Time-of flight

Time-of-flight (TOF) mass analyzers are the simplest mass spectrometers, where ions that have the same kinetic energy 

Time-of-fiight instruments are configured as either a stand-alone TOF mass analyzer (TOF MS) or as a hybrid quadrupole time-of-fiight (QqTOF) mass spectrometer the latter consists of a quadmpole front-end and an orthogonal acceleration TOF back-end for MS/MS experiments (Fig. 6.10). The orthogonal design minimizes the ions initial velocity spread as they are accelerated into the TOF by a pulsed potential. A QqTOF can be operated as a TOF mass analyzer (QqTOF MS, full-scan) or a quadmpole TOF tandem mass spectrometer (QqTOF MS/MS, product ion scan). Compared to a TOF MS, the 

5270) (Fig. 6.11 plots A2 and C2) and 3 IPs from two fragments at m/z 772.4506 (theoretical mass 772.4483) and 174.1131 (theoretical mass 174.1130) (Fig. 6.11 plot C2). In this example, CID with low and high collision energy was applied to acquire fragment-rich spectra, and additional IPs were assigned for confirmation. 

Time of flight (TOF) is undoubtedly the technique of choice for measuring the mobility in low mobility materials. The principle of the TOF experiment, which was first described by Le Blanc in 1960 [46], is depicted in Fig. 10. 

L is the distance between the electrodes, Fthe electric in the organic layer, and V the external voltage across the sample. 

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. 

Continuous ion sources, such as ESI, can be connected to the TOF analyzers through orthogonal acceleration (oa-TOF) [205-209]. In oa-TOF, the ions generated by the source enter the TOF analyzer perpendicular to its main axis (Fig. 2.11). The acceleration potential is initially set to zero and the start pulse is generated instantaneously as the potential is raised and the ions are accelerated into the field-free flight tube. 

Time Focusing Devices. The resolution of the TOF analyzer is limited by the initial velocity spread of the ions. However, there are powerful devices that can compensate for this velocity distribution, and the most widespread techniques at present are the electrostatic ion reflector (electrostatic mirror) and time-lag focusing (delayed extraction). 

The mass accuracy is highly dependent on the mode the instrument is operating in. In the reflector mode, with time-lag focusing, the best MALDI-TOF and oa-TOF instruments are capable of achieving 5 ppm with internal standards, provided that the isotopes are resolved. In many cases it is not possible to add internal calibrants, and then the error in mass accuracy is often increased to 50-100 ppm. Operation of an instalment in a linear mode will typically decrease the mass accuracy. 

The quantification capability is normally limited by the detector and/or the ion source. The MCP that is often utilized in TOF instruments cannot fully handle the ion currents that are produced in MALDI and are often saturated to some extent. With other ion sources, such as SIMS, the detection system is less strained so the detector is less limiting. Instead the ion source will limit the quality in quantification. Magnetic sectors and also qudmpoles are more often utilized when quantification is important. 

In a time-of-flight (TOF) analyzer the time of flight of ions between the ion source and the detector is measured [61]. This requires that the time at which the ions leave the ion source is well-defined. Therefore, ions are either formed by a pulsed ionization method or various kinds of rapid electric field switching. The single discontinuous laser pulses at distinct time points used in MALDI can be ideally combined with time-of-flight mass separation. TOF analyzers thus received increasing interest with the development of MALDI MS. The schematic draw of a linear MALDI-TOF MS is shown in Fig. (9). 

In this expression the presence of a built-in field of V JL has been taken into account. 

In the ideal case, as described above, the photocurrent transient drops abruptly to zero at f = T. The following equation holds 

The best way to determine the hole drift mobility ju is to construct a plot of 1/t 

In reality, the interpretation of a time-of-flight experiment is more complex than outlined above. There are several aspects that have been neglected so far. Some important aspects are discussed below. 

It has already been mentioned that the condition Ag g, must hold to ensure that the electric field strength is not affected by the optically-generated charge carriers. If this condition is not fulfilled, the electric field strength is not constant and can become a function of space and time. This will complicate the interpretation of TOF signals. 

