Mass analyzers transmission efficiency

Mass bias, or the instrumental mass fractionation, is the variable transmission of the ion beam into the mass spectrometer. A variety of phenomena create conditions that lead to variable transmission of ion beams. For modem instmments, the transmission in the flight tube and the efficiency of ion conversion to electrons at the collector are almost quantitative. Most fractionation processes, therefore, take place within the source, namely in the area where the analyte is introduced into the mass spectrometer and ionized, or at the interface between the source and the mass analyzer. 

According to the above definition, sensitivity does not only depend on the ionization efficiency of El or any other ionization method. Also relevant are the extraction of ions from the ion source, the mass range acquired during the experiment, and the transmission of the mass analyzer. Therefore, the complete experimental conditions have to be stated with sensitivity data. 

The atom-probe field ion microscope is a device which combines an FIM, a probe-hole, and a mass spectrometer of single ion detection sensitivity. With this device, not only can the atomic structure of a surface be imaged with the same atomic resolution as with an FIM, but the chemical species of surface atoms of one s choice, chosen from the field ion image and the probe-hole, can also be identified one by one by mass spectrometry. In principle, any type of mass analyzer can be used as long as the overall detection efficiency of the mass analyzer, which includes the detection efficiency of the ion detector used and the transmission coefficient of the system, has to be close to unity. 

Another significant challenge remains in further increasing the efficiency with which sample species are utilized through increased ion throughput and duty factor. Currently many of the ions generated by continuous ionization sources are lost because of the pulsed nature of the instrument. Realization of higher duty factor and the unit transmission efficiency of which TOF-MS is capable could propel the TOF-MS into a sensitivity realm well beyond that of current mass analyzers. 

Table 8.2 compares the main parameters of three mass analyzers. From Table 8.2 we can understand the reasons that the ToF analyzer has become so popular for static SIMS it provides high resolution, high transmission and high sensitivity. The major shortcoming of the ToF analyzer is its use of pulse primary ions. The ratio of primary beam on- to off-time is only about 10-4. Thus, it is not efficient for analysis such as depth profiling of chemical elements. 

The three most common types of mass analyzers in SIMS systems are (1) double focusing magnetic sector instmments, (2) time of flight (TOF) mass spectrometers, and (3) quadru-pole mass spectrometers. The choice of mass analyzer depends on whether dynamic or static SIMS is needed, on the requirements of mass range and resolution, and on transmission efficiency, among other factors. The mass analyzers have been discussed in Chapter 9 in detail and this chapter should be reviewed as necessary. 

The problem of the detection efficiency of product ions has two aspects. Do the ions escape from the collision chamber, and, if so, do they enter the mass analyzer Considering the second aspect first, it is exceedingly difficult to measure the transmission efficiency through the focusing and accelerating system without angular analysis. Conceptually, the most effective way to avoid the problem is to measure the differential cross... 

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