Topic 19 modern analytical techniques 2 Flashcards
What is mass spectroscopy?
Used to find Mr of a compound
M/Z ratio is just the mass of a fragment / by the charge
Peaks show fragments of the original peak. M+ peak is last peak. This is the same as the Mr of the molecule
What is high resolution mass spectroscopy?
Useful when identifying different molecules with the same Mr rounded to the nearest whole number.
High-resolution ones measure the Mr to several DPs unlike the standard low resolution which is only to the nearest whole number
NMR spectroscopy
Nuclear magnetic resonance spectroscopy is used to help determine the structure of a molecule
What are the 2 main types of NMR?
- 13C NMR - tells us info on how the carbon atoms can be arranged
- High-resolution HNMR - tells us how hydrogen atoms are arranged
Number of nucleons and how it affects spin
If an atomic nucleus has an odd number of nucleons (protons and neutrons) then it has a nuclear spin
This nuclear spin creates a weak magnetic field. NMR detects how the magnetic fields are affected by a larger external magnetic field.
Hydrogen has 1 proton so does have a nuclear spin. Carbon has 6 protons and 6 neutrons however approx 1% of carbons are 13C which has 7 neutrons so it does have a nuclear spin
The nuclei spin in random directions however when an external magnetic field is applied, they align in 2 directions.
Nuclei spin either in the direction of the external magnetic field or against it. Those that spin in the direction of the magnetic field have lower energy.
LOOK AT PAPER FOR DIAGRAM
How does NMR work?
NMR fires out random radio waves. At a specific frequency, the nuclei that are aligned with the mag field absorb the energy and flip to the higher level
Those with higher energies can also drop to lower energy and emit radio waves
Initially, there are more nuclei aligned with the magnetic field so overall more energy is absorbed than emitted.
NMR measures the amount of energy absorbed
What is an environment?
The energy absorbed by nuclei is dependent on the environment it is in. A nucleus can be shielded from an external magnetic field by electrons surrounding the nucleus. A lot of shielding protects more.
Atoms and groups of atoms adjacent to the nucleus affect the level of electron shielding. For example, EN elements such as oxygen near a C atom will reduce the electron shielding on the C atom.
The magnetic field will be felt by the nuclei differently depending on the environment it is in as they absorb different amounts of energy and various frequencies. It is this difference that NMR frequency picks up.
The environment is determined by the groups of atoms that exist near to the nuclei being examined. We look along the full chain NOT just the atom immediately bonded to the atoms being examined.
For an atom to be in the same environment, it must be bonded to an atom or group of atoms that are identical
LOOK AT PAPER FOR CARBON 13 AND HYDROGEN ENVIRONMENTS
What is tetramethyl silane (TMS)?
TMS is a chemical used as a standard when looking at chemical shift in NMR spectra.
TMS has 12 hydrogens all in identical environments. This will produce a large, single peak well away from sample peaks. It is also inert, non-toxic and volatile so easy to remove from your sample.
The difference between the TMS peak and peaks produced by the substance under test is called the chemical shift. W measure this in parts per million (PPM) because it is used as a standard to measure against. We give this peak the chemical shift on the furthest right of 0. This is as TMS is used to calibrate NMR machines when analysing samples
13C NMR spectroscopy
It tells us how many different carbon environments there are in the sample being tested
The peaks tell is the number of different carbon environments.
Look at paper
Peak furthest left is the one that is closest to the EN element which means the electron shielding is lower and so the chemical shift is higher
13C NMR spectroscopy for cyclic compounds
In these cases, we look for symmetry.
Look at paper
There is a data book with all the values. We can match up the position of the peaks in the spectrum to the table to work out what carbon environments exist.
Some issues:
Peak at 190 suggests a carbonyl group but we can’t be sure if it is aldehyde or ketone
There are overlaps too so a peak at 60 could be an amine or alcohol/ester/ether
1H(proton spectroscopy)
Tells us how many different H environments there are and how many Hs in each environment. The peaks tells us the number of different H environments.
The numbers above the peak tells us the ratio of the areas under the peak. This allows us to work out the relative number of hydrogens in each environment. Sometimes these can be decimals.
If there is a 1:3 ratio of peaks this could mean 1H on one environment and 3 on another environment.
The red line is an integration trace which helps to show the area ratio of these peals.
Proton NMR spectrum has peaks that split. It allows us to determine the structure.
Peaks that split into smaller peaks is known as a splitting pattern. The number of smaller peaks corresponds to the number of hydrogen atoms on the adjacent carbon+1 which is also known as spin-spin coupling. This is called the n+1 rule.
Singlet peak = 0 hydrogens on neighbouring carbon
Doublet peak = 1 hydrogen on neighbouring carbon
Triplet peak = 2 hydrogens on neighbouring carbon
Quartet peak = 3 hydrogens on neighbouring carbon
Integration traces show the area under a peak more clearly. This helps to work out the hydrogen ratio.
When we have split peaks it is difficult to work out the area under the peak. An integration trace is used and the height ratio of the trace corresponds to the area ratio.
In practice, we use a ruler to measure the vertical parts of the trace. Write the lengths down to come up with a ratio.
Look at paper for examples
Elemental analysis
It is a method by which we can determine the structure of the sample under test
It helps us to determine % composition of the mass of elements that make up a compound
If we know the molecular formula, we can work out the structure of the compound
TLC
Allows us to separate and identify compounds
Uses a stationary phase of silica or alumina mounted on a glass/metal plate. A pencil baseline is drawn and drops of mixture are added
Place the plate in a solvent - base line must be above the solvent level
Leave until the solvent has moved up to near the top of the plate. Remove, mark the solvent front and allow to dry
It works by the mixture spots dissolving in the solvent. Some chemicals may not dissolve as much and stick to the stationary phase quickly. We are left with a chromatogram
Rf values
Distance travelled by spot / distance travelled by solvent
Number of spots on the plate can tell you how many spots make up the mixture
Gas chromatography
Useful to separate a mixture of liquids that are volatile and hence can be identified
In GC you have a very thin column that is wound up inside an oven to save spcae. The column is lined with a solid or viscous liquid that acts as the stationary phase
The sample is injected into the machine and carried by an inert gas which is the mobile phase
Each substance takes a different amount of time to travel through the column and reac the detector. The lenght of time it takes is called the retention time.
The time it takes for the sample to travel through varies as some molecules spend more time stuck to the stationary phase and some spend more time travelling in the mobile phase
