THE I~ETEKMINATION OF N_4NOGlL4M AMOUNTS OF CHROMIUM IN URINE 13Y X-RAY I~LUORESCENCE SPIXTROSCOl'Y * Chromium lias been recognized as an essential trace elcmentl in glucose metabolism. Recausc cl~romiuin is normally present in biological materials at concentrations of IO--100 ppb.,
sensitive and specific methods are required for its determination. A review of the analytical chemistry of chromium~~3 points out the difficulty of obtaining good analytical data at these low levels.
Practically all separation procedures suggested in the literature require that clnomium be hexavalent, a state that is less stable than the tervalent state. The tcrvalent state forms a stable chelate on boiling with an excess of S-quinolinol~+; the chelate is readily extracted into chloroform. \Vliile all other oxine chelates, except cobalt(III), are readily clecomposed by 3 M hydrochloric acid, the chromium-(III) &elate is unaffected and provides a ready means for separation of chromium from other oxinates. This preconcentration method is used in combination with the X-ray milliprobe to determine nanogram amounts of chromium in urine.
Although it is known that for small samples S-ray fluorescence spectroscopy with curved crystal optics is about an order of magnitucle more sensitive than flat crystal optics, only a few applications for the actual determination of submicrogram amounts have been published 81". The reasons for this limited applicability are twofold. First, the sample must be confined to the area of focus, generally 2-4 mm in diameter. Second, the element must be nearly free of dilution by any reagent. The latter restriction requires the removal of excess of reagent after separation has been accomplished.
EXPERIMENTAL
Reagents
All chemicals were of reagent-grade purity. Water, nitric acid, sulfuric acid, hydrochloric acid, acetic acid, ammonia and chloroform were redistilled with all-glass stills and stored in new glassware to ensure that there was no contact with chromic acid. The S-quinolinol (oxine) was used in solid form.