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In the context of an epidemiological study, urinary concentrations of nine phthalic diester metabolites (monoethyl-, mono-(3-carboxypropyl)-, mono-n-butyl-, monoisobutyl-, monobenzyl-, mono-(2-ethylhexyl)-, mono-(5-hydroxy-2- ethylhexyl)-, mono-(5-oxo-2-ethylhexyl)- and mono-(5-carboxy-2-ethylpentyl)-phthalate) were quantified via LC-MS/ MS. As in the majority of epidemiological studies only single spot samples were available for urine analysis, the implicit assumption in this case is, that exposure data obtained from single spot samples are representative for a longer exposure period. To validate the relevance of single spot analyses we quantified the respective intra-individual variances of urine samples collected from ten volunteers once daily over a period of 30 days. Using the values for the daily variances, approximate values for the underlying population variances in the cohort samples representing the differences between the average individual metabolite levels were calculated. For most of the volunteers, daily metabolites variations were lower, than the variations observed in the epidemiological setup. The results showed that by accounting for the contribution of daily variance, the standard deviations of the log-transformed phthalate values of the cohort samples are reduced (14% to 28%) but still larger (3% to 66%) than daily standard deviation values, with the exception of MCPrP concentrations.
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