A very sensitive method for the determination of xylose in plasma has been developed, which involves condensation of phloroglucinol with the furfural released from xylose by the action of hot hydrochloric acid-acetic acid. In the standard method duplicate analyses of plasma samples containing 0·667–2·667 mmol l–1 of xylose can be achieved by using 0·2-ml samples. A modified method using 0·1 ml of whole blood can also be used. The absorbance obtained on analysing solutions containing 1·333 nmol l–1 of xylose is 0·372 at 554 nm using cuvettes of 10-mm light path. This value corresponds to a molar absorption of approximately 33 000. By using hydrobromic acid-acetic acid an even higher molar absorption of approximately 39 000 is obtained. Glucose gives only a weak reaction under the test conditions, 417 nmol of glucose giving an absorbance equivalent to 7·6 nmol of xylose.
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