Validation of deuterium and oxygen18 in urine and saliva samples from children using on‐line continuous‐flow isotope ratio mass spectrometry

The doubly labelled water method is valuable for measuring energy expenditure in humans. It usually involves blood or urine sampling, which might be difficult in neonates and children with cerebral palsy or other disabilities. We therefore aimed to validate a method making use of saliva samples analyzed by automated thermal conversion elemental analyzer in combination with isotope ratio mass spectrometry (TC‐EA/IRMS). The subjects received labelled water orally and urine and saliva samples were collected and analyzed. Deuterium as well as oxygen¹⁸ was measured in one single run using a peak jump method. Excellent linearity was found for measurement of enrichments of deuterium (R² = 0.9999) and oxygen¹⁸ (R² = 0.9999). The intra‐assay precision and the inter‐assay precision of the measurement of two standards were good for both deuterium and oxygen¹⁸. The variation between urine and saliva samples was small (4.83% for deuterium and 2.33% for oxygen¹⁸ n = 40). Saliva sampling is to be preferred, therefore, as it can be easily collected and is non‐invasive. Moreover, its time of production is almost exactly known. The TC‐EA/IRMS method is a good alternative to the more laborious off‐line IRMS measurements. Copyright © 2009 John Wiley & Sons, Ltd.