Sample arrangement

One important issue concerning fire prevention is the early detection of overheated wire insulation, a common source of smoldering fires. Therefore model experiments were performed to examine how far early indications of overheated insulation could be recognized by the gradient microarray and its simple sampling arrangement. The tests were carried out in a closed box with KAMINA placed next to a cable overheated by a current overload. The experiments were performed... 

Finally, after you have studied these piles and answered these questions for yourself, group your note cards into a third sample arrangement. Try to sort through aU of them now, regardless of the source of the material, or their topic headings, and put together three basic stacks by asking yourself ... 

Figure 6 Sample arrangement for impedance measurements. Note the fused silica cover slides (0.1 mm) inserted between the electrodes and the sample...

The Fourier transform Raman spectrometer is constructed around an interferometer (see Figure 4.20) [57], Normally, a continuous wave Nd YAG laser (1064nm) is used for the sample excitation. In relation to the sample arrangement inside the spectrometer, there are two fundamental geometries in which a sample is tested in Raman spectroscopy, that is, the 90° geometry, where the laser beam... 

Sample Arrange the following atoms in order of increasing atomic radius S, P, O... 

Infrared and Raman spectrometers usually combine a radiation source, a sample arrangement, a device for spectral dispersion or selection of radiation, and a radiation detector, connected to appropriate recording and evaluation facilities. An ideal spectrometer records completely resolved spectra with a maximum signal-to-noise ratio. It requires a minimum amount of sample which is measured nondestructively in a minimum time, and it provides facilities for storing and evaluating the spectra. It also supplies information concerning composition, constitution, and other physical properties. In practice, spectrometers do not entirely meet all of these conditions. Depending on the application, a compromise has to be found. 

This section provides a general description of the components of a spectrometer. The special features of spectrometers constructed for different methods of vibrational spectroscopy, especially the specific sample arrangements, are described in Secs. 3.4 and 3.5. 

There are several means of lowering the detection limit in vibrational spectroscopy. The intensity of a Raman band is for a given spectrometer and sample arrangement proportional to the flux of the exciting radiation the concentration c (in molecules per cm ), the effective molecular Raman scattering cross section and the effective thickness of the scattering sample d ... 

Michelson interferometers also have the advantage of being much more tolerant to misadjustment of the sample arrangement, since they have circular Jacquinot stops, compared to the straight small entrance slits of grating spectrometers (Hirschfeld, 1977 b). 

Sample arrangements have to fulfill the following conditions ... 

The sample arrangement of an interferometer is usually placed between the interferometer and the detector. Cuvettes with windows made of IR-transmitting material (Table 3.4-1) contain either a pure liquid sample or a solution usually in a thickne.ss of 5. .. 200 pm - defined by means of a spacer - sometimes, without a spacer ( capillary ) it may be even less. If the available amount of sample is very small or the concentration of the substance to be analyzed is very low, then the sample arrangement has to be optimized for this purpose as micro or trace arrangement respectively. 

A sample arrangement should make it possible to record Raman spectra within a short time period with a high SNR by using a small amount of sample. In order to optimize the... 

This is a useful prerequisite for the optimization of sample arrangements the low intensity of the Raman radiation can be considerably enhanced by utilizing multiple reflections of the exciting and the emerging Raman radiation at the sample and an external spherical mirror. 

Thus, the optimum sample arrangement for Raman spectroscopy of crystal pow ders with a low absorption coefficient is a forward-scattering (0°) arrangement of coarse crystallites with an optimum thickness in a multiple scattering arrangement. These are the conditions for the investigation of colorless samples in the visible range of the spectrum. 

Figure 3.5-5 Sample arrangement for Raman spectroscopy. Optimal illumination of a a grating spectrometer in the 90° arrangement b of an interferometer in the 180° arrangement.

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 ... 

Figure 3.5-10 Scanning micro arrangements for Raman spectroscopy a normal sample arrangement b arrangement using a microscope and fiber optics c scanning of surfaces with liber optics and half-spheric mirror, which reflects the part of the exciting and Raman radiation back to the sample which is not directly collected by the fiber bundle (Schrader, 1990).

There are several possibilities of adapting a micro sample to a spectrometer. The most straightforward approach uses the ordinary 180° sample arrangement (Fig. 3..5-10 a). The spatial resolution is given by the irradiated volume of the sample and the resolution of... 

If the spatial resolving power has to be high, then the Raman radiation must be observed through microscope objectives (Fig. 3.5-10 b). Unfortunately, these objectives have a somewhat lower optical conductance than the regular sample arrangement (Schrader, 1990). As a result, the observed Raman spectrum is also considerably weaker. A microscope may be connected to the spectrometer by a mirror system or by optical fibers, as shown in Fig. 3.5-10 b. Optical fibers are e.specially useful for NIR FT Raman spectroscopy, because the transmission of the fibers may be at its maximum exactly in the range of a Raman spectrum excited by a Nd YAG laser (Fig. 3.3-5). 

Fig. 3.5-10 c shows another sample arrangement which makes use of a fiber-optical connection from the laser to the sample and back to the spectrometer. It is specially designed for the scanning of surface layers, e.g., of precious prints or paintings. The half spheric concave mirror reflects the portion of exciting radiation and Raman radiation back to the sample which has been. scattered by the sample and is not collected by the optical fiber. Thus the mirror as a component of a multiple reflection system enhances the observed intensity of the Raman lines by a factor of 2 to 8, depending on the properties of the sample. 

Sample arrangements for Raman spectroscopy 143 Micro and 2D scanning arrangements 148 The radiant power of Raman lines 151... 

Figure 2. Sample arrangements for VUV side (left) and VUV- AO (right).

---