A simple rapid method to precisely determine 13 C/12 C ratios of plant methoxyl groups

Stable isotope ratios of individual plant components have become a valuable tool for the determination of the geographical origin and authenticity of foodstuff. A recently published method with considerable potential in this context is the measurement of the deuterium/hydrogen (D/H) isotope ratios of plant matter methoxyl groups. The method entailed cleavage of methyl ethers or esters with hydriodic acid (HI) to form gaseous methyl iodide (CH3I) and then measurement of the d 2 H value of this gas. Here, as a follow up to a previous study, we describe a method for the rapid and precise d 13 C analysis of plant matter methoxyl groups using gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). Conditions for sample preparation were investigated for isotope discrimination effects, the GC conditions optimized, the reproducibility of the measurement of standards undertaken, and the precision of the method defined. The reproducibility of the d 13 C value determined for a CH3I standard on 20 consecutive measurements was found to be 0.17%. The method was also tested on four methoxyl-rich plant components: vanillin, lignin, wood and pectin. The analytical precision obtained, expressed as the average standard deviation, for these compounds was found to be better than 0.13%. The described procedure which is simple and rapid, allowing preparation and analysis of a sample within 1h, produces accurate and reproducible isotopic measurements. We suggest that this validated d 13 C method when employed together with the recently published d 2 H method for two-dimensional stable isotope studies of organic matter