Determination of gold in natural waters at the parts-per-trillion (pg cm −3) level

Abstract

A combination of solvent extraction and flameless atomic absorption spectrometry was used to determine gold at the parts-per-trillion (pg cm −3) level in natural waters. The method consisted of a two-step extraction procedure where Au was complexed as HAuCl 4 in the presence of hydrochloric acid and extracted into a small volume of methylisobutyl ketone (MIBK), where a favourable distribution ratio ( D = 1250) exists. In stage 1 of the extraction, 2 l of water was acidified with 70 cm 3 of 6 M redistilled HCl and shaken with 55 cm 3 of redistilled MIBK. At equilibrium, the volume of solvent was reduced to ∼ 5 cm 3 and extracted 76% of the Au from the water. In the second extraction stage, the MIBK extracts were further concentrated by evaporating to dryness with a rotary evaporator in the presence of ∼ 5 cm 3 of the original extracted water. Standards were prepared in the same manner as the samples of natural waters. The Au was re-extracted into 0.2 cm 3 of MIBK. With a total concentration factor of ∼ 10 4, coupled with the high sensitivity of flameless atomic absorption, it was possible to determine Au levels as low as 1 pg cm −3. The technique was used to determine Au levels in unfiltered river-water samples draining Au-bearing Cambro-Ordovician sediments of the Meguma Group of eastern Nova Scotia, between Halifax and Sherbrooke. Au levels tended to decrease in a northeasterly direction and were highest (7 ppt) in the Little Salmon River draining Lake Major which is adjacent to the Montague Gold Mines.