Unit 1 Section 1 Mass Spectrometry

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Measuring Atomic Mass

Mass spectrometry can be used to find the relative atomic mass, relative molecular mass, relative isotopic abundance and molecular structure.

There are 5 steps in a mass spectrometer:

1. Vaporisation: The sample is tyrned into gas (vaporised) using an electrical heater.

2. Ionisation: The gas particles are bombarded with high-energy electrons to ionise them. Electrons are knocked off of the particles, leaving positve ions.

3. Acceleration: The positive ions are accelerated by an electric field.

4. Defelction: The positive ions' paths are altered with a magnetic field. Lighter ions have less momentum and are deflected more than heavier ions. For a given magnetic field, only ions with a particular mass/charge ration make it to the detector.

5. Detection: The magnetic field strength is slowly increased. As this happens, different ions (ones with a higher mass/ratio) can reach the detector. As ions hit the detector they cause a current to flow. The bigger the current produced the more of that isotope was present in the sample.

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Interpreting Mass Spectra

If the sample passed through the mass spectrometer is an element, each line will represent a different isotope of the element.

The y-axis gives the abundance of ions, often as a percentage.

For an element, the heigthof each peak gives the relative isotopic abundance

The x-axis units are given as a 'mass/charge' ratio.

Since the charge on the ions is mostly 1+, you can often assume the x-axis is simply the relative isotopic mass.

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Calculating Relative Atomic Mass

1. For each peak, read the % relative isotopic abundance from the y-axis and the relative isotopic mass from the x-axis. Multiply them together to get the total mass for each isotope.

2. Add up these totals.

3. Divide by 100 since % were used.

4. If the relative abundance is not given as a %, the total abundance may not add up to 100. In this case just divide by the total relative abundance instead of 100.

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Calcualting Relative Molecular Mass

A molecualr ion, M+(g), is formed when the bombarding electrons remove 1 electron from the molecule.

This gives the peak in the spectrum with the highest mass and furthest to the right beside isotopes.

The mass of M+ gives Mr for the molecule.

But bombarding with electrons makes some molecules break up into fragments.

These all show up on the mass spectrum, making a fragmentation pattern.

Fragmentation patterns can be used to identify molecules and even their structure.

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Glossary

Mass spectrum: A chart produced by a mass spectrometer giving information on relative isotopic mass and relative abundance of isotopes.

Relative isotopic abundance: The amount of each isotope present in a sample.

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