CHEMISTRY 2A 2 MASS SPECTROMETRY AND RELATIVE MASSES OF ATOMS, ISOTOPES AND MOLECULES

What is relative atomic mass?

1. Relative atomic mass is the weighted average mass of an element compared to one-twelfth of the mass of a carbon-12 atom.

What is the relative isotopic mass?

1. It is the mass of isotopes compared to one-twelfth the mass of one carbon atom

What should we note when calculating for relative atomic mass using relative isotopic masses?

1. The relative atomic mass will always be a number somewhere between the relative isotopic masses.

What is a mass spectrometer?

1. It is a machine that measures the masses of atoms and molecules.

Describe lightly how a mass spectrometer works.

1. A mass spectrometer works by produces positive ions that are deflected by a magnetic field according to their mass to charge ratio.

2. It also calculates the relative abundance of each positive ion and displays this as a percentage.

The positive ions could be positively charged ayoms, positively charged molecules or positively charged fragments of molecules.

Go into detail how a mass spectrometer works.

1. The sample must be in a gaseous state so the particles can move through the machine. The sample is injected into the mass spectrometer, where it is vaporized.

2. The vapour is bombarded with electrons, and these electrons collide with the atoms, removing one or more electrons, forming positive ions.

3. An electric field causes the ions to accelerate.

4. The positive ions enter, and are deflected by, a uniform magnetic field. The amount of deflection depends on the mass-to-charge ratio of the ions. The larger the mass to charge ratio, the less the deflection. If all ions have the same charge, usually 1+, the extent of the deflection is inversely proportional to their mass.

5. The deflected ions pass through a narrow slit and are collected on a metallic plate connected to an amplifier. For a given strength of magnetic field, only ions with a certain m/z ratio pass thrugh the slit and are detected. For example, an ion with a mass of 28 and a charge of 1+ will be detected at the same time as an ion with a mass of 56 and a charge of 2+. Both ions have a m/z ratio of 28.

6. The strength of the magnetic field is then changed to detect positive ions with other m/z ratios.

How do we determine relative isotopic and atomic masses using data from a mass spectrometer?

1. Percentage abundance x relative isotopic mass + …

What should we watch out for when anayzing organic compounds in a mass spec.?

1. There is always some carbon-13 isotope, M+1 peak.

2. It get smore significant as the molecule has a larger mass.

Describe the difference between relative atomic mass and relative isotopic mass.

1. Relative atomic mass refers to the average mass of an atom in a mixture of isotopes.

2. Relative isotopic mass refers to the mass of a single isotope.

Explain why the relative atomic masses of many elements are not exact whole numbers.

1. The mass calculated is an average mass of more than one isotope with different masses and the relative abundances of these are rarely whole numbers.