The Time-of-Flight Analyzers

Analyzers The mass analyzer separates the ions according to their miz values. The most common analyzers are listed in Table 31-3. The most common analyzers for GC/MS are the quadrupole mass filter and the ion trap. High-resolution mass spectrometers use the double-focusing analyzer, the ion-cyclotron resonance analyzer, or the time-of-flight analyzer. 

QTOF systems (also called QqTOF) are versatile configurations. The quadrupole may be used in either a wide or narrow band pass mode to determine which ions are passed into the collision region (q) and then into the time-of-flight analyzer (Figure 2.36). Full-scan MS data are obtained in the basic mode when the quadrupole... 

The term Q/TOF is used to describe a type of hybrid mass spectrometer system in which a quadrupole analyzer (Q) is used in conjunction with a time-of-flight analyzer (TOP). The use of two analyzers together (hybridized) provides distinct advantages that cannot be achieved by either analyzer individually. In the Q/TOF, the quadrupole is used in one of two modes to select the ions to be examined, and the TOF analyzer measures the actual mass spectrum. Hexapole assemblies are also used to help collimate the ion beams. The hybrid orthogonal Q/TOF instrument is illustrated in Figure 23.1. 

Upon emerging from the quadrupole, the ions are accelerated through about 40 V and focused into the time-of-flight (TOF) analyzer. A pusher electrode is sited alongside this focused ion beam. Application of a pulse of high electric potential (about 1 kV) to the pusher electrode over a period of about 3 ps causes a short section of the ion beam to be detached and accelerated into the TOF analyzer. A positive potential is used to accelerate positively charged ions and vice versa. 

Mass spectrometer configuration. Multianalyzer instruments should be named for the analyzers in the sequence in which they are traversed by the ion beam, where B is a magnetic analyzer, E is an electrostatic analyzer, Q is a quadrupole analyzer, TOP is a time-of-flight analyzer, and ICR is an ion cyclotron resonance analyzer. For example BE mass spectrometer (reversed-geometry double-focusing instrument), BQ mass spectrometer (hybrid sector and quadrupole instrument), EBQ (double-focusing instrument followed by a quadrupole). 

Time-of-flight analyzer. A device that measures the flight time of ions with an equivalent kinetic energy over a fixed distance. 

Magnetic and electrostatic sectors, quadrupole, and time of flight analyzers belong to the first group, while ion trap, Orbitrap and Fourier transform ion cyclotron resonance analyzers separate ions in time. 

With TOF, the ions from an ion source are accelerated linearly down a chamber containing an electrical field. The flight chamber is at very low pressure that facilitates the flight of the peptide ions with minimal collisions with other molecules. The ions travel in a linear trajectory until they impact a detector at the other end of the tube. The heavier ions travel more slowly than the lower molecular weight ions and reach the detector later. Hence, TOF analyzers derive their name from the concept that the time of flight of an ion is related to its m/z ratio and velocity within a fixed distance. Linear mode TOF analyzers contain single chambers and are not favored for proteomics applications because of their lower mass accuracy. 

A time-of-flight (TOF) mass analyzer separates ions according to the time difference between a start signal and the pulse generated when an ion hits the detector, that is, the time of flight. 

The TOF mass analyzer has a low duty cycle, and the combination with an ion accumulation device such as an ion trap is therefore very advantageous. It offers also MS capabilities with accurate mass measurement. In all acquisition modes, the ions are accelerated into the time of flight for mass analysis. Various other hybrid mass spectrometers with TOF have been described, including quadrupole ion trap [70] and linear ion trap [58]. High energy tandem mass spectrometry can be performed on TOF-TOF mass spectrometers [71, 72]. 

C. Eckers,N. Haskins and J. Langridge.The use of liquid chromatography combined with a quadrupole time-of-flight analyzer for the identification of trace impurities in drug substance. Rapid Communications in Mass Spectrometry, 1997,11(17), 1916-1922. 

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