Monochromatic detection

Monochromatic detection. A schematic of a monochromatic absorbance detector is given in Fig. 3.12. It is composed of a mercury or deuterium light source, a monochromator used to isolate a narrow bandwidth (10 nm) or spectral line (i.e. 254 nm for Hg), a flow cell with a volume of a few pi (optical path 0.1 to 1 cm) and a means of optical detection. This system is an example of a selective detector the intensity of absorption depends on the analyte molar absorption coefficient (see Fig. 3.13). It is thus possible to calculate the concentration of the analytes by measuring directly the peak areas without taking into account the specific absorption coefficients. For compounds that do not possess a significant absorption spectrum, it is possible to perform derivatisation of the analytes prior to detection. 

Monochromatic detection. The basic model comprises a deuterium or mercury vapour light source, a monochromator for isolating a narrow bandwidth (10 nm) or a characteristic spectral line (e.g. 254 nm if the source is a mercury lamp), a flow cell with a volume of a few pL (optical path of 0.1 to 1 cm) and a means of optical detection. 

This specific technique allows the determination of the different reactants and metabolites for different reaction conditions. However, in many cases the rates of photoreaction jM ocesses are not compar le to those in solution due to the photocatalytic behavicmr of the soibent matmal. Detectivity and limit of determination can be optimised by pixel bunching of the diode array. It measures concentraticms as low as in the case of monochromatic detection using commercial equipment [108]. 

There are basically two types of spectrophotometers that differ by position of the monochromator with respect to the sample. Sample illumination is either monochromatic or polychromatic. Each has its own characteristic response to fluorescent materials. Measurement data from both types of spectrophotometers are presented here. The ideal type of instrument for fluorescent samples would use both monochromatic illumination and monochromatic detection. The design is difficult to realize in practice, especially with respect to diffuse transmittance measurements and integrating spheres. 

The second type of spectrophotometer design uses polychromatic illumination of the sample and monochromatic detection. Placing the monochromator after the sample separates both A, and Af before they are detected. The division of wavelengths can be distributed spatially as depicted in Fig. 13. 

A strong point of EELS is that it detects losses in a very broad energy range, which comprises the entire infrared regime and extends even to electronic transitions at several electron volts. EELS spectrometers have to satisfy a number of stringent requirements. First, the primary electrons should be monochromatic. Second,... 

Lasers are used to deliver a focused, high density of monochromatic radiation to a sample target, which is vaporized and ionized. The ions are detected in the usual way by any suitable mass spectrometer to produce a mass spectrum. The yield of ions is often increased by using a secondary ion source or a matrix. 

In outline, the method used is to pass the monochromatic radiation through the gaseous sample and disperse and detect the scattered radiation. Usually, this radiation is collected in directions normal to the incident radiation in order to avoid this incident radiation passing to the detector. 

Photomultipliers are appreciably more sensitive sensors than the eye in their response to line or continuum sources. Monochromators are fitted to the light beam in order to be able to operate as substance-speciflcally as possible [5]. Additional filter combinations (monochromatic and cut-off filters) are needed for the measurement of fluorescence. Appropriate instruments are not only suitable for the qualitative detection of separated substances (scanning absorption or fluorescence along the chromatogram) but also for characterization of the substance (recording of spectra in addition to hR and for quantitative determinations. 

In the hrst case, the degree of self coherence depends on the spectral characteristics of the source. The coherence time Tc represents the time scale over which a held remains correlated this hme is inversely proportional to the spectral bandwidth Au) of the detected light. A more quantitative dehnition of quasi-monochromatic conditions is based on the coherence time all relevant delays within the interferometer should be much shorter than the coherence length CTc. A practical way to measure temporal coherence is to use a Michel-son interferometer. As we shall see, in the second case the spatial coherence depends on the apparent extent of a source. 

The first Raman and infrared studies on orthorhombic sulfur date back to the 1930s. The older literature has been reviewed before [78, 92-94]. Only after the normal coordinate treatment of the Sg molecule by Scott et al. [78] was it possible to improve the earlier assignments, especially of the lattice vibrations and crystal components of the intramolecular vibrations. In addition, two technical achievements stimulated the efforts in vibrational spectroscopy since late 1960s the invention of the laser as an intense monochromatic light source for Raman spectroscopy and the development of Fourier transform interferometry in infrared spectroscopy. Both techniques allowed to record vibrational spectra of higher resolution and to detect bands of lower intensity. 

Funk et al. have used a low-pressure mercury lamp without filter to liberate inorganic tin ions from thin-layer chromatographically separated organotin compounds these were then reacted with 3-hydroxyflavone to yield blue fluorescent chromatogram zones on a yellow fluorescent background [22]. Quantitative analysis was also possible here (XoK = 405 nm, Xji = 436 nm, monochromatic filter). After treatment of the chromatogram with Triton X-100 (fluorescence amplification by a factor of 5) the detection limits for various organotin compoimds were between 200 and 500 pg (calculated as tin). 

Fig. 1 Comparison of the detection sensitivity after derivatization of three purine derivatives with chloramine T - sulfuric acid (A) and chloramine T - hydrochloric acid (B). Measurement X. (. = 365 nm, A.(, = 440 nm (monochromatic filter M 440) 1 = theophylline, 2 = theobromine, 3 = caffeine.

A monochromatic beam of X-rays with about 1 eV bandwidth is produced by the standard beamline equipment, the undulator and the high-heat-load premonochromator being the most important parts among them. Further monochromatiza-tion down to approximately the millielectronvolt bandwidth is achieved with the high-resolution monochromator. The width of a band of a millielectronvolt, however, is much more than the inherent linewidth of the Fe y-radiation, F 10 eV, or the full range of hyperfine-split Mossbauer lines, A m 10 eV. Yet, NFS is detectable because the coherent excitation of the nuclei is caused in the... 

The experimental apparatus is illustrated schematically in Figure 1.8. Monochromatic light emitted from the point source S is focused by a lens L onto a detection or observation screen D. Between L and D is an opaque screen with two closely spaced slits A and B, each of which may be independently opened or closed. 

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