Monochromator optimization

Fluorescence spectra were measured at wavelength scanning of tunable dye-laser. In spite of the monochromic excitation the fluorescence spectmm has quite complex composition. Such variety of wavelengths allows to optimize fluorescence excitation and registration for any technological conditions. 

Ultrasensitive Equipment In recent years all components of Raman equipment (laser, sampling optics, filtering, monochromator, and detector) have been clearly improved. This has led to an enormous increase in sensitivity and has enabled direct observation of adsorbed molecules with carefully optimized instruments without the need for further enhancement or resonance effects. 

The light source can be a xenon lamp associated with a monochromator. The optical configuration should be carefully optimized because the electro-optic modulator (usually a Pockel s cell) must work with a parallel light beam. The advantages are the low cost of the system and the wide availability of excitation wavelengths. In terms of light intensity and modulation, it is preferable to use a cw laser, which costs less than mode-locked pulsed lasers. 

Interfering compounds will vary considerably from source to source, and samples may require a variety of cleanup steps to reach required method detection limits. The emission wavelengths used (EPA 8310) are not optimal for sensitivity of the small ring compounds. With modem electronically controlled monochromator, wavelength programs can be used which tune excitation and emission wavelengths to maximize sensitivity and/or selectivity for a specific analyte in its retention time window. 

The real limitation of detection in spectrofluorimetry is not the sensitivity of the detector, but rather the stray light which result from imperfections of the monochromators and emissions by impurities in the solvents. The limiting quantum yields of luminescence detection are of about 10-4 in optimal conditions. 

Figure 10. Excitation (left) and emission (right) spectra optimized for aleurone tissue showing intensity differences between aleurone, endosperm, and pericarp tissues. The emission monochromator was set at 445 nm for excitation spectral scans and the excitation monochromator was set at 350 nm for emission spectral scans. RFI = relative fluorescence intensity. (From [29])...

For the photon-counting mode, an optimal resolution can be maintained by coordinating integration time with monochromator scan rate or interval. For each Raman apparatus, careful optimization of the PM tube s high voltage and pulse discriminator levels based on S/N ratios must be performed. 

The function of the monochromator in AES is to isolate the determinant spectral wavelength of interest from the emission from all concomitant matrix emitting elemental or molecular species. This frequently means that a narrow spectral bandpass must be selected. It is however generally slightly easier to make sure in AES than in AAS that the optimal wavelength is being employed since emission spectra often may be scanned directly. 

Silicate, nickel, and cobalt tend to interfere in the air-acetylene flame, although nickel and cobalt are rarely present in sufficient excess to cause a problem. Silicate interference may be eliminated at modest excesses by the use of lanthanum as a releasing agent or by using a nitrous oxide-acetylene flame. Very careful optimization is sometimes necessary, for example in the analysis of freshwaters, when concentrations are very low. It is important to use a narrow spectral bandpass and to make sure that the correct line is being used, because the hollow cathode lamp emission spectrum of iron is extremely complex. If you have any doubts about monochromator calibration, check the sensitivity at adjacent lines ... 

Optimization in Flame AAS Source-related Parameters Effect of Lamp Current Effect of Lamp Warm Up Time Lamp Alignment Lamp Deterioration Choice of Lamp Atomizer-related Parameters Choice of Atomizer Effect of Fuel-to-oxidant Ratio Optimization of Burner Position Burner Design, Warm Up, and Cleanliness Gas Flow Stability Monochromator-related Parameters Choice of Slit Width Choice of Wavelength Optimization in Flame AFS Source-related Parameters Lamp Operating Parameters Lamp Alignment Atomizer-related Parameters Monochromator-related Parameters Optimization in Flame AES... 

Conventional Raman spectroscopy utilizes rectangular or cylindrical cuvettes. A given spectrometer collects maximum intensity of Raman radiation of a sample, if the sample is placed in the focal region of a laser beam and if a maximum amount of the Raman radiation emerging from this sample is collected by a sample optics of the spectrometer within a maximum solid angle (Schrader, 1980). As mentioned in Sec. 3.1, the optical conductance of the entrance optics should have the same value as that of the interferometer or monochromator. Inspection of conventional sample arrangements shows that these conditions were often not fulfilled optimally ... 

Double focussing, mirror-monochromator cameras are optimized for maximum flux at the sample. This type of camera is hence mainly used for real time diffraction studies on biological samples and polymers (see Sect. 4). Such a camera is shown in Fig. 23. The first optical element could only be placed at 20 m... 

Iron also has a very complex spectrum and most other points that have been made in regard to manganese apply to this metal. The choice of the most sensitive among the many absorption lines again was made with the help of the photographic technique by Allan (A7). The strongest line is that at 2483.3 A at which sensitivity limits of 0.1 ppm have been obtained, but the line at 3720 A still permits the detection of iron at the 1 ppm level. Especially narrow slit width and high resolution monochromators are necessary for optimal results, because the resonance... 

Many modern scanners have a computer-controlled motor-driven monochromator that allovre automatic recording of in situ absorption and fluorescence excitation spectra. These spectra can aid compound identification by comparison with stored standard spectra, test for identity by superimposition of spectra from different zones on a plate, and check zone purity by superimposition of spectra from different areas of a single zone. The spectral maximum determined from the in situ spectrum is usually the optimal wavelength for scanning standard and sample areas for quantitative analysis. 

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