A Superior Method of Mulling Samples for Infrared Spectroscopy I n mulling solids for infrared spectroscopy, i t is desirable, in t h e interest of highest resoltition and minimum light scattering, t o reduce the sample t o the smallest particle size consistent with economy of time and effort. I t is often necessary t o compromise these requirementwin order to protect an air- or moisturesensitive sample from pralonged exposme when t h e a~bst,aneeis mulled with a mort,ar and pestle or with a spatola. The procedure of crushing and mulling samples directly between salt plat,es results so frequently in salt plates being scored by abrasive samples or imporities, broken by uneven pressure, and/or fogged by handling t h a t it,s widespread w e is astonishing. I t is possible t o mull aolids quickly and easily with complete protection from air and moisture (and a t no hazard t o t h e salt plates) by grinding with t h e mulling agent (mineral oil, perfluorokerosene, hexachlorohutadiene, etc.) between ground glass . plate*. Two ordinary pieces of glass, such a. those commonly used in general chemistry laboratories t o cover gas collection bottles, may he used. Plates approximately 8-9 cm square have been found curlvenient to handle. They may be ground against each other by hand t o asnfficiently close fit ill about 10 min using 400or 000-mesh C a r h w m ~ d n mwspended in water or in aqueous glyocnJ. T h c lit is complete when rinsing dincloses all weas of tach p1at.e t o have a t~niformlyfrosted appearance.
I n use, a few milligrams of t h e solid sample and a small drop of the mulling agent are placed in the center of one of t h e ground plates, covered with tho other, and the sandwich ground with a twisting motion between the hands. The user quickly learns t o .C of nrrd fcx more ~ w o g u w rhy f r d b11d X ~ ~ C X ~ H I I (ind~ > c r a ~ f - d from theieparated glass plates wiih a razor blade and wiped onto a salt plate where i t is readily spread miformly by t h e second salt plate wibh only slight pressure and motion. I n almost every cme, this method produces uniform molls of such fine particle size t h a t fine print can easily be read through t h e salt plate sandwich; t h e spectra. produced commonly display resolution comparable wit,h t h a t expected from solution spectra. Only infrequently are difficulties encount,ered when certain solid8 tend t o form goma tmder the slight heat and pressure of mulling. These are usually substances, however, which behave in this same way under any of the other commou mulling techniques and would probably be best handled as KBr pellets. If i t is necessary t h a t such a sample be examined as a m d l , the method described above still is preferable to others.
Volume 46, Number 5, Moy 1969
/
301
