Infrared Spectroscopy transmittance

A critical study has been carried out in order to evaluate the capabilities of Near Infrared spectroscopy for the analysis of commercial pesticide formulations using transmittance measurements. In this sense, it has been evaluated the determination of active ingredients in agrochemical formulations after extraction with an appropriate solvent. 

In ultraviolet spectroscopy the absorption bands appeared as maxima but in infrared spectroscopy the wave length is plotted against transmittance and hence the peaks appear as dips. In ultraviolet the absorbance is plotted against wave length while in the infrared transmittance is plotted wave length. 

P. Merckle and K.A. Kovar, Assay of effervescent tablets by near-infrared spectroscopy in transmittance and reflectance mode acetylsalicylic acid in mono and combination formulations, J. Pharm. Biomed. Anal, 17, 365-374 (1998). 

Quite early in the history of infrared spectroscopy, reflections in form of reststrahlen bands were observed indicating strong absorption. Nowadays infrared spectroscopy is widely applied to organic compounds which are comparably weak absorbers. For their studies such reflection methods are favoured, whose results are interpretable in almost the same way as transmittance spectra are. However, there is an increasing interest in applying other reflection techniques also to such samples. Advantageous aspects are ... 

Infrared spectroscopy has not very often been used to check the deuterium purity of alkynes, with the exception of the case of compounds with the C=CD group. For these compounds a quantitative method, analogous to the one used to determine the deuterium content of deuterium oxide by n.m.r., has been described . Known quantities of the non-deuterated hydrocarbon RC=CH are successively added to the corresponding RC=CD compound of unknown deuterium purity, and the transmittance of the v(=CH) or K=CD) band is followed i>s. the added amounts of RC CH. The method is long but it does not require precise knowledge of the coefficient of molar extinction, e, of the v(=CH) band. Its main advantage is the elimination of errors due to molecular associations. It is not applicable to volatile compounds. 

As several researchers have shown empirically, the use of —log(reflectance) can provide, analogous to a transmittance measurement, a linear relationship between the transformed reflectance and concentration, if the matrix is not strongly absorbing as can be found for many samples studied by near-infrared spectroscopy. This issue is presented in detail below. A different approach based on a physical model was considered for UV/VIS measurements and later also applied within the mid-infrared. A theory was derived by Kubelka and Munk for a simple, onedimensional, two-flux model, although it must be noted that Arthur Schuster (1905) had already come up with a reflectance function for isotropic scattering. A detailed description of theoretical and practical aspects was given by Korttim. The optical absorption... 

Infrared spectra may be obtained for gases, liquids, orsolids. For transmittance infrared spectroscopy, the sampling techniques may involve a solution, a film, amull, or a pellet, depending on the type of sample. Reflectance spectroscopy differs from transmittance spectroscopy in that infrared radiation reflected from the surface of a material is studied. With a proper sampling accessory (obtainable from commercial sources), the materials analyzed by reflectance techniques normally require little or no sample preparation. The method is non-destructive, non-invasive, and very useful for analyzing materials that are too thick or have too much absorbance to be analyzed by transmittance spectroscopy. 

Infrared and Raman spectroscopy, coupled with optical microscopy, provide vibrational data that allow us to chemically characterise geochemical sediments and weathered samples with lateral resolutions of 10-20 pm and 1-2 pm respectively. Fourier transform infrared spectroscopy involves the absorption of IR radiation, where the intensity of the beam is measured before and after it enters the sample as a function of the light frequency. Fourier transform infrared is very sensitive, fast and provides good resolution, very small samples can be analysed and information on molecular structure can be obtained. Weak signals can be measured with high precision from, for example, samples that are poor reflectors or transmitters or have low concentrations of active species, which is often the case for geochemical sediments and weathered materials. Samples of unknown... 

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 ... 

FT-IR Infrared spectroscopy was used to determine the acidity of the MHCK catalysts. The apparatus was a Perkin Elmer 2865 with Fourier transform capabilities. The typical plots of absorbance as a function of increasing wavelength were obtained for each catalyst and reported in Fig. 3. To improve the plot, 4% and 8% of transmittance were added to the... 

Acid sites and hvdroxvl distribution were evaluated by FT-IR (Fourier Transform-InfraRed) spectroscopy (mod. 2000 Perkin-Elmer). The spectra were recorded in transmittance at 21°C after in situ treatments of pure wafers of calcined samples. The acid sites distribution was determined by pyridine adsorption at 200°C and stepwise desorption (in the 200-500°C range, 1 h, dynamic vacuum 2-10 mbar) on pure pellets, after evacuation (500°C, 1 h, dynamic vacuum 2T0 mbar). 

Infrared spectra are represented in terms of a plot of percentage transmittance versus wavenumber (cm-1). In its most common form, infrared spectroscopy makes use of Fourier transformation, a procedure for interconverting frequency functions and time or distance functions. Fourier-transform IR (FTIR) spectroscopy allows the rapid scanning of spectra, with great sensitivity, coupled with... 

Isaksson T, Kowalski BR, Piece-wise multiplicative scatter correction applied to near-infrared diffuse transmittance data from meat products, Applied Spectroscopy, 1993, 47, 702-709. 

The chemical modification of the clay can be studied with the help of Fourier transform infrared spectroscopy (FTIR). In FTIR spectra of co-treated and mono-treated clays, the transmittance bands at 2922 cm and 2854 cm correspond to asymmetric and symmetric stretching vibration of methylene groups (Figure 9.5). Carbonyl stretching shows a band at 1715 cm. Both the mono-treated and co-treated montmorillonites have the absorbance bands of methylene, which is due to the successful tethering of octodecylammonium and aminoimdecanoic acid to the clay platelets [24]. 

Figure 2.8 Transmittance spectrum of lactic acid. From Stuart, B., Biological Applications of Infrared Spectroscopy, ACOL Series, Wiley, Chichester, UK, 1997. University of Greenwich, and reproduced by permission of the University of Greenwich.

K. H. Norris and J. T. Kuenstner, Rapid Measurement of Analytes in Whole Blood with NIR Transmittance, in Leaping Ahead with Near Infrared Spectroscopy, ed. G. D. Batten, P. C. Flinn, L. A. Welsh, and A. B. Blakeney, NIR Spectroscopy Group, RACI, Australia, 1995, p. 431. 

The excellent transmittance of quartz in the near-infrared range has led to a further enhancement of the potential of near-infrared spectroscopy by the introduction of fiber optics. Fiber-optic waveguides are used to transfer light from the spectrometer to the sample and. after transmission or reflection, back to the spectrometer. Most fiber optic cables consist of three concentric compo-... 

Another method used in determining the wavelength accuracy of NIR spectrophotometers is to use the overtone peaks of polystyrene in the transmittance mode, much as is done in mid-infrared spectroscopy. It has been suggested that polystyrene can also be used in the reflectance mode to provide a similar calibration standard. 

O. Hermansson. Prediction of new constituents in peas using near infrared transmittance In Making Light Work, Proceedings of the 4th. International conference on Near Infrared Spectroscopy, I. Murray, I. Cowe, co-eds. Ian Michael Puhhcations, Chichester, U.K, 1991, p. 236-238. 

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