The Triple Quadrupole Mass Analyzer

A quadrupole mass analyzer consists of four metal rods arranged in parallel (Fig. 7A). Direct current and radiofrequency voltages applied to the rods create a magnetic field that causes ions to follow a corkscrew trajectory as they proceed down the axis between the rods. Depending on the voltage applied to the rods, ions of a specific m/z value will pass through the quadrupole, whereas ions of greater or lesser m/z values will fly outwards and fail to pass through the quadrupole. By sweeping the radiofrequency voltages on the rods, ions of increasing m/z values can be analyzed.

The triple quad is composed of two of these quadrupoles (Q1 and Q3, Fig. 7B). These are separated by a somewhat different quadrupole (q2; the lower case designation is widely accepted convention), which is governed by radiofrequency voltages only. The middle quadrupole q2 serves as a collision cell, in which collisions between ions and neutral gas atoms lead to peptide ion fragmentation. The detector is placed after Q3.

The triple quadrupole operates in two general ways. In the first, ions from the source are analyzed by rapid scanning of Q1, such that m/z values of all ions coming from the source at any given moment are recorded (Fig. 7C). This is referred to as "full-scan" analysis and yields signals for all the ions (e.g., singly, doubly, triply charged, etc.) coming from the source. This can be considered a "snapshot" of the peptide ions entering the source over the time interval of the scan (typically about 1 s). The other way in which the triple quad is operated is to use Q1 as a mass filter, in which the voltage settings are fixed to allow only ions of a specific m/z value to pass through (Fig. 7D). Those peptide ions then enter q2, where they collide with argon gas atoms and undergo fragmentation. The fragment ions thus produced are analyzed on the basis of their m/z by Q3, which scans repeatedly over a designated mass range to detect the fragment ions. This latter mode of operation is how the triple quad acquires

Triple Quadrupol

LC ESI Q1 q2 Q3 detector source

Fig. 7. Schematic representation of a triple quadrupole MS instrument. (A) A quadrupole mass analyzer; (B) the trajectories of an ion of the selected m/z with that of ions of other m/z; (C) operation of the triple quad in full-scan mode; (D) operation of the triple quad in MS-MS mode.

LC ESI Q1 q2 Q3 detector source

Fig. 7. Schematic representation of a triple quadrupole MS instrument. (A) A quadrupole mass analyzer; (B) the trajectories of an ion of the selected m/z with that of ions of other m/z; (C) operation of the triple quad in full-scan mode; (D) operation of the triple quad in MS-MS mode.

tandem MS data. The efficiency of MS-MS analysis by a triple quad depends on the properties of the peptide ions being analyzed and on instrument settings, including the pressure of Ar gas in q2 and the energy settings used for CID. In most MS-MS on triple quads, only a fraction of the precursor ions that enter q2 actually undergo f ragmenta-tion. Moreover, the fragmentation that does occur is sometimes more extensive than in an ion trap (see below). Thus, optimal MS-MS performance of a triple quad requires careful adjustment of instrument parameters in order to obtain an optimum degree of peptide fragmentation.

Triple quads were the original instruments used for tandem MS in proteomics studies. The accuracy of quadrupole mass analyzers allows selection of specific peptide ions (by Q1) and analysis of fragment ions from MS-MS (by Q3) to within at least ±0.5 amu of their true m/z values. This degree of mass accuracy is sufficient to allow direct interpretation of amino acid sequence from peptide MS-MS data obtained with triple quads. Moreover, these measurements of fragment ion m/z values are sufficiently accurate to permit peptide sequences by algorithms that correlate the MS-MS spectra with protein sequences obtained from databases (see below).

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  • leon
    Which is mass analyzers better for peptide?
    8 years ago

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