*2.7. Instrument Analysis*

The samples were analyzed using a 7890A gas chromatograph with a 5977B mass spectrometer. A DB-5ms capillary column was used for separation. Helium (99.9999% purity) was used as a carrier gas, with a 0.8 mL/min constant flow rate. Injection and ion source temperatures were set at 230 ◦C. The oven temperature program was as follows: initial temperature of 80 ◦C, 10 ◦C/min increase to 180 ◦C, 3 /min increase to 240 ◦C, and finally a post run at 250 ◦C for 15 min. The 1.0-μL sample volume was injected in splitless mode.

The mass spectrometer was operated in its SIM mode for quantification. The monitored fragment ions had m/z values of 121, 135, 149, and 163 for NP, m/z values of 107 and 220 for 4-n-NP, and m/z values of 113 and 226 for 4-n-NP RING-13C6.

The retention time and the qualifier and quantifier ions of each compound are listed in Table 1. The total ion chromatogram of analytes and the chromatogram of selected ions for 4-n-NP and 4-n-NP 13C6 are shown in Figures 1 and 2, respectively.

**Table 1.** Mass spectral parameter of the analyzed compounds.


\* Internal Standard.

**Figure 1.** GC/MS total ion chromatogram of analytes. Peaks: 1, NPs; 2, 4-n-NP and 4-n-NP 13C6.

**Figure 2.** Selected ion chromatogram of quantifier ions for 4-n-NP and 4-n-NP 13C6.
