Reflection mode

Another method of collecting IR microspectra is that of reflection mode. Samples probed in reflection mode are most often (i) highly reflective or polished samples 

460 I 74 Biomedical Applications of Irfrared Microspectroscopy Using Synchrotron Radiation 

Semi-reflective and polished samples are also probed in reflection mode. As the SNR of the spectra relies heavily on collection of the reflected light back into the IR objective, it is important that these samples have a smooth, flat surface and are correctly oriented. Samples with smooth surfaces that are not flat can be mounted into a micro-goiniometer, so as to adjust the tilt of the sample with respect to the incoming beam. Even simpler, samples can also be pressed into a small sphere of putty so that the sample surface is parallel to the microscope stage. 

While thin sections are often easier to prepare on IR-reflective substrates, the use of IR-reflective substrates does come at a cost to the IR data collection process, and even spectral quality. As noted above, the incident flux in reflection mode is reduced by almost 50% compared to transmission mode, as only half of the focusing objective is used to direct the beam onto the sample, while the second half is used for collecting the reflected beam. In addition, any inhomogeneities in the thin section can cause interference effects (e.g., oscillations) in the background of the IR spectra. These artifacts can alter peak shapes, intensities and frequencies. Thus, care must be taken with sample preparation, and only certain (generally homogeneous) samples can be investigated well in this mode. 

Infrared microspectroscopy was used to examine numerous plant and animal tissues long before the union of the IR microscope and the synchrotron source [30]. For complex samples such as human tissues, an IR spectrum can provide a direct indication of sample biochemistry. 

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Hiller, D., and Ermert, H., System Analysis of Ultrasound Reflection Mode Computerized Tomography, IEEE Trans. Sonic Ultrasonic SU-31, pp 240-250, (1984). 

Fischer U Ch, Durig U T and Pohl D W 1988 Near-field scanning microscopy in reflection Appl. Phys. Lett. 52 249 Cline J A, Barshatzky FI and Isaacson M 1991 Scanned-tip reflection-mode near-field scanning optical microscopy Ultramicroscopy 38 299... 

Electron Probe Microanalysis, EPMA, as performed in an electron microprobe combines EDS and WDX to give quantitative compositional analysis in the reflection mode from solid surfaces together with the morphological imaging of SEM. The spatial resolution is restricted by the interaction volume below the surface, varying from about 0.2 pm to 5 pm. Flat samples are needed for the best quantitative accuracy. Compositional mapping over a 100 x 100 micron area can be done in 15 minutes for major components Z> 11), several hours for minor components, and about 10 hours for trace elements. 

The case of Gai jfAlj(As alloy determination is an example of the importance of the reflectance mode in relation to transmittance. In almost all cases the Gai jfAljf As material is an epitajual film (O-l-lpm) grown on a GaAs substrate (-0.5 mm thick). Since the band gap of GaAs is smaller than that of Gai. Al As, the reflectance mode must be used. 

Infrared microscopes can focus the beam down to a 20-pm spot size for microprobing in either the transmission or reflection mode. Trace analysis, microparticle analysis, and spatial profiling can be performed routinely. 

In situ quantitation The photometric determination was made in reflectance mode at 2 = 550 nm (Fig. 1). 

In situ quantitation The absorption-photometric determination in a reflectance mode was performed at A = 330 nm (detection limit ca. 40 ng per chromatogram zone). The fluorimetric analysis was carried out at =313 nm and An > 560 nm (detection limits ca. 10 ng per chromatogram zone) (Fig. 1). 

Initial Situation A product contains three active components that up to a certain point in time were identified using TLC. Quantitation was done by means of extraction/photometry. Trials to circumvent the time-consuming extraction steps by quantitative TLC (diffuse reflection mode) had been started but were discontinued due to reproducibility problems. The following options were deemed worthy of consideration ... 

Transmission infrared spectroscopy is very popular for studying the adsorption of gases on supported catalysts and for studying the decomposition of infrared active catalyst precursors during catalyst preparation. Infrared spectroscopy is an in situ technique that is applicable in transmission or diffuse reflection mode on real catalysts. 

