Please help a struggling non-chemist! As part of a joint archaeological/geological research project, I am trying to detect (amongst other things) copper in lake sediments using ion chromatography and am coming up against some problems. I can find no reference for retention times of copper and there is no clearly defined peak when I run standards. I do have a peak very soon after the system peaks (at about 2.5 mins), but the software doesn't pick this up. Perhaps it's interpreted as a second system peak? It only appears when I know there is copper present.

I am using a "Metrohm 792 Basic" instrument with an eluent of 5mMol tartaric acid, 0.09mMol dipicolinic acid and 1mMol ascorbic acid.

Can anyone give me any tips?

Cheers,

Adam

Sounds to me like you just need to tweak your data system parameters to integrate the peak of interest.

shoot two levels of known concentrations of Cu and see if the suspected peak is copper. Do you have a spectrophotometer? There are good colorimetric methods for low level copper.

Is the lake sediment anerobic? If so there will be sulfide, and if there is sulfide the copper will be present as copper sulfide which is insoluble in all but strong acids. Copper also may be bound to humic stuff and will need to be released by digestion with acid. In fact, the more I think about this problem the more complicated it becomes. do you want total copper in sediment? If so some sample treatment to release it is essential. Soluble copper in sediment makes no sense as it wouldn't be "sediment"! You need a chemist on this project. I would volunteer but I am headed to Colorado hiking tomorrow.

Adam,

I checked the Metrohm web site myself and was surprised to find that there was no mention of copper separations using the analytical system commonly applied to metals termination. I suspect this is because of the properties of copper with the eluent system generally utilized. The second formation constant for the copper-PDCA complex is 16.52! This is significantly larger than for the other metal ions commonly analyzed with in this eluent system. Since the complex is an anion and the formation constant is large, this will result in a fairly low retention time on a cation exchange column. The early peak you report may well be the copper ion retained because of the fact that the complex is an equilibrium with free copper ion. I suspect you can correct the problem by eliminating the PDCA from your eluent system. You might need to offset the removal of this potent chelating agent by increasing the amount of tartaric acid modestly. I suspect, however, that you'll have a difficult time detecting copper at real world concentrations using this analytical system. Generally, post column addition of PAR to form a colored complex with metal ions prior to visible wavelength detection is the preferred method of detecting metal ions via ion chromatography.

I have never seen a geology lab without a flame AA unit. This would be the the preferred method for the system that you are suggesting unless speciation is the goal of the analysis.

Many thanks for the helpful responses.....have now ditched the IC idea and will use a spectrophotometer for copper.

Cheers,

Adam