Infrared absorbance methods

The primary reference method used for measuring carbon monoxide in the United States is based on nondispersive infrared (NDIR) photometry (1, 2). The principle involved is the preferential absorption of infrared radiation by carbon monoxide. Figure 14-1 is a schematic representation of an NDIR analyzer. The analyzer has a hot filament source of infrared radiation, a chopper, a sample cell, reference cell, and a detector. The reference cell is filled with a non-infrared-absorbing gas, and the sample cell is continuously flushed with ambient air containing an unknown amount of CO. The detector cell is divided into two compartments by a flexible membrane, with each compartment filled with CO. Movement of the membrane causes a change in electrical capacitance in a control circuit whose signal is processed and fed to a recorder. 

Infrared spectroelectrochemical methods, particularly those based on Fourier transform infrared (FTIR) spectroscopy can provide structural information that UV-visible absorbance techniques do not. FTIR spectroelectrochemistry has thus been fruitful in the characterization of reactions occurring on electrode surfaces. The technique requires very thin cells to overcome solvent absorption problems. 

The outcome of this is that although not easily interpretable by inspection (as for example in mid-infrared absorbance spectra) the NIR spectra of hydrocarbon streams possess a very high information content. Use of suitable chemometric methods allows for the reduction of this information content into extremely powerful predictive models for not only chemical compositional properties, but also bulk physical and fuel properties of hydrocarbon process streams and prodncts. ... 

These are GC-IR (5), LC-IR (6), and diffuse reflectance (7). On-the-fly GC-IR systems are commercially available, and lower detection limits are being continually reported. While GC-IR may not replace GC/MS in residue and metabolism work, it can provide valuable data in these areas. On-the-fly LC-IR systems have been developed and are also commercially available. The major problem in these systems is the strong infrared absorbence of many common LC solvents. However, with proper selection of solvents and the development of LC conditions specifically designed for the LC-IR experiment, these problems may be overcome. Recent reports on diffuse reflectance measurements by FTIR indicate the technique may provide a method of examining formulated material or TLC spots with no sample preparation. While this technique is still in the development stage, it may become quite significant in the future. 

C02 in the gas phase can be determined by means of its significant infrared absorbance (Fig. 10) at wave lengths (A) < 15 pm, particularly at 4.3 pm [289], or by acoustic means. Integrated photoacoustic spectroscopy and magnetoacoustic (PAS/MA) technology for combined C02 and 02 analysis has rapid response time and a small sample volume is sufficient. The acoustic methods are accurate, stable over long periods and very simple to use. 

The heterocyclic alkyl quaternary salts used for preparing spiropyrans are usually obtained by N-alkylating the heterocyclic bases by standard methods. The common bases having an active methyl group of the indolenine, benzothiazole, benzoxazole, and quinoline series some of their substitution products, and a few of their quaternary salts are commercially available. The pyrylium, thiopyrylium, 2-azaazulenium and bcnz[c,d]indolenium salts needed for potentially infrared-absorbing spiropyrans are not (as of the time of writing). 

Collect not less than 64 spectral scans of the standards. Constmct a calibration matrix containing infrared absorbance values for unsaturation types in the standards and their known concentrations. Confirm the validity of the calibration matrix model as recommended in the software manual. A recommended method is cross-validation for all standards by sequentially excluding one of the standards from the calibration matrix, then using the remaining standards to predict the concentrations. After validation, determine the optimum number of factors, or loading vectors, needed to minimize the deviation between actual and predicted concentrations. This determination is automated in most multicomponent analysis... 

A new analytical technique, fluorine combustion analysis, is based on the fact that fluorides of elements in high oxidation states are thermodynamically stable, often volatile, and very strong infrared absorbers (Jacob, 1989). This method is especially suited to the... 

Vashburn ry has reported that the potency of erythromycin may be readily determined by infrared absorbance at 10.46 p. Investigation of bands located at 7.29, 9 02, 9 88, and 10.46 p showed that only the latter consistently agreed with activities obtained by the microbiological methods. 

Mrythrornycin estolate has also been quantitated by determining infrared absorbance at 9.9 > , provided that anhydroerythromycin is absent. This compound, if present, increases the apparent potency of erythromycin. In this method, 120.0 mg of sample is added to a 125 ml separatory funnel containing 50 ml IN. iaOH and 50 ml chloroform. After shaking for 1 minute, the chloroform layer is collected. 

