Time spectrometer

Birch D J S and Imhof R E 1977 A single-photon counting fluorescence decay-time spectrometer J. Phys. E Sol. Instrum. 10 1044-9... 

Tandcm-in-Time Spectrometers. Tandem-in-iime instruments form the ions in a certain spatial region and then at a later lime expel the unwanted ions and leave the selected ions to be dissociated and mass analyzed in the same spatial region. This process can be repeated many limes over to perform not only MS/MS experiments, but also MS/MS/MS and MS" experiments. Fourier transform ICR and quadrupole ion-trap instruments are well suited lor performing MS" cxperimenls. In principle, tandem-in-time spectrometers can perform M.S/MS experiments much more simply than tandem-in-space instruments because of the dilTiculty in providing different ion focal positions in the latter. Although tandem-in-time spectrometers can readily provide product ion scans, other scans, such as precursor ion scans and neui ral loss scans, are much more difficult to perform than they arc with tandem in space instruments. 

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. 

The first requirement is a source of infrared radiation that emits all frequencies of the spectral range being studied. This polychromatic beam is analyzed by a monochromator, formerly a system of prisms, today diffraction gratings. The movement of the monochromator causes the spectrum from the source to scan across an exit slit onto the detector. This kind of spectrometer in which the range of wavelengths is swept as a function of time and monochromator movement is called the dispersive type. 

Gas chromatography is not an identification method the components must be identified after their separation by capillary column. This is done by coupling to the column a mass spectrometer by which the components can be identified with the aid of spectra libraries. However tbe analysis takes a long time (a gasoline contains aboutTwo hundred components) so it is not practical to repeat it regularly. Furthermore, analysts have developed te hpiques for identifying... 

A microwave pulse from a tunable oscillator is injected into the cavity by an anteima, and creates a coherent superposition of rotational states. In the absence of collisions, this superposition emits a free-mduction decay signal, which is detected with an anteima-coupled microwave mixer similar to those used in molecular astrophysics. The data are collected in the time domain and Fourier transfomied to yield the spectrum whose bandwidth is detemimed by the quality factor of the cavity. Hence, such instruments are called Fourier transfomi microwave (FTMW) spectrometers (or Flygare-Balle spectrometers, after the inventors). FTMW instruments are extraordinarily sensitive, and can be used to examine a wide range of stable molecules as well as highly transient or reactive species such as hydrogen-bonded or refractory clusters [29, 30]. 

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. 

Probably the simplest mass spectrometer is the time-of-fiight (TOP) instrument [36]. Aside from magnetic deflection instruments, these were among the first mass spectrometers developed. The mass range is theoretically infinite, though in practice there are upper limits that are governed by electronics and ion source considerations. In chemical physics and physical chemistry, TOP instniments often are operated at lower resolving power than analytical instniments. Because of their simplicity, they have been used in many spectroscopic apparatus as detectors for electrons and ions. Many of these teclmiques are included as chapters unto themselves in this book, and they will only be briefly described here. 

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). 

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

The original method employed was to scan eitiier the frequency of the exciting oscillator or to scan the applied magnetic field until resonant absorption occiined. Flowever, compared to simultaneous excitation of a wide range of frequencies by a short RF pulse, the scanned approach is a very time-inefficient way of recording the spectrum. Flence, with the advent of computers that could be dedicated to spectrometers and efficient Fourier transfomi (FT) algoritluns, pulsed FT NMR became the nomial mode of operation. 

One of the principal experimental advantages of this method of detemiining relaxation times is that it may be carried out with standard EPR spectrometers using CW-detected EPR lines [9, 10]. A discussion of more direct measurements of and T2 using time-resolved EPR techniques is deferred to a later point (see sections bl. 15.4 and Bl. 15.6.3(b)). 

M continually decreases under the influence of spin-spin relaxation which destroys the initial phase coherence of the spin motion within they z-plane. In solid-state TREPR, where large inliomogeneous EPR linewidths due to anisotropic magnetic interactions persist, the long-time behaviour of the spectrometer output, S(t), is given by... 

The low MW power levels conuuonly employed in TREPR spectroscopy do not require any precautions to avoid detector overload and, therefore, the fiill time development of the transient magnetization is obtained undiminished by any MW detection deadtime. (3) Standard CW EPR equipment can be used for TREPR requiring only moderate efforts to adapt the MW detection part of the spectrometer for the observation of the transient response to a pulsed light excitation with high time resolution. (4) TREPR spectroscopy proved to be a suitable teclmique for observing a variety of spin coherence phenomena, such as transient nutations [16], quantum beats [17] and nuclear modulations [18], that have been usefi.il to interpret EPR data on light-mduced spm-correlated radical pairs. 

The design of a pulsed EPR spectrometer depends heavily on tlie required pulse lengdi and pulse power which in turn are mainly dictated by the relaxation times of tlie paramagnetic species to be studied, but also by the type of experiment perfomied. When pulses of the order of a few nanoseconds are required (either to compete... 

Pulsed, or time-domain, EPR spectrometers have also been developed at higher frequencies up to 140 GHz [55. 56]. They are generally low-power units with characteristically long pulse lengths (typically 50 ns for a n/2-pulse) due to tire limited MW powers available at millimetre wavelengths and the lack of fast-switching... 

Figure Bl.23.10. Schematic diagram of a scattering and recoiling imaging spectrometer (SARIS). A large-area (95 X 75 nnn ), time-resolving, position-sensitive microchannel plate (MCP) detector captures a large...

Grizzi O, Shi M, Bu H, and Rabalais J W 1990 Time-of-flight scattering and recoiling spectrometer (TOF-SARS) for surface analysis Rev. Sc/. Instrum. 61 740-52... 

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