Mass spectrometer is a detector which sort ions based on their mass-to-charge ratios (m/z) in vacuum. The quadrupole is the most widely used analyzer having good linearity for quantitative analysis, mass range coverage, good resolution and quality of mass spectra.
It’s composed of two pair of rods, one set of rods works on positive potential and other set at negative potential at the same time. A combination of RF and DC voltage applied on each set of rods with 180 degree phase difference. QQQ systems has three set of analyzers where different vendor has different terminology and design. SQD systems has only one analyzer. The first and third analyzers or quadrupoles works as filter and middle one works as collision cell.
V(t) = – Vdc – Vrf cosΩt V(t) = Vdc + Vrf cosΩt
The positive pair of rods is acting as a high mass filter the other pair is acting as a low mass filter. The resolution depends on the dc value in relationship to the RF value. The quads are operated at constant resolution, which means that the rf/dc ratio is maintained constant. For a given amplitude of the dc and RF voltages, only the ions of a given m/z (mass to charge) ratio will resonate, have a stable trajectory to pass the quadrupole and be detected. Other ions will be de-stabilized and hit the rods. The performance (i.e. ability to separate two adjacent masses across the applicable range) depends on the quad geometry, on the electronics, on the voltage settings and on the quality of the manufacturing. Increasing the resolution means that fewer ions will reach the detector, and consequently impacts the sensitivity.
As per Mathieu stability diagram the quadrupole is scanned with A/Q = constant; the resolution depends on the slope of the scan line. If the continuous voltage DC is switched off, the scan line is the Q axis: We have now a transfer only device like the hexapole or octopole used to transfer and focus the ions into the mass spectrometer optics.
QQQ Mass Spectrometers are the Standard Analytical tool for LCMS/MS Applications, in particular when accurate quantification is desired where in a triple Quadrupole mass spectrometer, there are several types of experiment can be performed including:
• Product ion scan
• Precursor ion scan
• Neutral loss scan
• Single Reaction Monitoring
• Multiple Reaction Monitoring. QQQ are used for applications where the customer wants to perform minimal sample preparation
• These usually involve high-throughput quantitation on a few compounds (rather than many)
• Some examples include:
1) Pesticides and herbicides in food.
2) Drugs of abuse in human fluids.
3) Pharmaceuticals in surface water.
4) Pharmaceuticals in biological matrices.
Basics of MSn
• Ions are filtered more than once
• Filtering may take place in time (Ion Trap) or space (Triple Quadrupole)
• New species of ions are created between filtering through Collision Induced Dissociation (CID)
• More filtering gives greater specificity
QQQ have the most sensitive and precise implementation of neutral loss, and precursor i d i th MS li ti ion scan mo des i n th e MS application space.
• QQQ can not do MSn with n=3,4…as with ion trap. Current Market players are Sciex, Agilent, Thermo, and Waters having various models worldwide