2.1.4 Determining Masses Using Mass Spectroscopy

Question 1

Identify three things that a mass spectrometer can be used to determine.

Answer 1

1. Identify an unknown compound;
2. Find the rel ABUNDANCE of each isotope of an element; (MOST IMPORTANT USE)
3. Structural info about molecules;

Question 2

How does a mass spectrometer determine the mass of a molecule of isotope?

Answer 2

By measuring the mass-to-charge ratio of ions:

1. It causes substances to become positive ions;
2. Separates positive ions according to charge and mass;
3. Computer analyses data in ions, produces mass spectrum;
4. This gives info about abundances of ions present.

Question 3

On a mass spectrum graph, what values are shown on the x and y axis?

Answer 3

1. x: mass to charge ratio - this typically shows ISOTOPIC MASS;
2. y: relative or percentage abundance;

Question 4

Why does the mass to charge along ratio, shown along the x axis of a mass spectrum graph typically show isotopic mass?

Answer 4

m - mass
z - the charge on the ion
m/z is the mass to charge ratio, seeing as CHARGE IS USUALLY 1, m/z is just m, which is isotopic mass, as we are dealing with isotopes.

Question 5

A question shows you a mass spectrometer graph, with m/z along the x axis and % abundance along the y, how do you find the percentage abundances of different isotopes?

Answer 5

1. Find the isotopes in the form
   Element-no shown along x axis
2. Find their rel abundances, these could be percentages, or relative, in which case BE CAREFUL OF THE TOTAL
3. If there is no value shown along the y axis, work this out from the fraction of their heights.

Question 6

How should one find the relative abundances in three different scenarios from a mass spec graph?

Answer 6

1. Scenario a:
   Read the percentage value from the y axis
2. Scenario b:
   Values along the y axis are not shown as percentages, work out the rel abundance by a ratio
3. Scenario c:
   There are no values shown along the y axis: measure them using a ruler and use ratio method;

Question 7

What is the equation that one would use if there were no values along the y axis?

Answer 7

% Abundance of isotope A = (height / total height of all of the others)*100%

Question 8

How should one find the relative atomic mass of an isotope from a mass spectrum of the element?

Answer 8

RAM= (%abundance in dec form * RAM of isotope A on x axis) + (%abundance in dec form * RAM of isotope B on x axis)

Do this sum for each isotope.

Question 9

What is the thought process of a chemist when they see a mass spec graph?

Answer 9

1. Find no of peaks in graph, this is the no of isotopes that make up that element;
2. Find the masses of these elements along the x axis;
3. The peaks give the relative abundances of the isotopes in the sample.