Direct quantitative determination of cyanamide by stable isotope dilution gas chromatography-mass spectrometry

Cyanamide is a multifunctional agrochemical used, for example, as a pesticide, herbicide, and fertilizer. Recent research has revealed that cyanamide is a natural product biosynthesized in a leguminous plant, hairy vetch (Vicia villosa). In the present study, gas chromatography-mass spectrometry (GC-MS) equipped with a capillary column for amines was used for direct quantitative determination of cyanamide. Quantitative signals for ( ¹⁴N₂)cyanamide, (¹⁵N₂)cyanamide (internal standard for stable isotope dilution method), and m-(trifluoromethyl) benzonitrile (internal standard for correcting errors in GC-MS analysis) were recorded as peak areas on mass chromatograms at m/z 42 (A₄₂), 44 (A₄₄), and 171 (A_IS), respectively. Total cyanamide content, (¹⁴N₂)cyanamide plus (¹⁵N ₂)cyanamide, was determined as a function of (A₄₂ + A ₄₄)/A_IS. Contents of (¹⁴N₂)cyanamide and (¹⁵N₂)cyanamide were then calculated by multiplying the total cyanamide content by A₄₂/(A₄₂ + A₄₄) and A₄₄/(A₄₂ + A₄₄), respectively. The limit of detection for the total cyanamide content by the GC-MS analysis was around 1 ng. The molar ratio of (¹⁴N₂)cyanamide to ( ¹⁵N₂)cyanamide in the injected sample was equal to the observed A₄₂/A₄₄ value in the range from 0.1 to 5. It was, therefore, possible to use the stable isotope dilution method to quantify the natural cyanamide content in samples; i.e., the natural cyanamide content was derived by subtracting the A₄₂/A₄₄ ratio of the internal standard from the A₄₂/A₄₄ ratio of sample spiked with internal standard, and then multiplying the resulting difference by the amount of added (¹⁵N₂)cyanamide (SID-GC-MS method). This method successfully gave a reasonable value for the natural cyanamide content in hairy vetch, concurring with the value obtained by a conventional method in which cyanamide was derivatized to a photometrically active compound 4-cyanimido-1,2-naphthoquinone and analyzed with reversed-phase HPLC (CNQ-HPLC method). The determination range of cyanamide in the SID-GC-MS method was almost the same as that in the CNQ-HPLC method; however, the SID-GC-MS method was much simpler than the CNQ-HPLC method.