Infrared analysis liquid samples

A gas-washing bottle (Figure 4.7B) may also be used for trapping. This technique is especially useful in conjunction with infrared analysis. The sample is simply bubbled through the anhydrous solvent as it exits the chromatographic column. The solution is then placed in a liquid sample infrared cell. A matching cell containing only the solvent is placed in the reference beam. An infrared spectrum of the sample may then be recorded. 

Infrared spectroscopy is routinely used for the analysis of samples in the gas, liquid, and solid states. Sample cells are made from materials, such as NaCl and KBr, that are transparent to infrared radiation. Gases are analyzed using a cell with a pathlength of approximately 10 cm. Longer pathlengths are obtained by using mirrors to pass the beam of radiation through the sample several times. 

Reaction of K3Co(CN) with PMMA. A 1.0 g sample of PMMA and 1.0g of the cobalt compound were combined in a standard vessel and pyrolyzed for 2 hrs at 375°C. The tube was removed from the oven and the contents of the tube were observed to be solid (PMMA is liquid at this temperature). The tube was reattached to the vacuum line via the break-seal and opened. Gases were determined by pressure-volume-temperature measurements on the vacuum line and identified by infrared spectroscopy. Recovered were 0.22g of methyl methacrylate and 0.11 g of CO and C02. The tube was then removed from the vacuum line and acetone was added. Filtration gave two fractions, 1.27g of acetone insoluble material and 0.30g of acetone soluble (some soluble material is always lost in the recovery process). The acetone insoluble fraction was then slurried with water, 0.11 g of material was insoluble in water. Infrared analysis of this insoluble material show both C-H and C-0 vibrations and are classified as char based upon infrared spectroscopy. Reactions were also performed at lower temperature, even at 260°C some char is evident in the insoluble fraction. 

Infrared analysis is usually used as a qualitative method to identify substances. Liquids are usually analyzed as pure substances in cells with very small optical path lengths of 0.1-1.0 mm. Usable spectra can be obtained by placing a drop of relatively non-volatile sample between two sodium chloride plates, allowing them to be held together by capillary action. 

Recently, polyethylene and Teflon mesh sample holders have been used. A drop of sample is placed on the mesh and spread to a relatively uniform thickness for analysis. These holders can often be rinsed and reused. A very convenient alternative to liquid sample holders is the technique called attenuated total reflection or ATR. The ATR cell is a crystal of gallium arsenide, GaAs and the infrared radiation enters one end of the trapezoidal crystal. With the angles adjusted to obtain total internal reflection, all of the IR radiation passes through the crystal and exits the other end as shown in Fig. 5.14. 

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. 

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. 

Near-infrared absorption is therefore essentially due to combination and overtone modes of higher energy fundamentals, such as C-H, N-H, and O-H stretches, which appear as lower overtones and lower order combination modes. Since the NIR absorption of polyatomic molecules thus mainly reflects vibrational contributions from very few functional groups, NIR spectroscopy is less suitable for detailed qualitative analysis than IR, which shows all (active) fundamentals and the overtones and combination modes of low-energy vibrations. On the other hand, since the vibrational intensities of near-infrared bands are considerably lower than those of corresponding infrared bands, optical layers of reasonable size (millimeters, centimeters) may be transmitted in the NIR, even in the case of liquid samples, compared to the layers of pm size which are detected in the infrared. This has important consequences for the direct quantitative study of chemical reactions, chemical equilibria, and phase equilibria via NIR spectroscopy. 

Liquid and gas samples do not need much preparation, but special cells to contain the samples are often necessary. The simplest method to prepare a liquid sample is to make a capillary thin film of the liquid. The capillary thin film is made by placing a drop of liquid on a KBr plate and sandwiching it with another KBr plate. This method, however, is not suitable for volatile liquids. Liquid cells can be used for volatile liquid and toxic liquid samples, particularly for quantitative analysis. The spacing between the bottom and the top of liquid cell is typically from 1 to 100 /u.m. The cell is made of an infrared-transparent material. Typically, KBr is used however, KBr should not be selected as the material for holding samples containing water because water dissolves KBr. Instead, ZeSe or AgCl should be used because they are infrared-transparent but not water soluble. Cells for gas samples are structurally similar to cells for liquid but the dimension is much larger. 

The first step in carrying out an infrared spectroscopic analysis is the preparation of the sample. The types of samples analyzed by infrared spectroscopy in crude oil chemistry are solid or liquid samples. 

