Mulling is the process of grinding up a sample into fine powder through

mortar and pestle A mortar and pestle is a set of two simple tools used to prepare ingredients or substances by compression (physics), crushing and shear force, grinding them into a fine Paste (rheology), paste or powder in the kitchen, laboratory, and pharmacy. ...

that is dispersed in a

paraffin Paraffin may refer to: Substances * Paraffin wax, a white or colorless soft solid (also in liquid form) that is used as a lubricant and for other applications * Liquid paraffin (drug), a very highly refined mineral oil used in cosmetics and for med ...

for

infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functio ...

. The resulting powder is sometimes called a mull.

Sample preparation

Using a nonporous ceramic

, a small quantity of the solid sample is ground up until the sample is exceedingly fine and has a glassy appearance. A drop of a mulling agent (see below) is added to the ground solid in the mortar. The mixture is further ground up to acquire a uniform paste with the consistency of toothpaste. The resulting paste is transferred to a salt plate (

sodium chloride Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...

) with a small flat spatula. Applying another flat surface and sandwiching the paste under gentle compression renders the sample ready for analysis.

Mulling agents

There are a variety of

mineral oil Mineral oil is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum, as distinct from usually edible vegetable oils. The name 'mineral oil' by itself is imprecise, ...

s used as mulling agents, their relevant differences being the absorption bands in their respective infrared spectra. The mineral oil most commonly used in mulling is

Nujol Nujol is a brand of light paraffin oil that was produced by Schering-Plough that was commonly used in infrared spectroscopy. As a paraffin oil it is largely chemically inert and has a relatively uncomplicated IR spectrum, with major peaks between ...

, which is essentially a liquid-paraffin-based solution. When it is used for mulling, its carbon-to-hydrogen bonds exhibit strong absorption in the infrared spectrum. These absorptions are so strong that they may mask those of any C-H bonds that may be present in the sample itself. Another mulling agent that is commonly used is Fluorolube, essentially a

fluorocarbon Fluorocarbons are chemical compounds with carbon-fluorine bonds. Compounds that contain many C-F bonds often have distinctive properties, e.g., enhanced stability, volatility, and hydrophobicity. Several fluorocarbons and their derivatives are ...

-based solution that exhibits strong carbon-to-fluorine bond absorptions from 1300 cm⁻¹ onwards to 400 cm⁻¹ in the

mid-infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of ...

spectrum. The useful range for observation of a sample in a mid-infrared spectrum when using Fluorolube as the mulling agent is 4000 cm⁻¹ to 1300 cm⁻¹. Because of these two agents’ complementary absorption processes, it is common to run a sample as both a Nujol mull and a Fluorolube mull separately. This allows for all of the spectral features of the sample to be seen in an infrared spectrum, since the regions masked by each specific mulling agent are unaffected by the other.

References

{{Reflist Infrared spectroscopy