Chapter 29 - Chromatography and Spectroscopy Flashcards
Why is chromatography used?
to separate indiv components in a mixture of substances
All forms of chromatgraphy?
have a mobile and stationary phase
TLC: stationary phase
either SiO2 (s) or Al2O3 on coat TLC plate. IONIC
TLC: what is the stationary phase held on ?
plastic or Al sheet
TLC: mobile phase?
mixture of solvents
TLC: origin line must be?
- in pencil
* above solvent
TLC: Rf equation?
Retardation factor: distance moved by component/ distance moved by solvent front (always <1)
• depends on the solvents and stationary phase used
TLC: smaller Rf value means?
the component bonds more strongly to the stationary phase (more polar/ charged)
in the developing tank, why is there filter paper?
- filter paper soaked in solvent to saturate the air inside w the solvent
- helps solvent travel against gravity up the plate
to determine which substances are in the compound from a chromatogram ?
look at spots on the same level
can use chromatogram to determine?
purity - only 1 spot
how to visualise spots on a chromatogram - not all are coloured?
- can use UV light/ chemicl stains
- Solvent needs to be evaporated b4
- use iodine chamber
- leave the plate in the chamber for at least 5 mins
- once removed, as the iodine fades the spots’ll become visible
- circle spots w pencil before they fade
how does iodine help to visualise?
- warm beaker so it vaporises- sublimes
* iodine is covalent so can bond to the surface of the components (covalent) so dark brown spots appear
gas chromatography: oven
- regulates temp
- needs to be correct temp - middle of teh BP range
- KEEPS COMPONENTS AS GASES
gas chromatography: gas cylinder
He or N - unreactive mobile phase
gas chromatography: gas flow container valve
- to control the gas flow time through the column
* controls rate at which gas flows thru the column
gas chromatography: sample injection
- if liquid, the sample needs to be heated to vaporise it as it enters the column
- volatile organic chemicals - have to be a gas - GAS C
gas chromatography: recorded and computer
Records the gas chromatogram as a series of peaks - higher peaks = higher quantity
gas chromatography: detector
detects the organic chemicals not N/He. Connected to a computer
gas chromatography: column
- either an oily liquid (alkane) coating the inside of the thin tube/ covering the surface of tiny unreactive beads
- stationary phase
What is gas chromatography used for?
separating volatile organic compounds in a mixture
GC: stationary phase?
oily hydrocarbon (usually long chain alkane) which is held on inert plastic tubing/ inert solid beads in column (usually silica beads)
GC: mobile phase?
unreactive gas - N or noble gas
GC: separation by
solubility in oily hydrocarbon - stationary phase
GC: retention time
number of peaks SUGGESTS how many components are in the mixture
GC: what is retention time?
time from injection of MIXTURE to detection of COMPONENT
GC: what are peak integration areas?
calculated by computer = area under each peak. Gives idea of amount of each component.
GC: smaller retention time means?
- that compound is less soluble in the stationary phase (alkane)
- thfr, the compound is likely to be more polar
GC: smaller peak integration areas means?
less of the component in the mixture
GC: negative!
• e.g. 3 peaks suggests 3 components
7 but, 2 components could be equally soluble in the SP so 1 peak could actually represent 2 components
smallest retention time =
least soluble
GC and MS
- gas/ liquid chromatography separates the components
- mass spec produces a distinctive fragmentation pattern for each component
- identification by a computer using a spectral database
when trying to figure out the solubility and RT of a substance think abt
SA - larger SA = shorter RT
NMR basics
- molecules placed in strong magnetic field and irritated wit radio frequency EM radiation
- At certain radio wave frequencies, the nuclei of some atoms in the molecules absorb the radio waves
- These absorptions called resonances (the nuclei are flipping w/against the MF) are used to identify the atoms, count them and work out their position in the molecule
- NMR focuses on the H/C environments in a compound
Use of NMR spectroscopy?
MRI scanners
which 2 isotopes are most commonly studied in organic chem using NMR?
H and C13 - both have an odd number of nucleons
why are EMs used in NMR?
Nuclei align w/against the MF
What is chemical shift?
- δ (measured in ppm)
- place in the spectrum where the nucleus absorbs radio wave energy
- This depends on how the atom is bonded in a molecule. This gives us the type of C&H.
what is TMS?
• stands for tetramethylsilane - the standard refrence signal
(CH3)4Si
What is special about MS in NMR?
- 12 equiv protons, 4 equiv carbons which gives a singal sharp peak, defined as δ =0 ppm (reference peak)
- It’s chemically unreactive so doesn’t interfere w the chemical being analysed
which H isotope doesn’t give a signal in proton NMR?
• Deuterium nucleus - 2 1 H (even no. of nucleons so no signal)
the solvent used in NMR?
- Deuterated trichloromethane - CDCl3
* which is usually used for both proton and carbon NMR
How is CDCl3 used in NMR?
• 13C spectrum run for the one peak, can be subtracted from the spectra
D2O is?
deuterium oxide
