Quadrupole mass analyzer principle, limits, benefits and applications

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 Quadrupole mass analyzer

The type of analyzer which is used to separate the ions on the basis of specified charge to mass ratio. An instrument for measuring the relative amounts of different gases in a mixture. The gas could be atmospheric air, could be evolved from a chemical process, might be trapped inside a small mineral sample, or even be dissolved in a liquid such as sea water.

Introduction

The quadrupole mass analyzer (QMS), also known as a transmission quadrupole mass spectrometer, quadrupole mass filter, or quadrupole mass spectrometer, is one type of mass analyzer used in mass spectrometry. As the name implies, it consists of four cylindrical rods, set parallel to each other. In a quadrupole mass spectrometer the quadrupole is the mass analyzer  the component of the instrument responsible for selecting sample ions based on their mass-to-charge ratio (m/z). Ions are separated in a quadrupole based on the stability of their trajectories in the oscillating electric fields that are applied to the rods.

Quadrupole mass analyzers are often used for targeted, quantitative applications and can be used either to measure one or a few given ions in a mixture, or to scan across a range of m/z values. Depending on the configuration, they can facilitate both MS and MS/MS applications. For instance, in a triple-quadrupole mass spectrometer, you can select ions for analysis in the first quadrupole, fragment them by collision with a gas in the second quadrupole, and analyze the resulting fragmentation ions in the third quadrupole.

Principle

It consists of four cylindrical rods, set parallel to each other provided by AC and DC source. Lets one mass pass through at a time. Can scan through all masses or sit at one fixed mass.

Quadrupole mass analyzer Specifications

  • Mass range 10–4000 daltons (amu)
  • Resolution, typically 1000 O Scan rate 5000 daltons/min
  • Accuracy .1–.2 daltons

Construction

It consists of three components

  1. The ion source
  2. Quadrupole mass filters
  3. The Detector

The ion source

The Ion Source Molecules from the gas sample are ionised by energetic electrons from a heated filament.

Quadrupole mass filters

The Quadrupole Mass Filter A mixture of a.c. and d.c. voltages applied to opposite pairs of cylindrical metal rods allow ions of a chosen mass to pass from the ion source to the detector. All others are rejected. Varying these voltages allows different masses to pass in turn and a scanned spectrum is produced.

The Detector

The Detector Chosen ions are detected and generate a measured current that is used to construct the mass spectrum. The DC bias will cause all the charged molecules to accelerate and move away from the center line, the rate being proportional to their charge to mass ratio. If their course goes off too far they will hit the metal rods or the sides of the container and be absorbed. So the DC bias acts like the magnetic field B of the mass spec and can be tuned to specific charge to mass ratios hitting the detector.

Quadrupole Limits

  • m/z limit: Depends on the maximum RF potential that can be applied to the quadrupole rods. Normally goes to about 7000 V. The practical limit is about 10,000 – 15,000 V for electronic and arcing reasons. 7000 V translates into a limit of about 5000 m/z.
  • Resolution: Not for high resolution mass spectra.
  • in a quadrupole, higher resolution = lower sensitivity. Sensitivity: Inversely proportional to resolution
  • Mass Accuracy: Partly depends on the precision with which you can control U and V which is highly accurate. Around .1 Da on new instruments

Benefits

  • Classical mass spectra
  • Good reproducibility
  • Relatively small and low-cost systems
  • Low-energy collision-induced dissociation (CID) MS/MS spectra in triple quadrupole and hybrid mass spectrometers have efficient conversion of precursor to product

Applications

  • Great for most organic chemistry applications.
  • useful is in liquid chromatography-mass spectrometry
  • Also used in gas chromatography-mass spectrometry where they serve as exceptionally high specificity detectors.
  • Used for measuring the relative amounts of different gases in a mixture.

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