You find the mass of an ion (and thus infer the mass of the original molecule or fragment) from a mass spectrum by reading the x-axis value corresponding to the peak. Each peak represents an ion with a specific mass-to-charge ratio (m/z). Since mass spectrometers typically measure ions with a charge of +1, the x-axis effectively represents the mass of the ion in atomic mass units (amu or Da).
Understanding Mass Spectrometry and Mass Determination
Mass spectrometry is an analytical technique used to identify and quantify molecules by measuring their mass-to-charge ratio. The process involves ionizing a sample, separating the ions based on their m/z, and detecting the abundance of each ion. The resulting data is displayed as a mass spectrum, which plots the relative abundance of each ion against its m/z value.
Steps to Determine Mass from a Mass Spectrum
-
Identify Peaks: Examine the mass spectrum for distinct peaks. Each peak represents an ion that has reached the detector.
-
Read the m/z Value: For each peak of interest, read the corresponding value on the x-axis. This value represents the mass-to-charge ratio (m/z) of the ion.
-
Determine Mass: Assuming the ions are singly charged (z = 1), the m/z value is effectively equal to the mass (m) of the ion in atomic mass units (amu) or Daltons (Da). If the charge state (z) is different from 1, you need to multiply the m/z value by the charge state to get the mass: m = (m/z) * z.
Identifying Elements and Isotopes
Mass spectrometry is often used to identify elements and their isotopes.
-
Identify the Major Isotope: Find the largest spectral line (the highest peak) on the mass spectrum. This represents the most abundant isotope.
-
Find the Mass of the Major Isotope: Read the corresponding value on the x-axis to determine the mass of the major isotope.
Example
Suppose a mass spectrum shows a prominent peak at m/z = 20. This suggests that an ion with a mass of approximately 20 amu/Da is present in the sample. If the ion is singly charged, its mass is 20 amu/Da. This could correspond to an isotope of neon (20Ne).
Considerations
- Isotopes: Most elements have multiple isotopes, each with a different mass. Mass spectrometry can differentiate between isotopes, resulting in multiple peaks for the same element.
- Molecular Fragmentation: In many mass spectrometry experiments, molecules fragment into smaller ions. This can provide structural information, but it also means that not every peak corresponds to the intact molecule.
- Resolution: The resolution of the mass spectrometer affects the accuracy of mass determination. High-resolution instruments can distinguish between ions with very similar masses.
By carefully analyzing the mass spectrum and understanding the principles of mass spectrometry, you can determine the mass of ions and gain valuable information about the composition and structure of molecules.
