Reference: Ph. Eur.
Infrared spectrophotometers are used for recording spectra in the region of 4000-650 cm (2.5-15.4 μm) or in some cases down to 200 cm (50 μm).
Apparatus
Spectrophotometers for recording spectra consist of a suitable light source, monochromator or interferometer, and detector. Fourier transform spectrophotometers use polychromatic radiation and calculate the spectrum in the frequency domain from the original data by Fourier transformation.
Spectrophotometers fitted with an optical system capable of producing monochromatic radiation in the measurement region may also De used. Normally the spectrum is given as a function of transmittance, the quotient of the intensity of the transmitted radiation and the incident radiation. It may also be given in absorbance.
The absorbance (A) is defined as the logarithm to base 10 of the reciprocal of the transmittance (T):
To,
the intensity of incident radiation,
intensity of transmitted radiation.
Preparation of the sample
For recording by transmission or absorption: Prepare the substance by one of the following methods.
Liquids: Examine a liquid either in the form of a film between 2 plates transparent to infrared radiation or in a cell of suitable path length, also transparent to infrared radiation.
Liquids or solids in solution: Prepare a solution in a suitable solvent. Choose a concentration and a path length of the cell which gives a satisfactory spectrum. Generally, good results are obtained with concentrations of path length of 0.5 - 0.1mm. Absorption due to the solvent is compensated by placing in the reference beam a similar cell containing the solvent used. If an FTIR instrument is used, the absorption is compensated by recording the spectra for the solvent and the sample successively. The solvent absorbance, corrected by a compensation factor is subtracted using calculation software.
Solids: Examine solids dispersed in a suitable liquid (mull) or in a solid (powder) Mull. Triturate a small quantity of the substance to be examined with the minimum quantity of liquid paraffin or other suitable liquid; 5 - 10 mg of the substance to be examined is usually sufficient to make an adequate mull using one drop of liquid paraffin. Compress the mull between 2 plates transparent to infrared radiation.
Powder: Triturate 1-2 mg of the substance to be examined with 300 - 400 mg, unless otherwise specified, of finely powdered and dried potassium bromide or potassium chloride. If the substance is a hydrochloride, it is recommended to use potassium chloride. Carefully grind the mixture, and spread it uniformly on a Sample cup and examine the reflectance spectrum.
Identification using reference substances
Prepare the substance to be examined and the reference substance by the same procedure and record the spectra between 4000-650 cm (2.5-15.4 μm) under the same operational conditions. The transmission minima (absorption maxima) in the spectrum obtained with the substance to be examined correspond in position and relative size to those in the spectrum obtained with the reference substance (CRS).
When the spectra recorded in the solid state show differences in the positions of the transmission minima (absorption maxima), treat the Substance to be examined and the reference substance in the same manner so that they crystallize or are produced in the same form, or proceed as prescribed in the monograph, then record the spectra.
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