1. MASS SPECTROMETRY AP Chemistry Big Idea 1
Allows for detection of atomic weights
Supplementing slide 22 from chpt 2 powerpoint

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. 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. Particles with a large m/z value are deflected least
MASS SPECTROMETRY
In a mass spectrometer, particles are turned into positive ions, accelerated and then deflected by an electric or magnetic field. The resulting path of ions depends on their ‘mass to charge’ ratio (m/z).
Particles with a large m/z value are deflected least
Particles with a low m/z value are deflected most.

5. PARTICLES MUST BE IONIZED SO THEY CAN BE ACCELERATED AND DEFLECTED
A MASS SPECTROMETER
DETECTOR
ION SOURCE
ANALYSER
A mass spectrometer consists of ... an ion source, an analyser and a detector.
PARTICLES MUST BE IONIZED SO THEY CAN BE ACCELERATED AND DEFLECTED

6. HOW DOES IT WORK? Step 1: IONIZATION
DETECTOR
ION SOURCE
ANALYSER
Step 1: IONIZATION
gaseous atoms are bombarded by electrons from an electron gun and are IONIZED
sufficient energy is given to form ions of 1+ charge

7. HOW DOES IT WORK? Step 1: IONIZATION Step 2: ACCELERATION
DETECTOR
ION SOURCE
ANALYSER
Step 1: IONIZATION
Step 2: ACCELERATION
ions are charged so can be ACCELERATED by an electric field
Some type of voltage

8. HOW DOES IT WORK? Step 1: IONIZATION Step 2: ACCELERATION
DETECTOR
ION SOURCE
ANALYSER
Step 1: IONIZATION
Step 2: ACCELERATION
Step 3: DEFLECTION
charged particles will be DEFLECTED by a magnetic or electric field
Usually passed through an evacuated tube where they come in contact with electric or magnetic field
The magnetic field causes the ions to be deflected by different amounts depending on their mass. The ions are then detected and shown in a graph called a mass spectrum.

9. HOW DOES IT WORK? Step 1: IONIZATION Step 2: ACCELERATION
DETECTOR
ION SOURCE
ANALYSER
Step 1: IONIZATION
Step 2: ACCELERATION
Step 3: DEFLECTION
Step 4: DETECTION
by electric or photographic methods

10. HOW DOES IT WORK? IONIZATION ACCELERATION DEFLECTION DETECTION
DETECTOR
ION SOURCE
ANALYSER
IONIZATION
ACCELERATION
DEFLECTION
DETECTION

11. HOW DOES IT WORK? - Deflection
20Ne
21Ne
22Ne
HEAVIER ISOTOPES ARE DEFLECTED LESS
the radius of the path depends on the value of the mass/charge ratio (m/z)
ions of heavier isotopes have larger m/z values so follow a larger radius curve
as most ions are 1+charged, the amount of separation depends on their mass

12. HOW DOES IT WORK? - Deflection
20Ne
m/z values
ABUNDANCE
1+ ions
2+ ions
20Ne
22Ne
Doubling the charge, halves the m/z value
Abundance stays the same
21Ne
22Ne
HEAVIER ISOTOPES ARE DEFLECTED LESS
if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved

13. WHAT IS A MASS SPECTRUM? MASS SPECTRUM OF NEON
20Ne %
21Ne 0.26%
22Ne 8.82%
MASS SPECTRUM OF NEON
In early research with a mass spectrograph, Aston (Nobel Prize, 1922) demonstrated that naturally occurring neon consisted of three isotopes Ne, 21Ne and 22Ne.
positions of the peaks gives atomic mass
peak intensity gives the relative abundance
highest abundance is scaled to 100% and other values are adjusted accordingly

14. CALCULATING AVERAGE ATOMIC MASS
Calculate the average relative atomic mass of neon using data on the previous page.
Out of every 100 atoms are 20Ne , are 21Ne and are 22Ne
Answer: 20.18 A
TIP In calculations of this type... multiply each relative mass by its abundance
add up the total of these values
divide the result by the sum of the abundances;
(100 in this case)
* if the question is based on percentage abundance, divide by 100 but if
it is based on heights of lines in a mass spectrum, add up the heights
of the lines and then divide by that number (see later).

15. CALCULATING AVERAGE ATOMIC MASS
Naturally occurring potassium consists of potassium-39 and potassium-41. Calculate the percentage of each isotope present if the average is 39.1.
Assume that there are n nuclei of 39K in every 100;
there will then be (100-n) of 41K. So
Answer: % 39K and 5% 41K

16. Closest Element: Zirconium (Zr, Z=40)
IDENTIFY THE ELEMENT
First, calculate average atomic mass
Then find the element with the closest mass on periodic table
Average Atomic Mass = 91.32
Closest Element: Zirconium (Zr, Z=40)