In particular, the ion motion in the z (axial) direction may be described as an harmonic oscillation and Eq. 2.18 showed the relationship between the axial frequency and the mJz (m/q) value of the trapped ion. By the same approach used for FT-ICR, in the case of Orbitrap ion detection is obtained by image current detection on the two outside electrodes, and by a FT algorithm the complex signal due to the copresence of ions of different m/z values (and hence exhibiting different coz values) is separated into its single m/z components. The typical mass resolution obtained by this analyzer is up to 105. 

Time of flight is surely the simplest mass analyzer (Wollnik, 1993). In its basic form, it consists of an ion accelerator and a flight tube under vacuum. Magnetic, electrostatic, and electrodynamic fields are no 

Equation 2.19 shows that ions of different mJz values will follow, after acceleration, linear pathways with different speeds. In other words, the mJz values are inversely related to the squared speed. 

If the ions follow the linear pathway inside a field-free region (drift tube) of length l, considering that v = llt= t = l/v, it follows that 

This equation shows that ions of different mJz values reach the detector, placed at the end of the drift tube, at different times, proportional to the square root of their m/z value. By this experiment, we will obtain arrival time spectrum of the ions, which can be transformed into the mass spectrum by the relationship expressed by Eq. 2.20. For this reason, this device is called TOF. 

The TOP analyzer uses an electrical field to accelerate the ions flying through the drift tube with the same potential and then determines the time taken for them to get to the detector. The principle of TOP analyzer is that if an analyte with a mass of m carries the number of charges (z), the kinetic energy rmP ll) of this compound should 

Therefore, the mass of an individual analyte can be derived from its determined flight time. For example, the heavier ions reach the detector later than the lighter ions [64], The figures of merit of TOF analyzers can be summarized as follows  

TOF mass analyzers have been broadly used for lipid analysis in lipidomics [33, 65, 66]. Although TOF techniques are improved and can measure masses with high mass accuracy/resolution, high sensitivity, and high efficiency, instruments constructed by TOF alone have difficulty in performing MS/MS experiments for lipid analysis. Therefore, hybrid instruments with quadrupoles (i.e., QqTOF) or liner trap (i.e., LIT-TOF) are required to overcome this difficulty. 

The development of hybrid instruments has improved product ion analysis to a great extent in comparison to both QqQ and ion-trap mass analyzers. For example, QqTOF instruments have good mass accuracy and resolving power for determining product ions, whereas QqLIT instruments allow for MS analysis in addition to NLS and PIS analyses [59]. It should be noted that QqTOF mass spectrometers are incapable of virtual NLS and PIS analyses, but can extract NLS- and PlS-like dataset from the array of product ion analysis data. 

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Before ehding this presentation on mass spectrometry, we should cite the existence of spectrometers for which the method of sorting ions coming from the source is different from the magnetic sector. These are mainly quadripolar analyzers and, to a lesser degree, analyzers measuring the ion s time of flight. 

We present a novel method, called VIGRAL, to size and position the reflecting surface of a flaw. The method operates on standard B-scan recorded with traditional transducers, to extract Time-of-Flight (ToF) information which is then back-projected to reconstruct the reflecting surface of the flaw and characterize its radiation pattern. The VIGRAL method locates and sizes flaws to within k/2, and differentiates between flat and volumetric defects. 

M.G.Silk, The use of diffraction based time-of-flight measurements to locate and size defects , Br.J.Non-Destr. Test., 1984, 26(4), 208-213. 

In an early study, Greenleaf et al. [4] reported reconstructions of ultrasonic velocity from time-of-flight profiles. Since then there has been periodic activity in using ultrasound to determine the transmission properties attenuation or refractive index. 

Greenleaf, J.F. Johnson S.A. Samayoa, W.F. and Duck, F.A. (1975). Algebraic reconstruction of spatial distributions of acoustic velocities in tissue from their time-of-flight profiles. In Acoustical Holography, Vol. 6, Ed. N. Booth, Plenum Press, 71-90. 

Fig.5. Energy cepstrum of HF signal and Time of flight measure.

