Mass spectrometry

Mass Spectrometry

Introduction to Mass Spectrometry

Mass spectrometry is an analytical technique used to determine the molecular mass of compounds and the
relative atomic mass of atoms based on isotope abundance. The instrument has several key parts:

Electron Gun: Uses high-energy electrons to bombard the sample.
Ion Accelerator: Uses positively charged electrodes to accelerate positive ions.
Magnet: Deflects ions.
Detector: Detects ions and sends data to a computer.


The Process of Mass Spectrometry

1. Vaporization: The sample is injected into the instrument and heated to become a vapor.
2. Ionization: The vaporized sample is bombarded by high-energy electrons from the electron gun. This
removes electrons from the sample's atoms, forming positive ions. This process is called ionization.
Atom + e^- → Ion⁺ + 2e^-
3. Acceleration: The newly formed positive ions are passed through an electric field created by
positively charged electrodes. This field accelerates the ions, causing them to move faster.
4. Deflection: The accelerated ions pass through a strong magnetic field. The magnetic field deflects the
ions.
Lighter ions are deflected more than heavier ions.
This separation occurs because ions with different masses experience different forces in the
magnetic field.
5. Detection: The deflected ions hit a detector, which is connected to a computer. The computer
records the data and generates a mass spectrum.


Interpreting Mass Spectra

A mass spectrum is a graph showing the relative abundance of ions versus their mass-to-charge ratio
(m/z ). In most cases relevant to this syllabus, the ions have a charge of +1, so the m/z value is effectively
the mass of the ion.


Analyzing Compounds

When analyzing a compound, a mass spectrometer can determine:



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, Molecular Mass: The peak with the highest m/z value corresponds to the intact molecule that has
been ionized. This is called the molecular ion or parent ion.
Fragment Masses: Other peaks in the spectrum represent fragments of the original molecule that
have broken apart during the process. These fragments provide information about the molecule's
structure.

Example: Pentane (C5 H12 ) ​ ​




The molecular formula for pentane is C5 H12 . ​ ​




The molecular mass of pentane is calculated as: (5 × atomic mass of C) + (12 × atomic mass of H) = (5
× 12.0) + (12 × 1.0) = 60 + 12 = 72.
= 72 represents the molecular ion.
In the mass spectrum of pentane, the peak at m/z
Other peaks, such as those at m/z = 57, m/z = 43, and m/z = 29, represent fragment ions
formed by the breaking of bonds within the pentane molecule.
The base peak is the peak with the highest relative abundance, indicating the most stable or most
frequently formed fragment ion. In the pentane spectrum, the base peak is at m/z = 43.

Example: Ethanol (C2 H5 O H )
​ ​




The molecular formula for ethanol is C2 H5 OH .
​ ​




The molecular mass of ethanol is calculated as: (2 × 12.0) + (6 × 1.0) + (1 × 16.0) = 24 + 6 + 16 = 46.
In the mass spectrum of ethanol, the peak at m/z = 46 represents the molecular ion.
Fragmentation can lead to ions like:
m/z = 45 (loss of H)
m/z = 31 (loss of CH3 ) - This is often the base peak for ethanol.
​




m/z = 29 (loss of OH)
m/z = 28 (loss of OH and H)

Analyzing Elements (Isotopes)

When analyzing an element, a mass spectrometer identifies its isotopes and their relative abundances.

Isotopes: Atoms of the same element with the same number of protons but different numbers of
neutrons, resulting in different masses.
The mass spectrum of an element will show peaks corresponding to the masses of its isotopes. The
height of each peak indicates the relative abundance of that isotope.

Example: Chlorine (Cl)

Chlorine has two main isotopes: $^{35}Cland^{37}$Cl.
A mass spectrum of chlorine would show two peaks: one at m/z = 35 and another at m/z = 37.



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