THE USE OF COTTON BALLS TO COLLECT INFANT URINE SAMPLES FOR H-2 H-1 AND O-18/O-16 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 ba lls with 1-, 5-, and 10-mL aliquots of urine that contained either nat ural abundances or one of two enriched levels of H-2 and O-18. The liq uids were then expressed from the cotton balls, either at once, or aft er 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 expre ssed 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 tim e on O-18. Our results indicate that at low urine volumes, ambient moi sture dilutes H-2 in the cotton balls, but isotope exchanges and evapo ration have little or no effect on the hydrogen and oxygen isotopic ab undances 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 u rine samples were within 1% of the theoretical values. Therefore, cott on balls are suitable for collection of infant urine samples for hydro gen 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.