In the preceding section, we presented principles of spectroscopy over the entire electromagnetic spectrum. The most important spectroscopic methods are those in the visible spectral region where food colorants can be perceived by the human eye. Human perception and the physical analysis of food colorants operate differently. The human perception with which we shall deal in Section 1.5 is difficult to normalize. However, the intention to standardize human color perception based on the abilities of most individuals led to a variety of protocols that regulate in detail how, with physical methods, human color perception can be simulated. In any case, a sophisticated instrumental set up is required. We present certain details related to optical spectroscopy here. For practical purposes, one must discriminate between measurements in the absorbance mode and those in the reflection mode. The latter mode is more important for direct measurement of colorants in food samples. To characterize pure or extracted food colorants the absorption mode should be used. 

The FTIR spectra of the NO molecules adsorbed on the catalyst were measured on a JEOL JIR-KX) spectrophotometer in a diffuse reflectance mode. After a back ground spectrum was measured in situ for a freshly sulfided catalyst, NO was introduced to the catalyst as 10 % NO/He pulses (5.1 cm ). The FTIR spectra were recorded after an introduction of 5 pulses. ... 

The complex obtained when L = 2,2 -bipyridine was used in the development of an optical fiber sensor that works in the reflection mode [138]. The sensor response for three different VOCs with four different concentrations each was studied and it was shown that the sensor responded to the three VOCs and that it was possible to distinguish between the different concentrations. 

Here, we demonstrate the usefulness of SFG spectroscopy in the study of water structure at electrode/electrolyte solution interfaces by showing the potential dependent SFG spectra in the OH-stretching vibration region at a Pt/thin film electrode/0.1 M HGIO4 solution interface in internal reflection mode. 

Figure 7.IS Some typical calibration curves for several substances measured by absorption in the reflectance mode. Substance identification 1 = practolol 2 azobenzene 3 - diphenyl-acetylene 4 alprenolol 5 = estrone and 6 - pamatolol. (Reproduced with permission from ref. 20. Copyright Elsevier Scientific Publishing Co.)...

Also, NIRS detection in reflectance mode can be used for TLC [762]. The main reason for studying the applicability of NIRS as an in situ detection tool in TLC is that adsorbents such as silica gel have no strong absorption in the NIR region background... 

Odom and Schueler (1990) describe the basic components of the instrument, known as LIMA 2A or LAMMA 1000, depending upon the particular manufacturer. Figure 3.11 illustrates a schematic diagram of a reflection mode instrument. 

ION EXTRACTION OPTICS AND LIGHT OBJECTIVE LENS (REFLECTION MODE)... 

Electronic spectroscopy, often referred to as UV/visible spectroscopy, is a useful instrumental technique for characterising the colours of dyes and pigments. These spectra may be obtained from appropriate samples either in transmission (absorption) or reflection mode. UY/visible absorption spectra of dyes in solution, such as that illustrated in Figure 2.3, provide important information to enable relationships between the colour and the molecular structure of the dyes to be developed. 

The compensation birefringence measurement is very easily coupled to optical microscopy in the transmission and reflection modes, thus allowing characterizing orientation with a spatial resolution of a few hundreds of nanometers [14]. Polarizing microscopes are widely available and are often used for birefringence studies even if spatial resolution is not required. Objectives specifically designed for cross-polarized microscopy are necessary to avoid artifacts. 

Unless the coverage of adsorbate is monitored simultaneously using spectroscopic methods with the electrochemical kinetics, the results will always be subject to uncertainties of interpretation. A second difficulty is that oxidation of methanol generates not just C02 but small quantities of other products. The measured current will show contributions from all these reactions but they are likely to go by different pathways and the primary interest is that pathway that leads only to C02. These difficulties were addressed in a recent paper by Christensen and co-workers (1993) who used in situ FT1R both to monitor CO coverage and simultaneously to measure the rate of C02 formation. Within the reflection mode of the IR technique used in this paper this is not a straightforward undertaking and the effects of diffusion had to be taken into account in order to help quantify the data obtained. 

Measures in UV reflectivity mode are thus possible, using a 400 nm peaked narrowband filter. However, because of the glass optics absorption below 400 nm and the low sensor sensitivity in the UV, the quality of the images in UV reflection mode is not particularly good, because of the long exposure time need for compensating the low illumination on the sensor. 

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