Time reflectron

As m increases, At becomes progressively smaller (compare the difference between the square roots of 1 and 2 (= 0.4) with the difference between 100 and 101 (= 0.05). Thus, the difference in arrival times of ions arriving at the detector become increasingly smaller and more difficult to differentiate as mass increases. This inherent problem is a severe restriction even without the second difficulty, which is that not all ions of any one given m/z value reach the same velocity after acceleration nor are they all formed at exactly the same point in the ion source. Therefore, even for any one m/z value, ions at each m/z reach the detector over an interval of time instead of all at one time. Clearly, where separation of flight times is very short, as with TOF instruments, the spread for individual ion m/z values means there will be overlap in arrival times between ions of closely similar m/z values. This effect (Figure 26.2) decreases available (theoretical) resolution, but it can be ameliorated by modifying the instrument to include a reflectron. 

In (a), a pulse of ions is formed but, for illustration purposes, all with the same m/z value. In (b), the ions have been accelerated but, because they were not all formed in the same space, they are separated in time and velocity, with some ions having more kinetic energy than others. In (c), the ions approach the ion mirror or reflectron, which they then penetrate to different depths, depending on their kinetic energies (d). The ones with greater kinetic energy penetrate furthest. In (e), the ions leave the reflectron and travel on to the detector (f), which they all reach at the same time. The path taken by the ions is indicated by the dotted line in (f). 

Time-of-flight (TOF) instmments utilize the times taken by ions to pass (fly) along an evacuated tube as a means of measuring m/z values and therefore of obtaining a mass spectmm. Often a reflectron is used to direct the ions back along the TOF tube. 

By use of an electrostatic ion mirror called a reflectron, arrival times of ions of the same m/z value at the detector can be made more nearly equal. The reflectron improves resolution of m/z values. 

Figure 3.7 Schematic of a time-of-flight mass analyser, involving the use of a reflectron .

In instrnments withont a reflectron (see Figure 3.7 above), both the precursor and prodnct ions reach the detector at the same time and are not separated. The reflectron, however, is an energy analyser and product ions with different energies, after passage through the reflectron, will have different flight times to the detector and may be separated and their m/z ratios determined. This is known as post-source decay (PSD) [11]. 

In this instrnment, the final stage of the triple quadrnpole is replaced by an orthogonal time-of-fiight (ToF) mass analyser, as shown in Fignre 3.10. The con-fignration is typical of the latest generation of ToF instrnments in which a nnmber of reflectrons, in this case two, are used to increase the flight path of the ions and thns increase the resolution that may be achieved. 

Reflectron An ion lens nsed in the time-of-flight mass analyser to increase the distance travelled by an ion and thereby increase the resolntion of the instmment. 

The chemical compositions of the isolated Au SR clusters were investigated by mass spectrometry [15,16,18, 22,32-35]. TEM was used to confirm that the species detected by the mass spectrometer represents the clusters in the sample. Figure 3a is a schematic representation of the top view of the mass spectrometer, which consists of five stages of differentially pumped vacuum chambers. The apparatus accommodates two t5 pes of ion sources, electrospray ionization (ESI) and laser-desorption ionization (EDI), and a time-of-flight (TOE) mass spectrometer with a reflectron. Details of the apparatus and the measurement protocols are described below. 

Figure 6.15 Schematic design of a reflectron time-of-flight mass spectrometer...

The ECAP incorporates an electrostatic lens in the time-of-flight spectrometer in order to improve the mass resolution by compensating for small spreads in the energies of the ions evaporated from the specimen under the pulsed electric field. A lens design by Poschenrieder or a reflectron type of electrostatic lens is used for this purpose, and is standard equipment for metallurgical or materials applications of APFIM. These typically improve the mass resolution at full width half maximum (FWHM) from m/Am 250 to better than 2000. 

Mamyrin, B. A. Karataev, V. I. Shmikk, D. V. Zagulin, V. A. Mass reflectron New nonmagnetic time-of-flight high-resolution mass spectrometer. Zh. Eksp. Teor. Fiz. 1973, 64, 82-89. 

Chien, B. M. Michael, S. M. Fubman, D. M. Enhancement of resolution in matrix-assisted laser desorption using an ion-trap storage/reflectron time-of-flight mass spectrometer. Rapid Comm. Mass Spectrom. 1993, 7,837-843. 

Figure 2.10 Time of flight analyzer drift tube (a) and reflectron (b)...

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