Contamination From Reagents

High Purity Water

Sample preparations using acid digestion, fusion, or ashing all typically use water as the primary reagent. Most water used in trace metal laboratories is produced by systems that use ion-exchange purification. This water is commonly referred to as "conductivity water" because its conductivity approaches the theoretical conductivity of water (0.055 microhm / cm {18.2 megohm water} at 25°C).

Under normal laboratory conditions, conductivity water never measures to be 18.2 megohm due to the presence of CO₂ (H₂CO₃HCO₃^- + H⁺). Furthermore, if you could find water giving a conductivity of 18.2 megohm, it is not necessarily free of trace elemental contaminants because only ionized compounds are detected by conductivity measurement.

Through carefully controlled experiments and measurements in clean room facilities, we have found that conductivity water will typically give readings closer to 16 megohm and that it is free of trace metallic impurities down to conventional ICP-MS and axial view ICP-OES detection limits. We have also found that sub-ppb level impurities that were once thought to be coming from the water, are in actuality from the atmosphere and the container materials (see earlier parts of this guide). We have found that the use of clean room facilities and high temperature nitric acid-leached LDPE bottles are necessary for reliably measuring common contaminant elements in water. Therefore, do not assume that your water has significant levels of elemental contaminants if it gives conductivity readings between 16 and 17 megohm and your ICP-MS or OES is detecting trace levels of the common environmental contaminants.

Was this helpful?  Tell a Friend | Let us know