Folks,
Please, don't forget that we are dealing with ASW samples, not NSW. Copper level in all ASWs going to be in 100-200 ppb range, while it's 0.2-0.3 ppb in NSW. The same goes for the rest of "trace" metals: we are going to get hundreds and thousands times higher concentrations of them in ALL salt mixes comparatively to natural sea water (see Atkinson and Bingman, 1999)! I find it difficult to believe that any salt mix which cost $50 per 50 lb (200 G bucket) would contain less heavy metals than analytical grade sodium chloride from 'Sigma', let's say, with the cheapest one being at $150/50 lb (this particular one would give us about 100 ppb of copper if dissolved to give seawater salinity).
Please, don't forget that we are dealing with ASW samples, not NSW. Copper level in all ASWs going to be in 100-200 ppb range, while it's 0.2-0.3 ppb in NSW. The same goes for the rest of "trace" metals: we are going to get hundreds and thousands times higher concentrations of them in ALL salt mixes comparatively to natural sea water (see Atkinson and Bingman, 1999)! I find it difficult to believe that any salt mix which cost $50 per 50 lb (200 G bucket) would contain less heavy metals than analytical grade sodium chloride from 'Sigma', let's say, with the cheapest one being at $150/50 lb (this particular one would give us about 100 ppb of copper if dissolved to give seawater salinity).