S1.2.2 - Mass Spectrometry

SPECTROMETERS IN SPACE

Space probes will carry spectrometers to identify elements in rocks.

E.g. Giotto Space Probe passing close to Halley’s comet

Remember: we need a vacuum for a mass spectrometer to work. This is easy for a space probe as space is a vacuum!!

DATA FOR HALLEY’S COMET

Greenburg J. M (2002). Cosmic Dust and Our Origins. Surface Science. 500: 793-822

WHAT IS A MASS SPECTROMETER?

An instrument to accurately determine the relative atomic mass

Relative atomic mass (Ar) is the average mass of atoms of an element relative to an atom of carbon – 12

Separates atoms or molecules according
to their charge and mass.

This can be used to identify substances

🡪 e.g. illegal drugs

SUMMARISING WHAT HAPPENS:

1. Vaporisation: Atoms are vaporised before injection.

2. Ionisation: Atoms are converted to ions

3. Acceleration: Ions are accelerated

4. Deflected: Deflected according to their mass & charge

5. Detection: They arrive at a detector

---- = heavy ions

---- = ions reaching detector

---- = light ions

THE LAYOUT
5 key stages:

Vaporisation
Ionisation
Acceleration
Deflection
Detection

CONDITIONS:

Vacuum 🡪 so ions do not collide with air molecules (might stop them reaching the detector)
Gaseous State 🡪 solids are vaporised before being injected

Mass Spectrometer Simulator

LOOK IN MORE DETAIL:

Stage 1: Vaporisation

The sample is injected into the instrument where it is heated and vaporised, producing gaseous atoms or molecules. The sample needs to be gaseous, otherwise it cannot accelerate through the electric field or be deflected through the magnetic field.

Stage 2: Ionisation

Beam of electrons knocks electrons from atoms or molecules in the sample.

X(g) + e- ---> X+(g) + 2e-

This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon, for example).

Nearly all lose just one electron (~5% will lose two)

Mass spectrometers always work with positive ions!!

Stage 3: Acceleration

The ions are accelerated so that they all have the same kinetic energy.

Stage 4: Deflection

The ions are then deflected by a magnetic field according to the ratio of their mass to charge (m/z), where z is the charge (usually +1)

Heavier ions are deflected less than light ones

2+ ions are deflected twice as much as 1+ ions

Stage 5: Detection

Magnetic field is gradually increased 🡪 increases deflection

This allows ions of increasing mass to enter the detector

On striking the detector ions accept electrons, lose their charge and create a current

Current created is proportional to the abundance of each ion

MASS SPECTRA OF ELEMENTS

From the strength of the magnetic field at which a particular ion hits the detector the value of the mass to charge ratio (called m/z) is calculated.

A graph is produced (mass spectra) showing the relative abundances of each ion type

Mass Spectra of Zirconium

MASS SPECTRA OF ELEMENTS

We can use the mass spectrometer to identify the different isotopes making up an element

Each isotope is detected separately because they have different masses

To calculate an elements relative atomic mass (which is given in the periodic table) you must take account of the relative abundances of each isotope

CALCULATING Ar OF ELEMENTS

This is the mass spectra for chlorine

We have 2 isotopes with relative isotopic masses of 35 and 37, detected in a ratio of 3:1 (or 75%:25%)

To calculate the relative atomic mass of chlorine:

CALCULATING RAM OF ELEMENTS

RAM of Cl = 35.5

Notice there is no line at 35.5 on the mass spectra. No atoms of Cl actually have this mass. It is the average of all the isotopes and their abundances!

STEPS:

Multiply the m/z value by the relative abundance % for each peak
Add results for each peak together
Divide by total relative abundance

CALCULATING RAM OF ELEMENTS

Calculate the RAM of the element from its mass spectra:

Mass Spectra of Boron (B)

Mass Spectra of Zirconium (Zr)

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