Mass spectrometry

Question 1

Give some uses of were it is used in terms of looking at substances.

Answer 1

• To identify unknown compounds - To determine the abundance of each isotope in an element. - To gain further information about the structure and chemical properties of molecules.

Question 2

Gives some examples of were mass spectrometry is used in industrial and medical applications.

Answer 2

Examples of its use today include: -Detecting banned substances such as steroids in athletics. - Analysing molecules in space - Detecting traces of toxic chemicals in contaminated marine life.

Question 3

Explain how a mass spectrometer works.

Answer 3

1) The sample is introduced via a sample inlet. 2) Sample molecules are converted into ions by an ionisation source. 3) The ions are propelled into a mass analyser. 4) The ions are separated according to their mass-to-charge ratio. Methods of separation include time of flight(measurement of how long it takes for ions of different mass to reach the detector.) 5) The ions are detected. A mass spectrum is generated by computer software and displayed on a screen or printed.

Question 4

Explain how a mass spectrum graph is used to calculate and determine the isotopes present in an element.

Answer 4

• A mass spectrum is a graph showing relative or percentage abundance on the y-axis and mass: charge ratio on the x-axis.

Question 5

What does m/z mean on the x-axis of a spectra graph?

Answer 5

The mass to charge ratio in all mass spectra is shown as m/z. - m is the mass - z is the charge on the ion.

Question 6

Show how the graph allows you to work out the relative atomic mass of magnesium.

Answer 6

• The heights of the peaks gives the relative abundances of the isotopes in the sample.

The spectrum tells us that:

• 79% is magnesium-24
• 10% is magnesium-25
• 11% is magnesium-26

Relative atomic mass=(24x79)+(25x10)+(26x11)/100 = 24.32

Question 7

What is a molecular ion?

Answer 7

Molecular ion,M⁺ is the positive ion formed in mass spectrometry when a molecule loses an electron.

Question 8

Define fragmentation

Answer 8

Fragmentation is the process in mass spectrometry that causes a positive ion to split into pieces, one of which is a positive fragment ion.

Question 9

What is electron impact?

Answer 9

The molecules being tested are bombarded with electrons. If an electron is dislodged from a molecule, a positive ion is formed. With ethanol, a C₂H₅OH molecule is ionised to form the molecular ion, C₂H₅OH⁺.

Question 10

Write out the equation for ethanol being bombarded with an electron to form an ion(not fragmented).

Answer 10

C₂H₅OH + e^- ——> C₂H₅OH⁺+ 2e^-

Question 11

Write out the equation for the fragmentation of the ethanol molecular ion into CH₃and another product.

Answer 11

C₂H₅OH⁺—–> CH₃ + CH₂OH⁺

Question 12

What can mass spectrometry be used to determine about an unknown compound?

Answer 12

What the compound is!

Question 13

How are molecular ion peaks of two isomers in mass spectroscopy diffirentiated?

Answer 13

• Although the molecular ion peak of two isomers will have the same m/z value, the fragmentation pattern will be different.
• Each organic compound produces a unique mass spectrum, which can be used as a fingerprint for identification.

Question 14

Explain how a spectral database makes the identification of unknown sample spectrums very easy.

Answer 14

When a mass spectrum for an unknown sample is produced, the spectral database is scanned automatically until a perfect match is found. This enables straightforward and immediate identification of an unknown compound.