Sampling Techniques for NIR Spectroscopy

NIR spectroscopy has a real advantage over many spectroscopic techniques in that many plastic and glass materials are transparent to NIR radiation at the common thicknesses encountered for films, packaging materials, and coatings. It is practical to take an NIR spectrum of a sample without even 

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With a fiber optic dip probe, many liquids and solutions can be analyzed with no sample preparation. The use of a dip probe for transmission measurements requires that the liquid or solution be free from small particles or turbidity. Suspended particles scatter light and reduce the sensitivity of the measurement. The already low absorptivity of NIR bands makes transmission measurements of limited use for liquid samples that are not clear. 

There are a number of solvents that can be used to prepare solutions for NIR measurements. Carbon tetrachloride and carbon disulfide are transparent over the entire NIR range. Many other organic solvents are transparent up to 2200 nm, with only a short region between 1700 and 1800 nm obscured by the solvent. Solvents as varied as acetonitrile, hexane, dimethyl sulfoxide (DMSO), and dibutyl ether fit into this category. Methylene chloride and chloroform can be used up to 2600 nm, with short gaps at about 1700 and 2300 nm. 

For reflectance measurements, most solids require no sample preparation. Powders, tablets, textiles, solid chunks of material, food, and many other solids are analyzed as is . Samples can be analyzed through plastic bags, glass vials, or in sample cups. Examples will be discussed in the applications section. NIR reflectance spectra of solids are often plotted as the second derivative of the spectrum. This format shows small differences between samples more clearly and eliminates scatter and slope in the baseline. 

Polymer films can be measured in either reflectance or transmission mode. For transmission, the film may be taped across an IR transmission card or cardboard slide mount. 

Gas samples are handled by filling the same type of gas cell used for mid-IR gas analysis, except that the cell windows are of quartz instead of salt. 

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