3.1.1.2 Mass number and isotopes

atomic number (Z) defined?

number of protons in the nucleus.

mass number (A) defined?

total number of protons and neutrons in the atom.

isotopes?

Atoms with the same number of protons but different numbers of neutrons.

Why do isotopes have similar chemical properties?

They have the same electronic structure.

purpose of a mass spectrometer?

· find the Mr of molecules in a sample

· Determine the abundance and relative mass of each isotope in an element

This type of mass spectrometry is called

Time of Flight Mass Spectrometry (TOF or TOFMS)

four essential steps in a mass spectrometer?

Ionisation (2 types), acceleration, drift, and detection.

BEFORE 1ST STEP OF MASS SPECTOMETRY

The liquid sample to be tested is injected in the machine

This is vaporised into a gas

Why is the sample vaporised?

Easier for someone to move through a machine

What is electron impact ionisation?

The sample molecules (M) is injected into the machine.

The sample is vaporised into a gas which moves through the

machine more easily.

The gaseous sample is passed through the path of an electron gun.

The sample is bombarded with high energy electrons

The molecules in the sample form a M^{+ _(g)} ion.

Sometimes an M2+!

This technique is better for small molecules.

Why does the electron gun cause molecules to ionise?

High energy electron knocks off one of the electrons in the sample

Limitation of electron impact ionisation

Fragmentation where parts of molecule are also knocked off by high energy electrons such as side chains

Why are samples ionised before they enter the machine?

Ions can interact with and be accelerated by electric field to move sample through the machine

Only ions create a current when hitting the detector so abundance can be measured

What is electro spray ionisation?

(PREFERABLE FOR LARGER ORGANIC MOLECULES)

The sample molecules (M) are dissolved in a volatile solvent

The sample is injected through a hypodermic needle to give a fine aerosol

The tip of the needle is attached to the positive terminal of a high voltage power supply.

The molecules are ionised by gaining a proton from the solvent as they leave the needle

This technique is suitable for high Mr compounds because limited

fragmentation! (Why it’s called a soft ionisation technique)

WARNING ABOUT ANALYSING DATA FROM ELECTROSPRAY IONISATION

Data given is M_r + 1 SO SUBTRACT 1 FROM UR DATAAA !!!

Why does mass spectrometer need to be under a vacuum?

Air particles would ionise and be registered on detector

What happens during the acceleration step in a mass spectrometer?

Positive ions are accelerated by an electric field

All ions in sample have same kinetic energy. KE =1/2mv²

The ions in the sample spread out by velocity

Lower molecular mass ions reach the detector first.

How does the velocity of particles relate to their mass in a mass spectrometer?

Lighter particles have a faster velocity, while heavier particles have a slower velocity.

What does the time of flight (t) depend on in a mass spectrometer?

depends on the distance (d) and the velocity (v) of the particle.

What is detected in the detection step of a mass spectrometer?

The detector is a negatively charged electric plate.

The positive ions hit the detector and gain an electron.

This movement of electrons means an electric current is flowing.

The abundance of the species is determined by the size of the current flowing in the detector (MUST REMEMBER WORD FOR WORD)

Flight tube

Where positive ions with smaller m/z velues have the same KE as those w/ larger m/z values, and therefore move faster

significance of the m/z ratio in mass spectrometry?

It represents the mass-to-charge ratio of ions, allowing identification of isotopes → mass spectrometer measures this m/z ratio

significance of the peak in a mass spectrum?

The peak with the largest m/z corresponds to the complete molecule and is equal to the relative molecular mass (Mr) of the molecule.

Where are mass spectrometers used?

They’ve been included in planetary space probes to identify elements on other planets → Elements on other planets can have a different composition of isotopes