Time-of-Flight Measurements with Shear Horizontal Waves. 

Shear Horizontal (SH) waves generated by Electromagnetic Acoustic Transducer (EMAT) have been used for sizing fatigue cracks and machined notches in steels by Time-of-Flight Diffraction (TOED) method. The used EMATs have been Phased Array-Probes and have been operated by State-of-the-art PC based phased array systems. Test and system parameters have been optimised to maximise defect detection and signal processing methods have been applied to improve accuracy in the transit time measurements. 

Figure 1 shows the schematic of the experimental set up used for the time-of-flight diffraction... 

Prompted by the success, TOFD measurements were conducted on a fatigue crack in a stainless steel compact tension specimen. Test and system parameters were optimised following the same procedure used for carbon steel specimens. A clear diffracted signal was observed with relatively good SNR and its depth as measured from the time-of-flight measurements matched exactly with the actual depth. 

In general a thickness measurement using ultrasound is done by measuring the time of flight of the ultrasonic pulse and calculating the thickness of the objeet multiplying the time and the known constant sound velocity in the material. 

The time of flight is measured and because of the fixed distanee of the transducers the actual sound velocity of... 

Second, the target should rather be a large surface, homothetic to the probe shape, producing the strongest back-reflected echo with no front wave distortion and with the same time of flight for all the elements. 

Once the probe is set into the target, the acquisitions consist of the peak to peak amplitude, the time of flight and the frequency response of the back-reflected echo. 

When converted into decibel, the sensitivity of elements should rather stand in the range of +/-3 dB. The time of flight should show very smooth variations corresponding to a mispositioning of the probe or of the active surface, if any. Acceptance criteria depends on the shape of the probe let us just say that for plane probes expected variations should be below... 

The PS-4 ultrasonic examination system provides many new features, which allows the operator to perform several inspections simultaneously. Both pulse-echo and time-of-flight-diffraction technique can be applied together with storage of digital A-scan data at the same time. 

Unfortunately, now that such methods have become available, such as the Time Of Flight Diffraction (TOFD) technique, this revolution does not happen. What we see instead is a much slower process towards quantitative NDT, in combination with adapted acceptance criteria for weld defects. 

Figure A3.9.3. Time-of-flight spectra for Ar scattered from Pt(l 11) at a surface temperature of 100 K [10], Points in the upper plot are actual experimental data. Curve tinough points is a fit to a model in which the bimodal distribution is composed of a sharp, fast moving (lienee short flight time), direct-inelastic (DI) component and a broad, slower moving, trapping-desorption (TD) component. These components are shown...

The chapter is divided into sections, one for each general class of mass spectrometer magnetic sector, quadnipole, time-of-flight and ion cyclotron resonance. The experiments perfonned by each are quite often unique and so have been discussed separately under each heading. 

In the simplest fomi, reflects the time of flight of the ions from the ion source to the detector. This time is proportional to the square root of the mass, i.e., as the masses of the ions increase, they become closer together in flight time. This is a limiting parameter when considering the mass resolution of the TOP instrument. 

The ion time of flight, as given by equation (B1.7.7), is oversimplified, however. There are a number of factors which change the final measured TOP. These are considered below. 

Figure Bl.7.17. (a) Schematic diagram of a single acceleration zone time-of-flight mass spectrometer, (b) Schematic diagram showing the time focusing of ions with different initial velocities (and hence initial kinetic energies) onto the detector by the use of a reflecting ion mirror, (c) Wiley-McLaren type two stage acceleration zone time-of-flight mass spectrometer.

The final total ion time of flight in the TOF mass spectrometer with a single accelerating region can be written in a smgle equation, taking all of the above factors into account. 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

Guilhaus M 1995 Principles and instrumentation in time-of-flight mass spectrometry physical and instrumental concepts J. Mass Spectrom. 30 1519-32... 

Wiley W C and McLaren I H 1955 Time-of-flight mass spectrometer with improved resolution Rev. Sc/. Instrum. 26 1150-7... 

Time-of-flight experiments are used to measure particle velocities and particle mass per charge. The typical experiment... 

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