Solid Phase Extraction Mechanistic Studies of the Ag(I)-DMABR Complex: Improving Efficiency of the C-SPE Standard Method of Analysis

Aqueous silver(I) is added at trace levels (0.1 – 1.0 mg/L) to spacecraft potable water as a biocide. Development of a method that can be deployed on orbit and in future Lunar and Mars missions is therefore central to maintenance of safe drinking water and crew health. To address this need, our laboratory has created an analytical technique that couples a selective sorption process based on solid phase extraction (SPE) with the quantitative measurement of the extract by a hand-held diffuse reflection spectrophotometer. This technique, referred to as colorimetric-solid phase extraction (C-SPE), enables the low level detection (limit of detection ∼5 ppb) of silver(I) by metering 1.0 mL of a water sample through a reagent-impregnated (i.e., 5-(p-dimethyl-aminobenzylidene)rhodanine, DMABR) SPE membrane. The total workup time for the analysis is only 60-90 s. Moreover, the effectiveness of this method has been demonstrated by microgravity simulations through comparisons to concurrent ground-based analyses of companion samples.

In keeping with the mission of our research, we are constantly refining the efficiency and effectiveness of our methodology. One pressing problem when using C-SPE in a microgravity environment is the presence of dispersed air bubbles that can lead to errors in exact measurement of sampling volumes. Our current protocol requires the withdrawal of more than the specified volume of analyte into a syringe and swinging the syringe in an arc to force the entrapped air to the tip. The air and excess sample are expelled into a waste container before ejecting the analyte through the extraction cartridge. The ideal situation would involve filling a syringe without concern for the exact sample volume, and this can be accomplished by applying the principles of negligible depletion (ND) in conjunction with C-SPE measurements. To determine if current C-SPE protocols could be manipulated to our advantage, a series of experiments were performed to gain a fuller understanding of the role of DMABR in complexing silver(I). Currently used SPE membranes were found to be highly efficient for retaining DMABR, suggesting that reagent loadings may be reduced without compromising extraction performance, posing the potential to adjust the amount of reagent in a membrane to manipulate ND volumes.