Transmittance and Diffuse Reflectance

The method of diffuse transmittance (DT) is based on measurement of the radiation component 7dt (Fig. 1.22) that passes diffusely through an inhomogeneous layer. This method was first applied to the IR spectroscopic analysis of thin films on samples in powder form by Tolstoy in 1985 [116, 117], who obtained DT spectra of water adsorbed onto silica gel. When used in conjunction with a FTIR spectrometer, the method is called diffiise-transmittance infrared Fourier transform spectroscopy (DTIFTS). DTIFTS is the most recently developed IR spectroscopic methods for studying powder surfaces and has already found application in high-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC) [118, 119]. Of increasing popularity are DTIFTS measurements of powders that use an IR microscope to collect radiation [112, 119] (Section 4.3). 

OPTIMUM CONDITIONS FOR RECORDING INFRARED SPECTRA OF ULTRATHIN FILMS 

For transparent particles of CaF2, KBr, ZnSe, and Si (Table A.2) having the size distribution of 50 90 tim, the effective penetration depths measured 

The effect of the particle size on the DRIFTS intensity for a monolayer of dodecyl amine adsorbed on quartz particles is shown in Figs. 2Ala, b. The spectral contrast is higher for the 5-p,m fraction than for the 38-150-p.m one. This is akin to the data for adsorption of a photosensitizer on silica gel [115]. The results of Kim and co-workers [168] indicate that contrast of the DRIFTS of 4-dimethylaminobenzoic acid adsorbed on 2-3.5-p,m Ag particles, for which the SEIRA phenomenon is expected, and 30-nm Ti02 powder is practically identical. For both DRIFTS and DTlF l S, the particle size should be smaller than the shortest wavelength in the spectral range, as particles of a size comparable to the 

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Willey R R 1976 Fourier transform infrared spectrophotometer for transmittance and diffuse reflectance measurements Appl. Spectrosc. 30 593-601... 

Figure 3.31 Fibre-optic-coupled insertion transmittance and diffuse reflectance probes. (Courtesy of Axiom Analytical Inc., www.goaxiom.com)...

Evangelou, V. P. and J. Wang. 1993. Infrared spectra differences of atrazine between transmittance and diffuse reflectance modes and practical implications. Spectrochim. Acta 49 291-295. 

If the 3 u) pulse of mode-locked Nd YAG laser and the picosecond continuum are used as an excitation and a monitoring pulses, respectively, just as in the transmittance and diffuse reflectance laser photolyses, the time-resolution should be improved up to 10 ps. In this case the pulse width of the picosecond continuum is less than 20 ps, so that the multichannel diode array without gating function was used. A demonstration experiment was performed for poly(methyl methacrylate) film containing 15 wt% benzophenone. The transient absorption spectrum at about 650 ps obtained with 6 = 59° is... 

Figure 6.4-18 Transmittance T and diffuse reflectance R spectra related to the same compound specified by its absorbance spectrum A K/S.

Fig. 2 Comparison of near-infrared spectra of crystalline anhydrous glucose. Spectrum measured in transmittance using the KBr pellet technique [spectral resolution 64cm intensity data in absorbance (top trace) and diffuse reflectance spectrum using a spectral resolution of 32 cm and intensity data transformed to —log R) (bottom trace)].

Fourier transform infrared spectroscopy (FT-IR) is useful for identifying organic and inorganic compounds by comparison with library references. Perkin Elmer System 2000 offers near IR, mid IR, far IR 15,000-15,030 cm, transmittance (T), specular reflectance (SR Ref. 6) and diffuse reflectance (DR), horizontal and vertical attenuated total reflectance (ATR) microscope (>10-gm spot, 10,000-10,580 cmy ... 

The mathematical relationship of incident and reflected light is complicated by such phenomena as diffuse reflectance, ordinary transmittance, and specular reflectance of hemispherically distributed incident light. Based on tin analogy to transmittance density in wet chemistry methods, a mathematical relationship that takes into account all such phenomena is as follows ... 

Reflectance techniques may be used for samples that are difficult to analyze by the conventional transmittance method. In all, reflectance techniques can be divided into two categories internal reflection and external reflection. In internal reflection method, interaction of the electromagnetic radiation on the interface between the sample and a meditnn with a higher refraction index is studied, while external reflectance techniques arise from the radiation reflected from the sample surface. External reflection covers two different types of reflection specular (regular) reflection and diffuse reflection. The former usually associated with reflection from smooth, polished surfaces Hke mirror, and the latter associated with the reflection from rough surfaces. 

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... 

There are also different and efficient solutions to locate solid samples inside the spectrophotometer to study their transmittance or their total reflectance. These housing systems allow to orient the sample at different angles with respect to the incident light to measure both the incident and diffuse reflectance. It is also possible to use the so called integrating spheres that completely surround the sample and are able to collect all the reflected light to measure the value of the total reflectance. 

T. J. W. Salisbury, L. S. Walter, N. Vergo, and D. M. D Aria, Infrared (2.1-25 im) Spectra of Minerals, Johns Hopkins University Press, Baltimore, MD, 1991. Spectra in transmittance (KBr disks), specular reflection, and diffuse reflection for 130 carefully characterized minerals. Typically 4 spectra/mineral, but linear in micrometers. Numerical data on a CD-ROM included. 