The oil and grease content of the refinery wastewater effluent C was determined by the partition infrared method (5520 C) according to Standard Methods (1995). A mixture, by volume, of 37.5% iso-octane, 37.5% hexadecane and 25.0% benzene was used as the reference oil and infrared absorbance was determined using a Perkin-Elmer 881 infrared spectrophotometer. 

The precision of absorbance measurements and the possibilities of storage and re-treatment of the spectra have favoured quantitative analysis by infrared. The method is widely used for the facility with which the absorption bands of a particular compound can be located even in a mixture and because efficient methods of statistical analysis are available for the near IR. However, the development of reliable ETIR instrumentation and strong computerized data-processing capabilities have greatly improved the performance of quantitative IR work. Thus,... 

The microstructures of the polybutadienes, butadiene-styrene copolymers, and polyisoprenes were determined by infrared spectroscopic methods (1,3). The spectra of alkali metal-catalyzed polybutadienes and polyisoprenes show that other reactions occur during polymerization in addition to those involving cis- and trans 1,4, 1,2, and 3,4 additions. For sodium and potassium polybutadienes and polyisoprenes, the absorbances of the bands arising from these additional structures could be taken into account satisfactorily by the methods described. No foreign structures are found in lithium-catalyzed polyisoprenes and the additional band foimd near 14.2 microns in polybutadiene spectra does not appear to affect the cis-1,4 band at 14.7 microns. (Cesium and rubidium, as well as additives such as dimethoxy-tetraglycol, affect the polymerization of butadiene so markedly that it was not possible to obtain satisfactory analyses of such polymers. The effect of these catalysts in isoprene polymerizations does not appear to be so marked and satisfactory analyses were obtained by the method described. 

Various types of low-temperature cells are available commercially. Rochkind (1968) has presented a new low-temperature (20°K) technique, which provides a practical and sensitive method of infrared quantitative analysis of all infrared-absorbing gases and volatile liquids. The method, called pseudomatrix isolation spectroscopy (PMl), also provides a tool for the analysis of complex gas mixtures. The PMI method distinguishes between molecular isotopes, for example. Figure 3.21 shows a PMl spectrum of a mixture of isotopic d2-ethylenes (condensed on a 20 K Csl window). Rochkind claims that equivalent distinguishability has not been demonstrated with gas chromatography. 

A method is claimed [54] for the preparation of 1,1-diphenyl-butadiene derivatives for intermediates for near infrared-absorbing leuco dyes in heat-sensitive recording the following example is given ... 

A classic method is to digest the PHB-containing cell culture with sodium hypochlorite and measure the turbidity of the suspension produced by release of the polymer granules. Alternatively the polymer can be extracted into chloroform solution and its infrared absorbance at 1728 cm determined quantitatively. Another spectroscopic method involves hydrolysing and dehydrating the polymer to crotonic acid with concentrated sulphuric acid and assaying this by conventional quantitative UV analysis. ... 

Peppas [24] showed that polycaprolactam (polyamide 6) crystallises in different crystalline forms, which show different characteristic infrared absorbance bands. Based on this he prepared phase diagrams. He evaluated the absorbance coefficients of the bands from the dependence of their intensity on temperature and specific volume of different polyamide 6 samples and described a method for the quantitative determination of the amounts of alpha and gamma modifications and of the amorphous phase. On the basis of these data the densities of the samples were calculated and shown to agree with the values determined experimentally. The specific volume of the amorphous phase was v = 0.917 cm /g, and was independent of any alpha or gamma modifications the samples may have contained. 

The infrared absorbance at 13.66. m is sufficiently sensitive to the ethylene incorporation to determine the wt% ethylene within 0.1-0.2% at the 95% confidence level. From the C-NMR data, it can be concluded that the propylene units occur in predominantly isotactic, head-to-tail sequences and that the ethylene units are incorporated as isolated units only. Thus, this structural prequisite is a requirement for application of this method because it has not been tested on copolymers containing propylene configurational irregularities or ethylene sequences two units and longer. 

An infrared spectroscopic method for the determination of bound ethylene in ethylene propylene copolymers is described in Method 91. This method utilizes the absorbance of the 732 cm infrared absorption band attributed to (gamma) (CH2)3 groups characteristic of an ethylene unit isolated between two head to tail propylene units ... 

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