N-15 SAMPLE PREPARATION FOR PROTEIN-TURNOVER MEASUREMENTS IN LARGE POPULATIONS

The major limitations to the adoption of whole body protein turnover measurements to assess protein metabolism in large groups have been the time-consuming nature of sample preparation and the cost and complexity of mass spectrometric tracer analysis. The introduction of continuous flow isotope ratio mass spectrometry has greatly improved sample throughput and reduced costs for N-15 analysis in purified metabolic products, such as ammonium. However, purification of urinary ammonium is not sufficiently simple or rapid for the large sample numbers generated. This study compares separation of urinary ammonium by using a new immobilised cation-exchange resin apparatus with Conway microdiffusion, using KHSO4 as an acidic ammonium trap that is compatible with automated combustion analysis. The alternative methods gave equally good analytical precision. It is concluded that microdiffusion would be superior in minimally equipped field laboratories, whereas the design and speed of the method using an immobolised cation resin would make this the method of choice in laboratories with a higher degree of automation.