Transmission efficiency

The transmission efficiency of the analyzer is just one component of the overall sampling efficiency of the instrument, defined as the fraction of analyte molecules delivered to the ion source that are ionized and delivered to the detector obviously the ionization efficiency of the ion source must be considered in addition to the transmission efficiency of the analyzer. Even this does not fully describe the overall efficiency of production of analyte derived signal, since detector response (a function of m/z. 

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Gas transmission efficiency factor, vaiies with line size and surface internal condition of pipe... 

The removal of plasma lines is normally effected by using conventional interference filters. Interference filters, however, have several drawbacks in that they reduce transmission efficiency and do not withstand the intense laser output power over a long period of time. 

The Cary 82 spectrometer employs an optical filtering system which is similar in some respects to the design by Claassen et al. 41). This optical filtering arrangement is shown in Fig. 22. The Cary 82 filter system has higher transmission efficiency than conventional interference filters (Table VII). 

The weakness of MC-ICPMS lies in the inefficiency by which ions are transferred from the plasma source into the mass spectrometer. Therefore, despite very high ionization efficiencies for nearly all elements, the overall sensitivity (defined as ionization plus transmission efficiencies) of first generation MC-ICPMS instruments is of the order of one to a few permil for the U-series nuclides. For most, this is comparable to what can be attained using TIMS. 

The energies of the Auger electrons leaving the sample are determined in a manner similar to that employed for photoelectrons already described in chapter 2 Section 4. Modern instruments nearly always incorporate cylindrical mirror analysers (CMA) because their high transmission efficiency leads to better signal-to-noise ratios than the CHA already described. 

The total contribution to the Auger electron signal is then dependent upon the attenuation length (kM) in the matrix before being inelastically scattered, and upon the transmission efficiency of the electron spectrometer as well as the efficiency of the electron detector. Calculated intensities of Auger peaks rarely give an accuracy better than 50%, and it is more reliable to adopt an approach which utilises standards, preferably obtained in the same instrument. 

Until recently the only satisfactory way to separate these molecular interferences has been on the basis of nuclear mass defects, i.e., the mass of molecules having the same mass number differs from that of the atoms of the same mass number. Figure 2 shows the resolution that is needed to resolve the molecular impurities present in the previous example. Clearly, an unambiguous identification can be made, and all molecular fragments can only be eliminated for an instrument with resolution M/AM approximately 20,000. Once again, the need for high resolution will cause the transmission efficiency to be low. 

When the incident light is horizontally polarized, the horizontal Ox axis is an axis of symmetry for the fluorescence intensity Iy = Iz. The fluorescence observed in the direction of this axis (i.e. at 90° in a horizontal plane) should thus be unpolarized (Figure 5.3). This configuration is of practical interest in checking the possible residual polarization due to imperfect optical tuning. When a monochromator is used for observation, the polarization observed is due to the dependence of its transmission efficiency on the polarization of light. Then, measurement of the polarization with a horizontally polarized incident beam permits correction to get the true emission anisotropy (see Section 6.1.6). 

Polarization effects The transmission efficiency of a monochromator depends on the polarization of light. This can easily be demonstrated by placing a polarizer between the sample and the emission monochromator it is observed that the position and shape of the fluorescence spectrum may significantly depend on the orientation of the polarizer. Consequently, the observed fluorescence intensity depends on the polarization of the emitted fluorescence, i.e. on the relative contribution of the vertically and horizontally polarized components. This problem can be circumvented in the following way. 

Let Ix, Iy and Iz be the intensity components of the fluorescence, respectively (Figure 6.3). If no polarizer is placed between the sample and the emission monochromator, the light intensity viewed by the monochromator is Iz + Iy, which is not proportional to the total fluorescence intensity (Ix + Iy + Iz). Moreover, the transmission efficiency of the monochromator depends on the polarization of the incident light and is thus not the same for Iz and Iy. To get a response proportional to the total fluorescence intensity, independently of the fluorescence polarization, polarizers must be used under magic angle conditions (see appendix, p. 196) a polarizer is introduced between the excitation monochromator and the sample and... 

Moreover, it is easy to show that, if the emission is observed without a polarizer, an excitation polarizer must be set at 0 = 35.3° (cos2 6 = 2/3). This arrangement is suitable when the fluorescence is detected through an optical filter (to reject scattering light) and not through a monochromator, because of the polarization dependence of the transmission efficiency of the latter. 

The resolving power of a quadrupole mass filter depends on the number of cycles experienced by an ion within the rf field, which in turn depends on its velocity. Thus, the resolution will increase with increasing mass, as ions of higher mass have lower velocity. However, the transmission efficiency will decrease, due to the longer time ions of higher masses spend in the quadrupole. 

We must also take into account two further factors. First, the fact that the transmission efficiency of the analyzer is a fimction of the kinetic energy (K.E.) of the photoelectrons in the ESCA-3 Vacumn Generators instrument the transmission is inversely proportional to the K.E. of the electrons (3a). Second, photoelectron yields must refer to total yield from a particular ionization process and this need not, for example, be just the area of the relevant peak. Account must be taken of all processes that divert electrons from the primary peak, e.g., shake-up, shake-oflF, and plasmon peaks. In some cases, e.g., emission from the Cu 2P3/2 level, the contribution of additional processes is small but in others, and emission from the Al(2p) shell is an example, the no-loss peak is substantially less than the true Al(2p) emission. 

The observed excitation spectrum is distorted because the light intensity of the excitation source is a function of the wavelength and the transmission efficiency of the excitation monochromator is a function of wavelength. The emission spectra are distorted by the wavelength-dependent efficiency of the emission monochromator and the photomultiplier (PMP) tubes. Thus both... 

Table 7.1 Reflection and Transmission Efficiencies for a Nonabsorbing Sphere with m = 1.33...

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