of the current is proportional to abundance
Describe the stages of electron impact mass spectrometry
Sample of atoms is injected into vacuum and evaporated
An electron gun fires high energy electrons at the samples, an electron is knocked off each atom to create positively charged ions
A negative electric field is applied to the sample, attracts the ions and causes them to accelerate
Travel through the spectrometer and reach the detector at different times depending on their charge and mass
Gain electrons at the detector to create a current that is read on an ammeter
Size of the current is proportional to abundance
In thin layer chromatography, what is the TLC lid used for?
To prevent the solvent from evaporating into the atmosphere
In TLC, why must you draw the solvent front immediately after talking it out of the solvent/beaker?
So it is drawn before the solvent evaporates and the line can no longer be seen
