mass spectrometery

Define mass spectrometry

An analytical technique used to identify different isotopes + find the overall relative atomic mass of an element → the mass spectrometer measures the relative mass of isotopes

What are some different uses of mass spectrometry

• In environmental analysis

• In pharmaceutical / clinical analysis

• In forensic analysis

What are the different stages of mass spectrometry

1. Ionisation

2. Acceleration

3. Ion drift

4. Ion detection

5. Data analysis

Why is a vacuum required for during mass spectrometry

So there’s no interference from particles in the air

Explain what happens in Stage 1 - electrospray ionisation method

• Sample is dissolved in a volatile polar solvent

• Injected through a needle at a high voltage

• Each particle gains a proton (H⁺) to form 1+ ions

What is the equation for the electrospray method

X(g) + H⁺→ XH⁺ (g)

What are the XH⁺ ions attracted to?

A negatively charged electric plate

Explain the electron impact method for Stage 1

• High energy electrons are fired from an electron gun

• These knock off an electron from each particle to form 1+ ions

What is the equation for the electron impact method

X(g) → X⁺(g) + e⁻

Explain Stage 2 - acceleration

• Positive ions are accelerated by an electric field to a constant kinetic energy

• Velocity of the ions depend on the mass of the ion → mass + velocity are inversely proportional

What happens in Stage 3 - ion drift

• Ions separate → all have a constant kinetic energy but different velocities - ions with a lower mass take a shorter time to reach detector

• Ions drift in a flight tube of a known distance + time it takes for ions to reach detector is measure → time of flight

What does the time of flight depend on?

The ion’s velocity which depend on its mass

What happens in stage 4 - ion detection

Positive ions hit detector (negatively charged plate) + pick up an electron → causes a current to flow

The current produced in ion detection depends on what?

The abundance of ions hitting the detector → greater the abundance, the bigger the current - current is directly proportional to abundance

What happens in Stage 5 - data analysis

• Detector is linked to an amplifier + then to a recorder which converts the current into a peak which is shown in a mass spectrum

• Height of peak is proportional to size of current generated, and therefore proportional to the abundance of that ion

• Peaks are displayed on a plot of mass / charge (m/z)

Why is it necessary to ionise molecules when measuring their mass in a mass spectrometer

• Ions are accelerated by an electric field

• Only ions will create a current when hitting a detector → can be detected

What are the units for ‘time of flight’ calculations

time of flight

length of flight tube / distance

velocity of particle

mass of particle

kinetic energy of particle

How do you convert the mass number into kg

What is the equation for kinetic energy

½ x mass x (velocity)²

Rearrange the kinetic energy formula to make:

m the subject

v the subject

What is the equation for velocity

velocity = distance / time

Explain how to find the mass of an ion

• Multiply the number of protons by its mass

• Do the same for neutrons + electrons

• Sum of all (mass x no.)

What is the formula you use if you’re calculating time or mass of an isotope and are only given the time and mass of another isotope

What are mass spectra

Graphs which give the following information about the positive ions produced from a sample in the mass spectrometer

m/z of each ion → gives mass of each isotope as ion has a single positive charge

abundance of each ion → shown above each peak

What do we use as the standard when measuring relative atomic mass of all other elements?

Carbon - 12 → its relative mass is given 12 exactly

What is the equation for calculating relative atomic mass, using data from mass spectrometer?

Relative atomic mass should always be rounded to..

1 d.p → just like in the periodic table unless specifically asked

Masses of individual isotopes should always be to the nearest…

whole number

During electron impact ionisation, what can happen to covalently bonded molecules?

They can have their bonds broken + break into fragments → this is called fragmentation

True or false? Fragmentation occurs to all molecules

False → only occurs to some

So because fragmentation only occurs to some molecules, what does this mean for the molecules that it doesn’t occur to?

They will remain intact + just gain a positive charge → these unfragmented + positively charged molecules give us the peak with the greatest m/z value which is called the molecular ion peak

What does the peak with the greatest m/z value tell us about the sample during electron impact ionisation

The sample’s Mr

What is the fragmentation like during electro spray ionisation?

Very little occurs so there are a lot less peaks in a mass spectrum

With electrospray ionisation, what is the Mr of the sample equal to + why?

To the peak with the greatest m/z value minus one because during electrospray ionisation, a proton is gained which has a mass of 1

Both ___ and ___ exist as a pair of isotopes

Bromine

Chlorine

Bromine contains what isotopes + in what ratio?

79Br and 81Br → 1 : 1

Chlorine contains what isotopes?

35Cl and 37Cl → 3 : 1

The existence of these isotopes can be seen in…

the mass spectrum of any molecules that contain these atoms → their presence results in multiple molecular ion peaks

Use probability to calculate the ratio of the abundances for these peaks

Use probability to calculate the ratio of the abundances for these peaks

What are the number of molecular ion peak + the ratio of peaks for :

1 chlorine atom

2 chlorine atoms

1 bromine atom

2 bromine atoms