342 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table II Mass Spectral Peak Assignments for Bis-(O-trimethylsilyl)NDELA ON-N 0 / CH2-CH2-O-C-CH3 CH2-CH2-O-C-CH 3 II o Relative Elemental Proposed Fragment mlZ Intensity Composition Structure 218 3.8 CsH14N205 M ß 188 7.5 CsH14NO4 M ß _ NO 175 11.0 C6H11 N204 M (• - CH3CO 145 48.7 C6H11NO3 e//CH-CH2-O-C-CH3 N II CH2CH2_OH O 116 58.6 CsH9NO2 '• •/CH2 CH2CH2-O-C-CH3 II o 103 18.1 C4H9NO2 H2N - CH2CH 2 -O - C- CH 3 II o / \ 87 100.0 C4H702 O-.•O 74 32.6 C3H502 [ O line observed for NDELA-OAc (Figure 5) may reflect an intrinsically more sensitive detector response for this derivative alternatively, it may suggest that the acetylation reaction was more complete than the silylation reaction under the conditions used in this study. In practice, either derivative will function satisfactorily for the assay of most samples. However, NDELA-TMS must be prepared immediately before analysis, while NDELA-OAc has the advantage of greater stability and, accordingly, can be stored with refrigeration for somewhat longer periods (i.e., overnight) without suffering appre- ciable degradation. Analogously, acetylated standards can also be stored under refriger- ated conditions with no subsequent detectable changes in analytical results.

ASSAY OF NDELA ,, 343 Figure 8. Chromatogram of NDELA (as NDELA-TMS) recovered from 100-ppb spiked lotion. GC column temperature: 130øC. Other operating conditions were as described in text. High levels of reactor by-products can overwhelm the ELCD when working at high sensitivity. Generally, if an interfering peak is encountered from a sample matrix which cannot be resolved by simply changing chromatographic conditions, preparation of the other derivative and use of appropriately modified chromatographic conditions will allow satisfactory resolution of the sample and interference peaks. A ceramic reaction tube was used in this study and is a relatively sturdy, low-cost component the use of a gold or quartz tube would be expected to give similar results. Certain pyridine-soluble, non-volatile polar compounds are left in the derivatized sample after cleanup. These materials may eventually degrade column performance, so the use of GC precolumns is recommended. This is particularly necessary if larger derivatized sample volumes are injected to increase sensitivity. Mass spectrometry is the best means of confirming the presence of nitrosamines in critical analyses. The use of UV radiation to cleave the N-NO bond, thereby destroying the nitrosamine, has been described as a confirmatory technique (48). The absence of a chromatographic peak following irradiation of the sample lends credence to the analysis. CONCLUSIONS The gas chromatograph equipped with an electrolytic conductivity detector (ELCD) is capable of high sensitivity and selectivity for NDELA. This work demonstrated the sensitivity for NDELA, as NDELA-OAc, was 0.25 ng in MeOH. Approximately 81%