Mr Cole Chemistry

Mass Spec Theory Questions

Mass Spectrometry

Worksheet

Ionisation of the Sample

Explain how electron impact ionisation is used to ionise a sample.
Answer

A sample is vaporised. A hot filament wire with a current running through it is used to bombard the sample with electrons. When the electrons collide with the sample molecules, one or more electrons are knocked off, resulting in a positive ion.

Write a symbol equation for the ionisation of a sample ‘M’ using electron impact.
Answer

M(g) → M+(g) + e–

OR

M(g) + e– → M+(g) + 2e–

Explain how electrospray ionisation is used to ionise a sample.
Answer

The sample is dissolved in a volatile solvent, which is then passed through a hypodermic needle. The needle is connected to the positive terminal of a high voltage circuit. As the sample passes through the needle, it picks up a proton.

Write a symbol equation for the ionisation of a sample ‘F’ using electrospray ionisation.
Answer

F + H+ → FH+

(Note the lack of state symbols here – we don’t know what the solvent is)

State when you would use each of the ionisation methods.
Answer

Electrospray is used for very large molecules, such as proteins or DNA. This is because electron impact causes fragmentation that, while useful in small molecules, makes it difficult to determine the mass of a large molecule.

Acceleration

Explain why particles of different masses reach the detector at different times.
Answer

The energy transferred to the particles is dependent on the strength of the field and the charge on the sample. Therefore, all the samples have the same kinetic energy. As KE = ½mv2, velocity is dependent on the mass of the particles.

Detection

Describe what happens when the ionised sample reaches the detector.
Answer

An electron is transferred from the detector to the positive ion when they come into contact. This is measured as a current.

Explain how the detector determines the abundance.
Answer

The more particles that reach the detector at any given moment, the more electrons are transferred at that moment. This means the current is proportional to the abundance of each mass.

A sample of rubidium is analysed. Peaks were detected at 85, 86 and 43. Explain the presence of each of these peaks.
Answer

85 and 86 are the two isotopes of Rb present in the sample. The peak at 43 is not an isotope. It is caused by the Rubidium-86 that has lost 2 electrons rather than 1.

Observe the mass spectrum of an organic molecule below. Suggest the likely Mr of the molecule and explain your reasoning. Mass spectrum of an organic molecule
Answer

The relative mass of the sample is 100. While there is a small peak at 101, this is caused by a small amount of C-13 in the compound.

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