Mass Analyzers

Mass Analyzer

• central component within the mass spec instrument that separates ions based on their m/z ratios

• Separates, filters, or traps ions based on their specific values using electric or magnetic fields

• Defines the resolution, mass range, and sensitivity of the mass spectrometer

Time of Flight

• a mass analyzer that separates ions based on how long they take to travel a fixed distance

• often paired with ion sources like MALDI (MALDI-TOF) or ESI

• all ions are given the same kinetic energy, so their speed depends on m/z ratio (e.g. ping pong vs bowling ball)

  • lower m/z (lighter ions) → move faster

  • higher m/z (heavier ions) → move slower

• done in a vacuum to prevent ions from colliding w/ gas molecules

  • collisions w/ gas molecules would sow ions down and change their flight time

Time of Flight: How it works

• ions are generated (ion source) and accelerated by an electric field

• ions travel thru a flight tube toward a detector

• a plate detects when ions hit it

• the instrument records the exact time ions are accelerated and the time they hit the detector, which gives TOF

  • ions arriving at the same time = same m/z

Time of Flight Requirements for Accuracy

• distance must be known

  • if flight path is longer, small differences in ion speed result in longer differences in arrival time → improves accuracy

• time measurement must be prescise

• operates under vacuum

• temperature control is important (prevents expansion affecting distance)

TOF Ion Mirror

• a region with increasing (+) electric potential

• acts like a mirror that slows, stops and reverses ions

• ions enter the reflectron w/ diff kinetic energies

  • faster ions penetrate deeper into the electric field; travel a longer path before turning around

  • slower ions penetrate less deeply; travel a shorter path

• ions w/ the same m/z but diff speeds would normally arrive at different times but are corrected to arrive closer together (signal is narrower)

  • e.g. ions starting at slightly diff positions in the source and the electric field isn’t perfectly uniform

TOF Ion Mirror

Quadropole Mass Analyzer

• mass analyzer that acts as a mass filter

• consists of 4 rods w/ applied voltages and opposite rods are paired:

  • two rods: + (U+RF voltage)

  • two rods: - (U+RF voltage, opposite sign)

• ions travel between the ოთხ rods

• a combination of DC voltage (U), oscillating RF voltage (V cos(ωt)) creates a dynamic electric field (electric field b/w the rods oscillates)

Quadropole Mass Analyzer: Key Principle

• only ions w/ a specific m/z will have a stable path and pass thru

• correct m/z → stable oscillation → passes thru

• incorrect m/z → unstable path → collides w/ rods

Quadropole: Voltages

• DC Voltage (U): a constant voltage, sets a steady electric field bias between opposite rods

  • Helps determine overall ion stability region

• RF Voltage (V cos(ωt)): A rapidly oscillating voltage; alternates in time at frequency ω

  • Continuously changes the electric field direction/strength

  • without it, ions would drift and crash into rods

• Combined effect: Rods carry a superposition of DC + RF voltage

  • This creates a time-dependent (dynamic) electric field

Quadropole: Changing the Voltages

• by changing U and V (but keeping a constant ratio), diff m/z values become stable

  • allows the instrument to scan across masses

Why keep ratio constant? Ensures the shape of the stability region stays the same

Quadropole Parameters: a/q

• a/q = 2U/V: determines ion stability in the quadropole

  • Combines: U = DC voltage (constant field contribution) and V = RF voltage amplitude (oscillating field contribution)

• a/q defines which m/z ions have stable trajectories

  • Only ions with the correct balance of DC vs RF influence pass through

Quadropole Parameters: ±Φ₀ (±F₀)

• Refers to the electrical potential on the rods

• Rod pairs are set to opposite values

Trapped Ion Mobility

• a technique that separates ions based on how they move thru a gas under an electric field

• uses a controlled gas environment inside a vacuum gradient (small amount of gas is introduced, but instrument is still under high vacuum)

• ions travel thru this region and repeatedly collide w/ gas molecules

• “trapped”: ions are temporarily held and separated in a mobility region and then released in a more separated and organized way

Trapped Ion Mobility: Key Principle

• ion motion depends on m/z and size/shape

• smaller/lower m/z ions collide more w/ gas and lose more velocity → slower overall

