Difficulties in Differentiating Natural from Synthetic Alkaloids by Isotope Ratio Monitoring using 13C Nuclear Magnetic Resonance Spectrometry

Forensic application of position-specific isotopic analysis of trinitrotoluene (TNT) by NMR to determine 13C and 15N intramolecular isotopic profiles

2,4,6-trinitrotoluene (TNT) is a molecule which is easily identified with current instrumental techniques but it is generally impossible to distinguish between sources of the same substance (TNT). To overcome this difficulty, we present a multi stable isotope approach using isotope ratio monitoring by mass spectrometry (irm-MS) and Nuclear Magnetic Resonance spectrometry (irm-NMR). In the one hand, irm-MS provides bulk isotopic composition at natural abundance in ¹³C and ¹⁵N. The range of variation between samples is rather small particularly for ¹³C. In the other hand, irm-¹³C NMR and irm-¹⁵N NMR enable the determination of positional intramolecular ¹³C/¹²C ratios (δ¹³C_i) and ¹⁵N/¹⁴N ratios (δ¹⁵N_i) with high precision that lead to larger variation between samples. The present work reports an application of the recent methodology using irm-¹⁵N NMR to determine position-specific ¹⁵N isotope content of TNT. The interest of this methodology is compared to irm-¹³C NMR and irm-MS (¹³C and ¹⁵N) in terms of TNT samples discrimination. Thanks to the use of irm-NMR the results show a unique isotopic fingerprint for each TNT which enable origin discrimination between the samples without ambiguity.