Pathlength selection

Mf quantitative analysis is being carried out, it is necessary to use a cell, of known pathlength. A guide to pathlength selection for different solution concentrations is shown in Table 3.2c. 

Table 2.3 Pathlength selection for solution cells. From Stuart, B., Modern Infrared Spectroscopy, ACOL Series, Wiley, Chichester, UK, 1996. University of Greenwich, and reproduced by permission of the UnivCTsity of Greenwich...

A variety of sample presentation methods are available to the analytical scientist. These include transmittance (straight and diffuse), reflectance (specular and diffuse), transflection (reflection and transmittance), and interactance (a combination of reflectance and transmittance). Pathlength selection... 

Sensitivity The sensitivity of a molecular absorption analysis is equivalent to the slope of a Beer s-law calibration curve and is determined by the product of the analyte s absorptivity and the pathlength of the sample cell. Sensitivity is improved by selecting a wavelength when absorbance is at a maximum or by increasing the pathlength. 

With this spectrometer, a difference mid-IR spectrum at a selected time after sample excitation is recorded by sweeping from 1640 to 940 cm in steps that may be as short as approximately equal to the spectral resolution of the spectrometer—in this case, 8 cm. The sample solution is pumped through a flow cell that has IR-transmitting Cap2 windows set with a 0.1-mm optical pathlength. The Bap2 windows have also been used for the sample cell. ... 

The amount of light absorbed is a function of the so-called absorption coefficient (A ) and of the optical pathlength in the atomiser cell (ft) k depends on the frequency of the selected analytical line and on the concentration of the analyte absorbing atoms. The general absorbance law (Lambert Beer Bouguer law) relates transmittance (and so measured intensities I and If) to k and b through the following equation ... 

The primary reason FTIR has not been utilized as a point detection system for trace level contamination in water is that the opacity of water limits the pathlength (L) to 25 pm or less. Based on a minimum CL value of 0.22 pg/cm for phosmet, the minimum detection level would be approximately 10 ppm. Therefore direct detection at low ppb levels in the aqueous phase is not possible. The avenue to achieve a lower detection limit is to increase the concentration (C) in the IR beam. The work presented in this paper has focused on synthesizing and modifying adsorbent materials that would serve as the above mentioned concentrating surface. The main hmdle is to incorporate these adsorbent materials into a sampling technique that will allow trace detection from aqueous systems, while utilizing the inherent selectivity of FTIR spectroscopy. 

The Fermi resonance absorption bands, which obviously are IR active, increase in intensity with density of the supercritical CO2 fluid (11). This absorption becomes the limiting factor in selecting the pathlength for the flow-through analysis cell. In order to use this region of the IR spectrum, there must be enough analytical radiation remaining after the supercritical fluid absorbance to obtain adequate sensitivity for a particular application. 

Liquids may be sampled as neat liquids or in solution. A mid-infrared transmission spectrum sufficient for chemical identification may often be recorded from a capillary layer of a nonvolatile, pure liquid. This may be prepared simply from a drop of the liquid that has been sandwiched between a pair of mid-infrared transparent windows clamped together, which is also resistant to attack by the liquid. A more reproducible (and safer) practice, however, is to use an appropriate pathlength cell. Whichever method is selected, the specimen examined must be free from bubbles. For strongly absorbing liquids and some quantitative applications, a more efficient approach may be to use an appropriate infrared internal reflection technique accessory. 

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