Quantitative mass spectrometry in clinical chemistry

As has been demonstrated, mass spectrometry provides a powerful analytical tool for the accurate measurement of small amounts of substances in a complex biological matrix. In our laboratory this technique is used as a reference method for measuring the routine clinical chemical parameters creatinine, uric acid, cholesterol, total glycerol and the hormones cortisol, testosterone, oestradiol-17β, oestriol, progesterone, aldosterone and thyroxine in human serum. In general, the analytical procedure for measuring a substance by isotope dilution mass spectrometry (IDMS) consists of the following steps:

1. Addition of a certain amount of the isotopically labeled analyte to the serum sample.
2. Isolation and purification of the labeled and the non-labeled endogenous analyte from the biological matrix.
3. Derivative formation of the isolated and purified labeled and non-labeled compound.
4. Selected ion recording of characteristicm/z values of the labeled and non-labeled analyte using combined gas chromatography-mass spectrometry (GCMS).
5. Calculation of the concentration of the analyte from the isotope ratio measured by GCMS.

The methods described here are now routinely in use for the quality control scheme of the Deutsche Gesellschaft für Klinische Chemie for assessing target values in external quality control sera. The reference method values obtained by IDMS provide a reliable basis for evaluating and comparing the results of collaborative surveys.