1. Mass Spectrometry

2. MASS SPECTROMETRY
The first mass spectrometer was built in 1918 by Francis W Aston, a student of J J Thomson, the man who discovered the electron.
Francis Aston

3. We now call these isotopes.
MASS SPECTROMETRY
Aston used the instrument to show that there were different forms of the same element.
We now call these isotopes.
Francis Aston

4. Label your diagram with these parts:
A MASS SPECTROMETER
DETECTOR
ION SOURCE
ANALYSER
Label your diagram with these parts:
ion source, analyser and detector.

5. gaseous atoms are bombarded by energy so that e̶ are knocked off
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
Step 1: IONIZATION
gaseous atoms are bombarded by energy so that e̶ are knocked off
Ions formed have charge of +1

6. Only charged particles can be ACCELERATED by an electric field
HOW DOES IT WORK?
DETECTOR
ION SOURCE
Step 1: IONIZATION
Step 2: ACCELERATION
Only charged particles can be ACCELERATED
by an electric field
Ions are ACCELERATED so all have same kinetic energy
ANALYSER

7. Ions will be DEFLECTED by a magnetic/electric field
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
Step 1: IONIZATION
Step 3: DEFLECTION
Ions will be DEFLECTED by a magnetic/electric field
Because all ions have the same charge, the degree of deflection depends only on mass

8. HOW DOES IT WORK? - Deflection HEAVIER ISOTOPES ARE DEFLECTED LESS
20Ne
Label your diagram
21Ne
22Ne
HEAVIER ISOTOPES ARE DEFLECTED LESS

9. Differences recorded using electric or photographic methods
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
Step 1: IONIZATION
gaseous atom sufficient energy is given to form ions of 1+ charges are bombarded by electrons from an electron gun and are IONIZED
Step 2: ACCELERATION
Step 4: DETECTION
Differences recorded using electric or photographic methods

10. SUMMARY
In a mass spectrometer, particles are turned into positive ions, accelerated, and then deflected by an electric or magnetic field.
Why do we care?
Graphs of mass spec data were used to determine the Average Atomic Mass of elements on the Periodic Table
Today, data can be used to solve crimes, etc.

11. A MASS SPECTRUM is a graph of Relative Abundace vs. Mass
20Ne %
21Ne 0.26%
22Ne 8.82%
MASS SPECTRUM OF NEON
Mass (amu)

12. positions of the peaks give atomic masses of isotopes
peak intensity (height) gives relative abundance
Sum of RA of all isotopes = 100%
20Ne %
21Ne 0.26%
22Ne 8.82%

13. CALCULATING AVERAGE ATOMIC MASS
20Ne %
21Ne 0.26%
22Ne 8.82%
Multiply each mass by RA as a decimal; add together; round to 0.1
20 x x (22 x )
Answer: 20.2 amu

14. Practice Problem 39K = 95% 41K = 5% 39K = x 41K = 1 - x
Naturally occurring potassium consists of potassium-39 and potassium Calculate the percent abundance of each isotope present if the average atomic mass is 39.1 amu.
39K = x 41K = 1 - x
(39 (x)) + (41 (1 – x)) = 39.1
39x – 41x = 39.1
- 2x =
x = 0.95
1 – x = 0.05
39K = 95%
41K = 5%

15. Identify the number of isotopes (count the peaks)
Calculate the average atomic mass
Identify the element

16. 1. Number of isotopes: 5

17. Average Atomic Mass = 91.3 amu
2. Calculate the average atomic mass
Average Atomic Mass = 91.3 amu

18. Closest Element: Zirconium (Zr, Z=40)
3. Identify the element (using PT)
Closest Element: Zirconium (Zr, Z=40)

19. Which isotope is most abundant?
Write its symbol.
40 90 Zr

20. Which isotope would be deflected the least?
Write its symbol
40 96 Zr

21. Questions?