Charge and Weight Principles of Mass-to-Charge Ratio Quiz

The m represents the actual mass of the ion in atomic mass units. In mass spectrometry we measure the mass of individual atoms or molecules rather than the bulk molar mass of a substance.

The z represents the net charge of the ion. Most ions produced in a standard mass spectrometer have a charge of plus 1 because they have lost exactly one electron during the ionization process.

The mass to charge ratio is calculated by dividing the mass by the charge. For an ion with a mass of 100 and a charge of 2 the calculation is 100 divided by 2 which equals 50. This demonstrates why the spectrometer measures a ratio rather than mass alone.

Heavier ions with high m z values have more momentum and are harder to move. Conversely lighter ions or those with higher charges are deflected more sharply by the magnetic field. This difference in deflection is what allows the instrument to separate different ions.

The path is a balance between the ions kinetic energy and the magnetic force acting upon it. The magnetic field exerts a force that curves the path based on the ions m z value. The color of the sample does not affect the physical behavior of the ion in a vacuum.

Neutral molecules cannot be accelerated or deflected by electromagnetic fields. By turning molecules into ions the instrument can use voltage to speed them up and magnetism to sort them based on their mass to charge ratio.

The Dalton or unified atomic mass unit is used because we are measuring single particles. One Dalton is defined as one twelfth the mass of a single Carbon 12 atom making it the standard unit for atomic and molecular scales.

It is possible for different ions to share an m z value. For example a molecule with mass 100 and charge plus 1 has an m z of 100 while a molecule with mass 200 and charge plus 2 also has an m z of 100. High resolution instruments are often needed to distinguish between such species.

A stronger magnetic field exerts a greater force on the moving ions causing them to bend more. By scanning the magnetic field strength the instrument can bring ions of different m z ratios into focus on the detector one at a time.

Carbon dioxide propane and nitrous oxide all have molecular weights of approximately 44. If they each lose one electron they will all appear at the same m z 44 position on a low resolution mass spectrum.

The mass analyzer is the heart of the instrument where the separation actually happens. Whether using a magnetic sector or a quadrupole the analyzer uses the physical principles of the m z ratio to filter ions.

In a Time of Flight analyzer all ions are given the same kinetic energy. Because they are lighter ions with lower m z ratios travel at higher velocities and cover the distance to the detector in a shorter amount of time.

A methyl group has a mass of 15 units. If the original molecular ion has a mass of 100 and it breaks to lose 15 units the resulting fragment will have a mass of 85. Assuming the charge remains plus 1 the m z ratio will be 85.

The magnetic force depends on the charge of the ion. The amount of curvature or deflection depends on the mass. Higher momentum ions which are faster or heavier resist the magnetic force more and follow a wider path.

Since z equals 1 the ratio m divided by 1 is simply m. This means for most common analyses the values on the horizontal axis of the spectrum correspond directly to the mass of the ions in atomic mass units.