SPECTROPHOTOMETRY AND MEDICINAL LIQUID PARAFFIN 5 E(1%, 1 cm.)= 0.100 the absorption ratio must be at least 2-0 for a stable material. This numerical relationship holds as long as at 2710 ,& the absorption intensity does not fall below E(1%, 1 cm.)= 0.010. We require, therefore, the combination of two figures, the absorption intensity at 2710 A and the ratio of the absortion intensities at the maximum and minimum in order to assess the stability of a particular liquid paraffin. The more highly refined a liquid paraffin is the lower is its content of alkylated octahydrophenanthrenes, so that with increasing degree of refining the band structure in the region between 2400 A and 2800 A becomes less pronounced. It disappears alto- gether for extremely pure products, and experience shows that in these cases E(1%, 1 cm.) must be smaller than 0.006 for a stable material. This does not mean that any liquid paraffin with an absorption at 2710 .i of E(1 ø/o. 1 cm.)-• 0.006 must be stable. Normally, in stable liquid paraffins the proportion between "Group A" and "Group B" compounds is ten to one. There are, however, cases where the excess of the "Group A" compounds is appreciably smaller than this, with the result that the wavelength position of the minimum is shifted from its usual position at 2490 i to some•vhere between 2500 )• and 2600 A (Fig. 2). This, then, is also a sign of instability. 022 018 014 010 2.400 SAMPLE p 248- GROUP C i i 2,500 •,600' •,700 WAVELENGTH IN 2,800 2,900 Fig. 2. Ultra-violet absorption spectrum of an unstable liquid paraffin.

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Even when the absorption is so low that the spectrum shows no band struc- ture and the condition is fulfilled that at 2710 A the absorption intensity is E(1%, 1 cm.) 0.006, instability is indicated by a displacement of the "knee" of the absorption curve to wavelengths between 2500 A and 2650 A (Fig. 3). 0016 0014 0012 o • 0010' ,.- ,.,., 0.008' 0 006 0.002 200 2,400 2,6ffi [ 2,8• 3,•0, 32• Fig. 3. Ultra-violet absorption spectra of two unstable liquid paraffins. Whilst the absorption intensity is very low in both cases at 2700 J•, the "knee" of the absorption curves is at the comparatively long wavelength of 2600 A. DAYLIGHT EXPOSURE OF LIQUID PARAFFIN The ultimate commercial criterion for liquid paraffin is that the user demands a product which is water-white and free from odour and taste. These are subjective criteria which are satisfied fairly easily with stable materials. When an unstable material "goes off," it assumes a pungent, nauseating odour. This odour is persistent and increases in strength with shelf-life. There is, however, an essentially different type of odour which people with a sensitive nose sometimes spot even with stable liquid paraffins. This is an evanescent odour which disappears very quickly on unstoppering the con- tainer and which does not increase with shelf-life. It is a petroleum odour which is inevitable when the material is kept in a warm place. When judging the quality and stability of liquid paraffin it is most important that one is capable of discriminating between this evanescent odour and the repulsive persistent odour of an unstable material. The U.S. Pharmacopoeia makes