Stable hydrogen isotope ratio measurements of specific plant components are increasingly used in numerous fields of research, including sample origin verification and climate research. A recently suggested method with considerable potential in this context is the D/H isotope ratio (delta(2)H value) analysis of plant matter methoxyl groups. The method entails ether or ester cleavage with hydriodic acid (HI) to form the gaseous compound methyl iodide (CH(3)I) and measurement of the delta(2)H value of this gas. Here we describe a method for the rapid and precise delta(2)H analysis of plant matter methoxyl groups using gas chromatography/pyrolysis/isotope ratio mass spectrometry (GC/P/IRMS). The conditions for sample preparation were investigated for isotope discrimination effects, the GC conditions were optimized, the reproducibility of the measurement of standards was studied, and the precision of the method was defined. The reproducibility of delta(2)H values determined for a CH(3)I standard on 20 consecutive measurements was found to be 2 per thousand. The method was also tested on four methoxyl-rich plant components: vanillin, lignin, wood and pectin. The analytical precision obtained, when expressed as the average standard deviation for these compounds, was better than 1.6 per thousand. The described method is rapid, allowing preparation and analysis of a sample within 1 h, and produces accurate and reproducible isotopic measurements.
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 (CH(3)I) and then measurement of the delta(2)H value of this gas. Here, as a follow up to a previous study, we describe a method for the rapid and precise delta(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 delta(13)C value determined for a CH(3)I standard on 20 consecutive measurements was found to be 0.17 per thousand. 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 per thousand. The described procedure which is simple and rapid, allowing preparation and analysis of a sample within 1 h, produces accurate and reproducible isotopic measurements. We suggest that this validated delta(13)C method when employed together with the recently published delta(2)H method for two-dimensional stable isotope studies of organic matter containing methoxyl groups will be of considerable value, e.g. for proving the authenticity of foodstuff.
The characteristics of the new chiral stationary phase heptakis(2,3-di-O-met hyl-6-0-tert-butyldimethylsilyl)-fl-cyclodextrin are outlined and compared with permethyl-and perethyl-P-cyclodextrins. Column C: . T b 7 20 25 min. 20 25 min.Figure 6 Separation of the enantiomers of lavandulyl acetate on columns A and C (coated with 15 and 5 0 % of DIME-6-TBDMS-P-CD in OV-1701-vi, respectively): conditions as for Figure 3.phases. Figure 7 shows a test for chiral GC separations on a micro preparative scale, the separation of 1-(2-methoxypheny1)ethanol on a 0.54 mm i.d. fused silica wide bore column. In this instance SE-52 was used as solvent for the modified cyclodextrin.
ConclusionThe introduction of the bulky TBDMS groups slightly changes, but does not reduce, the chiral selectivity, and it is, furthermore, evident that the chiral selectivity and versatility of this new chiral stationary phase are at least as high as those of PME-P-CD.
Gas chromatography/pyrolysis/isotope ratio mass spectrometry (GC/P/IRMS) is a relatively new method for on-line determination of (2)H/(1)H isotope ratios. The influence of different parameters on the (2)H/(1)H isotope ratios obtained in GC/P/IRMS has been thoroughly studied using several flavor compounds, such as 5-nonanone, linalool, menthol, linalyl acetate and 4-decanolide. The requirement of "conditioning" the pyrolysis reactor to obtain reliable delta(2)H(V-SMOW) values is discussed. Furthermore, the influence of the carrier gas flow of the gas chromatograph on the completeness of pyrolysis and subsequently on the delta(2)H(V-SMOW) values is investigated in detail. The linear range of the compounds investigated is determined. The results show that calibrating the GC/P/IRMS system with secondary standard substances is absolutely necessary in order to obtain reliable delta(2)H(V-SMOW) values. In view of interlaboratory comparability, validation procedures are recommended.
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