The stability of a sull)hosuccinated monoethanolamide 347 working standard which would not have been satisfactory since its long term stability was unknown. Results obtained with this procedure over several months have shown that in routine use the 95•o confidence limits (2 ,) are q- 0-05•o w/w DSUM at a nominal 15/o w/w concentration. From the results of the preliminary studies it was apparent that the colorimetric assay procedure is specific for DSUM in the presence of its decomposition products formed by alkaline hydrolysis. If acidic hydrolysis occurs, the method may not produce valid results as the decomposition may proceed only as far as the formation of (IV) and (V), (Fig. 1), the latter containing an ester group which would respond to the colorimetric assay in a similar way to DSUM. This aspect of method specificity was investigated in conjunction with a storage test using TLC to detect degradation products. /SO3No C H::,: CH- (C H ::, )8-CO ß NH. C 2H.qO::,C. CH::, 'CH T \CO:,No /SO• N• X•ux CH•:CH. (CH2)•-COzH + H::,N' C:,H,,,.O•,C- CH::,.CH CH::,=CH-(CH:, )•.CO.NH.C•.H4OH T•- • \COBH 3M HCI reflux SO No H::,N.C•,H.q.OH + HO::,C'CH. ,'CH• X7T TITa CObH + /S O. ....3No No O2C. CH ,- C HXCO• No Figure 1. Routes of breakdown of DSUM by hydrolysis. Storage test oJ'shampoos The second part of the investigation took the form of a storage test during which the colorimetric assay was used in conjunction with TLC screening. The stability of DSUM (1.05/o w/w) in citrate buffered shampoo containing 175/o triethanolamine lauryl sulphate was monitored as a function of apparent pH and of temperature. Control samples con- taining no DSUM were also monitored to provide 'blank' values. The DSUM assay results are presented in diagrammatic form in Fig. 2 (a) and (b) and show clearly the dependance of stability on apparent pH. The loss of DSUM follows a first order rate equation. The concentration axes in Fig. 2 (a) and (b) are logarithmic and the plots are therefore linear within the limits imposed by the assay procedure. At an apparent pH of 7 DSUM is rather unstable, 255/o decomposing in 12 weeks at 20 ø, while at 37 ø 585/o was lost over the same period. Confirmation of the validity of these results was obtained from the TLC screening test. A typical chromatogram is shown schematically in Fig. 3. The separated components are seen as coloured fluorescent zones. In addition to un- identified spots at R, zero, 0.4-0.6 and 0.95 a clearly separated spot, having a pink

348 D. W. Whymark i loo 90 '• 80 ._ ,• 70 _o 6o •. 50 :2). I _ (o) _ (b) • • ! • I ' I I 4 8 12 4 8 12 Time/weeks Figure 2. The loss of DSUM from shampoos at various pH values (a) at 20øC, (b) at 37øC. Key: I pH 5.0 O pH 6.5 ApH 7.0. RF 0'95 0'90 0'85 0'75 0'4 to 0'6 0'14 A B C D E Solvent front Origin Figure 3. Schematic representation of developed TLC plate viewed under a UV lamp (365 nm). :., The extent of shading of the spots indicates approximate visual intensity. (A) Extract from .:: shampoo containing DSUM and its breakdown products. (B) Solution of II--product of alka- ß line hydrolysis. (C) Solution of IV--product of acidic hydrolysis. (D) Extract from blank :. shampoo. (E) Extract from aqueous DSUM solution. fluorescence, was seen at RF {•14. This corresponds in colour and mobility to II, •:: undecylenic monoethanolamide, prepared from DSUM as described above. Trace •:: amounts of the products of acidic hydrolysis were found in all samples including the original DSUM raw material, but only II was seen to increase in concentration on storage. Some approximate results obtained by TLC spot intensity comparisons as described above are shown in Table L From the chromatograms it was immediately evident that the shampoos at an ap- ß parent pH of 7 and stored at 37øC contained much more II than those at lower apparent .•: pH. Furthermore the estimated quantities of II (Table I) corresponded quite closely with i:i: i .