Mass Spectrometry

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

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

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

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

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

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

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

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

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.

A MASS SPECTRUM is a graph of Relative Abundace vs. Mass 19 20 21 22 23 20Ne 90.92% 21Ne 0.26% 22Ne 8.82% MASS SPECTRUM OF NEON Mass (amu)

positions of the peaks give atomic masses of isotopes peak intensity (height) gives relative abundance Sum of RA of all isotopes = 100% 19 20 21 22 23 20Ne 90.92% 21Ne 0.26% 22Ne 8.82%

CALCULATING AVERAGE ATOMIC MASS 19 20 21 22 23 20Ne 90.92% 21Ne 0.26% 22Ne 8.82% Multiply each mass by RA as a decimal; add together; round to 0.1 20 x 0.9092 + 21 x 0.0026 +(22 x 0.0882) Answer: 20.2 amu

Practice Problem 39K = 95% 41K = 5% 39K = x 41K = 1 - x Naturally occurring potassium consists of potassium-39 and potassium-41. 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 + 41 – 41x = 39.1 - 2x = - 1.9 x = 0.95 1 – x = 0.05 39K = 95% 41K = 5%

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

1. Number of isotopes: 5

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

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

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

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

Questions?