Solid samples infrared

A solvent free, fast and environmentally friendly near infrared-based methodology was developed for the determination and quality control of 11 pesticides in commercially available formulations. This methodology was based on the direct measurement of the diffuse reflectance spectra of solid samples inside glass vials and a multivariate calibration model to determine the active principle concentration in agrochemicals. The proposed PLS model was made using 11 known commercial and 22 doped samples (11 under and 11 over dosed) for calibration and 22 different formulations as the validation set. For Buprofezin, Chlorsulfuron, Cyromazine, Daminozide, Diuron and Iprodione determination, the information in the spectral range between 1618 and 2630 nm of the reflectance spectra was employed. On the other hand, for Bensulfuron, Fenoxycarb, Metalaxyl, Procymidone and Tricyclazole determination, the first order derivative spectra in the range between 1618 and 2630 nm was used. In both cases, a linear remove correction was applied. Mean accuracy errors between 0.5 and 3.1% were obtained for the validation set. 

Mancozeb is a dithiocarbamate pesticide with a very low solubility in organic and inorganic solvent. In this work we have developed a solvent free, accurate and fast photoacoustic FTIR-based methodology for Mancozeb determination in commercial fungicides. The proposed procedure was based on the direct measurement of the solid samples in the middle infrared region using a photoacoustic detector. A multivariate calibration approach based on the use of partial least squares (PLS) was employed to determine the pesticide content in commercially available formulations. 

Infrared spectra are commonly used for characterizing glycosaminoglycans, especially solid samples thereof. However, being dominated by strong and broad bands due to polar and ionized groups (OH, C02 , and S03 ), the i.r. spectrum of heparin often contains concealed bands from contaminants.77 More-reproducible spectra, obtainable for solutions in H20 (Ref. 79) and D20 (Ref. 80) provide a better characterization, as well as quantitative information about such functional groups as carboxyl, acetamido, and sulfate groups. 

Another technique that has been employed for studying certain types of changes in solids is infrared spectroscopy, in which the sample is contained in a cell that can be heated. By monitoring the infrared spectrum at several temperatures, it is possible to follow changes in bonding modes as the sample is heated. This technique is useful for observing phase transitions and isomerizations. When used in combination, techniques such as TGA, DSC, and variable-temperature spectroscopy make it possible to learn a great deal about dynamic processes in solids. 

Figure 3. Photoacoustic infrared spectrum of gaseous CO2 obtained in a microphonic PAS cell for solid samples when the operator exhaled once into the cell before closing. 100 scans, 0.5 cm-1 resolution. This illustrates the large photoacoustic signal arising from gas phase samples and the high resolution attainable.

Infrared spectrometry is one of the few analytical techniques that can measure both dissolved and undissolved solids. Thus there are some unique and interesting solid sampling methods. 

The most straightforward method for analyzing a solid material by infrared spectrometry is to dissolve it in a suitable solvent and then to measure this solution using a liquid sampling cell such as one of the several described in Section 8.8. Thus it becomes a liquid sampling problem, the experimental details of which have already been discussed (Section 8.8). It is the only method of solid sampling suitable for quantitative analysis because it is the only one that has a defined and reproduced pathlength. 

A Nujol mull is a mixture of a solid sample with Nujol, or mineral oil, for the purpose of more conveniently obtaining an infrared spectrum of the solid sample. 

Normal glass will only transmit radiation between about 350 nm and 3 /rm and, as a result, its use is restricted to the visible and near infrared regions of the spectrum. Materials suitable for the ultraviolet region include quartz and fused silica (Figure 2.28). The choice of materials for use in the infrared region presents some problems and most are alkali metal halides or alkaline earth metal halides, which are soft and susceptible to attack by water, e.g. rock salt and potassium bromide. Samples are often dissolved in suitable organic solvents, e.g. carbon tetrachloride or carbon disulphide, but when this is not possible or convenient, a mixture of the solid sample with potassium bromide is prepared and pressed into a disc-shaped pellet which is placed in the light path. 

Both LDI and MALDI make use of the absorption of laser light by a solid sample layer. The energy uptake upon laser irradiation then causes evaporation and ionization of the sample. Wavelengths ranging from ultraviolet (UV) to infrared (IR) have been employed, e.g., nitrogen lasers (337 nm), excimer lasers (193, 248, 308 and 351 nm), Q-switched, frequency-tripled and quadrupled Nd Yag lasers (355 and 266 nm, respectively), [24] Er Yag lasers (2.94 pm) [24,25] and TEA-CO2 lasers (10.6 pm). [16,26]... 

Samples of both fulvic and humic acids were suspended in methanol and methylated with diazomethane. Both H and spectra of the free acids were obtained, at 299.94 MHz and 75.42 MHz respectively, on a Varian XL-300 spectrometer having a Nicolet TT-100 PET accessory. Spectra were obtained in D2O, in a 12-mm tube, with deuterated TSP (sodium 3-(trimethylsilyl)propionate-, , 3,3- 4) added as internal reference. GC/MS of methylated acids was conducted on a Hewlett-Packard Model No 5995 GC/MS/DA system equipped with a fused silica capillary column (12 m x. 020 mm ID, Hewlett Packard) internally coated with crosslinked methylene silicone. Infrared spectra were obtained with solid samples dispersed in KBr pellets, by using a Beckman IR-33 spectrophotometer. The various absorption peaks in IR and NMR were interpreted conventionally (9-10). 

Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT).6 When IR radiation is directed onto the surface of a solid sample, two types of energy reflectance can occur specular and diffuse. The specular component is the radiation that reflects directly off the sample surface (i.e., not absorbed by the sample). Diffuse reflectance is the radiation that penetrates into the sample and then emerges. Diffuse reflectance accessories are designed to optimize the diffuse reflected energy and suppress the specular component. The optics therefore selectively directs the scattered radiation to the IR detector. 

For infrared measurements, cells are commonly constructed of NaCI or KBr. For the 400 to 50 cm 1 far-infrared region, polyethylene is a transparent window. Solid samples are commonly ground to a fine powder, which can be added to mineral oil (a viscous hydrocarbon also called Nujol) to give a dispersion that is called a mull and is pressed between two KBr plates. The analyte spectrum is obscured in a few regions in which the mineral oil absorbs infrared radiation. Alternatively, a 1 wt% mixture of solid sample with KBr can be ground to a fine powder and pressed into a translucent pellet at a pressure of —60 MPa (600 bar). Solids and powders can also be examined by diffuse reflectance, in which reflected infrared radiation, instead of transmitted infrared radiation, is observed. Wavelengths absorbed by the sample are not reflected as well as other wavelengths. This technique is sensitive only to the surface of the sample. 

In the MALDI technique a pulsed laser beam strikes a solid sample and heats, vaporizes, and ionizes compounds with little decomposition.201-209 Proteins or other biopolymers are mixed with a "matrix" Fiat absorbs the heat of Fie laser beam. The protein sample together with Fie matrix is dried. Most proteins form crystals and Fie laser beam is directed toward individual protein crystals or aggregates. Various materials are used for the matrix. Compounds as simple as glycerol, succinic acid, or urea can be used with an infrared laser. For proteins an ultraviolet nitrogen laser tuned to 337 nm is usually employed with an ultraviolet light-absorbing matrix such as hydroxy-benzoic acid, 2,5-dihydroxybenzoic acid, a-hydroxy-... 

In practice, infrared spectra can be obtained with gaseous, liquid, or solid samples. The sample containers (cells) and the optical parts of the instrument are made of rock salt (NaCl) or similar material that transmits infrared radiation (glass is opaque). 

Diffuse reflection involves reflecting the infrared beam off of a solid sample, as in specular reflection, but it is the nonspecular portion of the reflected radiation that is collected. When an ftir spectrometer is used, diffuse reflection is called DRIFTS (diffuse reflectance infrared Fourier-transform... 

A versatile low-temperature infrared cell is presented in Fig. 9.16.20 It is suitable for mull or pressed-disk infrared spectra of solids, solid state infrared spectra of condensed vapors, and spectra of products from solid-gas reactions. Mulled or powdered samples are supported between alkali halide windows in the copper block B, which is cooled by refrigerant in the cold finger A. Before this refrigerant is added, the cell must be evacuated through the needle valve. When the cell is used for low-temperature spectra of condensables, the vapors are bled through the needle valve and squirted directly onto one surface of a cold alkali halide plate held in block B. 

Infrared spectra were determined on a Unicam S P-100 prism-grating spectrometer. Spectra of solid samples were measured by the KBr disc technique in discs of 16 mm. diameter. Each disc contained 1.6 mg. of sample in 400 mg. of KBr. 

Stewart, J. E. Use of partially oriented solid samples and unpolarized radiant energy in the analysis of infrared absorption spectra. J. chem. Phys. 23, 986—987 (1955). 

Solids. Conventional near-infrared reflectance analyzers use a variety of methods to position the sample into the incident collimated beam and collect reproducibly the diffusely reflected radiation to measure the absorption which takes place in the body of the sample traversed. Solid samples are ground with care to achieve reproducible and reasonably uniform granulation for calibration and analysis measurements. The overall scattering characteristics of the sample which shift the reflectance baseline and control the depth of sample penetration and opportunity for absorption become a part of the method and the empirical analytical equation (6,9). Solid sampling is summarized on the following table. 

The pressed-salt method has attained wide application in studies of the infrared spectra of solids. In this method the solid sample is mixed with a powdered halide salt such as KI or KBr and the mixture is pressed into a disk at high pressures 53-55). This method reduces scattering because solid-gas interfaces are replaced by solid-salt interfaces. When this method is used,-the particle size of the solid is not of critical importance and most ordinary silica or alumina catalysts can be used without the necessity of any particle-size separation. Although it is simple experimentally, the pressed-salt method will probably never attain a major importance in catalytic work, because once the sample is embedded in the salt, it cannot be subjected to further treatment. 

Samples for analysis may be solids, liquids, or gases, or any forms in between and in combination, such as slurries, gels, and gas inclusions in solids. Samples can be clear or opaque, highly viscous or liquids with lots of suspended solids. While this variety is easy for Raman spectroscopy, it would be challenging for mid-IR and near-infrared (NIR) spectroscopy and numerous non-spectroscopic approaches. 

The introduction of Fourier Transform Infrared Spectroscopy (FTIR) brought along a number of typical solid sample techniques. DRIFTS (Diffuse Reflectance Fourier Transform Infrared Spectroscopy) is probably most commonly known. Another technique, developed specifically for measuring solid, opaque samples is PAS (Photo Acoustic Spectroscopy). This accessory is less known, probably due to its high cost and its rather difficult modus operandi. 

Infrared spectra were obtained using a Perkin-Elmer Model 283 Spectrometer. Spectra of liquid samples were obtained from smear coating on KBr cells and KBr pellets were used for solid samples. 

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