Standards for multi-element IRMS within a single run

Full text: The fulfilment of the precautions for the traceability of food and other products within the global market rapid and safe methods must be available. One has not only to confirm or reject the geographical origin, but in most cases to verify the identity of different batches. The bid companies order large quantities at low prices which can often not be supplied, in many cases not within the range of quality expected. The increasing market of bio-products from organic (ecological) production necessitates control also. The natural variation of the stable isotopes within their natural biogeochemical cycles and local turnovers results in regional and local patterns, which are compared to other methods as 'isotopic fingerprint'. This means that the material has a property given by nature during its production which remains unaltered. It is a physical property not influenced by chemical reactions. To use this property as an instrument of usually applied control it has not to fit international accepted methods, ring tests and proficiency test, but must be able to handle a large amount of samples quickly and safely. Two bottlenecks will be observed in daily routine: the sample preparation and the measurement itself. The sample preparation has be done carefully for only small quantities of material within the range of milligram are used for the measurement (including the generation of the sample gas and the isotope ratio mass spectrometry). For the modern open-split method (combination of elemental analyser or high temperature combustion) which has a rapid turnover of samples, only small quantities of gas are needed for the preparation site under normal pressure to the analyser under high vacuum are connected by a capillary only, replacing the former valve inlet system. Thus the aliquot measured has really to represent the whole sample. Consequently each standard has to be very homogeneous, more or less a fine powder. Then static problems during weighing out may occur. Another improvement is the opportunity to determine up to three isotopic ratios within a single run. This saves much time of labour (to weigh out the sample and to measure it). There has one problem to be solved. The portion of the element forming the biomass differ. Biomass consists of about 50 % carbon, but only 2 % nitrogen and less sulphur. Introducing the original outflow from the GC-column into the analyser will lead to exceeding peaks for at least one element. This problem can be solved by a diluting the gas outflow, e.g. that of carbon dioxide. Usually the IT programmes are also not suitable for multi-element measurements and must be adapted. Another problem are the standards. The number of standards to be distributed internationally and regularly is limited. The former idea to have a pair of standards for each matrix (food, juice, wine, etc.) available cannot be realized, because of many limitations, as homogeneity, storage, spoiling. One should also have standards which can be used as well for isotope ratio determinations as for the elemental analysis (content of an element of the sample). Therefore we will present our experience with a synthetic standard which has been calibrated against the international standards. In this case we have only one material, for further application would should use a pair of different isotopic composition. Their range should fit the natural scale. The material tested is 2,5.Bis-(-tert.-butyl-benzoxazol-2-yl)-thiophen. It sounds to be a complicated molecule but is not only available in larger quantities, is a homogeneous powder, not static charging, stable, non-poisonous and most important: It contains all the elements in quantities close to natural compounds except sulphur. But to hit the natural sulphur content the molecule should be much larger and perhaps not easily to synthesize. The elementary composition is: 72.46% carbon, 6.5 % nitrogen, 7.43% oxygen, 7.45% sulphur and 6.02% hydrogen. It has been tested as well in an EA-IRMS run including the stable isotope ratios of carbon, Nitrogen and sulphur within one run lasting about 12 minutes as in one run for organic hydrogen, organic oxygen and nitrogen by pyrolysis. Except for sulphur the isotope ratios are within the range of natural variation. A second problem is the lack of a second standard for 18O-enriched water to calibrate the scale depression. This standard is needed for metabolic studies, e.g. in humans. (author)