A Field Method For Assessing Extracellular Water By Portable Xrf Bromine Analysis:
Validation Against Instrumental
Neutron Activation

Joseph J. Kehayias and Ion E. Stamatelatos^*

USDA Human Nutrition Research Center on Aging at Tufts University

^*Institute of Nuclear Technology and Radiation Protection, NCSR Democritos, Athens, Greece

Nutritional status can be evaluated by monitoring changes in body composition, including the depletion of protein and muscle (in vivo neutron activation analysis), adipose tissue distribution (in vivo fast neutron inelastic scattering), changes in hydration status (isotope dilution), bone mineral (photon absorption) and cell mass (whole body K-40 gamma counting).

Extracellular water space (ECW) is expanded in acute illness or in catabolic states. This change in extracellular hydration represents loss of cell mass and muscle. It provides a simple indirect method for monitoring the nutrition status of the elderly and diseased. ECW can be measured by Br dilution. A small amount of natural Br (typically 17-23mg per Kg of body weight) is given orally. The concentration of Br in plasma is measured prior and 3 hours following the administration. Plasma is analyzed for Br either by ion-exchange chromatography, or by non-destructive methods, such as instrumental neutron activation analysis (INAA) or X-ray fluorescence (XRF), which require no sample preparation.

We developed a method for the rapid analysis of Br in plasma on the field with a portable, hand-held, battery operated XRF instrument consisting of a 30mCi Cd-109 source and an electronically-cooled solid-state X-ray detector. 128 plasma samples were analyzed (64 baseline and 64 post-dosing) from 64 volunteers aged 19 to 84y. The samples were stored in 1cc polyethylene vials and were measured first with the XRF instrument and then by INAA without transferring the plasma or even opening the vials.

The nuclear reactor of the research center “Democritos” was used to measure plasma Br concentration by the ⁷⁹Br(n,g)⁸⁰Br neutron capture reaction. The samples were transferred to the irradiation position by a pneumatic system and were exposed to 2x10¹³n/cm²/s for 3 min. Detection of the ⁸⁰Br (t_1/2= 17.8 min) gamma-ray (616.2 KeV) followed 3 min after the irradiation using a liquid nitrogen-cooled purified Ge photon detector.

Using the INAA as the reference technique we found the XRF instrument to be highly correlated with the neutron activation measurements (R= 1.000 for 7 standards, R = 0.997 for human plasma samples). We also observed that, in our group of volunteers, ECW was able to assess nutrition status as defined by the quality of lean body mass principle (potassium content of fat-free mass). We concluded that portable XRF provides an acceptable non-destructive method for rapid clinical ECW assessment, without the complications associated with sample handling and preparation. This simple method makes body composition measurements widely available as the main outcome of nutritional and pharmaceutical anabolic interventions.