The use of cotton balls to collect infant urine samples for 2H/1H and 18O/16O isotope ratio measurements.

To test the validity of cotton balls as a urine collection medium for hydrogen and oxygen isotope ratio measurements, we moistened cotton balls with 1-, 5-, and 10-mL aliquots of urine that contained either natural abundances or one of two enriched levels of H-2 and O-18. The liquids were then expressed from the cotton balls, either at once, or after 30 or 60 min of atmospheric exposure, and analyzed for their H-2/H-1 and O-18/O-16 isotope ratios. The H-2 abundances of the fluids expressed from the cotton balls were significantly lower than the original values. This dilution of H-2, however, diminished in the samples that had a greater volume of urine. We observed no effects of volume or time on O-18. Our results indicate that at low urine volumes, ambient moisture dilutes H-2 in the cotton balls, but isotope exchanges and evaporation have little or no effect on the hydrogen and oxygen isotopic abundances of the urine samples. Total body water and energy expenditure values calculated from the H-2 and O-18 enrichments of 5- and 10-mL urine samples were within 1% of the theoretical values. Therefore, cotton balls are suitable for collection of infant urine samples for hydrogen and oxygen isotope ratio measurements, if the volume of urine that can be expressed from a cotton ball is greater-than-or-equal-to 5 mL.