Time-of-flight mode

Crystal analyser instruments at pulsed sources are used in the time-of-flight mode. There are two main types of instrument those that use a variable final energy and those that use a fixed final energy. There is only one working example of the former t)q)e at present, PRISMA [22] at ISIS. This is used exclusively for coherent inelastie neutron scattering and so will not be considered further here. 

Mass spectrometric analysis Spectra were recorded on a Bruker Reflex 111 time-of-flight mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany). This mass spectrometer was equipped with a Scout MTP ion source with delayed extraction. Spectra were recorded in positive ion linear time-of-flight mode. Typical acceleration potentials were 18 kV. For delayed extraction, the acceleration potential was switched with a delay of 200 ns. 

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. 

Another form of array is called a microchannel plate detector. A time-of-flight (TOP) mass spectrometer collects ions sequentially in time and can use a point detector, but increasingly, the TOP instrument uses a microchannel plate, most particularly in an orthogonal TOP mode. Because the arrays and microchannel plates are both essentially arrays or assemblies of small electron multipliers, there may be confusion over their roles. This chapter illustrates the differences between the two arrays. 

To differentiate tteir functions and modes of operation, the array collector of spatially dispersed m/z values is still called an array collector for historical reasons, but the other multipoint detector of a temporally dispersed range of m/z values is called a microchannel plate (typically used in time-of-flight instruments). 

The most common modes of operation for ms/ms systems include daughter scan, parent ion scan, neutral loss scan, and selected reaction monitoring. The mode chosen depends on the information required. Stmctural identification is generally obtained using daughter or parent ion scan. The mass analyzers commonly used in tandem systems include quadmpole, magnetic-sector, electric-sector, time-of-flight, and ion cyclotron resonance. Some instmments add a third analyzer such as the triple quadmpole ms (27). 

The time-of-flight mass spectrometer was usually operated in the positive mode in order to detect the cationic sodium adducts of the analyte molecules. The anions of H-acidic substances were detected in the negative mode. This mode significantly enhanced the signals of acidic compounds, e.g. free fatty acids compared with ester signals in the mass spectra of beeswax and tung oil. 

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. 

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