strength on the extraction of gold and silver was studied by varying
the concentration of NaNO3 within the range of 0–0.5 mol L−1. The results
showed that addition of salt has no significant effect on extraction
efficiency.
3.6. Effect of sample volume and IL amount
Enrichment factor (EF) is one of the most important aspects in
preconcentration procedure. It determines the sensitivity of preconcentration
system. Sample volume has a unique role to achieve high
EF and depends on the amount of extracting phase ([Hmim][PF6]).
Due to the fact that the IL is partially solvable in aqueous phase, for
a quantitative extraction, if sample volume increases, more amount
of IL is required. Melting point, density and water solubility of some
imidazolium-ILs containing PF6 ¯ are listed in Table 2. The effect of
different sample volumes on extraction of gold and silver at constant
amount of IL (250 μL) was examined at optimum conditions. The
results showed that the quantitative recovery (>95%) was obtained
for sample volume up to 25 mL and then decreased with further increase
in sample volume. It seems that the decrease in the extraction
recovery is due to non-reproducible solubility of IL. Based on the final
extraction volume (500 μL) and the maximum sample volume that
extraction was quantitative (25.0 mL), an enrichment factor of 50
could be obtained. Furthermore, based on the slope ratio of calibration
curves with and without microextraction (sample volume:
25 mL), an enhancement factor of 48.7 and 48.3 for gold and silver
were determined, respectively. However, a sample volume of 20 mL
and 250 μL IL were selected for subsequent experiments.