Mass Spectrometry

Ionization: The sample is bombarded with high-energy electrons, knocking an electron off the molecule to form a positive radical cation: $M + e^- \rightarrow M^{\bullet+} + 2e^-$.

Acceleration: Positive ions are accelerated by an electric field toward a magnetic region, ensuring all ions have the same kinetic energy.

Deflection: A magnetic field exerts a force on the moving ions, causing them to travel in a curved path; the radius of curvature depends on the ion's mass and charge.

Detection: Ions that successfully navigate the curve hit a detector, which records the current produced, proportional to the abundance of that specific ion.

Identifying the Molecular Ion: Locate the peak with the highest $m/z$ value (ignoring small $M+1$ or $M+2$ peaks) to determine the molar mass.

Analyzing Fragmentation: Molecules break at their weakest bonds or to form stable carbocations; identifying these 'pieces' helps reconstruct the original structure.

The Base Peak: The tallest peak in the spectrum is assigned an abundance of 100%; it represents the most stable ion formed during fragmentation.

Isotopic Analysis: The presence of $M+1$ peaks (from $^{13}C$) or $M+2$ peaks (from $^{37}Cl$ or $^{81}Br$) allows for the determination of the elemental composition.

The Rule of 13: If you have the molecular mass, divide by 13 to estimate the number of Carbons and Hydrogens in a hydrocarbon ($C_n H_{n+r}$).

Halogen Signatures: Always check the $M$ and $M+2$ ratio; a $3:1$ ratio indicates one Chlorine atom, while a $1:1$ ratio indicates one Bromine atom.

Common Fragment Losses: Look for specific mass drops from the $M^+$ peak, such as a loss of 18 (water, $H_2O$), 15 (methyl group, $CH_3$), or 29 (ethyl group, $C_2H_5$).

Verification: Ensure that the proposed structure's total mass matches the $M^+$ peak and that all major fragment peaks can be logically derived from that structure.

Neutral Fragments: Students often forget that mass spectrometers only detect positively charged ions; neutral radicals or molecules formed during fragmentation do not appear on the spectrum.

Misidentifying the M Peak: Do not mistake the $M+1$ peak for the molecular ion peak; the $M$ peak is the one representing the most common isotopes (e.g., $^{12}C$, $^1H$).

Charge State: While most ions in simple mass spectrometry are $1+$, an ion with a $2+$ charge will appear at half its actual mass ($m/2$), which can lead to misinterpretation if not considered.