# Mass Spectrometry Chapter 2 Pg 48

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Mass Spectrometry Chapter 2 Pg 48

Isotopes Atoms of the same element (same number of protons) that differ in the number of neutrons. Ex: Neon-20, Neon-21, Neon-22 Atomic Mass- average mass of all the isotopes But it’s not a normal average!!!

Mass Spectrometer The most direct and accurate means for determining atomic and molecular weights. Use the gas form of the element or compound.

Basic Idea Suppose you had a cannonball travelling past you and you wanted to deflect it as it went by you. All you've got is a jet of water from a hose- pipe that you can squirt at it. Frankly, its not going to make a lot of difference! Because the cannonball is so heavy, it will hardly be deflected at all from its original course.

Basic Idea But suppose instead, you tried to deflect a table tennis ball travelling at the same speed as the cannonball using the same jet of water. Because this ball is so light, you will get a huge deflection. The same idea can be applied to atomic sized particles. The lighter the atom, the more it’s deflected and so on.

An outline of what happens in a mass spectrometer
Stage 1: Ionisation The atom is ionised by knocking one or more electrons off to give a positive ion. Mass spectrometers always work with positive ions.

An outline of what happens in a mass spectrometer
Stage 2: Acceleration The ions are accelerated so that they all have the same kinetic energy.

An outline of what happens in a mass spectrometer
Stage 3: Deflection The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.

An outline of what happens in a mass spectrometer
Stage 4: Detection The beam of ions passing through the machine is detected electrically.

Mass Spec

Mass Spectrometer

What does the output look like?
Relative Abundance for Molybdenum (of the different isotopes)

Relative Abundance of Molybdenum
As you will see from the diagram, the most common ion has a mass/charge ratio of 98. That means that molybdenum consists of 7 different isotopes.

Mass Spec of Zirconium

The Number of Isotopes The 5 peaks in the mass spectrum shows that there are 5 isotopes of zirconium - with relative isotopic masses of 90, 91, 92, 94 and 96 on the 12C scale.

The Abundance of the Isotopes
This time, the relative abundances are given as percentages. zirconium-90 (51.5%), zirconium- 91 (11.2%), zirconium-92 (17.1 %)zirconium-94 (17.4%), zirconium- 96 (2.8%)

How to Calculate! To calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products. The total mass of these 100 typical atoms would be (0.515 x 90) + (0.112 x 91) + (0.171 x 92) + ( x 94) + (0.028 x 96) = 91.3

Practice Naturally occurring magnesium has the following isotopic abundances: What is the average atomic mass of Magnesium? Sketch the mass spectrum of Mg. Isotope Abundance Atomic Mass (amu) Magnsium-24 78.99% Magnesium-25 10.00% Magnesium-26 11.01%

Did you know that mass spectrometry is used to...
Detect and identify the use of steroids in athletes Monitor the breath of patients by anesthesiologists during surgery Determine the composition of molecular species found in space Determine whether honey is adulterated with corn syrup Locate oil deposits by measuring petroleum precursors in rock Monitor fermentation processes for the biotechnology industry Detect dioxins in contaminated fish Determine gene damage from environmental causes Establish the elemental composition of semiconductor materials