Monitors, monochrome

Sohd-state multi-element detector arrays in the focal planes of simple grating monochromators can simultaneously monitor several absorption features. These devices were first used for uv—vis spectroscopy. Infrared coverage is limited (see Table 3), but research continues to extend the response to longer wavelengths. Less expensive nir array detectors have been appHed to on-line process instmmentation (125) (see Photodetectors). 

Direct-reading polychromators (Figure 3b) have a number of exit slits and photomultiplier tube detectors, which allows one to view emission from many lines simultaneously. More than 40 elements can be determined in less than one minute. The choice of emission lines in the polychromator must be made before the instrument is purchased. The polychromator can be used to monitor transient signals (if the appropriate electronics and software are available) because unlike slew-scan systems it can be set stably to the peak emission wavelength. Background emission cannot be measured simultaneously at a wavelength close to the line for each element of interest. For maximum speed and flexibility both a direct-reading polychromator and a slew-scan monochromator can be used to view emission from the plasma simultaneously. 

Figure 2. Schematic of the SLM 8000 fluorometer. Excitation occurs through the excitation monochromator, and light emitted from the sample is observed in as many as four different positions. Photomultiplier tubes (PMTs) A, B, and C can be used to monitor fluorescence or right-angle light scatter through the monochromator (PMT A) or through filters (PUT B and C), and position D measures transmittance. Three channels can be monitored simultaneously with measurements being acquired at intervals of 1 s or less. The data are stored by the computer for subsequent manipulation.

Quin-2 and PHPA are the only combination we have foimd that allows us to look at Ca and oxidant production simultaneously. For this experiment, Quin-2 is detected at 490 nm through a bandpass filter and PHPA is monitored at 400 nm through the monochromator. Under... 

Scattering on the Triple-Axis-Diffractometer [1,2] at the HASYLAB high-energy beamline BW5 is performed in the horizontal plane using an Eulerian cradle as sample stage and a germanium solid-state detector. The beam is monochromatized by a singlecrystal monochromator (e.g. Si 111, FWHM 5.8 ), focused by various slit systems (Huber, Riso) and iron collimators and monitorized by a scintillation counter. The instrument is controlled by a p-VAX computer via CAMAC. 

Sx, Ti -> Tx). Figures 3.2 and 3.3 illustrate the principle of flash spectroscopy/65 If the second light source is continuous, the change in optical density due to the transient species can be monitored as a function of time at a particular wavelength selected on a monochromator. This type of system is illustrated in Figure 3.4. 

Optical devices are placed in the light path in order to shape the primary beam. Beam-position monitors, shutters, slits, monochromators, stabilizers, absorbers, and mirrors are utilized for this purpose. The effective beam shape and its flux are defined by these components. In particular, if mirrors are cooled, vibration must be avoided and thermal expansion should be compensated. 

Automatic adjustment of mirrors and monochromators by intensity monitors and phase-locked loops is called stabilization. If the feedback breaks down, the beam will slowly move as the temperature of the optical part is changing. 

A continuously monitoring detector of high sensitivity is required and those that measure absorption in the ultraviolet are probably the most popular. These may operate at fixed wavelengths selected by interference filters but the variable wavelength instruments with monochromators are more useful. Wavelengths in the range of 190-350 nm are frequently used and this obviously means that a mobile phase must not absorb at those wavelengths. 

A schematic diagram of the apparatus used in the energy transfer experiments is shown in Figure 8.22. The particles are produced and levitated in an electrodynamic levitator as described previously. Excitation is provided by the filtered output of either a Xe or Hg-Xe high-pressure arc. The intensity produced at the particle was found to be 10-50 mW/cm2. The fluorescence emitted from each of the levitated particles was monitored at 90° to the exciting beam using //3 optics, dispersed with a j-m monochromator, and detected with an optical multichannel analyzer. The levitator could be... 

Figure 1.9 The energy-level and transition schemes and possible luminescence spectra of a three-level ideal phosphor (a) the absorption spectrum (b, c) emission spectra under excitation with light of photon energies hvi and /iV2, respectively (d, e) Excitation spectra monitoring emission energies at /i( V2 — vi) and at /i vi, respectively. Arrows mark the absorption/emission transitions involved in each spectrum. Eixed indicates that the excitation or emission monochromator is fixed at the energy (wavelength) corresponding to this transition.

---