Mass Analyzers | Mass Spectrometer | Orbitrap | FTICR | QMS | TOF | Dr. Nagendra Singh | PENS#71

Summary

The lecture explains how mass analyzers separate ions by their mass-to-charge ratio ((m/z)) and reviews the operating principles and uses of several common analyzer types.

Where the mass analyzer fits

In a mass spectrometer, a sample enters the instrument, is ionized, and then passes to a mass analyzer. The analyzer separates or filters ions according to their (m/z) values. A detector records the separated ions, producing a mass spectrum. Different analyzers can be paired with different ionization methods.

Two broad analyzer types

• Continuous analyzers process a steady stream of ions and pass selected ions to a detector. Examples include quadrupole and magnetic-sector analyzers.
• Pulsed analyzers receive a packet of ions and resolve the ions from that pulse into a mass spectrum. Examples include time-of-flight (TOF), ion-trap, and ion-cyclotron-resonance analyzers.

Analyzer types and how they work

Magnetic-sector analyzer

• An ion beam enters a magnetic field and curves rather than traveling in a straight line.

• The radius of its path depends on its mass, charge, velocity, and the magnetic-field strength. The lecture gives the relationships: [ r = \frac{mv}{qB} ] and, after accounting for acceleration, [ r = \frac{1}{B}\sqrt{\frac{2mV}{q}}. ]

• A smaller-(m/z) ion is deflected more; a larger-(m/z) ion is deflected less.

• At a fixed magnetic-field strength, only ions with a particular trajectory reach the detector. Varying the field allows ions with different (m/z) values to be detected in turn.

Dual-focusing analyzer

• Combines an electrostatic sector with a magnetic sector.
• The electrostatic sector focuses ions according to differences in kinetic energy; the magnetic sector separates them according to (m/z).
• The lecture identifies Nier–Johnson and Mattauch–Herzog geometries and notes that dual-focusing instruments provide better resolution than a magnetic sector alone.

Quadrupole mass filter

• Uses four parallel rods with direct-current and radio-frequency electric fields.
• The fields are adjusted so that ions with a selected (m/z) have stable paths through the rods and reach the detector; other ions are filtered out.
• The lecture characterizes quadrupoles as economical and easy to connect to different input systems, but lower in resolution than dual-focusing instruments.

Triple quadrupole (Q1–Q2–Q3)

• Q1 selects ions by (m/z).
• Q2 acts as a collision-induced dissociation cell: inert gas, such as nitrogen or argon, collides with the selected ions and fragments them.
• Q3 analyzes the resulting fragment ions.

This tandem-MS arrangement is used to obtain structural information, including for peptide sequencing and nucleotide analysis.

Time-of-flight (TOF) analyzer

• Ions are accelerated and travel through a flight tube to a detector. Their travel times are used to determine (m/z).

• The lecture relates ion kinetic energy to the accelerating voltage and gives: [ t = d\sqrt{\frac{m}{2zeV}}, ] so, for a known tube length and voltage, flight time can be converted to (m/z).

• Lower-(m/z) ions travel faster and arrive sooner; higher-(m/z) ions travel more slowly and arrive later.

• The lecture highlights TOF’s speed and sensitivity, and its ability to analyze large proteins.
• Linear TOF detects ions at the far end of the tube.
• Reflectron TOF uses an ion mirror to compensate for differences in kinetic energy among ions with the same (m/z). This brings their arrival times closer together and improves resolution compared with linear TOF.

FT-ICR (Fourier transform ion cyclotron resonance)

• Ions move in circular orbits inside a cell held in a strong magnetic field. Their cyclotron frequency depends on (m/z).
• Radio-frequency excitation can bring ions into resonance and enlarge their orbits so they induce a measurable image current at detector plates.
• The resulting time-based signal, or free induction decay (FID), is a combination of oscillations from the ions present. A Fourier transform converts it to frequency data, which is calibrated to produce a mass spectrum.
• The lecturer describes FT-ICR as offering exceptionally high resolution and separation efficiency, while also being larger and more expensive than quadrupole, magnetic-sector, and TOF instruments.

Orbitrap

• Uses a spindle-shaped central electrode surrounded by cup-shaped electrodes; unlike FT-ICR, it does not use a magnetic field.
• Ions oscillate around the central electrode. Their axial oscillation frequency is measured through an induced image current at the outer electrodes.
• As with FT-ICR, a time-based signal is Fourier transformed and converted into a mass spectrum.
• The lecture gives the frequency relationship as proportional to (\sqrt{K/(m/z)}), where (K) is an instrument-dependent constant, and notes that resolving power decreases more slowly as (m/z) increases than it does for FT-ICR.

Overall comparison

The lecture presents quadrupoles as the least expensive option, while FT-ICR and Orbitrap are advanced analyzer types that measure ion-induced signals as well as distinguish ions by (m/z). FT-ICR is described as particularly high in resolution, but also costly and relatively large. The analyzer choice depends on factors such as resolution, sensitivity, speed, cost, and the type of ions or structural information being sought.

Speakers and sources featured

• Dr. Nagendra Singh — lecturer, as identified in the video title.
• No other speakers or sources are featured in the subtitles.