PHAR 307 - Lecture 12: Mass analyzers

sensitivity

lowest concentration of analyte detected

range

what range of m/z values can the analyzer measure

accuracy/resolution

how many decimal points can m/z value be reliably measured to

accuracy =

(measured MW - theoretical MW)/theoretical MW x 10^6 ppm

lower PPM =

higher accuracy

resolution =

m/z / full width half max

quadrupole analyzers

4 metal rods arranged symmetrically around axis, combo of direct current and radio frequency is applied to rods = oscillating field which changes trajectory of atoms

ionic gate

certain m/z ions at certain voltage in quadrupole can pass and be detected while others deflect off rods

single ion monitoring

focus filter range on single m/z value

single ion monitoring result

higher sensitivity, poor resolution, better cost, low mz range

reason for poor resolution in single ion monitoring

voltage can’t be tuned precisely enough to separate ions with similar m/z

time of flight mass analyzers

ions accelerate to same kinetic energy by constant electromagnetic field, released down vacuum tube, time taken to reach detector is measured and m/z determined

linear TOF

ions are detected after passing through one vacuum tube

reflection TOF

ions are reflected, pass through second tube and detected

TOF advantages

high resolution, no upper limit m/z, fast analysis

TOF disadvantages

more expensive, complex to operate and maintain (compared to quadrupole

ion trap analyzer

ions are generated and trapped using electromagnetic fields, voltage change ejects ions with different m/z

ion trap advantage

cost effective, small footprint

ion trap disadvantages

poor resolution and accuracy, low m/z range

magnetic sector mass analyzer

first type, ions accelerated and enter magnetic field, force makes them follow a curved path, radius of circular motion is dependent on the m/z

magnetic sector advantages

high resolution

magnetic sector disadvantage

slower scanning speed

electron multiplier (EM) detectors

amplifies ion signal through dynodes that generate secondary electrons for each ion that hits it, provides measurable current, destructive = ions lost in process of detection

microchannel plates (MCP)

used by modern ms, made up of parallel channels across a plate, increased surface area allows many ions to be detected

fourier transform (FT) ion trap types

ion cyclotron resonance (ICR), orbitrap

FT ion trap

like QIT and LIT but analyzer is also the detector

orbitrap

outer electrodes and central electrode make electrostatic field, ions oscillate around central electrode and between 2 outer, measures frequency of oscillation

image current

produced when rotating ion passes outer electrodes and induces transient movement of charge in electrode

getting mass spectra from FT

image current is detected, digitalized, and converted into frequency domain then mass spectra

Fourier transform

decompses orbitrap sinusoidal wave frequency into m/z values (oscillation into lines)

orbitrap advantages

high resolution and accuracy (repeated measurement of image current over many oscillations), non destructive = ions can be used further

Fourier-transform ion cyclotron resonance (FT-ICR)

strong magnetic field forms oscillation of ions, frequency of oscillation detected by image current

analyzer with highest frequency

FT-ICR

magnets used in FT-ICR and advantage

super-conducting magnets, boosts magnetic field strength and increases frequency of all ion rotation and resolution