Artifact removal and/or linearization. A common form of artifact removal is baseline correction of a spectrum or chromatogram. Common linearizations are the conversion of spectral transmittance into spectral absorbance and the multiplicative scatter correction for diffuse reflectance spectra. We must be very careful when attempting to remove artifacts. If we do not remove them correctly, we can actually introduce other artifacts that are worse than the ones we are trying to remove. But, for every artifact that we can correctly remove from the data, we make available additional degrees-of-freedom that the model can use to fit the relationship between the concentrations and the absorbances. This translates into greater precision and robustness of the calibration. Thus, if we can do it properly, it is always better to remove an artifact than to rely on the calibration to fit it. Similar reasoning applies to data linearization. 

Figure 8.18. Intensities of forward and backward fluorescence of a scattering layer as a function of the layer thickness (ff = 1 cm-1, 5° = S = 100 cm-1). The corresponding curves of diffuse reflectance and diffuse transmittance at Xa are also given.

Figure 8.19. Normalized intensities of forward and backward fluorescence, diffuse reflectance and transmittance of a scattering layer (S = 50 cm"1, d = 1 cm) as a function of the absorption coefficient of the fluorophore.

Spectra of solid samples are usually recorded in the units of reflectance (R) or percent reflectance (%/ ), which is analogous to percent transmittance in that reflectance equals the ratio of the reflected radiation to the incident radiation. With diffuse reflectance, the reflected signal is attenuated by two phenomena absorption (coefficient k) and scattering (coefficient s). Lollowing the Kubelka-Munk theory, these two coefficients are related to the reflectance of an infinitely thick sample, by... 

An infrared spectrum is a plot of percent radiation absorbed versus the frequency of the incident radiation given in wavenumbers (cm ) or in wave length ( xm). A variation of this method, diffuse reflectance spectroscopy, is used for samples with poor transmittance, e.g. cubic hematite crystals. Increased resolution and sensitivity as well as more rapid collection of data is provided by Fourier-transform-IR (FTIR), which averages a large number of spectra. Another IR technique makes use of attenuated total reflectance FTIR (ATR-FTIR) often using a cylindrical internal reflectance cell (CIR) (e.g. Tejedor-Tejedor Anderson, 1986). ATR enables wet systems and adsorbing species to be studied in situ. 

Modern infrared (IR) spectroscopy is a versatile tool applied to the qualitative and quantitative determination of molecular species of all types. Its applications fall into three categories based on the spectral regions considered. Mid-IR (MIR) is by far the most widely used, with absorption, reflection, and emission spectra being employed for both qualitative and quantitative analysis. The NIR region is particularly used for routine quantitative determinations in complex samples, which is of interest in agriculture, food and feed, and, more recently, pharmaceutical industries. Determinations are usually based on diffuse reflectance measurements of untreated solid or liquid samples or, in some cases, on transmittance studies. Far-IR (FIR) is used primarily for absorption measurements of inorganic and metal-organic samples. 

FIGURE 10 Diffuse reflectance, transflectance, and transmittance measurements. 

Many techniques are based on this principle and can be used for the analysis of all types of samples. The spectrum obtained from reflected light is not identical to that obtained by transmittance. The spectral composition of the reflected beam depends on the variation of the refractive index of the compound with wavelength. This can lead to specular reflection, diffuse reflection or attenuated total reflection. Each device is designed to favour only one of the above. The recorded spectrum must be corrected using computer software. 

LP-CVD ZnO Total and diffuse transmittance (TT and DT) for boron-doped LP-CVD ZnO films are shown in Fig. 6.17 as a function of the film thickness. TT remains superior to 85% in the spectral range [500-900 nm] for ZnO layers with d < 1.5 pm. Because 15% of the incident light is reflected due to the change of refractive index at the air/ZnO and glass/air interfaces, this means that, for d < 1.5 pm, the absorption of the LP-CVD ZnO B itself is too low to be measured by the spectrometer. TT is reduced by only about 5% for a thickness d = 3 pm. This means that 3 pm-thick ZnO films still have a high transparency, in spite of their relatively high thickness. For A > 900 nm,... 

Modern NIR equipment is generally robust and precise and can be operated easily by unskilled personnel [51]. Commercial instruments which have been used for bioprocess analyses include the Nicolet 740 Fourier transform infrared spectrometer [52, 53] and NIRSystems, Inc. Biotech System [54, 55]. Off-line bioprocess analysis most often involves manually placing the sample in a cuvette with optical pathlengths of 0.5 mm to 2.0 mm, although automatic sampling and transport to the spectrometer by means of tubing pump has been used (Yano and Harata, 1994). A number of different spectral acquisition methods have been successfully applied, including reflectance [55], absorbance [56], and diffuse transmittance [51]. 

It is usually considered more difficult to evaluate and quantify diffuse reflectance data than transmission data, because the reflectance is determined by two sample properties, namely, the scattering and the absorption coefficient, whereas the transmission is assumed to be determined only by the absorption coefficient. The absorbance is a linear function of the absorption coefficient, but its counterpart in reflection spectroscopy, the Kubelka-Munk function (sometimes also called remission2 function), depends on both the scattering and the absorption coefficient. Often, researchers list a number of prerequisites for application of the Kubelka-Munk function, but, in contrast, transmittance is routinely converted without comment into absorbance. 

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