Question

1. Can you perform quantitative analysis by IR? How? what difficulties will you encounter? how could you deal with these?

2. Explain ATR sampling method and are any corrections necessary to be performed on ATR spectra? why?

Answer 1

1 Infrared spectroscopy is most often used for qualitative identification. An unknown material can be determined by comparing the infrared spectrum acquired on this sample to the spectra of known compounds. For a conclusive identification, all features of both spectra that are more intense than instrumental noise must match. Alternatively, IR spectral lines may be interpreted to provide clues to the structure of an unknown. This approach is rarely conclusive, but must be supplemented with other types of evidence.

The infrared spectrum is rich in structural information. The spectral lines are produced by the absorption of incident radiation by the vibrational modes of functional groups in the molecule. The absorptions adhere to Beer's law. Thus, analysis of the infrared spectral band intensities as a function of solute concentration provides a straightforward means for determining the concentration mixture components.

2 An attenuated total reflection accessory operates by measuring the changes that occur in a totally internally reflected infrared beam when the beam comes into contact with a sample. An infrared beam is directed onto an optically dense crystal with a high refractive index at a certain angle. This internal reflectance creates an evanescent wave that extends beyond the surface of the crystal into the sample held in contact with the crystal. It can be easier to think of this evanescent wave as a bubble of infrared that sits on the surface of the crystal. This evanescent wave protrudes only a few microns (0.5 µ - 5 µ) beyond the crystal surface and into the sample. Consequently, there must be good contact between the sample and the crystal surface. In regions of the infrared spectrum where the sample absorbs energy, the evanescent wave will be attenuated or altered. The attenuated energy from each evanescent wave is passed back to the IR beam, which then exits the opposite end of the crystal and is passed to the detector in the IR spectrometer. The system then generates an infrared spectrum.

For the technique to be successful, the following two requirements must be met:

• The sample must be in direct contact with the ATR crystal, because the evanescent wave or bubble only extends beyond the crystal 0.5 µ - 5 µ.

• The refractive index of the crystal must be significantly greater than that of the sample or else internal reflectance will not occur – the light will be transmitted rather than internally reflected in the crystal. Typically, ATR crystals have refractive index values between 2.38 and 4.01 at 2000 cm-1. It is safe to assume that the majority of solids and liquids have much lower refractive indices.