It is very useful scientific investigations and in clinical evaluations to determine with a high degree of accuracy the concentration or relative concentrations of isotopes.  For example, by substituting a tracer isotope for a naturally occurring isotope in a molecule the effect of subsequent processes on the molecule can be investigated.  Such studies require precise measurements of isotopes. 
The predominant method of isotope measurement had used radioactive isotopes.  The drawbacks of that method include a limitation on the isotopes available for used, the effects of radioactivity on subjects exposed to that material, and the problem of radioactive waste disposal. 
A variety of conditions that can be examined with harmless stable isotopes suggest the great potential of the present invention to clinical applications.  A few, but by no means complete, examples of clinical tests taken from literature to which the present invention can be applied are:
A. Fat malabsorption and the underlying cause (intestinal mucosa defects, bilary obstruction, liver disease, pancreatic enzyme deficiency)--monitoring expired CO.sub.2 after oral administration of labeled fat and/or labeled free fatty acid. 
B. Ileal Dysfunction--monitoring breath CO.sub.2 following injection of labeled bile acid. 
C. Small-intestine Bacterial Overgrowth--increased labeled CO.sub.2 excretion following intake of labeled .sup.13 C-xylose indicates this condition. 
D. Liver Dysfunction (cirrhosis) and Liver Cancer--test based on the conversion of labeled .sup.13 C-galactose to expired CO.sub.2. 
It is evident that stable isotopes could beneficially be used in place of radioactive isotopes if there were a suitable measurement method and apparatus for stable isotopes.  Indeed the use of stable isotopes in biomedical as well as general scientific and engineering applications has been hindered by the lack of a direct, simple and inexpensive measuring method.  The present isotope ratio mass spectrometry is lacking in, specificity, ease of operation and cost effectiveness. 
To take advantage of the benefits of stable isotope analysis there is need for a simple inexpensive portable clinical system that can be used for routine diagnosis.  Such a system would for the first time enable most hospitals to perform stable isotopic tracer analysis.