2.7.1 Principle of Mass Spectrometry

This topic explains the fundamental principle behind mass spectrometry — how it works and what information it provides.

Definition

Mass Spectrometry: An analytical technique used to determine the molecular composition and structure of a sample by measuring the mass-to-charge ratio of ions.

Principle of Mass Spectrometry: The sample is ionized, the ions are separated based on their mass-to-charge ratio (mz), and then detected to generate a mass spectrum.

Main Idea

Mass spectrometry involves three fundamental steps: ionization, mass analysis, and detection. The sample is first ionized, converting its molecules or atoms into charged ions. These ions are then separated based on their mass-to-charge ratio (mz) and detected to generate a mass spectrum. The mass spectrum provides information about the composition and abundance of the sample's molecules or the isotopic composition of an element.

Core Concept

The mass spectrometer separates ions based on their mass-to-charge ratio (mz). The resulting mass spectrum shows peaks at different mz values, with the height of each peak representing the relative abundance of that ion.

Detailed Explanation

The Three Fundamental Steps

• The sample is converted into charged ions.
• This is necessary because the mass analyzer can only manipulate charged particles.
• Methods include:
• EI (Electron Ionization): Bombarding the sample with high-energy electrons.
• ESI (Electrospray Ionization): Introducing the sample to a high-voltage field.
• MALDI (Matrix-Assisted Laser Desorption/Ionization): Using a laser to vaporize the sample.

• The ions are separated based on their **mass-to-charge ratio (mz)**.
• Ions with different masses are focused and detected at different times.
• The mass analyzer is the component that performs this separation.

• The separated ions hit the detector.
• The detector records the time it takes for each ion to reach it.
• This data is used to calculate the **mz ratio** of each ion.

What Information Does a Mass Spectrum Provide?

The mass spectrum provides information about:

• The mz values tell us the masses of the ions.
• The peak heights tell us the relative abundance of each ion.

• Different isotopes of an element have different masses.
• The mass spectrum shows peaks for each isotope.
• The relative heights of these peaks give the relative abundance of each isotope.

How the Mass Spectrum is Constructed

• The **x-axis** of the spectrum represents the **mz ratio**.
• The **y-axis** represents the **abundance** of ions at each mz ratio.
• Peaks in the spectrum correspond to different ions present in the sample.
• The position of each peak reveals the mass of that ion.
• The height of each peak reveals the relative abundance of that ion.

Example: Chlorine Isotopes

• **Chlorine-35** (mz = 35)
• **Chlorine-37** (mz = 37)

• One at mz = 35 (height = 75.76%)
• One at mz = 37 (height = 24.24%)

• Chlorine-35 is more abundant (75.76%).
• Chlorine-37 is less abundant (24.24%).

From this data, we can calculate the average atomic mass of chlorine: Average atomic mass = (35 × 75.76) + (37 × 24.24)100 = 35.48 amu

Intermediate Level

Principle

Mass spectrometry involves three fundamental steps:

What the Mass Spectrum Shows

• **x-axis:** mz ratio
• y-axis: Relative abundance
• Peaks: Correspond to different ions
• Position of peak: Mass of the ion
• Height of peak: Relative abundance of the ion

Information Provided

• The mz values tell us the masses of the ions.
• The peak heights tell us the relative abundance of each ion.

• Different isotopes have different masses.
• The mass spectrum shows peaks for each isotope.
• The relative heights give the relative abundance.

Example: Chlorine

Average atomic mass = (35 × 75.76) + (37 × 24.24)100 = 35.48 amu

Important Points

• 1. Mass spectrometry measures the mass-to-charge ratio (mz) of ions.
• 2. Three steps: Ionization, mass analysis, detection.
• 3. Mass spectrum: x-axis = mz, y-axis = abundance.
• 4. Applications: Isotope abundance, unknown compounds, molecular structures, biomolecules.
• 5. Average atomic mass = weighted average of isotope masses.
• 6. Example: Chlorine → 35.48 amu.

Remember

• - **Mass spectrometry:** mz ratio
• - Steps: Ionization → Mass analysis → Detection
• - **Mass spectrum:** x-axis = mz, y-axis = abundance
• - Average atomic mass: ∑ (Mass No. × Relative Abundance)100
• - Examples: Chlorine (35.48 amu)

Common Mistakes

• 1. Forgetting to divide by 100: Relative abundance is in percentage.
• 2. Mixing up mz ratio with mass: mz is mass-to-charge ratio.
• 3. Ignoring the charge: Ions with different charges have different mz ratios.
• 4. Confusing mass spectrum peaks with isotopes: Each peak corresponds to a specific ion.

Quick Revision

• Mass spectrometry measures the mass-to-charge ratio (mz) of ions.
• Three steps: Ionization, mass analysis, detection.
• Mass spectrum: x-axis = mz, y-axis = abundance.
• Applications: Isotope abundance, unknown compounds, molecular structures, biomolecules.
• Average atomic mass = weighted average of isotope masses.
• Example: Chlorine → 35.48 amu.

60-Second Revision

• **Mass spectrometry:** mz ratio
• Steps: Ionization → Mass analysis → Detection
• **Mass spectrum:** x-axis = mz, y-axis = abundance
• Average atomic mass: ∑ (Mass No. × Relative Abundance)100
• Examples: Chlorine (35.48 amu)

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