j. Soc. Cosmet. Chem., 30, 73-79 (March/April 1979) The analysis of 2,2'-dithiobis-pyridine-l,l'.dioxide and related compounds in clear antidandruff shampoos via reverse-phase liquid chromatography ROBERTJ. FENN and DAVID A. CSEJKA Olin Research Center, 275 [F inchester Ave., New Haven, CT 06511. Received August 8, 1979. Synopsis A REVERSE-PHASE LIQUID CHROMATOGRAPHIC METHOD is described for the ANALYSIS of 2,2'-DITHIOBIS-PYRIDINE-I,I'-DIOXIDE (I) and related compounds in some CLEAR ANTIDANDRUFF SHAMPOO formulations. Other compounds simultaneously quantitated are: 2-pyridinethiol, 2-pyridinesulfonic acid and 2-pyridinesulfonic acid 1-oxide. Another related compound, 2,2'-dithiobispyridine, is analyzed separately. The analysis of (I) offers increased precision and accuracy and decreased analysis time compared to the commonly used polarographic method. In addition, some of the other compounds of interest cannot be conveniently analyzed polarographically, but are easily quantitated by liquid chromatography. INTRODUCTION The magnesium sulfate adduct of 2,2'-dithiobis-pyridine-l,l'-dioxide (OMADINE © MDS Registered Trademark of Olin Corp., Stamford, Conn.), is an excellent antimicrobial agent and is presently used as an antidandruff ingredient in numerous clear shampoo formulations. A typical concentration of 2,2'-dithiobis-pyridine-l,l'- dioxide (OMDS) is 1%. A fast, accurate analytical method for the analysis of this compound is of obvious interest, both as a quality control tool and as a means of determining its stability in various formulations. Particularly since applications are in personal care products such as shampoos, the analysis of the commonly known degradation products of OMDS is also of interest. These compounds include 2-pyridinethiol-l-oxide (OM), 2-pyridine- thiol (2MP), 2,2'-dithiobis-pyridine (PDS), 2-pyridinesulfonic acid 1-oxide (OMSA) and 2-pyridinesulfonic acid (PSA). With regard to specific analytical methods for these compounds, the most commonly used at present are polarographic (1-3) iodimetric (4) and, in the case of OM, colorimetric (5). In this laboratory polarographic methods have, until recently, been used almost exclusively. Several problems are encountered with these methods, 73

74 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS however. For example, polarographic analysis of 2MP in the presence of a large excess of OM, or of PDS in the presence of a large excess of OMDS (for which additional problems exist) are difficult due to the close proximity of the corresponding polarographic half-wave potentials. Accurate analysis of low concentrations of 2MP and PDS in OMDS shampoos is thus not feasible polarographically. The iodimetric determination of OMDS in clear shampoos involves the prior conversion of OMDS to OM and is, therefore, subject to interference from this compound. On the other hand, in the colorimetric analysis of OM, PDS is an interferent. The present study describes the analysis by reverse-phase LC of the compounds listed above, with the exception of OM. This compound, because of its strong complexing ability, ease of oxidation and poor ultraviolet stability, presents severe obstacles to direct LC analysis. Reverse-phase LC with UV detection is particularly attractive for the present application for several reasons: The compounds of interest are water soluble and are present in a largely aqueous medium. There are very few UV-active components present in most clear shampoo formulations and the separation of functionally very similar compounds (e.g., OMDS and PDS, OMSA and PSA), which are difficult to distinguish by other means, are achievable. Those other UV-absorbing components, such as fragrance and color components, present in the shampoos studied here are easily separated from the compounds of interest. In addition, LC analyses can be easily automated using automatic sample injectors. EXPERIMENTAL APPARATUS Chromatograms were obtained using a Waters Associates Model ALC/GPC 244 high pressure liquid chromatograph equipped with a Model U6K injector and Model 440 ultraviolet absorbance detector operated at 254 nm. A Waters Associates/.t Bondapak C•, reverse-phase column was used for all separations. REAGENTS 2-Pyridinethiol, 2,2'-dithiobis-pyridine, 2-pyridinesulfonic acid, 2-pyridinesulfonic acid 1-oxide and 2,2'-dithiobispyridine-l,l'-dioxide were prepared and purified by the Biocides Group, Chemicals Division, Olin Corporation. Acetonitrile (UV) was obtained from Burdick & Jackson Laboratories and the tetrabutylammonium hydrox- ide, 40% aqueous, from Aldrich Chemicals. All other reagents were analytical reagent grade. PROCEDURES Mobile phase was prepared by dissolving in 1 1 of the appropriate acetonitrile/water mixture (as discussed below): 5 ml of a 40% aqueous tetrabutylammonium hydroxide solution 4 ml of glacial acetic acid and 0.5 ml of a saturated Na2EDTA solution. Standard solutions were prepared in the appropriate mobile phase. All shampoo samples were prepared by weighing approximately 1 g to __+ 0.1 mg of shampoo into a 10-ml volumetric flask and bringing to volume with mobile phase. Quantitation was accomplished by peak height measurements for 10/.tl injections in all analyses.