Reflectrons

The reflectron is situated behind the field-free region opposed to the ion source. The detector is positioned on the source side of the ion mirror to capture the arrival of ions after they are reflected. There are two common methods of positioning the detector. It may be coaxial with the initial direction of the ion beam. This detector has a central hole to transmit the ions leaving the source. However, the most common instrument geometry has the detector off-axis with respect to the initial direction of the ion beam. Indeed, adjusting the reflectron at a small angle in respect to the ions leaving the source allows the detector to be positioned adjacent to the ion source. 

If the potential of the reflectron VR and its length D lead to an electric field in the reflectron E = V r/D, an ion of charge q with a kinetic energy k will enter with a velocity vu and penetrate the reflectron to a depth d such that 

Its speed vx along the x axis will then be zero, and its mean velocity into the reflectron will be equal to vlx/2. The time needed to penetrate at a distance d will be thus 

Schematic description of a TOF instrument equipped with a reflectron. = ions of a given mass with correct kinetic energy = ions of the same mass but with a kinetic energy that is too low. The latter reach the reflectron later, but come out with the same kinetic energy as before (see text). With properly chosen voltages, path lengths and fields, both kinds of ions reach the detector simultaneously. 

Then the ion will be symmetrically repelled outside of the reflectron, so that its kinetic energy will be restored to the same absolute value as before, but the velocity will be in the opposite direction. The total flight length in the reflectron will be 2d, and the total time tT in the reflectron will be 

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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. 

The reflectron consists of a series of ring electrodes, on each of which is placed an electric potential. The first ring has the lowest potential and the last ring the highest to produce an electrostatic field that increases from the front end of the electron to the back. 

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. 

The reflectron increases the spatial separation of the ions of different m/z values by making them travel up and down the flight tube, so the distance traveled is twice what it would be if the ions simply passed once along the tube from one end to the other. The reflectron also narrows the energy spread for individual m/z values, thus improving mass resolution. TOP analyzers are not necessarily equipped with a reflectron. 

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. 

After reflection in the reflectron, the ions must again pass along the length of the analyzer to reach the detector. 

The improvement in resolution with the reflectron is achieved at the expense of some loss in overall sensitivity due to loss of ions in the reflectron and in the second length of analyzer. 

For very high mass, when sensitivity is frequently critical, the reflectron is not used and lower resolution is accepted. 

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. 

We use laser photofragment spectroscopy to study the vibrational and electronic spectroscopy of ions. Our photofragment spectrometer is shown schematically in Eig. 2. Ions are formed by laser ablation of a metal rod, followed by ion molecule reactions, cool in a supersonic expansion and are accelerated into a dual TOE mass spectrometer. When they reach the reflectron, the mass-selected ions of interest are irradiated using one or more lasers operating in the infrared (IR), visible, or UV. Ions that absorb light can photodissociate, producing fragment ions that are mass analyzed and detected. Each of these steps will be discussed in more detail below, with particular emphasis on the ions of interest. 

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