• larger/higher m/z ions have more momentum and are less effected by collisions → move faster

Trapped Ion Mobility + Time of Flight

• Trapped ion mobility has limited resolution

• it spreads out ions based on mobility and then feeds them into TOF mass spec, which improve separation and increases overall resolution

Quadropole Ion Traps

• use radio-frequency (RF) electric fields to create regions were ions can be stably trapped in space depending on their m/z

• instrument gradually changes RF conditions, and ions become unstable at specific m/z thresholds

  • unstable ions are ejected from the trap, they hit the detector and the signal recorded

• stable ions stay trapped

3D Quadropole Ion Trap

• has a ring electrode (middle), and two end-cap electrodes (top and bottom)

• RF voltage are applied to create a 3D trapping field

  • the voltages continuously oscillates in polarity over time

• ions are confined to a single central trapping region

2D Quadropole Ion Trap

• four rods arranged like a quadropole

• walls at both ends (end electrodes)

• RF field trap ions in a line along the center axis

• ions are confined radially, but an move along the axis unless capped

Fourier Transform-Ion Cyclotron Resonance: Key Principle

• ions are trapped in a strong magentic field and forced into circular motion (cyclotron motion)

• the frequency of this motion depends on mass-to-charge ratio (m/z)

• a Fourier Transform (mathematical tool) converts detected signals into a mass spectrum

Fourier Transform-Ion Cyclotron Resonance: Why ions move in a circle

• in a magnetic field, moving charge particles experience a force

• this forces causes ions to move in a circular orbit

• smaller m/z → faster motion

• larger m/z → slower motion

Fourier Transform-Ion Cyclotron Resonance: Ion Excitation

• Ion excitation: process where electrons within an atom or ion absorb energy, causing them to move to higher energy levels without leaving the particle

• ions are excited in FT-ICR using a radiofrequency (RF) pulse

• this increases their orbital radius and synchronizes motion

  • motion frequency depends on m/z

  • creates a stronger signal that can be detected

Fourier Transform-Ion Cyclotron Resonance: Vacuum Conditions

• requires the lowest (most extreme) vacuum among mass analyzers

• prevents ion collisions

• allows ions to orbit for long periods → high resolution

  • the only thing that limits the time period is collisions with gas molecules

  • when ions collide, they lose energy and their motion becomes less coherent → signal decays faster

Fourier Transform-Ion Cyclotron Resonance: Detector Plates

• metal plates placed on either side of ion motion

• detect induced current as ions pass by; their motion causes a change in electric field, which induces a small current in the plates

Cyclotron Frequency Equation

• frequency depends on magnetic field (B): ↑ B => ↑ frequency & mass to charge (m/z): ↑ m/z = ↓ frequency

  • frequency = how fast an ion completes one full circle

• the detector measures a wave (frequency signal), and that frequency is plugged into the equation to determine m/z

FT-ICR Figure

Orbitrap: Core Idea

• mass analyzer

• ions are trapped in an electric field (no magnet)

• they oscillate in space (back and forth along central spindle axis, while also rotating it) at frequency that depends only on m/z

• that motion produces a detectable current signal

• easier to use than FT-ICR b/c no need for strong magnet, and still achieves high resolution via frequency detection

Orbitrap: Structure

• central spindle-shaped electrode with a surrounding outer electrode

• ions are injected into the space between them

• the electrodes are typically oppositely charged

Orbitrap: Signal Generation

• as ions oscillate, they induce a current in the outer electrode

• this produces a time-domain decay signal (called an image current)

• the signal decays b/c ions spread out in phase (exact position of each ion in its oscillation at a given moment) over time and don’t all oscillate perfectly together forever

  • at first, ions are in phase (moving tgt in sync), but over time, tiny differences make them drift apart

• when ions are in phase their signals add up strongly

• when they become out of phase, their signals start cancelling each other out

Orbitrap: Are they high yield?

Yes! The measured frequency depends only on [figure]

• lower m/z → higher frequency

• higher m/z → lower frequency

Orbitrap: What determines resolution in Orbitrap?

• how long the ion signal can be measured before it decays

• longer measurement → more precise frequency → higher resolution

Orbitrap Figures