Infrared spectrophotometry calibration

Hydrogen fluoride (HF) is determined quantitatively by near infrared spectrophotometry. The spectrum of HF is shown in Figure 6. It was obtained on a Beckman DK-2 spectrophotometer. A calibration... 

Fourier transform infrared spectrophotometry is used widely in the semiconductor industry for the routine determination of the interstitial oxygen content of production silicon wafers. However, the lack of interlaboratory reproducibility in this method has forced the use of ad-hoc calibration methods. The sources of this lack of reproducibility are just beginning to be understood. As investigation of this problem continues and wider acceptance is gained for improved experimental and analytical techniques, a greater degree of reproducibility should be achieved. Furthermore, new standard test methods and standard reference materials being developed by the ASTM (71 ),... 

Total hydrocarbons in the air may be determined using infrared spectrophotometry. The hydrocarbons are collected in a condensation trap immersed in liquid oxygen. The hydrocarbons absorb in the 3- to A-jxnx region of the infrared spectrum using a 20-m pathlength cell. They are expressed as parts per million hexane, and the instrument is calibrated using a hexane standard. 

T oluene diisocyanate in particular can be obtained in a range of mixtures of the 2,4- and the 2,6-isomers. The proportion of the two isomers present can be determined by infrared spectrophotometry. The method is based on the quantitative intensity measurement of absorption bands at 12-35//m and 12-80/im for the 2,4- and 2,6-isomers, respectively. As the heights of the absorption bands do not vary linearly with concentration (Beer s law) it is necessary to construct calibration curves. 

The vanadium eluate was slowly evaporated under an infrared lamp, the residue dissolved in 6 M hydrochloric acid (10 ml) containing 1 ml of the aluminium chloride solution [603], and vanadium was determined by atomic absorption spectrophotometry. For calibration, suitable standard solutions were aspirated before and after each batch of samples. 

Always based on the use of IR spectrophotometry, a novel attenuated total reflection-Fourier-transform infrared (ATR-FTIR) sensor [42] was proposed for the on-line monitoring of a dechlorination process. Organohalogenated compounds such as trichloroethylene (TCE), tetrachloroethylene (PCE) and carbon tetrachloride (CT) were detected with a limit of a few milligrams per litre, after extraction on the ATR internal-reflection element coated with a hydro-phobic polymer. As for all IR techniques, partial least squares (PLS) calibration models are needed. As previously, this system is promising for bioprocess control and optimization. 

Near Infrared Reflectance Analysis (NIRA) is in use at over 5000 sites for the analysis of multiple constituents in food and other products. The technology is based upon correlation transform spectroscopy, which combines NIR spectrophotometry and computerized analysis of a "learning set" of samples to obtain calibrations without the need for detailed spectroscopic knowledge of factors being analyzed. The computer can obtain spectral characteristics of the analyte (based upon a correlation with data from an accepted reference analysis) without separation of the sample s constituents. 

The overtones of CH, OH, and NH valence vibrations appear in the near IR region. Therefore, foods give a large number of absorption bands that can be assigned to definite components and have intensities that correlate with the amounts of the constituents. As an example. Fig. 15.21 shows the absorption of wheat in the near IR region. The sample containing water absorbs at 1.94 pm in addition. Therefore, after subtraction of the absorption of dried wheat and after calibration, the water content can be determined. Other con-situents which can be determined in food by near-infrared (NIR) spectrophotometry are listed in Table 15.34. 

Infrared spectroscopy has originally mainly been used as a qualitative tool, as opposed to UV spectrophotometry, but this situation is now slowly changing. Quantitation requires a calibration curve and/or multivariate analysis in case of mixtures. In view of the frequently low additive concentrations only the most intense bands (e.g. carbonyl bands) can be used for quantitation. 

Current spectrometric monographs include calibration requirements for infrared (IR), near infrared (NIR), ultraviolet and visible (UV-visible), nuclear magnetic resonance (NMR), circular dichroism and polarimetry. Monographs for atomic spectrophotometry, emission and absorption, fluorescence and X-ray fluorescence are available in the European... 

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