Overview
This lecture explains mass spectrometry using pentane as an example, focusing on how fragments form, their detection, and interpretation of the resulting mass spectrum.
Basics of Mass Spectrometry
- A sample is vaporized and ionized by an electron beam in a mass spectrometer.
- Only positively charged ions (cations) are detected; radicals are not measured.
- The y-axis of a mass spectrum shows the relative abundance of detected fragments; the x-axis shows the mass-to-charge ratio (m/z).
- For singly charged ions (z = 1), the m/z value equals the fragment’s mass.
Fragmentation of Pentane
- Breaking the C1–C2 bond yields a methyl (CH3, mass 15) and a butyl group (mass 57).
- Only the cation fragments (e.g., methyl cation) reach the detector, explaining the peak at m/z = 15.
- The peak at m/z = 57 comes from a butyl cation fragment.
- The parent or M+ peak at m/z = 72 represents pentane’s molecular ion (original molecule minus one electron).
- Breaking the C2–C3 bond yields ethyl cation (m/z = 29) and propyl cation (m/z = 43).
- The base peak at m/z = 43 is usually the most abundant and corresponds to the propyl cation.
Stability and Abundance of Fragments
- Base peak (m/z = 43, propyl cation) is most abundant due to easier formation of stable carbocations after breaking C2–C3 bond.
- Breaking C1–C2 bond gives less stable methyl carbocation; thus, this process is less favored.
Other Notable Peaks
- The peak at m/z = 41 is due to a propyl cation losing two hydrogen atoms, forming an allylic carbocation stabilized by resonance.
- Primary carbocations can undergo hydride shifts to form more stable secondary carbocations, especially notable for the propyl cation.
Key Terms & Definitions
- Mass-to-Charge Ratio (m/z) — The value measured in mass spectrometry, representing the mass of the ion divided by its charge (usually z = 1).
- Base Peak — The most abundant (highest) peak in a mass spectrum.
- Parent (M+) Peak — Peak representing the molecular ion (intact molecule minus one electron).
- Carbocation — A positively charged carbon ion.
- Allylic Carbocation — A carbocation adjacent to a double bond and stabilized by resonance.
Action Items / Next Steps
- Review mass spectra of other hydrocarbons to practice fragment identification.
- Learn carbocation stability rules for predicting abundant fragments.
- Complete any assigned readings on mass spectrometry from the textbook.