6.3 Analysis Flashcards
Types of chromatography
TLC - thin layer
GC - gas
Describe chromatography
A separation technique to identify chemicals in a mixture with a mobile phase (liquid/gas to carry mixture through system) and stationary phase (solid/solid-supported liquid)
How does the mobile phase move components at varying speeds?
Solubility
How does the stationary phase absorb components differently?
Affinity
What is the stationary phase in TLC?
Typically, silica gel or aluminium oxide
How does the solvent move up the plate in TLC?
By capillary action
Name the labels on a TLC diagram
Solvent front
Baseline
Distance travelled by component
Distance travelled by solvent
What is Rf?
Retardation factor - ranges from 0 to 1 and can be used to identify substances when compared to database values
How do you calculate Rf?
Component distance travelled ÷ Solvent distance travelled
What is the stationary phase in GC?
A liquid hydrocarbon with a high boiling point coating in a coiled tube kept at constant temperature
What is the mobile phase in GC?
An inert carrier gas, such as Helium or Nitrogen
What is GC used for?
To separate volatile components in a mixture
How are GC results presented?
Forms a graph (absorption y/time x) with peaks
How is a GC graph analysed?
The number of peaks is the number of components
The area under a peak is proportional to its concentration
To find the % composition of a component, find the area under its peak as a % of the total area under all peaks
What is the equivalent of Rf for GC?
Retention time
How does GC differ from TLC?
More sensitive - shows what chemicals are in a mixture and their concentrations
Often connected to a mass spectrometer so components can under further analysis
Test for haloalkanes
Aqueous silver nitrate and ethanol
Forms different colour precipitates (Cl, Br, I)
Test for alcohols
Acidified potassium dichromate turns from orange to green
Varying results based on 1°, 2°, 3° and distillation or reflux
What isotopes are NMR active?
¹H and ¹³C (due to spin)
Describe NMR
A non-destructive analytical technique based on ¹H and ¹³C spin alignments
What is the reference chemical for NMR and why?
TMS - tetramethylsilane Si(CH3)4
Contains both C and H so can be used for both NMRs
Produces 1 sharp signal as all H’s are in same environment, and only has 1 C
Non-toxic, inert, and volatile (low b.p) so can be easily distilled off
Importance of Deuterium in NMR
²H isotope is not NMR active
Solid molecules undergoing NMR need to be dissolved in a deuterated solvent to prevent swamping of graph
What solvents are used in NMR?
Deuterated solvents
CDCl3 - heavy trichloromethane
CCl4 is an option but poses environmental risks
How do you answer a ¹³C NMR question?
A) n of environments and ratio
B) describe functional groups indicated by chemical shift ranges
–> downfield means further left, carbons in same env are ‘equivalents’, describe height of peaks
How do you answer a ¹H NMR question?
Make table of:
A) functional group
B) chemical shift
C) H number in environment
D) splitting pattern
E) H number adjacent
D2O shake
Due to broad and wide-ranging -OH and -NH peaks
D does not produce peak
D2O vigorous shake removes -OH or -NH peak to identify them
Why are -OH and -NH peaks broad and wide ranging?
Due to small traces of water forming H bonds with them
