Detector, atomic spectrometer noise

Thus atoms with thermal energy of about 0.02 eV have X = 1 A and can readily diffract from surfaces. A beam of atoms is chopped with a variable frequency chopper before striking the surface. This way, an alternating intensity beam signal is generated at the mass spectrometer detector, that is readily separated from the noise due to helium atoms in the background. 

If the flame background emission intensity is reduced considerably by use of an inert gas-sheathed (separated) flame, then an interference filter may be used rather than a monochromator, to give a non-dispersive atomic fluorescence spectrometer as illustrated in Figure 14.36-38 Noise levels are often further reduced by employing a solar blind photomultiplier as a detector of fluorescence emission at UV wavelengths. Such detectors do not respond to visible light. The excitation source is generally placed at 90° to the monochromator or detector. Surface-silvered or quartz mirrors and lenses are often used to increase the amount of fluorescence emission seen by the detector. 

These two-dimensional detectors [63] are ideally suited for coupling with an echelle spectrometer, which is state of the art in modem spectrometers for ICP atomic emission spectrometry as well as for atomic absorption spectrometers. As for CCDs the sensitivity is high and along with the signal-to-noise ratios achievable, they have become real alternatives to photomultipliers for optical atomic spectrometry (Table 3) and will replace them more and more. 

A primary source is used which emits the element-specific radiation. Originally continuous sources were used and the primary radiation required was isolated with a high-resolution spectrometer. However, owing to the low radiant densities of these sources, detector noise limitations were encounterd or the spectral bandwidth was too large to obtain a sufficiently high sensitivity. Indeed, as the width of atomic spectral lines at atmospheric pressure is of the order of 2 pm, one would need for a spectral line with 7. = 400 nm a practical resolving power of 200 000 in order to obtain primary radiation that was as narrow as the absorption profile. This is absolutely necessary to realize the full sensitivity and power of detection of AAS. Therefore, it is generally more attractive to use a source which emits possibly only a few and usually narrow atomic spectral lines. Then low-cost monochromators can be used to isolate the radiation. 

The major components of the monochromator are the slits and the dispersion element. The source radiation falls on the entrance slit and is directed to the dispersion element that is based on either reflection or refraction. Only the light of the desired wavelength passes through the exit slit to fall onto the detector. The monochromator plays an important role in determining the baseline noise in an atomic absorption spectrometer since it defines the amount of light energy reaching the detector. 

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