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 (AIS), respectively. Total cyanamide content, (14N2)cyanamide plus (15N2)cyanamide, was determined as a function of (A42 + A44)/AIS. 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.