Use of tap water samples for monitoring the geographical variation of stable isotopes used in authenticity studies
1997
Foerstel, H. | Hoube, J. | Huetzen, H.
Analysis of tap water samples is the best and most easily obtained information for estimating the pattern of stable isotope ratios in water across large areas. Usually such samples are not supplied continuosly, but have to be collected and therefore stored. Storage of such water samples of about 20 ml volume in high-density polyethylene (HDPE) vials for 15 years affected the 18(O) / 16(O) ratio, depending on the tightness of closure, thus resulting in enrichments of up to about + 0,2% . Initially a set of 900 samples across the old federal states was collected and measured, first in 1979 and then a selected number were measured again in 1990 and in 1995. The selected set of water samples was also used to determine the D/Hratio in 1995. The D/H ratio did not change in those stored samples, which maintained an only slightly altered 18(O)/16(O) ratio.Therefore, the D/H/ratio is more suitable for characterizing the origin of stored water samples. Additional fresh samples from identical or comparable sites were supplied after 15 years, confirming the overall constancy of the isotopic pattern in the water supply of the old federal states. Since the geographical pattern of the water intake has changed, about half of the samples were not of identical, but only of comparable, origin. Nevertheless they showed a similar pattern to samples of identical origins collected 15 years previously. Thus, a set of limited tap water samples will be sufficient to represent the isotopicvariation of water across a large area if influences of the origin (e.g. river bank filtrate) and treatment (blending) can be checked. Since tap water is commonly used by the local food industry, its isotope composition can be used to trace water-containing material to its origin in order to check its authenticity.
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