6.3 Analysis Flashcards
what are the two types of chromatography
Gas
Thin layer chromatography
How does chromatography work
Works on the basis that components have different affinities for a stationary and mobile phase
define stationary phase
fixed in place such as the paper in TLC
define mobile phase
moves in a definite direction - solvent in TLC
Describe separation by adsorption
component molecules bind to the surface of the solid stationary phase, the stronger the adsorption to the stationary phase the more the components molecules are slowed down
describe separation by absorption
components dissolve in the liquid stationary phase, the greater the solubility in the stationary phase the more the components molecules are slowed down
What are the stationary and mobile phases in TLC
Stationary - silica gel SiO2
mobile - solvent
How would you produce a chromatogram
dissolve the sample
draw a pencil line and spot the sample using a capillary tube and allow it to dry
place the plate in a tank of solvent - which must be below the line
separation is by adsorption so allow the solvent to almost reach the top and draw a line at the solvent front
How would you find rf values if the spots are colourless
ninhydrin or UV lamp
How do you calculate rf value
distance moved by solvent/ distance moved by solvent
what are the limitations of chromatography
similar compounds have very similar rf values
unknown compounds have unknown rf values
if the components have little solubility they will hardly move
how does gas chromatography work
the stationary phase is a solid or liquid coating the coiled tube; usually a hydrocarbon with a high boiling point
The mobile phase is an inert gas carrier such as helium or nitrogen
The coiled tube is often connected to a mass spectrometer for further analysis
How would you interpret the results from a gas chromatogram
the chart will produce a graph of absorption against time
The x-axis is the retention time of a chemical and the value can be compared against a data book
the area under the absorption peak is proportional to the concentration of each component
Define retention time
the time taken for a component to travel from the inlet to the detector in gas chromatography
define chemical shift
the scale that compares the frequency of NMR absorption with the frequency of the reference peak of TMS
What is TMS
an internal standard for both carbon and proton NMR - tetramethylsilane (CH3)4Si has a value of 0ppm
Define deuterium
an isotope of hydrogen and does. not produce a signal in the proton NMR spectrum
What is the purpose of NMR
the spectrum can confirm the formula mass of an organic chemical and give clues to the structure of the chemical
How does NMR work
H1 and C13 have a property known as spin. These isotopes are put in a large magnetic field and the direction of the spin aligns with the magnetic field.
Energy in the form of radio waves is used to force the spins to flip and change direction. The energy required to change the alignment depends on the environment that the atom is in and can be compared to a standard
Why is TMS a good internal standard?
contains both carbon and hydrogen so can be used for both NMRs
hydrogens are all in the same environment so only one peak is produced
inert and non-toxic
low boiling point so it can be removed easily from the sample
What would the chemical shift of a molecule similar to TMS look like
more upshifted
What solvents are used for NMR
organic solvents that contain hydrogen isotopes that are not NMR active
deuterated solvents such as CH3Cl would become the heavy CDCl3
define equivalent protons
hydrogen atoms bonded to the same atoms that therefore experience the same magnetic field in the NMR spectrometer
describe spin-spin coupling
The protons on the neighbouring carbon atoms affect the magnetic field and the alignment of protons and causes a split in the signal - different numbers of adjacent protons will cause different numbers of splits
how would you figure out the number of peaks in the splitting pattern
number of protons adjacent on carbon atoms + 1
describe what impacts rf value or retention time
in gas chromatography separation is by relative solubility whereas in tlc it is adsorption
in gas chromatography if the stationary phase is non-polar, non-polar compounds will interact the most, polar compounds will interact the least and have shorter retention times
how would you interpret a gas chromatogram
area of peak/ total area of all peaks
an area of peaks is proportional to the amount of a component in a sample
how can concentration of a component be found
by using a calibration curve
prepare standard solutions of compound under investigation
obtain gas chromatogram for each concentration
com pare area under curve for the compound under investigation with calibration curve
