Treatment D. MW sample digestion was done by mixing maximally
0.4 g of dry sample, 3 mL of H2O (when using wet samples the water
content has to be into consideration), 0.75 mL of concentrated HNO3
and 0.15 mL of concentrated HCl in a Teflon vial. This exceeds the 2
M HNO3 minimal limit as described by Walter et al. (21) and the use
of similar acid content/sample ratio as recommended by Knapp et al.
(22). The sample was digested to a transparent solution in the MW
oven by a continuos temperature program reaching 180 °C in 15 min;
this temperature was kept constant for 20 min. After this treatment the
sample was cooled to room temperature, and the sample was decanted
into a test tube with the 10 mL volume level premarked. The Teflon
vial was washed, and the washing water and sample liquid were pooled
in the test tube and diluted to a final volume of 10 mL. Before injection,
0.1 mL of ascorbic acid (20 g/L) was added to 0.9 mL of sample. If
necessary to reduce the negative consequences of the acid content for
the chromatographic performance, the sample was dried and dissolved.
It was dried to complete dryness by streaming 0.22 ím of filtered air
overnight. To the dried sample were added 0.1 mL of 0.5 M HCl, 0.1
mL of ascorbic acid (20 g/L), and 0.8 mL of H2O. This solution was
sonicated for 15-30 min. The sample was transferred to a 1.7 mL
microcentrifuge tube, and finally centrifugation at 12000g during 5 min
completed the treatment before injection. Normally 50 íL of sample
was injected for analysis.
The development of the sample treatments aimed at maximizing the
possible injection volume while maintaining an excellent chromatography.
This objective was accomplished by maximizing the sample
amount and minimizing the amount of acid for sample preparations.
For samples high in mineral content, a simple water dilution can be
applied prior to injection to reduce the effects of acid concentration.
Standard solutions originally of 1000 ppm concentration were prepared
by diluting the standards to 1000-5 ppb in the solution of 0.05 M
HCl and 2 g of ascorbic acid/L.