6.3.1 Chromatography and Qualitative Analysis Flashcards
what is chromatography used for
used to separate individual components from a mixture of substances
- e.g. for analysis of drugs, plastics, air samples, flavourings and forensics
what are the 2 phases of chromatography
- stationary
- mobile
what is the stationary phase of chromatography
- does not move
- usually a solid
- or liquid supported on a solid
what is the mobile phase in chromatography
- does move
- usually a liquid or gas
what does TLC stand for
thin layer chromatography
why is TLC a good technique of chromatography
quick and inexpensive, and is able to indicate how many components are in a mixture
what is a TLC plate
a plastic sheet or glass coated with a thin layer of a solid, adsorbent substance, e.g. silica gel
what is the stationary phase in TLC
the adsorbent, e.g. silica gel
what is adsorption
the process by which the solid silica holds the different substances in the mixture to its surface
why do components separate in TLC
- the different components in the mixture have different affinities for the adsorbent, and bind with differing strengths to its surface
- so the components separate due to the relative adsorptions of the substances with the stationary phase
PRACTICAL: how can you carry out TLC
1) add a base line on the TLC plate using a pencil, about 1cm from one end
2) use a capillary tube to add a small spot of a sample to the base line
3) add solvent to a beaker, about 0.5cm depth and add a lid on top, e.g. using a watch glass
4) place your TLC plate in the solvent, making sure the solvent doesn’t go above the base line, and add the lid
5) allow the solvent to rise until it is near the top, and then remove the plate and mark the solvent font straight away using a pencil, then allow to dry
6) if any spots are visible, mark them with a pencil. could also use UV lamp to see those invisible to the naked eye, or spray on dyes/chemicals to show the spots too
what is Rf value
retention factor
how can Rf values tell what components are present in a TLC plate
can be compared to known Rf values, which have already been recorded
- MAKE SURE values compared to also used the SAME solvent system and SAME adsorbent
how do you calculate Rf
distance moved by component/distance moves by solvent font
how can you run TLC to identify pure samples
- run TLC of sample alongside the pure sample
- see if the same component is present
what is gas chromatography used for
separating and identifying volatile organic compounds in a mixture
what is the stationary phase in gas chromatography
a high boiling liquid adsorbed onto an inert, solid support
what is the mobile phase of gas chromatography
the inert carrier gas, e.g. helium or neon
what is the apparatus called in gas chromatography
gas chromatogram
what is contained in the capillary column of the gas chromatogram
the liquid stationary phase adsorbed onto the solid support
what are the stages of gas chromatography, and why di the components separate
1) a small amount of the volatile mixture is injected into the apparatus
2) the mobile carrier gas carries the components through the capillary column
3) the components slow down as the interact with the liquid stationary phase inside the column
- the more soluble the component is in the liquid
- the slower it moves
- means that the components are separated, relative to their solubility
4) each component reaches the detector at different times depending on their interactions with the stationary phase
- the compound retained in the column for the shortest time
- has the lowest retention time
- so is detected first
what is retention time
the time taken for each component to travel through the column
how is each peak detected on a chromatogram
as a peak
what do the peaks on a gas chromatogram tell us
- the retention time = IDENTIFY THE COMPONENT
- the peak integrations(area) = CONCENTRATION OF COMPONENTS
what does the retention times of chromatogram peaks tell us, and how
- the peak can be compared to known values of retention times
- and can be compared, to identify the component
what does the peak integration of a gas chromatogram tell us
- can identify the concentration of a component in the sample
- can compare its peak integration value
- with those obtained from standard solutions
how can you carry out external calibration to identify the concentration of a component in a gas chromatogram
- prepare standard solutions of known concentrations of a compound
- obtain the gas chromatograms for each
- plot a calibration curve of peak area against concentration = EXTERNAL CALLIBRATION
- this means you can convert peak area into concentration
- obtain gas chromatogram of the compound being investigated under the same conditions
- use calibration curve to measure the concentration of the compound
