Detection of metal vapor atoms in bubbles at room temperature

One of the largest obstacles in miniaturizing traditional atomic spectroscopic sources is the need for a thermal/electrical source for free atom production. A single article in the literature has demonstrated atomic absorption detection of Ag, Cu, and Pd in aqueous solution at room temperature for atoms in the gas phase, which may ultimately permit miniaturization. Unfortunately, several laboratories have found that reproducing the phenomenon has been difficult. Without a sound fundamental explanation of the processes leading to the signal, one must conclude that it can be done, but some unsuspected and unknown design/methodological nuances are responsible for only a single reported success. Gas phase atoms could exist at room temperature “in solution” if the atoms were trapped in very small bubbles. A simpler system containing Hg vapor within a single 500 µL bubble was first studied using atomic absorption measurements. The use of experimental data and computer simulations revealed that metal transport out of bubbles suffers from slow diffusion through solution and limited solubility of the elemental species. Absorption signals for Hg vapor decayed over thousands of seconds, with slower decay rates associated with solutions higher in metal concentration and reducing power. Submicron sized bubbles were created in a flow-through cell during mixing of a 20% ethanol solution containing a reducing agent with Pd in 2% HCl. A repeatable atomic absorption signal was produced using this method. Replacement of ethanol with 1-propanol and use of a surfactant enhanced the signal through generation of more bubbles with lower internal pressures present. Limits of detection of ca. 100 ppb in Pd were achieved, although it is estimated that about 0.4% of the Pd initially added is contained within the bubbles as gaseous atoms. Further discussion includes exploration of the fundamental processes present in a procedure that delivers a repeatable signal.