Transmission spectrometry

Transmission, Absorption, and Beer s Law. The majority of infrared spectrometry is stiU done by the classic method of transmission spectrometry the intensity of an infrared beam passing completely through a sample is measured. The standard description of how much radiation passes through the sample is that of Beet s law (or the Bouguer-Beer-Lambertlaw) ... 

In emission spectrometry, the sample is the infrared source. Materials emit infrared radiation by virtue of their temperature. KirchhofF s law states that the amounts of infrared radiation emitted and absorbed by a body in thermal equilibrium must be equal at each wavelength. A blackbody, which is a body having infinite absorptivity, must therefore produce a smooth emission spectrum that has the maximum possible emission intensity of any body at the same temperature. The emissivity, 8, of a sample is the ratio of its emission to that of a blackbody at the same temperature. Infrared-opaque bodies have the same emissivity at all wavelengths so they emit smooth, blackbody-like spectra. On the other hand, any sample dilute or thin enough for transmission spectrometry produces a structured emission spectrum that is analogous to its transmission spectrum because the emissivity is proportional to the absorptivity at each wavelength. The emissivity is calculated from the sample emission spectrum, E, by the relation... 

Consistent sampling conditions are a necessity for careful quantitative work. In transmission spectrometry it is critical to fill the entire accessory aperture with the sample. If the actual sample area is smaller than the beam that passes through the aperture, stray light or excess radiation will reach the detector (Figure 9.2). The radiation that does not pass through the sample provides a spurious signal to the detector, and this excess radiation establishes a minimum transmittance. That is, no matter how strongly a sample absorbs, no band can have a transmittance below the minimum... 

Measurement of the Fresnel reflectance spectrum is a very useful way of obtaining the spectrum of solids with flat surfaces when sample preparation is not possible. For example, to measure the spectrum of an oriented polymer, the sample cannot be melted, dissolved, or finely ground. A microscopic sample of a hard polymer may be available that is too thick for transmission spectrometry. If the sample is so hard, rough, and/or thick that a good transmission or attenuated total reflection (see Chapter 15) spectrum cannot be measured, Fresnel reflection spectrometry presents a very useful means of obtaining the spectrum. 

By analogy to transmission spectrometry, — og Rs/RQ) for p- and 5-polarized radiation are usually denoted as Ap and A, respectively. When — og RslRQ) is calculated for thin films on both dielectric and metallic substrates, the maximum absorbance is always on the order of 0.001 AU, but the signal measured from dielectric substrates is often very low because of the very low reflectance of the substrate, Rq. It is important to note that the sign of Ap is positive or negative depending on whether 0 is greater or less than 0. ... 

Vibrational microspectrometry will undoubtedly be applied to medical diagnosis in the near future. One particularly important application of microspectrometry is for the characterization of tissue samples. Tissue samples can be mounted on a water-insoluble infrared-transparent window such as ZnSe, but these windows are expensive and not conducive to visual examination (e.g., after staining of the tissue). A convenient alternative to transmission spectrometry is the measurement of the transflectance spectrum (see Section 13.5) of tissue samples mounted on low-emissivity glass slides [4]. These slides are transparent to visible light but highly reflective to mid-infrared radiation. 

In practice, the value of dp calculated from Eq. 15.4 is only an approximation of the effective penetration depth, de, which is the thickness of a sample measured by transmission spectrometry that is required to match the corresponding absorbance in a single-reflection ATR spectrum [2]. The value of de depends on the polarization... 

Like transmission spectra, DR spectra must be converted to a different form in order to convert R(v) to a parameter that varies linearly with concentration. By analogy to transmission spectrometry, most practitioners of DR near-infrared (NIR) spectrometry convert R(v) to log]o[l/R(v)]. Plots of logio[l/R(v)] versus concentration are not linear over wide concentration ranges but are perfectly adequate for multicomponent quantitative analysis provided a) that the concentration of each analyte does not vary by much more than a factor of 2, and (h) that the absorptiv-ities of the analytical bands are fairly low. These criteria are often obeyed for the determination of the components of commodities by DR near-infrared spectrometry, but are not usually valid for mid-infrared spectra, where absorptivities are one or two orders of magnitude higher than in the near infrared. 

Figure 16.1. Spectra of cholic acid measured (a) by diffuse reflection and plotted linear in/(/ oo). ( ) by transmission spectrometry as a poorly prepared KBr disk, and (c) by transmission spectrometry as a well-prepared KBr disk (b) and (c) were plotted linear in absorbance.

If the reflectance of a sample is low, as it is with gaseous samples, e(v), is approximately equal to 1 — r(. Thus, for any sample for which a transmittance spectrum with discrete absorption bands can be measured, the emittance spectmm should yield equivalent information. As a result, qualitative analysis of the components of hot gases by infrared emission spectroscopy can be as easy as it is by transmission spectrometry. The problem of obtaining quantitative information by infrared emission spectroscopy is more difficult, since not only must the temperature of the sample be known if the radiant power from the blackbody is to be calculated, but the instrument response function must also be taken into account [1]. 

The alternative technique to TIRES is transient infrared transmission spectrometry (TIRTS). This technique is analogous to TIRES, but instead of the sample being at ambient temperature and being heated by the gas jet, the sample is above the ambient temperature and is cooled by a narrow jet of cold helium. Were the sample... 

Figure 20.2. Interferograms of methanol vapor measured by (a) transmission spectrometry and (b) photoacoustic spectrometry (c, d) corresponding spectra. (Reproduced from [2], by permission of Elsevier Publishing Co. copyright 1978.)...

In the same way that variations in somce intensity, beamsplitter efficiency, and detector response are compensated in transmission spectrometry by ratioing the single-beam spectra measured with and without the sample in the beam, in PA spectrometry the single-beam spectrum of the sample is ratioed against the spectrum of an optically opaque and thermally thick reference. As noted above, carbon black and heavily carbon-filled polymers make excellent reference materials for PA spectrometry. 

Figure 20.6. (a) Library spectrum of polycarbonate measured by transmission spectrometry (b) photoacoustic spectrum of a polycarbonate sheet. (Reproduced from [3], by permission of CRC Press copyright 1993.)... 

As described above, the reflection-absorbance Ajj is analogous to the absorbance A in transmission spectrometry. Nonetheless, Ar is a quantity characteristic of reflection spectrometry. The most significant characteristic of Ar is that it can be negative. In a transmission measurement, the transmittance always decreases after passing through the absorbing sample that is, T = < 1. By contrast, in ER measurements, the reflectance of the... 

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