DIFFERENT FIELDS OF APPLICATIONS - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 332

After either reaction 5.1 or 5.2, the hexafluorosilicic acid is decomposed
according to

H 2 SiF 6 Ç SiF 4 2HF (5.3)

The volatile fluoride is evaporated by further heating the PTFE vessel at
100 ° C for 2 h, thereby reducing the volume of the solution. After evaporation
to dryness and cooling, the residue is taken up with 1 ml diluted HNO 3 (10%,
suprapure) and spiked with an internal standard (Rb or Ga). A portion of
10-50 μ l is transferred to a cleaned but not siliconized quartz-glass carrier,
dried, and analyzed by TXRF.
If impurities of TiO 2 or Al 2 O 3 are present, the dissolution will be
incomplete. But at any rate, it will lead to a usable fine suspension (grain
size < 1 μ m). Table 5.5 shows the results found for the certified reference
material BCS-CRM 4 313/1“High-Purity Silica”[118]. The TXRF values agree
quite well with the reference values. Beyond that, about 20 additional elements
could be detected. Detection limits were estimated from the blanks and
reached a level of 0.1 μ g/g. Only about 1 μ g silicon of the original matrix of
100 mg was found to be left. Thus, trace elements were enriched by a factor of
about 10 5 .

5.4.3UltrapureAluminum

For the technical evaluation of ultrapure aluminum, about 20 trace elements
needed to be determined in the range of ng/g to μ g/g. To this end, a multistage
procedure had to be applied consisting of the digestion of the solid, the
adsorption of trace metals as their hexamethylene-dithiocarbamates
(HMDTC) on a reversed-phase cellulose, and the elution of the collected
trace metals [116]. Detection limits of some 10 ng/g could be reached by
subsequent TXRF.
In this method, 2 g of aluminum is first dissolved in 25 ml HCl (30%,
suprapure) by heating at 100 ° C for 4 h. The resultant solution is diluted
with ultrapure water to a volume of 75 ml. The pH value is adjusted to
2.5-3 by addition of 5 ml NaOH in order to prevent hydroxide precipitation.
This solution is mixed with 5 mg HMDTC salt (E. Merck, Darmstadt,
Germany) and dissolved in 100 μ l methanol p.a. The final solution is pumped
through a small glass column (inner diameter 5 mm) filled with a suspension of
0.1 g acetylated cellulose in 10 ml ultrapure water. A peristaltic pump is used at
a rate of 2 ml/min, so that leaching requires 40 min.
Afterward, the trace metals loaded on the column are slowly eluted by 0.1 ml
methanol, by 1 ml nitric acid (25%), and lastly by 0.9 ml ultrapure water. The
eluate of 2 ml is spiked with an internal standard, for example, gallium at

4 BCS stands for British Chemical Standard - Certified Reference Material.