C29 - Chromatography and Spectroscopy Flashcards
(37 cards)
What do all forms of chromatography consist of?
Mobile phase - moves. Normally a liquid or gas.
Stationary phase - doesn’t move. Normally a solid or a liquid supported on a solid.
What does TLC stand for?
Thin layer chromatography
What happens in TLC?
Thin layer chromatography
A TLC plate is used which is usually a plastic sheet or glass, coated with a thin layer of a solid adsorbent substance (usually silica).
The adsorbent is the stationary phase.
Different components in the mixture have different affinities for the adsorbent and bind with different strengths to the surface.
What’s adsorption?
The adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent.
(The process by which the solid silica binds to and holds different substances in the mixture to it’s surface.
How is the Rf (retention factor) value calculated?
Rf = distance moved by the component / distance moved by the solvent front
What’s GC and what’s it used for?
Gas chromatography - used for separating and identifying volatile organic compounds in a mixture.
(The stationary phase is a high boiling liquid adsorbed onto an inert solid support. The mobile phase is an inert carrier gas e.g. helium or nitrogen.)
What is retention time?
The time taken for each component to travel through the column, in the oven, and reach the detector in gas chromatography.
How are compounds separated in GC?
They’re separated depending on their solubility (in the liquid stationary phase).
The more volatile, the shorter the retention time.
How are gas chromatograms interpreted?
Retention times - used to identify the components present in the sample by comparing these to retention times for known components.
Peak integrations - (areas under each peak) can be used to determine the concentrations of components in the sample.
What’s the procedure to identify the concentration of components in a sample, with gas chromatography?
1) Prepare standard solutions of known concentrations of the compound being investigated.
2) Obtain gas chromatograms for each standard solution.
3) Plot a calibration curve of peak area against concentration.
4) Obtain a gas chromatogram of the compound being investigated under the same conditions.
5) Use the calibration curve to measure the concentration of the compound.
What’s chemical shift in spectroscopy measured in?
Parts per million
ppm
TMS (used as the standard/calibration) has a 0ppm shift.
Why are deuterated solvents used?
The use hydrogen isotope deuterium D (H2).
This produces no NMR signal so wouldn’t produce a peak on the spectrum.
What information does a carbon-13 NMR spectrum provide?
The number of different carbon environments (from the number of peaks)
The types of carbon environment present (from the chemical shift)
How do the groups bonded to carbon atoms affect the number of peaks in the NMR spectrum?
C atoms bonded to different atoms/groups of atoms have different environments so will absorb at different chemical shifts.
However, if 2 C atoms are positioned symmetrically within a molecule, then they’re equivalent and have the same chemical environment.
They will absorb radiation at the same chemical shift and produce one peak.
What information does a proton NMR spectrum provide?
The number of different proton environments (from the number of peaks)
The types of proton environment present (from the chemical shift)
How does proton equivalence affect the NMR spectrum?
If 2+ protons are equivalent, they’ll absorb at the same chemical shift and produce one peak (increasing size of peak).
Protons of different types have different chemical environments and are non-equivalent. They absorb at different chemical shifts, producing separate peaks.
What does an integration trace on a proton NMR spectrum show?
The area under each peak, showing the number of protons in that environment.
What’s spin coupling?
The splitting of a main peak into sub peaks (in a proton NMR spectrum).
The number of sub peaks is one less than the number of adjacent protons.
What’s the n+1 rule?
For a proton with n protons attaches to an adjacent carbon atom, the number of sub peaks in a splitting pattern is n+1.
(The number of sub peaks is one less than the number of adjacent protons).
What does a singlet show, in a proton NMR spectrum?
A single peak, indicating no protons on adjacent atoms.
What does a doublet show, in a proton NMR spectrum?
Two peaks, indicating an adjacent CH / a single proton.
What does a triplet show, in a proton NMR spectrum?
Three peaks, indicating an adjacent CH2 / 2 adjacent protons.
What does a quartet show, in a proton NMR spectrum?
Four peaks, indicating an adjacent CH3 / 3 adjacent protons.
What does a multiplet show, in a proton NMR spectrum?
Multiple (more than 4) peaks, indicating multiple (possible unsymmetrical) adjacent protons.
