Mass Spectrometry

Electric Field Acceleration: Once formed, the positive ions are accelerated through an electric field to ensure they all possess the same kinetic energy before entering the analyzer.

Magnetic Deflection: The ions pass through a magnetic field where they are deflected into curved paths; the degree of deflection is inversely proportional to the mass-to-charge ratio ($m/z$).

Trajectory Factors: Lighter ions (lower mass) or ions with higher positive charges (e.g., $2+$) experience greater deflection than heavier or less charged ions.

Detection: A detector records the intensity of the ion beam at specific $m/z$ values, which is then translated into a mass spectrum showing relative abundance.

Fragmentation Mechanism: The high energy of the initial electron bombardment often causes the molecular ion to break apart into smaller fragments, including smaller positive ions, neutral molecules, and radicals.

Detection Limitation: It is critical to note that the mass spectrometer only detects positively charged ions; neutral radicals and molecules do not interact with the magnetic field and are lost.

The Base Peak: The tallest peak in the spectrum is called the base peak, representing the most stable and abundant fragment ion, and is assigned a relative intensity of $100$.

Structural Fingerprinting: Specific mass losses (e.g., a loss of $18$ for $H_2O$ or $15$ for a $CH_3$ group) allow chemists to deduce the arrangement of atoms within the original molecule.

Carbon-13 Influence: The $[M+1]$ peak is a small peak found one mass unit higher than the main molecular ion peak, caused by the natural $1.1\%$ abundance of the $^{13}C$ isotope.

Predicting Carbon Count: The height of the $[M+1]$ peak relative to the $M$ peak increases as the number of carbon atoms in the molecule increases, providing a diagnostic tool for molecular formula determination.

Halogen Isotopes: Elements like Chlorine ($^{35}Cl$ and $^{37}Cl$) and Bromine ($^{79}Br$ and $^{81}Br$) produce distinct patterns (e.g., $3:1$ or $1:1$ peak ratios) due to their significant natural isotope distributions.

Identifying the M Peak: Always look for the furthest right major peak to find the molecular mass, but be careful not to mistake the tiny $[M+1]$ peak for the actual molecular ion.

Charge Awareness: Remember that $m/z$ is a ratio; if an ion has a $2+$ charge, its peak will appear at half its actual mass (e.g., a mass of $56$ with $2+$ charge appears at $m/z = 28$).

Fragment Logic: When analyzing fragments, calculate the 'mass loss' from the molecular ion (e.g., $M - 15$ suggests the loss of a methyl group) rather than just trying to identify the fragment itself.

Radical Exclusion: Never draw a radical (e.g., $\cdot CH_3$) as the species responsible for a peak; only the charged species (e.g., $[CH_3]^+$) can be detected.