The quadrupole is the most widely used analyzer due to its ease of use, mass range covered, good linearity for quantitative work, resolution and quality of mass spectra. Reasonably priced.
The main characteristics are:
- Working mass range: 10 to 4000 A.M.U.
- Resolution: usually operated at a resolution = 1000, but resolution can be reasonably pushed up to 4000
- Mass accuracy: 0.1 to 0.2 A.M.U.
- Scan speed: up to 5000 A.M.U per second
A quadrupole can be operated in RF-only mode, which allows ions of any m/z ratio to pass through, or in scanning mode, where a potential difference is applied and the instrument acts as a mass filter.
A triple quadrupole has three quadrupoles arranged in a series. It can be set either for the analysis of intact peptides or their fragment ions.
- Q1 is used to scan across a preset m/z range and select an ion of interest.
- Q2 focuses and transmits the ions while introducing a collision gas into the flight path of the selected ion.
- Q3 serves to analyze ion fragments.
Quadrupole separates ions in space.
Quadrupoles offer 3 main advantages:
- tolerate relatively high pressures
- have a significant mass range (up to 4000 m/z)
- Relatively low cost
U is voltage, V is RF. Mass spectrum is generated by increasing U and V at a constant ratio.
Operation mode and dwell time
The quadrupole can be used in two modes: SIM (single ion monitoring) or Scan. In SIM mode, the parameters (amplitude of the DC and RF voltages) are set to observe only a specific mass, or a selection of specific masses. This mode provides the highest sensitivity for users interested in specific ions or fragments, since more time can be spent on each mass. That time can be adjusted; it is called the dwell time.
The mass window for observing an ion in SIM mode can be adjusted, in order to compensate small mass calibration shift. This is the span factor.
In Scan mode, the amplitude of the DC and RF voltages are ramped (while keeping a constant RF/DC ratio), to obtain the mass spectrum over the required mass range. The sensitivity is a function of the scanned mass range, scan speed, and resolution.
Scanning Mode in Tandem MS
Single scan. Pass ion sources from ionization source to detector without collision. Gives molecular wt. information.
(1) Product ion scan. The precursor ion is focused in Q1 and transferred into Q2 - the collision cell - where it interacts with a collision gas and fragments. The fragments are then measured by scanning Q3. This results in the typical MS/MS spectrum and is the method most commonly employed with ESI ionization and/or LC-MS. Q1 fixed, Q3 scan. Gives structural information.
(2) Precursor ion scan. Q3 is held to measure the occurrence of a particular fragment ion and Q1 is scanned. This results in a spectrum of precursor ions that result in that particular product ion. Goal is to find all occurrence of a certain ion. Especially useful for EI and CI ionization. Q1 scan, Q3 fixed. Structural information & screening for analogues.
(3) Neutral loss scan. Q1 is scanned as in (2) but this time Q3 is also scanned to produce a spectrum of precursor ions that undergo a particular neutral loss. Again this mode is especially useful for EI and CI ionization. Q1 scan, Q3 scan – neutral offset. Structural information & screening for conjugates.
(4) Selected Reaction Monitoring. Q1 and Q3 are set to fixed masses. Goal is to detect a specific reaction. Which peptide would fragment into which fragments. –Both Q1 & Q2 are fixed. Target analysis & highest sensitivity.