Chapter 29 Chromatography And Spectroscopy Flashcards
What is TLC Chromatography?
Separating components within a mixture by their difference in attraction to the stationary or mobile phase.
What is the stationary phase of TLC?
Solid silica (SiO2) or alumina AlO3 on an inert support material like glass or plastic
What is the mobile phase of TLC?
Liquid solvent
How fast does the component of the mixture move if it has a stronger affinity to the mobile phase?
The component will move faster and further up the TLC plate
How fast does the component of the mixture move if it has a stronger affinity to the stationary phase?
The component will move slower and stop closer to the start of the TLC plate
How does separation occur during TLC?
Relative adsorption
The components in the mixture adsorb differently to the surface of the stationary phase
State an example of polar solvents that may be used in TLC?
Alcohol or water
State an example of a non polar solvents that may be used during TLC?
Alkanes, hexane
What quantitative measurement can be gained from TLC plates?
Retention factors (RF) of each component can be calculated and compared to known database values of pure samples
How is Rf calculated?
Distance travelled by component
————————————————
Solvent front distance
What are the limitations of TLC?
-Difficult to measure the exact centre of component spots
-Similar compounds can have similar Rf values so difficult to distinguish which one it may be
-Reference chromatogram may not exist on the database
-One solvent may not dissolve all components in sample or cause a component to be too soluble meaning it does not show up
What is Gas liquid Chromatography?
Used to separate volatile components in a mixture, so is useful to analyse organic compounds with low boiling points
What is the mobile phase of Gas liquid chromatography?
Inert carrier gas
What is the stationary phase of Gas liquids chromatography?
High boiling point liquid adsorbed onto the surface of a solid support
How is Gas liquid chromatography separated?
Relative solubility
What quantitive measurement can be gained from Gas liquid chromatography?
Retention time which can be compared to a database of known pure samples
What is Retention time?
The time taken for a compound to travel through the column to detector
What does a longer retention time suggest?
The greater interaction between the component and stationary phase, the slower the component moves through the column
The component is more soluble in stationary phase than mobile
What does a shorter retention time suggest?
The weaker the interaction between the component and stationary phase the, the faster the component moves through the column
The component is more soluble in mobile phase
Suggest approximations for the retention times of a non polar or polar compound being separated using Gas liquid chromatography using a non polar liquid stationary phase
The non polar compound will be more soluble in the non polar stationary phase, so will move slower and have a longer retention time.
While polar compounds will be more soluble in the mobile phase, interacting less with the stationary phase allowing it to move faster and have a shorter retention time.
What graph does the Gas liquid chromatogram produce?
Gas Chromatogram
What does each peak in the Gas chromatogram represent?
Each compound in the mixture will produce a peak and the area under each peak is proportional to the relative amount of the compound present in the sample, however it does not give an exact concentration.
How can % component of mixture be calculated from the Gas chromatogram?
Area under the peak
——————————- X 100
Total area of all peaks
What is the calibration curve used to determine?
Determines the concentration of a substance analysed by Gas chromatography
Describe how the calibration curve is calculated
-Prepare standard solutions of the compound under investigation
-Obtain a gas chromatogram for each standard solution and measure the area under each curve
-Plot a calibration curve of peak area against concentration
-Compare the area under the curves for the compound under investigation with calibration curve
What are the limitations of Gas Chromatography?
-Compounds can have the same Retention factor
-A small amount of one component may hide behind a component of higher concentration
-Unknown compounds may have no retention time data to compare against
State another form of analysis that is often used alongside Gas chromatography
Mass spectrometry
Why is mass spectrometry used with gas chromatography?
Gas chromatography is used to separate the components within a mixture
While mass spectrometry is used to confirm the identity of each component within the mixture from accurate measurements of Mr and unique fragmentation.
How is a sample prepared before NMR is carried out?
The sample to be studied is dissolved in a solvent (which contains no H1 atoms as this could hide the signal of the actual compound by appearing in the spectra)
State the different solvents that can be used to dissolve the sample
CCl4
CDCl3
C6D6
Why is deuterium used as a solvent?
Because it has no spin
What molecule is used as reference to compare other compounds with in NMR?
TMS or Si(CH3)4 is used to provide the internal standard conditions of zero that other peaks can therefore be compared against to work out the spectra.
State the different factors on a H-NMR spectrum that are used to identify the overall compound
-The number of different Hydrogen environments can be found from the number of peaks
-The type of Proton environments can be found from chemical shift
-The relative numbers of each proton can be found from the area under peak or integration value.
-The number of protons adjacent to a given proton (within 3 bonds) can be found from the splitting pattern
How can the number of Hydrogen environments in the NMR spectrum be determined?
Number of peaks
How can the type of proton environment be determined from the NMR spectra?
Chemical shift table on data sheet
How can the relative numbers of each proton be determined from NMR spectra?
Peak area or integration value
How can the number of protons adjacent to a given proton (within 3 bonds) be determined from the NMR spectra?
Splitting pattern
How can the integration values be calculated to determine a ratio for the number of protons involved in the compound?
Each intensity is divided by the smallest intensity before multiplying out to give a whole number= number of H atoms
NOTE: ratio may be in simplest form so check the number of Hydrogens against a larger ratio.
What is the rule to calculate adjacent protons from splitting patterns?
Rule = n + 1
Where N = number of lines
How many protons would be adjacent to a singlet signal?
The H causing the signal will have no adjacent H within 3 bonds
How many protons would be adjacent to a doublet signal?
The H causing signal will be adjacent to CH group
How many protons would be adjacent to a triplet signal?
The H causing the signal will be adjacent to a CH2 group
How many protons would be adjacent to a quartet signal?
The H will be adjacent to a CH3 group
How does a OH or NH group always appear as on the NMR spectra?
Always will appear as a singlet
How can OH and NH be determined within the sample?
By adding D2O to the sample and rerunning the spectra causes the groups to disappear.
