Analytical Methods:IR Spectroscopy

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Questions
Infrared spectroscopy has been used to identify clay minerals and humic matter in soils. It analyses the absorption of infrared radiation by the sample under investigation. The infrared region of the electromagnetic spectrum is located between the visible and the radio wavelength regions, in the wavelength band between 750nm and 1mm. The absorption of infrared radiation depends on the vibrations of the atoms within the substance. These vibrations produce periodic displacement of the atoms with respect to each other, resulting in a change in interatomic distance. When the vibrations are accompanied by a change in dipole moment, and when the frequency of vibration coincides with that of the infrared light, the vibration will absorb infrared radiation. The advantages of infrared techniques over other analytical techniques include:
 * 1) minimal sample preparation
 * 2) a short turn around time at the laboratory
 * 3) the need for only basic infrastructure
 * 4) minimal training of staff
 * 5) simultaneous determination of several constituents in every sample and
 * 6) the ability to analyse samples remotely. ie spectra are acquired electronically and can therefore be transported electronically.

All of these advantages contribute to a reduced cost of analysis.

Case studies where infrared spectroscopy has been used in archaeology include:

Samples
Samples can be prepared in two major ways. The first is to crush the sample with a mulling agent in a marble or agate mortar, with a pestle. A thin film of the mull is applied onto salt plates and measured. The second method is to finely grind the sample with a specially purified salt to remove scattering effects from large crystals. This powder mixture is then crushed in a mechanical die press to form a translucent pellet through which the beam of the spectrometer can pass.

Analysis
Infrared light energy is focused onto the surface of the air-dry soil sample. Some of this light is absorbed by the soil, but the rest is reflected back into the spectrophotometer, detected and analysed. The resulting spectrum is determined by the nature of the soil; specific vibrational signatures can be seen for organic matter (OM) and for minerals such as quartz, kaolinite and smectite, carbonates, gypsum, and iron and aluminium oxides.

Data and interpretation
Infrared spectroscopy can predict soil physical, chemical, and biological properties. Quantitative predictions of several important soil properties have been made. These properties are important in assessing soil fertility, agricultural practices and land degradation. They include:
 * 1) organic carbon: Soil organic carbon is an indication of soil organic matter content, which acts as both a source and sink for nutrients. Soil organic carbon is linked to soil chemical, physical and biological health, and is strongly correlated with soil nitrogen supply.
 * pH: Techniques that promote the measurement of soil pH, the determination of the rate of lime required to achieve an acceptable pH, and the quality of lime products, will greatly aid the management of soil acidity.
 * 1) iron and aluminium oxide content: Soil iron and aluminium oxides bind phosphate that may otherwise be displaced from the soil rooting depth. Displaced phosphate is not only a loss in potential crop productivity, but in many regions results in the eutrophication of wetlands and waterways.

Other soil properties that have been predicted with infrared technology are total nitrogen, carbonate, lime requirement, cation exchange capacity and soil texture.