Optical systems wavelength selection

Both lamps discussed above produce continuous emissions of all wavelengths within their range. Therefore, a spectrophotometer must have an optical system to select monochromatic light. Modem instruments use a diffraction grating usually to produce the desired wavelengths. 

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. 

Stray light passing through the optical system is generally not absorbed by the sample to the same extent as that of the selected wavelength. Its presence leads to negative deviations. 

The tail of the plasma formed at the tip of the torch is the spectroscopic source, where the analyte atoms and their ions are thermally ionized and produce emission spectra. The spectra of various elements are detected either sequentially or simultaneously. The optical system of a sequential instrument consists of a single grating spectrometer with a scanning monochromator that provides the sequential detection of the emission spectra lines. Simultaneous optical systems use multichannel detectors and diode arrays that allow the monitoring of multiple emission lines. Sequential instruments have a greater wavelength selection, while simultaneous ones have a better sample throughput. The intensities of each element s characteristic spectral lines, which are proportional to the number of element s atoms, are recorded, and the concentrations are calculated with reference to a calibration standard. 

The cardinal element of a spectrometer, the spectral apparatus which is necessary for the selection or discrimination of radiation of different wavelengths, has been discussed above. It is used in connection with elements which generate, transport, and detect radiation. These components of an optical system are ... 

The functions now realized by microprocessors include the control of the optical system (lamp and analytical wavelength selection), selection of the kind of data collected (e.g., absorbance, concentration), zero-adjustment, autocalibration and control of measurement parameters [21]. The microprocessor determines the equation of the regression curve and provides statistical processing of the results. It can also be programmed to measure the absorbance, the % transmittance at a selected wavelength, or the concentration based on the relationship (linear or non-linear) established between the measured absorbance and the concentration. 

The principle of a small angle X-ray scattering experiment using synchrotron radiation (SR) is illustrated in Fig. 2. The optical system selects X-rays with a wavelength of 0.15 nm and a narrow band-width (A V ) 5 10 . This beam is focused on the detector with an adequate cross section at the sample position. The incident beam intensity I, which follows the slow decay of the current in the storage... 

Fig. 2. The layout of synchrotron X-ray small angle scattering measiurements. The optical system selects X-rays with a narrow band-width from the continous wavelength distribution of synchrotron radiation. Intensity of the primary>beam at the sample lo is monitored by an ion chamber. ly is the transmitted beam, and, I(s), the scattered intensity is recorded with a position sensitive detector. Scattering patterns of fibres are presented as Log (sl(s)) vs s plots where s = 2 sin 0/X is the scattering vector. 26 is the scattering angle and X is the wavelength.

A crucial point of detector selection is whether or not an accurate IRF can be recorded in the given optical system. IRF recording is often a problem in micro-seopes or other systems that use the same beam path for excitation and detection. Reflection and scattering makes it difficult to record an accurate IRF in these systems. In two-photon microscopes the detector may not even be sensitive at the laser wavelength, or the laser wavelength may be blocked by filters. If an accurate IRF is not available, lifetimes much shorter than the detector IRF cannot be reliably deconvoluted. The rule of thumb is to use a detector with an IRF width shorter than the shortest lifetime to be measured. 

Simple optical systems using filters for wavelength selection and a photoelectric detector are called photometers. Photometers are used for both the visible and the UV region. For example, UV photometers were commonly used as detectors in HPLC but have been superceded by PDAs. HPLC detectors will be discussed in greater detail in Chapter 13. 

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