these metals. Solid samples were weighed (approx. 0.250 g) into
pre-cleaned glass beakers, and then aqua regia (5 mL) added. The
beakers were then covered with clean watch glasses and placed on
a hotplate and boiled for at least 1.5 h. If the sample neared dryness,
a further volume (2 mL) of aqua regia was added. Sediment
and soil samples usually contain high concentrations of iron, which
could potentially saturate the column and hence lead to low recovery
of the analytes. Alternatively, at some pH values the iron may
precipitate which may also lead to the coprecipitation of analyte
ions [28]. Therefore, to obtain reliable results, the Fe must be prevented
from being retained on the column and from precipitating.
Sufficient NaF (1% m/v in the final volume of 25 mL) was therefore
added to the samples to prevent precipitation of Fe(OH)3 and
to form stable complexes with the Fe, hence preventing it from
being retained on the resin [8,9]. The samples were then filtered
to remove undigested material and diluted to volume with ammonium
acetate buffer to give a pH of 6.0, prior to being analysed using
the proposed method. The results obtained were in good agreement
with the certified values and the precisions (RSDs) for the
analytes examined were in the ranges 0.14–20.36%. The results are
summarised in Table 3.Those analytes that were at a sufficiently
high concentration such that preconcentration was not necessary
(Cu, Ni and Pb in LGC 6156 and Cu in MESS-3) were not determined
because they could be measured directly using FAAS. The results of
the experiments demonstrated that as well as providing a robust
and reliable method for the determination of ultra-trace analytes,
these resins are equally applicable to the determination of trace
analytes in digests of solid materials.
When a comparison of more than two treatment or sample
means is required, the null hypothesis to be tested is usually that
the t-treatment or sample means are the same, and that the alternative
hypothesis is that they are not [29]. Analysis of variance
(ANOVA) is a useful technique for making decisions about hypotheses.
In ANOVA it is actually the variation in the t-treatment or
sample responses that is used to decide whether or not treatment
effects are significant. The one-way ANOVA statistical method was
applied to the results obtained using the resins to decide whether
or not there are any significant effects between the different resins
and whether those effects are significant [29]. The SPSS 10.0 for
windows program (Woking, Surrey, UK) was therefore used as a
chemometric package programme. According to the ANOVA analysis,
the effect of resins on the preconcentration of the elements
were not found to be significant (P > 0.01), i.e. there was no significant
difference between the results obtained and the certified
values.