Topic 7: Mass Spectrometry

Question 1

What is a mass spectrometer?

Answer 1

A machine that weighs ions. it tells us the mass of each ion and su abundance

Mass spectrometry differentiates between similar molecules by looking at molecule fragments.

In mass spectrometers you compare the sample you put in to a built-in database of samples to help identify the molecule

Question 2

How does a mass spectrometer work?

Answer 2

At the top of the mass spectrometer, there is a sample inlet. This is where your sample goes in, and it needs to be vaporised.

Once vaporised, the sample is ionised- it is bombarded with high energy electrons, knocking off e to form cations: X(g) + e- ——> X+(g) + 2e-

These ions are accelerated through the chamber by negatively charged plates because they attract the cations. At the bottom of the chamber hay a MF which will deflect ions. Lighter ions are deflected more than heavy ions.

A screen detects signals, recording the ions and their relative intensity. This is turned into the mass spectra showing % abundance and m/z ratio.

Question 3

How do you calculate Ar from mass spectra?

Answer 3

the x axis should have the masses. the peaks should show the relative abundance. take the abundance and masses and find the Ar for the element

Question 4

How do you find the Mr in mass spectra?

Answer 4

The mass spectrometer also finds the Mr. M(g) + e- –> M+(g) + 2e-.

M+ is called the molecular ion, w the same Mr as the molecule. The e has been knocked off, but the e has a negligible mass.

To find the M+, find the peak with the the highest m/z ratio- this is furthest right and must be a significant peak- it can’t be tiny!!

Question 5

What can happen to the molecular ion?

Answer 5

The molecular ion can breakdown: M+(g) —–> O+ + L.(g)

It forms a positive ion (O+) and a radical (L.)

We wouldn’t be able to pick up the radical on the mass spectrometer- only the cation. The process is fragmentation, and forms fragment ions.

Question 6

How many times can fragmentation occur?

Answer 6

Fragmentation can happen multiple times, creating an array of cations. These show up according to their m/z ratio and abundance on the mass spectra.

We can use the m/z ratio of those tiny fragment peaks to work out the structural formula of the molecule.

Question 7

What do we look for in a mass spectra?

Answer 7

In the mass spectra, look for common fragments. A common fragment that can break off from a molecule is a CH3 group. This has a m/z ratio of 15, so the difference in mass is 15. This is because the Mr of CH3 is 15.

Question 8

List some other common fragments and their group

Answer 8

CH3+ 15

OH 17

C=O 28

C2H5+ 29

COOH+ 45

COCH3+ 43

Question 9

What must you do when writing organic groups and fragment ions?

Answer 9

When writing the groups that organic, you must draw a + It doesn’t matter where the positive charge is. Also draw square brackets around fragment ions.

Question 10

Here is the mass spectra for ethyl ethanoate. Identify the ions responsible for the peaks at 88 and 43

Answer 10

Question 11

Here is the mass spectra for ethyl ethanoate. Write an equation to show the fragmentation when the ion at 88 is converted to the ion at 43

Answer 11

Question 12

Why is there sometimes a peak at M +2 in a mass spectra? Eg for 2-chloropropane

Answer 12

Question 13

What happens to some covalent molecules when they absorb infrared radiation?

Answer 13

Some covalent bonds in molecules absorb energy from IR radiation to make bonds vibrate w either a bending or a stretching motion. Every bond vibrates at its own unique Hz.

This frequency is measured in wave numbers between 300 cm-1 - 4000 cm-1

The Hz depends on the length and strength of the bond, the mass of each bonded atom and the environment the atom is in.

Question 14

Which molecules can absorb infrared?

Answer 14

Only molecules able to switch polarity can absorb infrared. This means diatomic molecules like Cl2, H2, O2, N2 will not absorb IR radiation.

If you have a bond with the same atom on either side of the bond, it will not absorb IR because they can’t switch polarity.

Question 15

What is the fingerprint region?

Answer 15

The fingerprint region is below about 1500 cm-1. It is contains too many signals, so picking out functional group signals are difficult. We do not analyse the fingerprint section by eye.

The fingerprint region is unique for every compound so is used to distinguish or identify compounds. This region can also check if a compound is pure- any extra peaks must be due to an impurity.

Question 16

What must you remember when talking about peaks?

Answer 16

When you talk about peaks, use is squiggly line to indicate that you were estimating.

Question 17

What do peaks on a IR spectra represent?

Answer 17

The peaks on an IR spectrum represent the absorbance of energy from IR that causes the vibration of the particular bond.

The precise position of the peak depends on the bond environment, so only wave number regions can be quoted

Question 18

Draw a table to show the peaks you would see for alcohols, carbonyls, carboxylics and esters

Answer 18

Question 19

Give uses for IR spectra

Answer 19

Forensics, drug analysis, perfume manufacture, monitoring polymers.

Question 20

What is this compound?

Answer 20

Question 21

Identify the unknown compound

Answer 21