Irradiation. The liquid formulation was applied with a calibrated bar onto either a KBr crystal for infrared analysis, or a glass plate for hardness and gel fraction measurements. 10 to 20 pm thick films were exposed to the radiation of a 80 W/inch medium pressure mercury lamp, in the presence of air, at a passing speed of 60 m/min, which corresponds to a 0.1 s exposure at each pass. The incident light intensity (Iq) at the sample position was measured by radiometry (International Light IL-390) and... 

For the direct analysis of in biological species, an interface is available [43]. The liquid samples are deposited into a bed of CuO powder which is held on a refractory support in an enclosed chamber. The CuO matrix is heated locally by an infrared laser, and the CO2 evolved is swept away by a flow of He into the ion source. 

Sample Handling. Enzymes Immobilized Enzymes Enzyme-Based Assays. Fluorescence Clinical and Drug Applications. Gas Chromatography Mass Spectrometry. Infrared Spectroscopy Near-Infrared. Isotope Dilution Analysis. Liquid Chromatography Column Technology Instrumentation. Sensors Overview. 

See alsa. Adhesives and Sealants. Food and Nutritional Analysis Oils and Fats. Infrared Spectroscopy Overview Sample Presentation Industrial Applications. Liquid Chromatography Food Applications. Plastics. Sensory Evaluation. 

In industry, time is at premium so the emphasis is always to carry out the analysis in the most efficient manner possible. In practice, this means that the analyst employs the sampling method that requires the least possible sample preparation. Historically, the great majority of liquid samples were run as thin Aims between infrared transparent plates (usually KBr) or in fixed pathlength cells. Similarly, most solid samples were run as KBr discs, as mulls or as a thin film deposited from solution onto a suitable substrate. Polymers were usually hot-pressed to make a self-supporting film, with a thickness typically between 0.020 and 1.0 mm the former being used for the identification of a bulk material and the latter for the identification of minor components, contaminants, or degradation products. 

See also Activation Analysis Neutron Activation. Atomic Absorption Spectrometry Principles and Instrumentation. Atomic Emission Spectrometry Principles and Instrumentation. Chromatography Overview Principles. Gas Chromatography Pyrolysis Mass Spectrometry. Headspace Analysis Static Purge and Trap. Infrared Spectroscopy Near-Infrared Industrial Applications. Liquid Chromatography Normal Phase Reversed Phase Size-Exclusion. Microscopy Techniques Scanning Electron Microscopy. Polymers Natural Rubber Synthetic. Process Analysis Chromatography. Sample Dissolution for Elemental Analysis Dry... 

Application of infrared spectroscopy to food analysis has made possible the highly automated determination of protein content in foodstuffs. Available commercial instruments allow the simultaneous determination of water, lipid, fiber, and protein content in solid or liquid samples with little if any sample preparation and impressive throughput. 

Installing additional accessories in front of the ion source can render analytes amenable to ionization and subsequent mass spectrometric analysis. On-line sample treatment is especially important when analyzing liquid-phase, complex, and/or concentrated samples. For example a thermal vaporizer was used to enable analysis of liquid samples by a process mass spectrometer designed for gas analysis [196], This system has been successfully implemented in the monitoring of an esterification reaction [197]. The obtained data were in a good agreement with those recorded by in-line mid-infrared spectrometry. The setup incorporated a magnetic sector analyzer with two detectors an electron multiplier detector... 

Preparation of solid samples for analysis by infrared analysis has tended to be much more labor intensive than analysis of liquids. The traditional methods involve preparation of a KBr pellet or a Nujol mull described in sections 25.5 B and 25.5 C. The use of an attenuated total reflectance accessory with an FT-IR instrument has dramatically improved the preparation time previously required. One simply places a small amount of a solid on the instrument. For these laboratories equipped with this accessory, it is strongly recommended for analysis of both solids and liquids (Section 25.1). 

Sloane and co-workers [129] described a specular reflectance system for the infrared analysis of micro-sized samples. They compare the advantages and limitations of this technique with other micro infrared techniques. Samples are mounted on small metal mirrors (Figure 4.20), which reflect the light beam back through the sample. A transmission spectrum is obtained but the effective path length is twice that of the actual sample thickness and a given absorption band consequently has twice the absorbance obtained by conventional transmission measurements. This system was applied successfully to gas chromatographic fractions, and is particularly useful for the examination of non-volatile liquids such as, for example, dioctyl phthalate. Crystalline solids are easily deposited and... 

With modem sampling techniques, good quantitative infrared analysis with virtually every type of sample is practicable however, liquids are ideal for this purpose, being measured in a liquid cell of fixed thickness, either as 100% sample or diluted with solvent. In this connection it should be taken into account that there are no ideal solvents for infrared spectroscopy [35]. In addition, because absorption bands and path length can be influenced by the temperature of the transmission cell, it is advisable to control the temperature. 

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