Topic 19 - Modern Analytical Techniques 2 Flashcards
What is high resolution mass spectrometry?
mass spectrometry which can measure Mr to 4 decimal places (more accurate)
What can high resolution mass spectrometry be used for?
to distinguish between compounds that appear to have same Mr when rounded to a whole number
What does NMR stand for?
nuclear magnetic resonance
What is NMR (nuclear magnetic resonance)?
an analytical technique which uses a magnetic field to align protons in the nucleus of hydrogen atoms and carbon-13 atoms
-is used to determine structure of a compound
What is used as a standard in NMR?
tetramethylsilane (TMS)
(CH3)4Si
Why is TMS used as a standard/comparative peak in NMR?
- all 12 of its hydrogen atoms are in the same environment (no splitting pattern)
- all of its hydrogen atoms are more heavily shielded in any other molecule (aka their e- lie closer to the hydrogens than in any other molecule)
- it is inert
- its peak is to the right so doesn’t overlap with other peaks
- it has a low boiling temperature/is volatile so can be easily removed
- gives a strong signal so only a small amount needed
What does C-13 NMR provide information about?
position of C-13 atoms in a molecule
What does proton NMR provide information about?
positions of H atoms in a molecle
What does low resolution NMR tell us?
- number of hydrogen environments
- number of hydrogen atoms in each hydrogen environment
What does high resolution NMR tell us?
- number of hydrogen environments
- number of hydrogen atoms in each hydrogen environment
- how many hydrogens are on adjacent carbon
What is the number of peaks on a NMR spectra equivalent to?
number of hydrogen environments
What is the height of a peak on a low resolution NMR spectra equivalent to?
how many hydrogen atoms are in that environment
What is the splitting pattern on a high resolution NMR spectra equivalent to?
the number of hydrogen atoms are on the adjacent carbon plus one
(N+1 rule)
What is the area under a cluster on a high resolution NMR spectra equivalent to?
number of hydrogen atoms in environment
How many hydrogens are on the adjacent carbon to a singlet?
0
How many hydrogens are on the adjacent carbon to a doublet?
1
How many hydrogens are on the adjacent carbon to a triplet?
2
How many hydrogens are on the adjacent carbon to a quartet?
3
How many hydrogens are on the adjacent carbon to a quintet?
4
What do you call the splitting pattern when there are 0 carbons on the adjacent carbon?
singlet
What do you call the splitting pattern when there is 1 carbon on the adjacent carbon?
doublet
What do you call the splitting pattern when there are 2 carbons on the adjacent carbon?
triplet
What do you call the splitting pattern when there are 3 carbons on the adjacent carbon?
quartet
What is the N+1 rule?
the number of subpeaks within a cluster on a high resolution NMR is equivalent to the umber of hydrogen atoms on the adjacent carbon plus one
What is the unit for chemical shift?
ppm
What is chromatography?
a technique used to separate components of a mixture so that they can be identified
-uses a mobile and stationary phase
What types of chromatography are there?
- thin layer chromatography (TLC)
- column chromatography
- high performance liquid chromatography (HPLC)
- gas chromatography (GC)
What does TLC stand for?
thin layer chromatography
What is the stationary phase in thin layer chromatography?
TLC plate coated in solid particles
What is the mobile phase in thin layer chromatography?
solvent
Rf =
distance moved by spot
_____________________
distance moved by solvent
What does the height a component move up thin layer chromatography depend on?
its interactions with the stationary phase
-ie. if its moved higher up, it has greater interactions with plate
What is the stationary phase in column chromatography?
solid (eg. silica) which is packed into column
What is the mobile phase in column chromatography?
solvent (which runs down column with gravity)
Name examples of a polar solvent that could be used with a non-polar silica in column chromatography
water
ethanol
ethanoic acid
Name examples of a non-polar solvent that could be used with a polar silica in column chromatography
hexane
cyclohexane
hydrocarbons
What does HPLC stand for
high performance/pressure liquid chromatography
What is the stationary phase in high performance liquid chromatography?
solid packed into column
What is the mobile phase in high performance liquid chromatography?
solvent
What happens in high performance liquid chromatography (HPLC)?
- solvent (and sample) are forced through horizontal column (packed with solid) under high pressure
- detector at the end of column detects each compound and calculates its retention time (Rt)
What does GC stand for?
gas chromatography
What is the stationary phase in gas chromatography?
solid with high mp which is packed into column
What is the mobile phase in gas chromatography?
inert gas (aka noble gas)
What happens in gas chromatography (GC)?
- an inert gas (eg. noble gas) acts as a solvent and is forced through column (packed with a solid) under high pressure by a pump
- sample is injected into the column as a gas (sample may have been heated and vaporised first depending on its standard state) and is forced through column
- detector detects each compound and calculates its retention time (Rt)
Why is the column in an oven in gas chromatography?
to ensure sample is maintained as a vapour
What must the gas used as a solvent in gas chromatography be?
inert/not reactive
ie. noble gas eg. helium
What must the solid used as the stationary phase in gas chromatography be?
have a high mp
What are the advantages of high performance liquid chromatography (HPLC) and gas chromatography (GC)?
- faster
- more accurate
- can use smaller quantities
What are the disadvantages of high performance liquid chromatography (HPLC) and gas chromatography (GC)?
-not so good at identifying components (∴ are used with mass spectrometry)
Name two examples of when chromatography (HPLC/GC) is used with mass spectrometry
- forensics
- drug testing
Why are chromatography (HPLC/GC) and mass spectrometry used together?
mass spectrometry can be used to identify the substances HPLC/GC have separated (instead of using a detector)
as HPLC/GC are not so good at identifying
