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Sample presentation

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  432

2026-11-14

31

Sample presentation

IR spectra can be measured in the gas, liquid or solid phase. However, most compounds of interest are solids at room temperature. In principle, an IR spectrum can be obtained from as little as 1 lg of a compound. From a practical point of view, quantities of the order of 200–1000 lg are much easier to handle. Very small quantities require greater sensitivity to be achieved with microcells, placing as much material as possible in the IR beam. Microcells require beam-condensing optics to focus as much light as possible through the microcell assembly. Glass and silica contain SiO-H and Si-O bonds that can vibrate and strongly absorb IR radiation. Therefore, cell windows need to be fabricated from ionic materials with bonds that do not have vibrations. Crystal lattice vibrations cause absorption in the far IR. Barium fluoride (and calcium fluoride) are excellent for aqueous media, although window absorption prevents measurements below 1000 cm^-1. Silver chloride windows are water resistant and will allow trans mission down to 400 cm1, but are friable. The most popular IR window material is sodium chloride, which is transparent down to 600 cm^-1. However, NaCl is a water-soluble material and caution needs to be exercised in solvent choice when preparing thin films from solution on this material.