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Triple Quadrupole Mass Analyzer

A triple quadrupole instrument (QqQ) is a combination of two mass quadrupole mass filters (tandem mass spectrometry) separated by a collision cell which is also a quadrupole operating in RF-only mode (Fig. 1.18). A common nomencla-

collision gas _i_

collision gas _i_

qO Q1_q2 Q3

qO Q1_q2 Q3

Mass Collision Mass analyser cell analyser

Fig. 1.18 Schematic of a triple quadrupole instrument. Stage q0: focusing quadrupole; Q1, Q3: mass analyzing quadrupoles; q2: collision cell. In the present configuration the collision energy (CE) is determined by the potential difference between q0 and q2.

ture is to use (Q) to describe a quadrupole which is operated in RF/DC mode and (q) for a quadrupole which is operated in RF only mode. Tandem mass spectrometry is particularly attractive to obtain additional mass spectral information. In a first step, a specific m/z ion (precursor ion) is selected in the first mass analyzer (Q1). Collision induced dissociation (CID) occurs in the collision cell (q2) which is filled with a neutral gas such as argon or nitrogen. The fragment ions (product ions) are then sorted according to their mass to charge ratio in the second mass analyzer (Q3) and recorded by the detector. This way to obtain MS/MS data is called MS/MS in space, contrasting with quadrupole ion traps where MS/MS experiments are performed in time. On triple quadrupole mass spectrometers the potentials used to carry out collision induced dissociation are in the range 0-250 V. The collision energy is defined in electrons volts (eV) and is therefore dependent on the charge of the ions. For a potential difference of 30 volts the collision energy for a singly charge precursor ion would be 30 eV, and 60 eV for a doubly charged precursor ion. The nature of the collision gas (N2 or Ar) does not affect the product ion spectrum. The gas pressure in the collision cell mainly influences the sensitivity while collision energy influences the nature of the spectrum.

Depending on how the mass analyzers are operated, various types of MS and MS/MS experiments can be performed on a QqQ and these are summarized in Table 1.3. To normalize the description of various MS/MS or multi-stage MSn experiments a symbolism has also been described [54, 55].

A product ion scan can obtain structural information of a given precursor ion while a precursor ion scan is more suited to find structural homologues in a complex mixture. Bosentan (Mr = 551, Fig. 1.19) has two metabolites corresponding to the tert-butyl hydroxylation product (Mr = 567) and the dealkylation of the me-thoxy group to form the phenol (Mr = 537). Bosentan (Tracleer, Actelion Phrama-ceuticals) is an oral duel endothelin receptor antagonist approved for the use in arterial hypertension [56]. Selection of the fragment at m/z 280 can fish out precursor ions corresponding only to bosentan and these two metabolites (Fig. 1.19C). A similar result is obtained with the constant-neutral loss scan mode (Fig. 1.19D) which is based on neutral loss of 44 units.

Table 1.3 Settings of the Q1 and Q3 quadrupoles for the various scan modes of a triple quadrupole mass spectrometer.

Mode Q1 quadrupole Q3 quadrupole

Full scan Q1/single ion monitoring (SIM) Q1 Full scan Q3/single ion monitoring (SIM) Q3 Product ion scan (PIS) Precursor ion scan (PC) Neutral loss (NL)

Selected reaction monitoring (SRM)

Scan/fixed Rf mode

Rf mode Scan/fixed

Fixed Scan

Scan Fixed

Scan Scan: neutral loss offset

Fixed Fixed

Fig. 1.19 (A) Q1 full-scan spectrum of bosentan [(M+H)+, m/z 552], its demethylated metabolite [(M+H)+, m/z 538] and its hydroxylated metabolite [(M+H)+, m/z 568], (B) product ion spectrum of bosentan, (C) precursor ion spectrum, (D) neutral loss spectrum. Electrospray ionization is in positive ion mode.

Fig. 1.19 (A) Q1 full-scan spectrum of bosentan [(M+H)+, m/z 552], its demethylated metabolite [(M+H)+, m/z 538] and its hydroxylated metabolite [(M+H)+, m/z 568], (B) product ion spectrum of bosentan, (C) precursor ion spectrum, (D) neutral loss spectrum. Electrospray ionization is in positive ion mode.

Precursor ion and neutral loss scans are efficient on QqQ to identify structurally related compounds in a mixture, using either a common fragment with the parent compound or the specific neutral loss such as glucuronid or sulfate for phase II metabolites. These selective scan modes do not require any knowledge of the molecular weight or the structure of the compounds. In the selected reaction monitoring (SRM) mode, Q1 is set at the mass of the precursor [M+H] + (m/z 552) and Q3 at m/z 202, which is the most important fragment of bosentan. Because in SRM mode both quadrupoles are not scanning, better detection limits can be achieved compared to full-scan acquisition. Therefore, this mode has become the working horse for quantitative analysis. Typical dwell times are in the range 5-250 ms. Because with quadrupole mass analyzers transmission is dependent on the mass resolution, it is always mandatory, in SRM mode, to indicate the mass resolution of quadrupole Q1 and Q3. In general, full width of the peak at half maximum (FWHM) is indicated. Analysis in single ion monitoring mode can also be performed on a QqQ either using Q1 and Q3. Generally when performing a SIM analysis in Q3 mode, the collision cell is filled with collision gas and serves as a further declustering device to improve signal-to-noise.

Source

Source

End Cap

End Cap

Ring Electrode [Donut Shape)

Ring Electrode [Donut Shape)

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