8.3 Mass analyzers (quadrupole, time-of-flight, ion trap)

Quadrupole, time-of-flight, and ion trap mass analyzers are key players in mass spectrometry. Each has unique strengths in separating and detecting ions based on their mass-to-charge ratios, offering different levels of resolution, sensitivity, and speed.

These analyzers form the backbone of modern mass spectrometry techniques. Understanding their principles and applications is crucial for selecting the right tool for various analytical challenges, from small molecule analysis to complex protein studies.

Principles of quadrupole mass analyzers

Quadrupole configuration and electric field

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• Quadrupole mass analyzers consist of four parallel cylindrical rods arranged in a square configuration
• Opposite rods are electrically connected in pairs
• A combination of direct current (DC) and radio frequency (RF) voltages is applied to the rod pairs
• The applied voltages create a quadrupolar electric field within the analyzer

Ion filtering based on mass-to-charge ratio (m/z)

• Ions entering the quadrupole oscillate in the x-y plane due to the electric field
• Only ions with a specific m/z ratio have a stable trajectory and pass through the analyzer to reach the detector
• Ions with unstable trajectories collide with the rods or are ejected from the analyzer
• The applied voltages can be varied to scan across a range of m/z values, allowing selective transmission and detection of ions
• Example: A quadrupole mass analyzer can be set to transmit only ions with an m/z of 500, while filtering out all other ions

Performance characteristics and applications

• Quadrupole mass analyzers offer fast scanning speeds, enabling rapid data acquisition
• They provide good sensitivity, allowing the detection of low-abundance analytes
• Quadrupoles have moderate resolution compared to high-resolution analyzers (FT-ICR, Orbitrap)
• Suitable for a wide range of applications (small molecule analysis, environmental monitoring, drug screening)

Time-of-flight mass analyzer functionality

Principle of ion separation based on velocity

• Time-of-flight (TOF) mass analyzers separate ions based on their velocities as they travel through a field-free drift region
• Ions are accelerated to a fixed kinetic energy by an electric field and then allowed to drift in a vacuum chamber
• The time taken for ions to reach the detector is proportional to their m/z ratio
• Lighter ions with lower m/z values have higher velocities and reach the detector faster than heavier ions with higher m/z values

Advantages of TOF analyzers

• TOF analyzers offer high sensitivity, enabling the detection of low-abundance species
• They provide a wide mass range, allowing the analysis of large molecules (proteins, polymers)
• TOF analyzers have fast acquisition times, making them suitable for high-throughput screening and the analysis of complex mixtures
• Example: MALDI-TOF is commonly used for the rapid identification of proteins in proteomic studies

Resolution enhancement with reflectrons

• The resolution of TOF analyzers can be improved by using reflectrons
• Reflectrons are ion mirrors that correct for small variations in initial ion velocities
• Ions with slightly higher velocities penetrate deeper into the reflectron's electric field and spend more time in the reflectron
• This time correction results in ions with the same m/z reaching the detector simultaneously, improving resolution
• Reflectrons also increase the effective flight path, further enhancing resolution

Ion trap mass analyzer mechanisms

Types of ion traps

• There are two main types of ion traps: quadrupole ion traps (QIT) and linear ion traps (LIT)
• QITs confine ions in a three-dimensional quadrupolar electric field created by applying RF voltages to a ring electrode and two end-cap electrodes
• LITs use a quadrupolar field to confine ions radially and a static electric field to confine them axially

Ion storage and ejection

• In a QIT, ions of a specific m/z ratio can be selectively trapped by applying appropriate RF voltages
• Trapped ions oscillate in stable orbits within the ion trap
• By ramping the RF voltage or applying supplementary AC voltages, ions of a specific m/z can be ejected from the trap for detection
• Example: An ion trap can isolate and fragment a precursor ion, then trap and analyze the resulting product ions (MS/MS)

Advantages and applications

• Ion traps offer high sensitivity due to their ability to accumulate and concentrate ions
• They can perform multiple stages of mass spectrometry (MS/MS, MSn), providing valuable structural information
• Ion traps are capable of studying ion-molecule reactions and performing gas-phase chemistry experiments
• Applications include proteomics, metabolomics, and the analysis of complex organic mixtures

Comparison of mass analyzer types

Resolution and mass accuracy

• Mass analyzers differ in their resolution, which is the ability to distinguish between ions with similar m/z values
• High-resolution analyzers (FT-ICR, Orbitrap) offer the highest resolving power, enabling the separation of ions with very close m/z ratios
• Mass accuracy refers to the closeness of the measured m/z value to the true value
• TOF and FT-ICR analyzers provide high mass accuracy, while quadrupoles and ion traps have lower mass accuracy

Sensitivity and scanning speed

• Sensitivity is the ability to detect low concentrations of analytes
• Triple quadrupole (QqQ) and ion trap analyzers are known for their high sensitivity, making them suitable for trace analysis
• Scanning speed refers to the rate at which the analyzer can acquire mass spectra
• Quadrupoles and TOF analyzers have fast scanning speeds, allowing for rapid data acquisition and high-throughput analyses
• Example: A triple quadrupole analyzer can quickly scan for multiple targeted compounds in a complex matrix, such as pesticides in food samples

Tandem mass spectrometry (MS/MS) capabilities

• MS/MS involves the fragmentation of selected ions to obtain structural information
• Ion traps and QqQ analyzers are commonly used for MS/MS experiments due to their ability to isolate and fragment ions efficiently
• QqQ analyzers are particularly well-suited for quantitative MS/MS analysis using selected reaction monitoring (SRM) or multiple reaction monitoring (MRM)
• Example: A QqQ analyzer can be used to quantify a specific drug compound in a biological sample by monitoring a characteristic precursor-to-product ion transition