The Positionalδ(18O) Values of Extracted and Synthetic Vanillin

Abstract

The positional d( 18 O) values of vanillin (1) of different origins have been determined from the global values of 2-methoxy-4-methylphenol (4), obtained from 1 upon Clemensen reduction, and of 3-methylanisole (5), obtained from 4 by removal of the phenolic O-atom. By these means, it is possible to differentiate samples of 1 of synthetic origin from those extracted from Vanilla plants or produced from lignin by chemical oxidation. The main difference between the samples derived from guaiacol and those possessing the aromatic moiety of natural origin is in the enrichment values of the O-atoms at C(3) and C(4), while the extractive materials from the pods are distinguished from the product from lignin on the basis of the carbonyl oxygen d( 18 O) values, ranging from 25.5 and 26.2 in the natural material to 19.7½ in the lignan-based sample. The values for the phenolic Oatom vary from 8.9 and 12½ of the synthetic materials to 6.5, 5.3, and 6.3½, respectively, of the sample from lignin and the two samples from Vanilla pods,whereas the MeO O-atoms show the following values for the same compounds: À 2.9, À 3.2, 3.5, 3.1, and 2.3½, respectively. This study indicates the significance of the positional d( 18 O) values of polyoxygenated compounds for the definition of their origin.Introduction. ± The distribution of stable isotopes within the atomic species constituting an organic molecule is not statistical but rather depends upon the way in which it was formed [1]. A relevant practical analytical application of this principle deals with the determination of the origin of food components and the authentication of the natural rather than natural-identical status of flavor materials [2] [3]. Most of the measurements performed to this end have, until now, concerned the quantification of 13 C and 2 H, and comparatively little attention has been dedicated to the pattern of Oisotopes in an organic molecule. This is most likely due to experimental difficulties in the analytical determinations. Indeed, a great deal of information [4] can be drawn from the O-isotope content of a molecule, since the three primary sources for the Oatoms incorporated into an organic material, i.e., CO 2 , atmospheric O 2 gas, and ground water, possess quite different isotope enrichments. The O-isotope enrichment of a molecule is expressed as the d( 18 O) value, which is the relative difference of the isotope ratio of a compound to that of an international standard (i.e., ocean water) in ½. Accordingly, the d( 18 O) values of CO 2 , atmospheric O 2 , and ground water, the infinite reservoirs that supply oxygen to organic compounds, are: 40.3 ± 42.5½, 23.5 ± 23.8½ and À 10 ± 0½, respectively [4]. Vanillin (1), perhaps, is the organic molecule whose isotopic content has been most extensively investigated [5] [6], mainly because there are two sources for this industrially important phenolic aldehyde. The first is supplied from the costly material

Site-specific carbon isotope measurements of vanillin reference materials by nuclear magnetic resonance spectrometry

“…Isotopic analysis of hydrogen, carbon, and oxygen has been used for the authentication of vanillin products [ 4 , 8 – 11 ], with the power of this tool increasing when hydrogen and carbon isotopic compositions are combined [ 10 – 12 ]. Despite this, IRMS provides access to bulk, not to the intramolecular 13 C composition of the analyte, resulting in some limitations in the authentication of bio- or chemical-synthetic vanillin obtained from the same precursor [ 9 , 11 ] and detection of the addition of synthetic vanillin to natural vanillin [ 13 ].…”

Carbon and Oxygen Isotopic Ratio Analysis by FT ICR MS for Natural Vanillin Authentication

“…Vanillin shares the common biosynthetic phenylpropanoid derivation from C 6 –C 3 p -coumaric acid. It possesses the additional feature of bearing adjacent to the O atom initially present at C(4) and additional O atom at C(3), possibly introduced into the aromatic moiety via different activation mechanisms . Fronza et al outlined positional 18 O/ 16 O of all oxygen functionalities, carbonyl, hydroxyl at C(4), and methoxyl at C(3) for vanillin of synthetic samples from guaiacol, lignin, and two natural samples from ex-pods, one of them from Madagascar.…”

“…The main differences between the vanillin derived from guaiacol and those possessing the aromatic moiety of natural origin are in the enrichment values of the O atoms at C(3) and C(4), while the extractive materials from the pods are distinguished from the product from lignin based on the carbonyl oxygen δ( 18 O) values ranging from +25.5 and +26.2 in the natural material to +19.7‰ in the lignin-based sample. 5 However, it has been reported that measuring the oxygen-18 content of guaiacol is a better authentication tool than δ 18 O of bulk vanillin because it does not suffer the drawback of being altered by the chemical exchange of the sp 2 oxygen atom with water in industrial or laboratory procedures. As outlined before, guaiacol can be easily obtained in a reasonable time, becoming an interesting isotopic probe for carbon and oxygen isotope ratio mass spectrometry determinations.…”

“…Early studies pointed out the great potential of information to determine the positional δ 18 O values of chemically identical materials of different origins. The main differences between the vanillin derived from guaiacol and those possessing the aromatic moiety of natural origin are in the enrichment values of the O atoms at C(3) and C(4), while the extractive materials from the pods are distinguished from the product from lignin based on the carbonyl oxygen δ­( 18 O) values ranging from +25.5 and +26.2 in the natural material to +19.7‰ in the lignin-based sample . However, it has been reported that measuring the oxygen-18 content of guaiacol is a better authentication tool than δ 18 O of bulk vanillin because it does not suffer the drawback of being altered by the chemical exchange of the sp 2 oxygen atom with water in industrial or laboratory procedures.…”

“…Vanillin is the organic molecule whose isotopic content has been most extensively investigated, mainly because there are several sources of this industrially important spice. 5 Several studies have been devoted to the authentication of vanilla, as the natural product extracted from beans of Vanilla planifolia is worth a lot more than its synthetic or semisynthetic counterparts. 6 Isotope ratio mass spectrometry (IRMS) was first used for distinguishing between natural vanillin and those originating from other sources since vanilla plant photosynthesizes following the Crassulacean acid metabolism (CAM).…”