Field Filtration as Best Practice
Co-Precipitation of Dissolved Metals with Iron
ALS Co-Precipitation Impact Study
ALS recently studied the impact of iron precipitation on the co-precipitation of other metals with a series of high-iron groundwater samples and found substantial losses of most dissolved metals (in comparison to field filtered metal concentrations), with losses up to 100% in several cases where filtration was delayed (by 6 to 10 days in these somewhat worst-case examples). Arsenic, Lead, and Cadmium were particularly impacted by co-precipitation, in addition to the expected loss of Iron, but most dissolved metals were impacted to some degree.
A summary of the ALS study results for Arsenic, Lead, Cadmium, and Iron are shown in Figure 1. For these 5 samples, Arsenic losses averaged over 80%, Lead losses averaged over 95%, and Cadmium losses varied significantly by sample, ranging from zero to 97% loss. The impact on sample results can clearly be seen, with many of the test samples potentially having key metal concentrations underestimated by a factor of 10 to 100 times if correct field filtration techniques were not employed. This study also highlights the necessity of field filtration for the analysis of dissolved Ferrous Iron.
Recommended Field Filtration Techniques
Disposable syringes and syringe filters that have been proofed for suitability and/or recommended by the laboratory are essential and low cost syringe filters and syringes are available that are also suitable for ultra-trace level metals analysis. Using a 60 mL disposable syringe and syringe filter is very convenient, especially when used with smaller 60 mL sample bottles, which minimizes the amount of sample to be filtered, and also reduces the likelihood of having to use multiple filters for highly turbid samples.
When larger volumes of water are required, inline filtration may be more cost and time effective, but careful attention to recommended rinsing volumes to reduce contamination from the filter itself is essential. ALS studies have shown that pre-rinsing these inline filters with a minimum of 1L greatly reduces the likelihood of contamination from the filter being seen at ultra-trace levels.