Mass Spectrometry
was used to identify the presence of isotopes
which modifies Dalton’s Atomic Theory
Mass Spectrometer
Is the ion formed positive or negative?
What does m/z mean and why is it
important that z = 1?
How does mass affect the angle of deflection? (show on the diagram which dotted line
represents the largest mass and which represents the lowest mass ion, then explain below)
http://www.bozemanscience.com/ap-chem-009-mass-spectrometry/
Image # / Step / What Happens1 / Inject and vaporize / Molecules/ atoms separate from each other
2 / Ionize / Fast moving electrons hit the atoms and cause an electron to come off
3 / Accelerate
4 / Deflection / Magnetic field – heavier isotopes get deflected less than lighter isotopes.
5 / Detection / Counts how many of each mass come through
Interpreting Mass Spectra
E g. the mass spectrum of rubidium Rb.
Height of peak
m/z 85 87
Notes:
1. Each peak represents a different isotope of the element being analyzed.
2. The height of each peak is proportional to the amount of each isotope present (i.e. it’s relative abundance).
3. The m/z ratio for each peak is found from the accelerating voltage for each peak. Many ions have a +1 charge so that the m/z ratio is numerically equal to mass m of the ion. That will be true for all the isotopes we will analyze.
Exercise 1
Refer to the diagram of the mass spectrum of rubidium previously to answer this question.
(a) Describe the two isotopes of rubidium using isotopic symbols.
8737Rb; 8537Rb
(b) What information can you get from the heights of the peaks on the mass spectrum?
The height of each peak is proportional to the relative abundance of the isotope it represents. In this case 8537Rb is more than twice as abundant as 8737Rb
Calculating the relative atomic mass of an element
1. Measure the height of each peak.
85 Rb =
87 Rb =
2. Calculate the percentage relative abundance
% abundance = amount of isotope x 100
total amount of all isotopes
% 85 Rb =
% 87 Rb =
3. Calculate the Average Atomic Mass of Rubidium:
Exercise 2
Use the mass spectrum shown below to calculate:
(a) the percentage of each isotope present in a sample of naturally occurring lithium;
(b) the average atomic mass of lithium.
3 4 5 6 7 8
mass/charge ratio
Exercise 3
The mass spectrum of neon consists of three lines corresponding to mass/charge ratios of 20, 21 and 22 with relative intensities of 0.910; 0.0026; 0.088 respectively. Calculate the percent abundance of each isotope and then calculate the average atomic mass of neon.
Exercise 4
How many isotopes exist for this element?
What is the percent abundance of each isotope?
Calculate the average atomic mass and identify this element.
Exercise 5
How many isotopes exist for this element?
What is the percent abundance of each isotope?
Calculate the average atomic mass and identify this element.
Exercise 6
There are two isotopes of silver, with masses of 107 (106.905097) and 109 amu (108.904752). Use the average atomic mass off the periodic table to determine the percent abundance of each isotope.
Exercise 7
Gallium has two naturally occurring isotopes. Ga-69 (68.9255736 amu) and Ga -71 (70.925 amu). Determine the percent abundance of each isotope.