ASSAY OF NDELA 339 •7.! :, \ •..• Figure 6. 10 ng bis-(O-trimethylsilyl)NDELA sample chromatogram. and allowed to sit in the dark overnight. A 1-1zL aliquot was removed by Hamilton syringe and injected into the GC system, with a column temperature of 170øC and other instrument parameters as described earlier. The sample concentration may be calculated from the standard curve or, proportionally, from a tandem standard sample peak area on the same order of magnitude as the sample. The derivatized NDELA present in the reaction flask can be calculated and the concentration in the sample matrix expressed as nanograms of NDELA found divided by the total beginning sample weight in grams, giving a concentration in ng/g or ppb. An identical sample should then be spiked with approximately the same concentration as was found in the original sample and analyzed in an identical manner to calculate the efficiency of the cleanup recovery procedure. For example, a 12-g sample of hand cream was spiked with 20 ppb of NDELA, extracted, derivatized with acetic anhydride, and chromatographed (Figure 9). The peak area is that of a 0.8-ng NDELA sample: (0.8-ng peak/ltzL analyzed volume) X 250-tzL reaction volume = 200 ng. The sample was spiked with 240 ng of NDELA (20 ng/g X 12-g sample) for a recovery efficiency of (200 ng recovered/240-ng spike) x 100 = 83%. Analysis of five similarly spiked hand cream samples yielded a mean recovery of 81% with a standard deviation of 7. DISCUSSION The procedure developed for cosmetic cleanup is based on that described by Black et al.

340 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 100_ 90- 8O 60- i 40- 30- I1"' 20 10 a o ..... '•b'" 87 74 NDELA-OAC 116 145 103[ 75 m/Z 100- 90- 80- z 7o- z 60- 133 50- i•• 40- 30- U.I r3 103 130 144 NDELA-TMS 0 263 100 150 200 300 D m/z Figure 7. Mass spectra of b/s-(O-acetyl)NDELA (a) and b/s-(O-trimethylsilyl)NDELA (b) derivatives.