Mass Spectrometry: Principles and Interpretation

Mass spectrometry (MS) is a powerful analytical technique used to identify and quantify the different molecules present in a sample. It works by measuring the mass-to-charge ratio (m/z) of ions. This allows for the determination of the elemental composition and molecular weight of substances.

Principles of Mass Spectrometry

Ionization

The first step in mass spectrometry is the ionization of the sample molecules. This process converts neutral molecules into ions, typically by removing electrons.

Common ionization methods include:

• Electron Ionization (EI): The sample is bombarded with high-energy electrons, causing ionization. This is a "hard" ionization technique, often leading to extensive fragmentation.
• Chemical Ionization (CI): The sample reacts with reagent ions (e.g., CH₅⁺) which then ionize the analyte molecules through proton transfer. This is a "softer" ionization technique, producing less fragmentation.
• Electrospray Ionization (ESI): The sample is dissolved in a solvent and sprayed through a charged capillary. This creates highly charged droplets that evaporate, leading to the formation of ions. Suitable for large biomolecules.
• Matrix-Assisted Laser Desorption/Ionization (MALDI): The sample is mixed with a matrix and then irradiated with a laser. The matrix absorbs the laser energy, causing the sample molecules to be desorbed and ionized.

Mass Analysis

Once the molecules are ionized, the ions are accelerated and separated based on their mass-to-charge ratio (m/z). This is achieved using different types of mass analyzers.

1. Magnetic Sector Analyzers: Ions are accelerated through a magnetic field, causing them to follow a curved path. The radius of the curve depends on the m/z ratio.
2. Quadrupole Analyzers: Four parallel rods with oscillating electrical fields are used to filter ions based on their m/z ratio.
3. Time-of-Flight (TOF) Analyzers: Ions are accelerated through a flight tube of a known length. The time it takes for the ions to reach the detector depends on their velocity, which is related to their m/z ratio.
4. Ion Trap Analyzers: Ions are trapped in a three-dimensional electric field and their m/z ratios are determined by manipulating the field.

Detection

The separated ions are detected by measuring their abundance. The detector generates an electrical signal proportional to the number of ions hitting it. This signal is then processed to produce a mass spectrum.

Mass Spectrum Interpretation

A mass spectrum is a plot of ion abundance versus m/z ratio. It provides information about the molecular weight of the sample and its elemental composition.

Key features of a mass spectrum include:

• Molecular Ion Peak (M⁺): Represents the molecular weight of the compound.
• Isotope Peaks: Due to the natural abundance of different isotopes of elements, there are peaks corresponding to the different isotopes of a given mass.
• Fragment Ions: Peaks at m/z values that are different from the molecular weight, resulting from the fragmentation of the molecule during ionization.

m/z	Ion	Abundance
M+	Molecular Ion
M - 1	Loss of an atom
M - 18	Loss of a water molecule
Isotope peaks (e.g., 13C)	Ions containing isotopes

Fragmentation Patterns: The pattern of fragment ions can provide valuable information about the structure of the molecule. Characteristic fragmentation patterns are often associated with specific functional groups.

Applications of Mass Spectrometry

Mass spectrometry has a wide range of applications, including:

• Identifying unknown compounds.
• Determining the purity of a compound.
• Quantitative analysis of compounds in a mixture.
• Proteomics (studying proteins).
• Drug discovery and development.
• Environmental monitoring.