Quantitation by Isotope Dilution

The contention that there is a lack of 26-hydroxylase activity in the liver of patients with CTX was recently supported by an investigation by Javitt et al. [194]. 26-Hydroxycholesterol in serum was quantitated by isotope dilution-mass spectrometry in normal subjects and in 5 subjects with CTX. The CTX patients had no detectable or markedly reduced levels of 26-hydroxycholesterol in their serum. 

Isotope dilution mass spectrometry (cf. (Heumann, 1992 Yu et al., 2002)) has two main requirements. The first is that the element being analyzed must have more than one isotope. The second is to have a well-characterized and pure tracer solution that has a significantly different isotopic composition from the element under analysis. In practice, a known amount of the tracer is added to the sample, which is then treated by any necessary chemical separations before being inserted into the mass spectrometer. The tracer must be isotopically equilibrated with the sample by forcing them into a common valence state, as discussed in Section 4.7. For elements with multiple valence states (such as uranium or plutonium) this is a crucial requirement. Failure to achieve isotopic equilibration will lead to erroneous results. Sample quantitation by isotope dilution can be determined by use of the following general equation ... 

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